1. The dynamicevolutionary process by which biological organisms develop characteristics that allow them to survive and reproduce within their environments.
2. The state or condition reached by apopulation during that process.
3. Any character orphenotypic trait with a functional role in an individual organism and which has evolved and is maintained throughnatural selection.
TheDarwinian view that many or most physiological and behavioral traits of organisms areadaptations that have evolved for specific functions or for specific reasons (as opposed to beingbyproducts of the evolution of other traits, consequences ofbiological constraints, or the result of random variation).
The simultaneous or near-simultaneousevolutionary divergence of multiple members of a singlephylogeneticlineage into a variety of different forms with differentadaptations, especially a diversification in the use of resources or habitats.[1]
agamospecies
A species that does not reproducesexually but rather by cloning.[2] Agamospecies are sometimes represented byspecies complexes that contain somediploid individuals and otherapomictic forms—in particular, plant species that can reproduce viaagamospermy.[3]
One of multiple alternative versions of an individualgene, each of which is a viableDNA sequence occupying a given position, orlocus, on achromosome. For example, in humans, one allele of the eye-color gene produces blue eyes and another allele of the eye-color gene produces brown eyes.
The relative frequency with which a particularallele of a givengene (as opposed to other alleles of the same gene) occurs at a particularlocus in the members of a population; more specifically, it is the proportion of all chromosomes within a population that carry a particular allele, expressed as a fraction or percentage. Allele frequency is distinct fromgenotype frequency, although they are related.
The isolation of two populations of a species due to a change in breeding periods. This isolation acts as a precursor toallochronic speciation, a type ofspeciation which results when two populations of a species become isolated due to differences in reproductive timing. An example is the periodical 13- and 17-yearMagicicada species.[3]
The comparative study of the relationship between the size of an organism's body (or of a specific organ, e.g. the brain) and various other biological characteristics, such as body shape, anatomy, physiology, or behavior.
A set ofmorphological structures in different organisms which have similar form or function but were not present in the organisms'last common ancestor. Thecladistic term for the same phenomenon ishomoplasy.
Also called anancestral character,primitive character, orprimitive trait.
For a givenclade, any trait or feature (e.g. a specificphenotype) that appears in the clade'scommon ancestor; the same trait may also appear in some or all of the lineal descendants included within the clade, indicating that it has undergone little or no significant change during the clade's evolutionary history and thus retained its "primitive" condition. Some but not all subgroups within the clade may containderived traits, in which the ancestral trait has changed significantly over evolutionary time such that the original ancestral condition no longer exists. Both terms are relative: an ancestral trait for one clade may be a derived trait for a different clade. The term "ancestral trait" is often used interchangeably with the more technical termplesiomorphy.
A phenomenon by which the symptoms of agenetic disorder become apparent (and often more severe) at an earlier age in affected individuals with each generation that inherits the disorder.
Aderived character state; i.e. the state or condition of a particular trait or feature (e.g. a specificphenotype) that is distinct from and derivative of anancestral character by virtue of its modification over time in one or more lineal descendants of a givenclade. Apomorphies are often viewed as evolutionary "innovations" which set thetaxa in which they appear apart from the clade'scommon ancestor, as well as from other clades;shared apomorphies are used to construct and define clades. The term is relative; a trait considered an apomorphy in one clade may not be considered an apomorphy in a different clade. Contrastplesiomorphy.
Anapomorphy is aderived trait present in one or more members of a clade but not the common ancestor; aplesiomorphy is anancestral trait present in the common ancestor of the clade and possibly some or all of its descendants.
aptation
Any character orphenotypic trait that is currently subject tonatural selection, whether its origin can be ascribed to selective processes (adaptation) or to processes other than selection or selection for a function that is different from the current function (exaptation).[8]
The phenomenon by which thelinkage of a neutral locus to a selectively maintainedpolymorphism causes theheterozygosity of the neutral locus to increase.[8]
Also calledpositive assortative mating andhomogamy.
Amating system in which individuals with similarphenotypesmate with each other more frequently than would be expected in a completely random mating system. Assortative mating usually has the effect of increasing genetic relatedness between members of the mating population. Contrastdisassortative mating.
A modification of a biological structure whereby anancestral trait suddenly reappears after having been lost through evolutionary change in previous generations.[9] Atavisms can occur in a number of different ways, including by the re-expression of latent genes for ancestral phenotypes as a result of mutation, or by the shortening or prolongation of the time allocated for theontogenesis of a particular trait during development.
Amutation that reverses the effect of a previous mutation which had inactivated a gene, thus restoringwild-type function.[10] See alsoreverse mutation.
The breeding of ahybrid organism with one of its parents or an individual genetically similar to one of its parents, often intentionally as a type ofselective breeding, with the aim of producingoffspring with a genetic identity which is closer to that of the parent. The reproductive event and the resulting progeny are both referred to as abackcross, often abbreviated in genetics shorthand with the symbolBC.
An evolutionary model of thegenetic incompatibility that occurs as a result of negativeepistatic interactions between two or moregenes oralleles with different evolutionary histories, which may meet when distinct populationshybridize. The incompatible genes or alleles themselves, referred to asDobzhansky–Muller incompatibilities, may be the result ofrandom orneutral mutations, or they may be specificadaptations driven bynatural selection. By preventing populations from successfully interbreeding, these incompatibilities can reinforcereproductive isolation and thereby increase the chance ofspeciation.
The scientific study of the spatial distributions of biological organisms, populations, and species. It includes the study of bothextinct andextant organisms.[11]
The ability of a population to consistently produce the samephenotype regardless of the variability of its environment or thegenetic variation within itsgenome. The concept is most often used indevelopmental biology to interpret the observation that developmental pathways are frequently shaped bynatural selection such that developing cell lineages are "guided" or "canalized" towards a single, definite fate, becoming progressively more resistant to any minor perturbations that may redirect development of the cells away from their initial course.
An individual who hasinherited arecessiveallele for a genetic trait or mutation but in whom the trait is not usually expressed or observable in thephenotype. Carriers are usuallyheterozygous for the recessive allele and therefore still able to pass the allele onto their offspring, where the associated phenotype may reappear if the offspring inherits another copy of the allele. The term is commonly used inmedical genetics in the context of a disease-causing recessive allele.
A variation ofperipatric speciation in which speciation occurs by geographic isolation, butreproductive isolation evolves in the larger population instead of the peripherally isolated population.[12]
Incentrifugal speciation, the range of an original population (green) expands and then contracts, leaving an isolated fragment population behind. In the absence of interbreeding, the central population (changed to blue) becomesreproductively isolated over time.
The phenomenon by which differences between similar species that occupy similarniches and have partially overlapping geographic distributions are accentuated in regions where the species co-occur but are minimized or lost where the species' distributions do not overlap. This occurs because competition between the similar species for one or more limited resources drives evolutionary change that differentiates the species in the common geographic areas such that they no longer occupy the same niche, thereby allowing them to coexist and avoidingcompetitive exclusion.
The presence of two or more populations of cells with distinctgenotypes within the body of an individual organism, known as achimera, which has developed from the fusion of cells originating from separatezygotes; each population of cells retains its own genome, such that the organism as a whole is a mixture of genetically non-identical tissues. Genetic chimerism may be inherited (e.g. by the fusion of multiple embryos during pregnancy) or acquired after birth (e.g. byallogeneic transplantation of cells, tissues, or organs from a genetically non-identical donor); in plants, it can result fromgrafting or errors in cell division. It is similar to but distinct frommosaicism.
AnuclearDNA molecule containing part or all of the genetic material of an organism. Chromosomes may be considered a sort of molecular "package" for carrying DNA within thenucleus ofcells and, in mosteukaryotes, are composed of long strands of DNA coiled withpackaging proteins which bind to andcondense the strands to prevent them from becoming an unmanageable tangle. Chromosomes are most easily distinguished and studied in their completely condensed forms, which only occur duringcell division. Some simple organisms have only one circular chromosome, while most eukaryotes have multiple linear chromosomes.
An approach to biological classification in which organisms are grouped inclades defined byshared ancestry; hypothesized relationships between organisms are typically based onshared derived characters which can be traced to themost recent common ancestor and are not present in more distant ancestors or unrelated groups.
The branch ofgenetics based solely on observation of the visible results of reproductive acts, as opposed to that made possible by the modern techniques and methodologies ofmolecular biology. Contrastmolecular genetics.
A measurable spatial gradient in a single biological character or trait of a species or population across its geographic range. The nature of a cline may be genotypic (e.g. variation inallele frequency) or phenotypic (e.g. variation in body size or pigmentation), and may show smooth, continuous gradation or abrupt changes between different geographic regions.
The process of producing, either naturally or artificially, individual organisms or cells which are genetically identical to each other. Clones are the result of all forms ofasexual reproduction, and cells that undergomitosis produce daughter cells that are clones of the parent cell and of each other. Cloning may also refer to biotechnology methods which artificially create copies of organisms or cells, or, inmolecular cloning, copies of DNA fragments or other molecules.
The mutualadaptation of organisms belonging to different populations or species, of different parts of the same organism, or ofgenes at differentloci in the samegenome, especially implying that adaptation in both entities is driven by the same evolutionary force.[13]
The process by which two or more distinctpopulations,species, or other groups of organisms, or two or more distinct traits within a species, reciprocally affect each other'sevolution throughnatural selection. Each party in a coevolutionary relationship exertsselective pressures upon the other, leading to the evolution of separate traits in each party.
An organism ortaxon (e.g. aspecies) which is hypothesized to be the lineal progenitor of two or more organisms or taxa which exist at a later point in evolutionary time. The concept of common descent is fundamental to the study ofevolution,phylogenetics, andcladistics; for instance, allclades, by definition, are rooted in a common ancestor. See alsomost recent common ancestor.
(of two or more individuals) Closely genetically related; sharing a recentcommon ancestor (usually no more than three or fourgenerations distant). The effect of consanguineous mating, also known asinbreeding, is to increase the probability that the progeny will behomozygous at any given pair of geneticloci.[14]
An interdisciplinary branch ofpopulation genetics which applies genetic methods and concepts in an effort to understand the dynamics of genes in populations, with a principal aim of avoidingextinctions and preserving and restoringbiodiversity.
The independentevolution of similar traits oradaptations in two or more different taxa from different periods or epochs in time, creatinganalogous structures that have similar form or function but were not present in thelast common ancestor of those taxa; e.g. structures enabling flight evolved independently in at least four distinct lineages: insects, birds, pterosaurs, and bats. Incladistics, the same phenomenon is termedhomoplasy. Contrastdivergent evolution.
The breeding ofpurebred parents belonging to two differentbreeds, varieties, or populations, often intentionally as a type ofselective breeding, with the aim of producing offspring which share traits of both parent lineages or which showheterosis. In animal breeding, the progeny of a cross between breeds of the same species is called acrossbreed, whereas the progeny of a cross between different species is called ahybrid.
The understanding of biologicalevolution as developed by the English naturalistCharles Darwin and others, which states that all biological organisms arise and develop through thenatural selection of small, inherited variations that increase the individual's ability to compete, survive, and reproduce. Colloquially, the term is sometimes used to refer more broadly tomodern evolutionary theory as a whole, though in scientific circles distinctions are usually made between Darwin's ideas and later additions to evolutionary biology.
A spontaneousmutation in thegenome of an individual organism that is new to that organism'slineage, having first appeared in agerm cell of one of the organism's parents or in the fertilized egg that develops into the organism; i.e. a mutation that was not present in either parent's genome.[8]
For a givenclade, any trait or feature (e.g. a specificphenotype) that is present within one or more subgroups of the clade but not in the clade'scommon ancestor. Derived traits show significant differences from the original "primitive" condition of anancestral trait found in the common ancestor, implying that the trait has undergone extensiveadaptation during the clade's evolutionary history to reach its derivative condition. Both terms are relative: a derived trait for one clade may be an ancestral trait for a different clade. The term "derived trait" is often used interchangeably with the more technical termapomorphy.
A mode ofnatural selection in which an extremetrait orphenotype is favored over other phenotypes, causingallele frequencies to shift over time in the direction of that trait or phenotype. This shift can occur whether or not the alleles governing the extreme phenotype aredominant or recessive.
Amating system in which individuals with dissimilarphenotypesmate with each other more frequently than would be expected in a completely random mating system. Disassortative mating usually has the effect of decreasing genetic relatedness between members of the mating population. Contrastassortative mating.
A mode ofnatural selection in which the extreme values of atrait orphenotype within a breeding population are favored over intermediate values, causingallele frequencies to shift over time away from the intermediate. This causes the variance in the trait to increase and results in the population dividing into two distinct groups, each with trait values at one end of the trait's distribution curve.
A relationship between thealleles of agene in which one allele produces an effect onphenotype that overpowers or "masks" the contribution of another allele at the samelocus; the first allele and its associated phenotypictrait are said to bedominant, and the second allele and its associated trait are said to berecessive. Often, the dominant allele codes for a functional protein while its recessive counterpart does not. Dominance is not an inherent property of any allele or phenotype, but simply describes its relationship to one or more other alleles or phenotypes; it is possible for one allele to be simultaneously dominant over a second allele, recessive to a third, andcodominant to a fourth. In genetics shorthand, dominant alleles are often represented by a single uppercase letter (e.g. "A", in contrast to the recessive "a").
Any mechanism by which organisms neutralize the large difference ingene dosage caused by the presence of differing numbers ofsex chromosomes in the different sexes, thereby equalizing theexpression of sex-linked genes so that the members of each sex receive the same or similar amounts of theproducts of such genes. An example isX-inactivation in female mammals.
The ecological state of a species being unique to a single geographic location, such as an island, nation, country, or any other clearly defined area, or to a singlehabitat type.
The collective action of multiple genes interacting duringgene expression. A form of gene action, epistasis can be either additive or multiplicative in its effects on specificphenotypic traits.
Theextinction of apopulation of organisms (insofar as the population can be defined by one or more identifiable characteristics) as a result of the excessive accumulation of genetic mutations, such that the population loses self-identity because all of its mutated descendants lack the identifiable characteristics.
The condition of a cell or organism having an abnormal number of complete sets ofchromosomes, possibly excluding thesex chromosomes. Euploidy differs fromaneuploidy, in which a cell or organism has an abnormal number of one or more specific individual chromosomes.
The phenomenon by which theheritable characteristics of biologicalpopulations change over successivegenerations. Evolution occurs when processes such asnatural selection andgenetic drift act on thevariation in characteristics that exists between members of a population, resulting in certain characteristics becoming more or less common within the population.
Thepositive feedback mechanism operating between competing sets ofco-evolvinggenes, traits, species, or other taxa which evolve specificadaptations and counter-adaptations due to each other's presence, which may be seen as analogous with an "arms race".
For a givengenotype associated with a variable non-binaryphenotype, the proportion of individuals with that genotype who show or express the phenotype to a specified extent, usually given as a percentage. Because of the many complex interactions that governgene expression, the sameallele may produce a wide variety of possible phenotypes of differing qualities or degrees in different individuals; in such cases, both the phenotype and genotype may be said to showvariable expressivity. Expressivity attempts to quantify the range of possible levels of phenotypic variation in a population of individuals expressing the phenotype of interest. Comparepenetrance.
Currently living or existing; still in existence and notextinct. The term is generally used to refer to the present-day state of existence of a particulartaxon (such as a family,genus,species, etc.).
The process by which a singleallele for a particulargene with multiple alleles increases infrequency in a given population such that it becomes permanently established as the only allele at thatlocus within the population'sgene pool. How long fixation takes depends onselection pressures andchance fluctuations in allele frequencies.
An experimental approach inmolecular genetics in which a researcher starts with a specific knownphenotype and attempts to determine the genetic basis of that phenotype by any of a variety of laboratory techniques, commonly byinducing randommutations in the organism's genome and thenscreening for changes in the phenotype of interest. Observed phenotypic changes are assumed to have resulted from the mutation(s) present in the screened sample, which can then bemapped to specific genomicloci and ultimately to one or more specificcandidate genes. This methodology contrasts withreverse genetics, in which a specific gene or its gene product is individually manipulated in order to identify the gene's function.
The loss ofgenetic variation that occurs when a new, physically isolatedpopulation is established by a very small number of individuals who have migrated from a larger population and are not fully representative of the larger population's genetic diversity. As a result, the new population is often distinctively different, bothgenotypically andphenotypically, from the parent population. Besides migration,population bottlenecks can also result in a type of founder effect; extreme founder effects can lead tospeciation.
A hypothesis that describes the evolutionary advantages of the first-arriving lineages in a new ecosystem.[17] An example could be when a species becomesreproductively isolated on an island, as inperipatric speciation.
fugitive species
Aspecies that only temporarily occupies environments or habitats (either because its members frequently migrate or because its environments frequently change) and so does not persist for manygenerations at any one site.[1]
Any segment or set of segments of anucleic acid molecule that contains the information necessary to produce a functionalRNA transcript in a controlled manner. Genes are often considered the fundamentalunits ofheredity and are typically encoded inDNA. A particular gene can have multiple different versions, oralleles, and a single gene may influence many differentphenotypes.
The number of copies of a particulargene present in agenome. Gene dosage directly influences the amount ofgene product a cell is able to express, though a variety of controls have evolved which tightlyregulategene expression. Changes in gene dosage caused by mutations includecopy-number variations.
The set of processes by which the information encoded in agene is used in the synthesis of agene product, such as a protein or non-coding RNA, or otherwise made available to influence one or morephenotypes. Expression at the molecular level canonically consists oftranscription andtranslation, though the information contained within a DNA sequence need not necessarily be transcribed and translated to exert an influence on molecular events, however; broader definitions encompass a huge variety of other ways in which genetic information can be expressed.
Gene flow is the transfer ofalleles from one population to another population through the interbreeding of individual organisms belonging to the populations.
1. In any given organism, a singlereproductive cycle, or the phase between two consecutive reproductive events, i.e. between an individual organism's reproduction and that of theprogeny of that reproduction; or the actual or average length of time required to complete a single reproductive cycle, either for a particularlineage or for a population or species as a whole.
2. In a given population, those individuals (often but not necessarily living contemporaneously) who are equally removed from a givencommon ancestor by virtue of the same number of reproductive events having occurred between them and the ancestor.[14]
The co-occurrence within a population of one or morealleles orgenotypes with a particularphenotypic trait more often than might be expected by chance alone; such statistical correlation may be used to infer that the alleles or genotypes are responsible for producing the given phenotype.
The process of advising individuals or families who are affected by or at risk of developinggenetic disorders in order to help them understand and adapt to the physiological, psychological, and familial implications of genetic contributions to disease. Genetic counseling integratesgenetic testing,genetic genealogy, andgenetic epidemiology.[18]
Any illness, disease, or other health problem directly caused by one or more abnormalities in an organism'sgenome which arecongenital (present at birth) and not acquired later in life. Causes may include amutation to one or moregenes, or achromosomal abnormality such as ananeuploidy of a particular chromosome. The mutation responsiblemay occur spontaneously during embryonic development or may beinherited from one or both parents, in which case the genetic disorder is also classified as ahereditary disorder. Though the abnormality itself is present before birth, the actual disease it causes may not develop until much later in life; some genetic disorders do not necessarily guarantee eventual disease but simplyincrease the risk of the organism developing it.
A measure of the genetic divergence between species, populations within a species, or individuals, used especially inphylogenetics to express either the time elapsed since the existence of acommon ancestor or the degree of differentiation in theDNA sequences comprising thegenomes of each population or individual.
The total number of genetictraits or characteristics in the genetic make-up of a population, species, or other group of organisms. It is often used as a measure of the adaptability of a group to changing environments. Genetic diversity is similar to, though distinct from,genetic variability.
Also calledallelic drift or theSewall Wright effect.
A change in thefrequency with which an existingallele occurs in a population due to random variation in the distribution of alleles from onegeneration to the next. It is often interpreted as the role thatrandom chance plays in determining whether a given allele becomes more or less common with each generation, irrespective of the influence ofnatural selection. Genetic drift may cause certain alleles, even otherwise advantageous ones, to disappear completely from thegene pool, thereby reducinggenetic variation, or it may cause initially rare alleles, even neutral or deleterious ones, to become much more frequent or evenfixed.
The study of the role played by genetic factors in determining health and disease, in particular through the interaction of genetic factors with environmental factors, and typically as observed in genetically related individuals, often families or lineages but also populations and subpopulations.
The use ofgenealogical DNA testing in combination with traditionalgenealogical methods to infer the level and type of genetic relationships between individuals, to find ancestors, and to constructfamily trees,genograms, or other genealogical charts.
A type oflinked selection by which the positive selection of anallele undergoing aselective sweep causes alleles for different genes at nearbyloci to changefrequency as well, allowing them to "hitchhike" tofixation along with the positively selected allele. If selection at the first locus is strong enough, neutral or even slightly deleterious alleles within the same linkage group may undergo the same positive selection because the physical distance between the nearby loci is small enough that arecombination event is unlikely to occur between them. Genetic hitchhiking is often considered the opposite ofbackground selection.
Any reduction in the meanfitness of apopulation owing to the existence of one or moregenotypes with lower fitness than that of the most fit genotype.[1]
A broad class of procedures used to identify features of an individual's particular chromosomes, genes, or proteins in order to determine parentage orancestry, diagnose vulnerabilities to heritable diseases, or detectmutant alleles associated with increased risks of developinggenetic disorders. Genetic testing is widely used in human medicine, agriculture, and biological research.
The genetic differences both within and betweenpopulations,species, or other groups of organisms. It is often visualized as the variety of differentalleles in thegene pools of different populations.
The formation or the presence of individuals differing ingenotype within a population or other group of organisms, as opposed to individuals with environmentally induced differences, which cause only temporary, non-heritable changes inphenotype. Barring other limitations, a population with high genetic variability has a greater potential for successful adaptation to changing environmental conditions than a population with low genetic variability. Genetic variability is similar to, though distinct from,genetic diversity.
The genetic differences both within and between populations, species, or other groups of organisms. It is often visualized as the variety of differentalleles in thegene pools of different populations.
A mechanism ofsex determination that depends upon the ratio of the number ofX chromosomes (X) to the number of sets ofautosomes (A). Males develop when the X/A ratio is 0.5 or less, females when it is 1.0 or more, and anintersex develops when it is between 0.5 and 1.0.[8]
genic selection
A type ofnatural selection that occurs at the level of individualgenes oralleles, in which thefrequency of an allele within a breedingpopulation is determined by itsfitness averaged over the variety ofgenotypes in which it occurs; the differential propagation of different alleles within a population as a consequence of properties borne by the alleles themselves, rather than by the genotypes in which they are found.[1]
The entire complement of genetic material contained within thechromosomes of an organism, organelle, orvirus. The term is also used to refer to the collective set of geneticloci shared by every member of a population or species, regardless of the differentalleles that may be present at these loci in different individuals.
Anycell that gives rise to thegametes of an organism that reproducessexually. Germ cells are the vessels for the genetic material which will ultimately be passed on to the organism's descendants and are usually distinguished fromsomatic cells, which are entirely separate from thegerm line.
1. In multicellular organisms, the population ofcells which are capable of passing on their genetic material to the organism's progeny and are therefore (at least theoretically) distinct fromsomatic cells, which cannot pass on their genetic material except to their own immediatemitotic daughter cells. Cells of the germ line are calledgerm cells.
2. Thelineage of germ cells, spanning many generations, that contains the genetic material which has been passed on to an individual from its ancestors.
A rule formulated byJ.B.S. Haldane which states that if one sex of thehybrid offspring resulting from a cross between twoincipient species is inviable or sterile, that sex is more likely to be theheterogametic sex (i.e. the one with two different sex chromosomes).[19]
Asex-determination system in which the sex of an individual organism is determined by thenumber of sets ofchromosomes it possesses: offspring which develop from fertilized eggs arediploid and female, while offspring which develop from unfertilized eggs arehaploid and male, with half as many chromosomes as the females. Haplodiploidy is common to all members of the insect orderHymenoptera and several other insect taxa.
A principle of population genetics which states thatallele andgenotype frequencies of a population will remain constant fromgeneration to generation in the absence of other evolutionary influences. In the simplest case of a randomly mating population of diploid organisms possessing a single locus with two alleles,A anda, with frequenciesf(A) =p andf(a) =q, respectively, the expected genotype frequencies aref(AA) =p2 forAAhomozygotes,f(aa) =q2 foraa homozygotes, andf(Aa) = 2pq forheterozygotes. In the absence of evolutionary forces such asnatural selection,mutation,assortative mating,gene flow, andgenetic drift,p andq will remain constant between generations, such that the population is said to be inHardy–Weinberg equilibrium with respect to the locus in question.
In adiploid organism, having just oneallele at a givengenetic locus (where there would ordinarily be two). Hemizygosity may be observed when only one copy of achromosome is present in a normally diploid cell or organism, or when a segment of a chromosome containing one copy of an allele isdeleted, or when a gene is located on asex chromosome in theheterogametic sex (in which the sex chromosomes do not exist in matching pairs); for example, in human males with normal chromosomes, almost allX-linked genes are said to be hemizygous because there is only oneX chromosome and few of the same genes exist on theY chromosome.
The storage, transfer, and expression of genetic information in biological organisms,[14] as manifested by the passing on ofphenotypic traits from parents to theiroffspring, either throughsexual orasexual reproduction. Offspring cells or organisms are said toinherit the genetic information of their parents.
2. A statistic used inquantitative genetics that estimates the proportion of variation within a givenphenotypic trait that is due togenetic variation between individuals in a particular population. Heritability is estimated by comparing the individualphenotypes of closely related individuals in the population.
The improved or increased function or quality of any biologicaltrait in ahybridoffspring, with respect to the same trait in its genetically distinct parents. If any one or more of the parents' traits are noticeably enhanced in the offspring as a result of the mixing of the parents' genetic contributions, the offspring is said to beheterotic.
In adiploid organism, having two differentalleles at a givengenetic locus. In genetics shorthand, heterozygousgenotypes are represented by a pair of non-matching letters or symbols, often an uppercase letter (indicating adominant allele) and a lowercase letter (indicating arecessive allele), such as "Aa" or "Bb". Contrasthomozygous.
homoallele
A mutantallele having a differentmutation at the same site as another allele. Intragenicrecombination between homoalleles cannot produce a functionalcistron.[8]
A set of two matchingchromosomes, one maternal and one paternal, which pair up with each other inside the nucleus duringmeiosis. They have the samegenes at the sameloci, but may have differentalleles.
In adiploid organism, having two identicalalleles at a givengenetic locus. In genetics shorthand, homozygousgenotypes are represented by a pair of matching letters or symbols, such as "AA" or "aa". Contrastheterozygous.
Any process by which genetic material is transferred between unicellular and/or multicellular organisms other than by vertical transmission from parent to offspring, e.g.bacterial conjugation.
Theoffspring that results from combining the qualities of two organisms of differentgenera,species,breeds, or varieties throughsexual reproduction. Hybrids may occur naturally or artificially, as duringselective breeding of domesticated animals and plants. Reproductive barriers typically prevent hybridization between distantly related organisms, or at least ensure that hybrid offspring are sterile, but fertile hybrids may result inspeciation.
A geographic area in which the ranges of two interbreeding species or populations overlap, allowing them to cross-fertilize and generatehybrid offspring. The formation of a hybrid zone is one of the four outcomes ofsecondary contact between divergent genetic lineages.
hybridization
The process by which ahybrid organism is produced from two parents of different genera, species, breeds, or varieties.
The exaggeration of one or morephenotypic features of a descendant organism compared to those of its ancestors due to an increase in the duration ofontogenetic development over evolutionary history.[1]
A mutantallele that permits a subnormal expression of the gene's normal phenotype, e.g. by encoding an unstable enzyme which degrades too quickly to fully serve its function but which nevertheless is functional in some limited capacity, being generated in quantities sufficient for its reaction to proceed slowly or at low levels.[8]
(of a gene or allele) Traceable back through an arbitrary number ofgenerations without mutation to acommon ancestor of the group of descendant organisms that carries the gene or allele.[1] A gene or allele present in a group of descendant organisms is said to be identical by descent to a gene or allele in a common ancestor of the group if both sequences are identical, indicating that the sequence has been passed down unmodified from the common ancestor to its descendants.
Anylineage of a particular species in which individuals are nearly or completely genetically identical to each other due to a long history of repeatedinbreeding, either by natural or artificial means. Lineages are typically considered inbred after at least 20generations of inbreeding (e.g. by self-fertilization or sib mating), at which point nearly allloci across the genome arehomozygous and all individuals can therefore effectively be treated asclones (despite the fact that individuals are still produced by sexual reproduction).
Sexual reproduction between breeds or individuals that are closelyrelated genetically. Inbreeding results inhomozygosity, which can increase both the probability of offspring being affected by deleteriousrecessive traits and the probability of fixing beneficial traits within the breeding population. The reproductive event and the resulting progeny may both be referred to as anincross, and the progeny is said to beinbred. Contrastoutbreeding.
The number ofoffspring equivalents that an individual organism rears, rescues, or otherwise supports through its behavior, regardless of whether or not the individual is actually a biological parent of the offspring equivalents. Inclusive fitness is one of two metrics of evolutionary success as defined byW.D. Hamilton in 1964, the other beingpersonal fitness.
The frequency of new occurrence of agenetic disorder (or more broadly any genetic condition ortrait, deleterious or otherwise) among the members of a particular population and within a particular period of time.[14]
Themovement of agene from thegene pool of one population or species into that of another population by the repeatedbackcrossing ofhybrids of the two populations with one of the parent populations. Introgression is a ubiquitous and important source ofgenetic variation in natural populations, but may also be practiced intentionally in the cultivation of domesticated plants and animals.
Two or moregenes that are equivalent and redundant in the sense that, despite coding for distinctgene products, they each result in the samephenotype when set within the samegenetic background. If several isomeric genes are present in a singlegenotype they may be either cumulative or non-cumulative in their contributions to the phenotype.[14]
The repeatedevolution of similarphenotypic characteristics or traits in different organisms at different times during the evolutionary history of aclade,[1] a phenomenon which can result in the seemingde-extinction of an organism previously consideredextinct.
A reproductive strategy characterized by multiple reproductive cycles during an individual organism's lifetime. Organisms that use such a strategy are said to beiteroparous. Iteroparity is usually contrasted withsemelparity.
In theKaneshiro model ofperipatric speciation, a sample of a larger population results in an isolated population with less males containing attractive traits. Over time, choosy females are selected against as the population increases.Sexual selection drives new traits to arise (green), thereby reproductively isolating the new population from the old one (blue).
A form ofgenic selection by whichalleles differ in their rates of propagation by influencing the survival or reproductive success of individuals who carry the same alleles bycommon descent (theirkin).[1]
An evolutionary hypothesis which proposes that duringsexual selection, organisms preferentially seek mates with a minimum of unusual ormutanttraits, e.g. in terms of functionality, appearance, or behavior. The hypothesis attempts to explain the clustering of sexual organisms into distinctspecies and other issues described byDarwin's dilemma.
Also called thelast universal cellular ancestor or simply thelast universal ancestor.
The most recentpopulation of organisms from which allextant organisms on Earth share acommon descent; i.e. themost recent common ancestor of all organisms now living. LUCA is not thought to have been the earliest life on Earth, but rather the only organism of its time to still have living descendants. Its existence is not known from any specific fossil record but is inferred fromphylogenetic comparisons of modern organisms, all of which are its descendants.
One of three fundamental principles ofMendelian inheritance, which states that differentalleles of the samegene may bedominant orrecessive relative to others, and that an organism with at least one dominant allele will uniformly display thephenotype associated with the dominant allele.
One of three fundamental principles ofMendelian inheritance, which states that genes responsible for different phenotypic traits are segregated independently duringmeiosis.Linked genes are a notable exception to this rule.
The average number ofrecessive deleterious genes existing in theheterozygous condition that is carried by a member of a population ofdiploid organisms, multiplied by the average probability that each such gene will cause prematurelethality whenhomozygous. For example, an organism carrying eight recessivesemilethal alleles, each of which produces only a 50% probability of premature death when homozygous, is said to carry a genetic burden of four "lethal equivalents".[8]
A linear evolutionary sequence connecting an ancestral cell, organism, or species to a particular descendant cell, organism, or species, including all intermediate organisms and spanning any number ofgenerations; the direct progression of reproductive events (i.e. the line of descent) between two individuals, including vertically related individuals, e.g. parent(s) and offspring, but usually excluding horizontally related individuals who did not themselves directly contribute genetic material to any of the included individuals, e.g. siblings.
lineage-splitting
Also calledlineage-branching.
Whengene flow between two populations is completely eliminated.[7]
The tendency of DNA sequences which are physically near to each other on the same chromosome to beinherited together duringmeiosis. Because the physical distance between them is relatively small, the chance that any two nearby parts of a DNA sequence (oftenloci orgenetic markers) will be separated on to differentchromatids duringchromosomal crossover is statistically very low; such loci are then said to be morelinked than loci that are farther apart. Loci that exist on entirely different chromosomes are said to be perfectlyunlinked. The standard unit for measuring genetic linkage is thecentimorgan (cM).
Evolutionary change as it occurs at a relatively large scale, at or above the level ofspecies, as opposed tomicroevolution, which occurs at a smaller scale. Macroevolution is often thought of as the compounded effects of microevolution.
Any nongenetic effect of the mother on thephenotype of heroffspring, owing to factors such as cytoplasmic inheritance, transmission of congenital disease, and the sharing of nutritional conditions.[1]
(of anoffspring) Resembling the female parent, genotypically or phenotypically, more closely than the male parent; derived from the mother. Contrastpatroclinous.
The branch of medicine and medical science that involves the study, diagnosis, and management ofhereditary disorders, and more broadly the application of knowledge about humangenetics to medical care.
A theory of biologicalinheritance based on a set of principles originally proposed byGregor Mendel in 1865 and 1866. Mendel derived three generalized laws about the genetic basis of inheritance which, together with several theories developed by later scientists, are considered the foundation ofclassical genetics. Contrastnon-Mendelian inheritance.
Evolutionary change as it occurs at a relatively small scale, typically within a particularspecies orpopulation, as opposed tomacroevolution, which occurs at a larger scale. Because of the convenience of observing and modeling small-scale changes inallele frequencies within discrete populations, the principles ofpopulation genetics are often conceptualized at microevolutionary scales.
The process by which an organism evolves to resemble another object, often an organism of another species. Mimicry can also occur between individuals of the same species. A type of adaptive signaling, mimicry evolves when a signal-receiver, known as thedupe, perceives the similarity between themimic and the object or organism it is mimicking, known as themodel, and as a result changes its behavior in a way that provides aselective advantage to the mimic; the model may also benefit from the shared resemblance, in which case there is amutualism, or the mimicry may be to the model's detriment, making itparasitic or competitive. The evolved resemblance may be visual, acoustic, chemical, tactile, or electrical, or any combination of sensory modalities. There are many varieties of mimicry, such asBatesian,Müllerian, andVavilovian.
Ineukaryoticcells, the part of thecell cycle during which the division of thenucleus takes place andreplicatedchromosomes are separated into two distinct nuclei. Mitosis is generally preceded by the S stage ofinterphase, when the cell'sDNA is replicated, and either occurs simultaneously with or is followed bycytokinesis, when thecytoplasm andcell membrane are divided into two new daughter cells.
A classification scheme ofspeciation processes based on the level ofgene flow between two populations.[22] The traditional terms for the three modes—allopatric,parapatric, andsympatric—are based on the spatial distributions of a species population.[23][22]
The evolutionary change of certain adaptive structures, traits, or other components of the phenotype at different times or different rates than others, either within a single species or between different species.[24]
mosaic hybrid zone
A zone in which two speciating lineages occur together in a patchy distribution–either by chance, random colonization, or low hybridfitness.[22]
mosaic sympatry
A case ofsympatry in which two populations overlapping in geographic distribution exhibit habitat specializations.[22]
The presence of two or more populations of cells with differentgenotypes in an individual organism which has developed from a single fertilizedegg. A mosaic organism can result from many kinds of genetic phenomena, includingnondisjunction of chromosomes,endoreduplication, or mutations in individualstem cell lineages during the early development of the embryo. Mosaicism is similar to but distinct fromchimerism.
Any permanent change in thenucleotide sequence of a strand ofDNA orRNA, or in theamino acid sequence of apeptide. Mutations play a role in both normal and abnormal biological processes; their natural occurrence is integral to the process ofevolution. They can result from errors inreplication, chemical damage, exposure to high-energy radiation, or manipulations bymobile genetic elements.Repair mechanisms have evolved in many organisms to correct them. By understanding the effect that a mutation has onphenotype, it is possible to establish the function of thegene or sequence in which it occurs.
mutation distance
The smallest number ofmutations required to derive one particular gene, sequence, or phenotype from another;[8] the minimum number of nucleobase insertions, deletions, or substitutions necessary to change one sequence into another.
The actual origin of a particularmutation in time and space; the instance of its original introduction into a genome, as opposed to that of its phenotypic manifestation, which may only occurgenerations after the fact.
The frequency of newmutations at a particularlocus or in a particulargene,sequence,genome, or organism over a specified period of time, e.g. during a singlegeneration. Mutation rates may be calculated for a specific class of mutation or for all types collectively; they vary widely by organism and with an organism's environment.
1. Anymutation of anucleic acid sequence that is neither beneficial nor detrimental to the ability of an organism to survive and reproduce.
2. Any mutation for whichnatural selection does not affect the spread of the mutation within a population.
nexus hypothesis
The hypothesis that each phenotypic trait is likely to be influenced by more than one gene, and conversely that most genes affect more than one phenotype.[24]
1. The ecological role of a particularspecies or othertaxon in a largercommunity, generally conceptualized as the multidimensional space, of which the coordinates are the various parameters representing the conditions which are necessary for the existence of the species in every aspect of its present form, to which a species is restricted by the presence of competitor species.[24]
2. A particular environment or environmental condition to which a species is matched; the variety of activities, behaviors, and ecological functions carried out by an organism or population in response to its environmental context, e.g. the distribution of resources and competitors, and the ways in which it in turn alters that same context. The term is sometimes used loosely as an equivalent ofmicrohabitat, in the sense of the physical space occupied by a species.[24] See alsofundamental niche andrealized niche.
A type ofmutation in which thesubstitution of onenucleotide base for another results, aftertranscription andtranslation, in an amino acid sequence that is different from that produced by the original unmutated gene. Because nonsynonymous mutations always result in a biological change in the organism, they are often subject to strongselection pressure. Contrastsynonymous mutation.
Anyallele made non-functional by way of a geneticmutation. The mutation may result in the complete failure to produce agene product or a gene product that does not function properly; in either case, the allele may be considered non-functional.
The condition of a cell or organism lacking all of the copies of a particularchromosome that are normal for itsploidy level; e.g. in adiploid organism, lacking both members of the normal pair. Nullisomy is frequently lethal early in development.
The origination andbiological development of an organism within its own lifetime, as opposed tophylogeny, which refers to the evolutionary history of the organism's ancestors. In sexually reproducing organisms, ontogeny is the study of the development of an organism from the time offertilization to the organism's reproductively mature form; the term may also be used to refer to the study of an organism's entire lifespan.
Any coding or non-coding DNA sequence which is derived from atandemmultigene family orcluster but is physically isolated from the other genes in the family because it is dispersed to a distantlocus in the genome. Orphons are usually non-functionalpseudogenes with highlyvariable copy numbers.[25]
One of a set ofgenes (or more generally any DNA sequences showinghomology) which are present in differentgenomes but are descended from the same ancestral sequence, i.e. they are directly related to one another by vertical descent from a single gene or sequence in themost recent common ancestor of those genomes. Such genes or sequences are said to beorthologous. Orthologs can be inferred to be related to each other based on the similarity of their sequences; though they may have evolved independently within separate genomes by mutation and natural selection, theirproducts may still retain similar structures, functions, or levels ofexpression across species and populations. The identification of orthologs has proven important in inferringphylogenetic relationships between organisms. Contrastparalog.
Sexual reproduction between different breeds or individuals, which has the potential to increasegenetic diversity by introducing unrelated genetic material into a breeding population. The reproductive event and the resulting progeny may both be referred to as anoutcross, and the progeny is said to beoutbred. Contrastinbreeding.
The independent evolution of similar or identicalderived traits or characters in relatedlineages, thought usually to be based on similar modifications of common developmental pathways.[1] Contrastconvergent evolution.
One of a set ofgenes (or, more generally, any DNA sequences showinghomology) which are directly related to each other via one or more geneticduplication events; such genes or sequences are said to beparalogous. Paralogs result from the duplication of a single sequence within a singlegenome and then the subsequentdivergence of the duplicated sequences by mutation and natural selection (either within the original genome, or, duringspeciation, in different genomes). Contrastortholog.
Parapatric speciation can occur when the members of a population subject to a selective gradient of phenotypic or genotypic frequencies (acline) experience different selective conditions at each end of the gradient (divergent selection). Reproductive isolation occurs upon the formation of a hybrid zone. In most cases, the hybrid zone is eliminated due to a selective disadvantage, which effectively completes the speciation process.
The principle of accounting for empirical observations by whichever hypothesis requires the fewest or the simplest assumptions for which there is limited or no evidence. In biological systematics,maximum parsimony is an optimality criterion which invokes a minimum of evolutionary changes to inferphylogenetic relationships; i.e. thephylogenetic tree that minimizes the total number of character-state changes is to be preferred.[1]
A type ofasexual reproduction in which the growth and development ofembryos occurs withoutfertilization. In animals which reproduce by parthenogenesis, an unfertilizedgamete of the female parent is capable of developing into an adult without any contribution from a male parent, resulting in offspring possessing only the mother's genetic material (the exact proportion of which depends on the parthenogenetic mechanism, of which there are numerous varieties). Some species reproduce exclusively by parthenogenesis, while others can switch betweensexual reproduction and parthenogenesis under certain environmental conditions.
(of anoffspring) Resembling the male parent, genotypically or phenotypically, more closely than the female parent;[8] derived from the father. Contrastmatroclinous.
The proportion of individuals with a givengenotype who express the associatedphenotype, usually given as a percentage. Because of the many complex interactions that governgene expression, the sameallele may produce an observable phenotype in one individual but not in another. If less than 100% of the individuals in a population carrying the genotype of interest also express the associated phenotype, both the genotype and phenotype may be said to showincomplete penetrance. Penetrance quantifies the probability that an allele will result in the expression of its associated phenotype in any form, i.e. to any extent that makes an individualcarrier different from individuals without the allele. Compareexpressivity.
pericentric
(of a gene or region of a chromosome) Positioned near thecentromere of thechromosome.
A variation of allopatric speciation where a new species forms from a small, peripheral isolated population.[26] It is sometimes referred to as centripetal speciation in contrast tocentrifugal speciation.
Inperipatric speciation, a small population becomes isolated on the periphery of the central population evolving reproductive isolation (blue) due to reducedgene flow.
The complete set ofphenotypes that are or can be expressed by agenome, cell, tissue, organism, or species; the sum of all of its manifest chemical, morphological, and behavioral characteristics or traits.
phenomic lag
A delay in thephenotypicexpression of a geneticmutation owing to the time required for the manifestation of changes in the affected biochemical pathways.[14]
The composite of the observable morphological, physiological, and behavioraltraits of anorganism that result from theexpression of the organism'sgenotype as well as the influence of environmental factors and the interactions between the two.
A model of evolution which theorizes that most speciation occurs slowly, uniformly, and gradually, and that there is seldom a clear line of demarcation between ancestral species and descendant species unless there is a sudden split which reproductively isolates members of the same population. The theory is often contrasted withpunctuated equilibrium.
A method used to infer the likelihood of specific traits being present in organisms whose phenotypes are incomplete or unknown based on their positions in aphylogenetic tree relative to ancestors, descendants, or contemporaneous organisms with more completely understood phenotypes. A major application of this method is inpaleontology, where extinct organisms known only from fossils are compared to their closest known relatives in order to infer the presence or absence of certain traits for which fossils provide limited or no evidence, such as soft tissues, integumentary structures, and physiological and behavioral traits, though the method is extremely sensitive to confounds fromconvergent evolution.
A graphical representation of aphylogeny, consisting of a branching, tree-like diagram showing the evolutionary relationships between biologicalspecies or othertaxa as inferred from similarities and differences in their morphological or genetic characteristics, and how they have all descended from acommon ancestor.
The study of the evolutionary history of and relationships between individuals or groups of organisms, such as species or populations, through methods that evaluate similarities and differences between observedheritabletraits, includingmorphological features andDNA sequences. The graphical presentation of data from such analyses is known as aphylogeny orphylogenetic tree.
The phenomenon by which onegene influences two or more seemingly unrelatedphenotypic traits, by any of several distinct but potentially overlapping mechanisms.
Anancestral character state; i.e. the state or condition of a particular trait or feature (e.g. a specificphenotype) that is present in thecommon ancestor of a givenclade. Plesiomorphies may or may not beshared by some or all descendants within the clade. The term is relative; a trait considered a plesiomorphy in one clade may not be considered a plesiomorphy in a different clade. Contrastapomorphy.
The regular and simultaneous occurrence in the same population of two or morealleles (orgenotypes) at the samelocus atfrequencies that cannot be accounted for by recurrent mutation alone (generally at least 1%), implying that the multiple alleles are being stably inherited by members of the population.[14]
The grouping of organisms which do not share an immediatecommon ancestor; such groups are said to bepolyphyletic. The term is often applied to groups of organisms that share characteristics which appear to be similar but are not actually closely related, frequently as a result ofconvergent evolution. The avoidance of polyphyletic groupings is often a stimulus for major revisions of biological classification schemes. Contrastmonophyly andparaphyly.
(of acell or organism) Having more than twohomologous copies of eachchromosome. Polyploidy may occur as a normal condition of chromosomes in certain cells or even entire organisms, or it may result from abnormal cell division or a mutation causing theduplication of the entire chromosome set. Contrasthaploid anddiploid; see alsoploidy.
A group of organisms of the samespecies which occupies a more or less well-defined geographic region and which exhibits reproductive continuity fromgeneration to generation. It is generally presumed that ecological and reproductive interactions occur more frequently among the members of the group than between them and members of other populations of the same species.[1]
A sharp, often sudden reduction in the size of a biologicalpopulation, often due to a major environmental event such as a flood, fire, volcanic eruption, drought, famine, or disease. Because only a small population with a narrower range ofgenetic diversity remains afterward to pass on genes to future generations, such events tend to reduce thegenetic variation in the population'sgene pool, and often lead to new and distinct populations throughfounder effects. Diversity increases again only whengene flow from another population occurs, or very slowly over time as random mutations accumulate.
Any effect on theexpression or functionality of agene or sequence that is a consequence of its location or position within achromosome or other DNA molecule. A sequence's precise location relative to other sequences and structures tends to strongly influence its activity and other properties, because differentloci on the same molecule can have substantially differentgenetic backgrounds and physical/chemical environments, which may also change over time. For example, thetranscription of a gene located very close to anucleosome,centromere, ortelomere is oftenrepressed or entirely prevented because the proteins that make up these structures block access to the DNA bytranscription factors, while the same gene is transcribed at a much higher rate when located ineuchromatin. Proximity topromoters,enhancers, and otherregulatory elements, as well as to regions of frequenttransposition bymobile elements, can also directly affect expression; being located near the end of a chromosomal arm or to commoncrossover points may affect whenreplication occurs and the likelihood ofrecombination. Position effects are a major focus of research in the field ofepigenetic inheritance.
Possession of the necessary properties to permit a shift into a newniche or habitat. A structure is said to bepreadapted if it canassume a new function before it itself becomes modified byselection.[1]
Any of two or more different genes or sequences which have the same or similar contributions tophenotype, and thus appear to be genuinealleles, but are not actually structurally allelic (i.e. they do not occupy homologousloci onhomologous chromosomes).[24]
A non-functional sequence of DNA that resembles a functionalgene. Pseudogenes are typically superfluous copies of functional genes which have beenduplicated by natural processes, except that they lackregulatory sequences necessary for propertranscription ortranslation or contain other defects such asframeshift mutations, prematurestop codons, or missingintrons.
pseudopolyploidy
1. The condition in which the number ofchromosomes in a chromosome set isdoubled (or tripled, etc.) but without a corresponding increase in the actual amount of genetic material (i.e. theploidy level). This occurs when the chromosomes of a normal chromosome complement (e.g.diploid) become fragmented into smaller pieces, increasing the total number of individual chromosomes but not creating additional homologous copies of those chromosomes (such that the cell remains diploid).[24]
2. Any numerical relationship between chromosome sets in groups of related organisms which suggests that some of those organisms arepolyploids of others when in fact they are not.[24]
A tabular diagram used to predict the possiblegenotypes that can be inherited by theoffspring of a particularcross or breeding experiment by summarizing all of the various combinations of maternalalleles with paternal alleles. The resulting table can then be used to determine the probabilities that the offspring will have a particular genotype. The usefulness of Punnett squares is limited to discretephenotypes inherited according to simpleMendelian patterns.
A specificnucleotide sequence suspected to be a functionalgene based on the identification of itsopen reading frame. The gene is said to be "putative" in the sense that no function has yet been described for itsproducts.
A branch ofpopulation genetics which studiesphenotypes that vary continuously (such as height or mass) as opposed to those that fall into discretely identifiable categories (such as eye color or the presence or absence of a particulartrait). Quantitative genetics employs statistical methods and concepts to link continuously distributed phenotypic values to specificgenotypes andgene products.
A chromosomal model of speciation that occurs rapidly when a cross-fertilizing plant species buds off from a larger population on the periphery, experiencing interbreeding and strong genetic drift that results in a new species.[27][28][29] The model is similar to that of Ernst Mayr'speripatric speciation.[30]
Thenatural selection of combinations of traits in organisms or species which appear to involve a trade-off between quantity and quality ofoffspring, whereby an organism or species may evolve to make use of either of two different reproductive strategies:r-strategists tend to produce many, low-quality offspring, yielding large numbers of progeny during their lifespan but investing little or no energy in nurturing or protecting them, whereasK-strategists tend to produce few, high-quality offspring, yielding small numbers of progeny but with a corresponding increase in parental investment. Which strategy evolves depends on which one results in greaterreproductive success, which itself often depends on the stability of the organism's environment. In an unstable environment, where the probability that any individual offspring will survive to maturity is low, investment in parental care may not be sensible, and the parent may be more likely to pass on its genetic material if it dedicates its metabolic energy to simply producing as many offspring as possible rather than to parenting. Conversely, in more stable environments where survival to maturity is relatively common, the parent may find greater success if it dedicates more time and energy to parental care, improving each individual offspring's likelihood of reproducing successfully. The different strategies are often accompanied by characteristic anatomical or physiological traits, e.g.r-selected species often have small body size, rapid development, and short lifespans.
A relationship between thealleles of agene in which one allele produces an effect onphenotype that is overpowered or "masked" by the contribution of another allele at the samelocus; the first allele and its associated phenotypictrait are said to berecessive, and the second allele and its associated trait are said to bedominant. Often, recessive alleles code for inefficient or dysfunctional proteins. Like dominance, recessiveness is not an inherent property of any allele or phenotype, but simply describes its relationship to one or more other alleles or phenotypes. In genetics shorthand, recessive alleles are often represented by a lowercase letter (e.g. "a", in contrast to the dominant "A").
Acrossbreeding experiment designed to test whether parental sex influences the inheritance of a particular trait. In crosses where the parents differ ingenotype orphenotype or both, and hence only one of the parents (either the male or female parent) expresses the trait of interest, the reciprocal cross is the inverse, in which the parent of the other sex expresses the trait of interest instead. For example, if in the first cross a male expressing the trait is crossed with a female not expressing it, then in the reciprocal cross a female expressing the trait is crossed with a male not expressing it. By observing theprogeny resulting from each cross, geneticists can make inferences about whichsex chromosome, if either, influences the trait's expression.
A type ofchromosomal translocation by which there is a reciprocal exchange of chromosome segments between two or more non-homologouschromosomes. When the exchange of material is evenly balanced, reciprocal translocations are usually harmless.
A species or population that is the last surviving representative of an otherwiseextinct group,taxon,lineage, orclade, or which has been left behind in a locality after extinction throughout most of a formerly larger geographic distribution.[1]
A graphical representation of the relationship between the number of individuals at a given stage of onegeneration and the number of individuals at the same stage in a previous generation.[24]
The condition in whichinterbreeding between two or morepopulations of organisms is prevented by intrinsic factors, such that the members of one population cannotmate with the members of another population and produce fertileoffspring. The evolution of reproductive isolation between members of different populations is usually considered the first step in the process ofspeciation, because it effectively preventsgene flow between the populations and thereby allows each to evolve independently; hence the existence of reproductive barriers is often used as a criterion by which to define species in variousspecies concepts. Isolation may occur when the populations are physically separated by environmental changes or migration such that members of the other population are simply inaccessible, or it may occur when anatomical or genetic differences make copulation between members of different populations impossible or at least ensure that any offspring that happen to develop are sterile, even though the populations are not physically separated from each other. Isolating mechanisms are typically classified asprezygotic (isolating barriers occurring before the formation of azygote) andpostzygotic (isolating barriers occurring after the formation of a zygote).
The successful production ofoffspring by an individual, often quantified as the number of offspring produced by the individual per reproductive event or during the individual's entire lifespan, or as the number of an individual's offspring that survive to reproductive maturity themselves or that are surviving at a given time.[24]
reproductivity effect
The decrease in the rate of reproduction of new individuals per colony member as colony size increases.[24]
rescue
The restoration of a defective cell, tissue, or organism to a healthy or normal condition,[8] or thereversion or recovery of amutant gene to its normal functionality, especially in the context of experimental genetics, where an experiment (e.g. a drug,cross, or gene transfer) resulting in such a restoration is said torescue the normalphenotype.
An experimental approach inmolecular genetics in which a researcher starts with a knowngene and attempts to determine its function or its effect onphenotype by any of a variety of laboratory techniques, commonly by deliberately mutating the gene's nucleic acid sequence or by repressing or silencing its expression and thenscreening the mutated organisms for obvious changes in phenotype. When thegene of interest is the only one in the genome whose expression has been manipulated, any observed phenotypic changes are assumed to be influenced by it. This is the opposite offorward genetics, in which a known phenotype is linked to one or more unknown genes.
Anymutation in a gene or DNA sequence which restores orrescues the original function or phenotype that was altered or destroyed by a previous mutation in the same sequence.[8] Contrastforward mutation; see alsosuppressor mutation.
A gene or allele in which areverse mutation occurs, or an organism bearing such a gene or allele.[8]
In aring species, individuals are able to successfully reproduce and exchange genes with members of their own species in adjacent populations occupying a suitable habitat around a geographic barrier. Individuals at the ends of the cline are unable to reproduce when they come into contact.
Connectedpopulations of the samespecies, each of which can interbreed with closely sited, closely related populations, but for which there exist at least two "end" populations in the series which are too distantly related to interbreed.
A type ofchromosomal translocation by whichdouble-strand breaks at or near thecentromeres of twoacrocentricchromosomes cause a reciprocal exchange of segments that gives rise to one largemetacentric chromosome (composed of thelong arms) and one extremely small chromosome (composed of theshort arms), the latter of which is often subsequently lost from the cell with little effect because it contains very few genes. The resultingkaryotype shows one fewer than the expected total number of chromosomes, because two previously distinct chromosomes have essentially fused together.Carriers of Robertsonian translocations are generally not associated with any phenotypic abnormalities, but do have an increased risk of generating meiotically unbalancedgametes.
The persistence of a certain phenotypic trait or characteristic in a biological system despite perturbations or conditions of uncertainty. Robustness is achieved through the combination of many genetic and molecular mechanisms which effectively preserve the integrity of a particularadaptation, and can evolve by direct or indirect selection.
A sudden and large mutational change from onegeneration to the next which is sufficient to cause rapid or immediatespeciation. Various forms of saltation, such as bypolyploidy in plants, have often historically been interpreted as evidence for certain theories ofmutationism, in contrast toDarwiniangradualism.
The process by which twoallopatrically distributed populations of a species are geographically reunited. Contact between divergent populations may renew the potential forgene flow between them, depending upon howreproductively isolated the populations have become.
The four outcomes ofsecondary contact: 1. Anextrinsic barrier separates a species population into two but they come into contact before reproductive isolation is sufficient to result in speciation. The two populations fuse back into one species. 2. Speciation by reinforcement. 3. Two separated populations stay genetically distinct whilehybrid swarms form in the zone of contact. 4.Genome recombination results in speciation of the two populations, with an additionalhybrid species. All three species are separated byintrinsic reproductive barriers.[31]
The process by which strongpositive selection of a new and beneficialmutation within a population causes the mutation to reachfixation so quickly that nearbylinked DNA sequences also become fixed viagenetic hitchhiking, thereby reducing or eliminating thegenetic variation of nearby loci within the population.
A reproductive strategy characterized by a single reproductive episode during an individual organism's lifetime, especially one in which the programmed death of the organism immediately after the reproductive event constitutes part of an overall strategy that includes putting all available resources into maximizing the probability of reproductive success, at the expense of the organism's future life. Organisms that use such a strategy are said to besemelparous. Semelparity is usually contrasted withiteroparity.
semi-geographic speciation
semipermeable species boundary
The idea that gene flow can occur between two species but that certainalleles at particularloci can exchange whereas others cannot.[22] It is often used to describe hybrid zones and has also been referred to asporous.[22]
One of several groups ofpopulations that are partially but not entirelyreproductively isolated from each other by biological isolating mechanisms,[1] and which are therefore neither easily definable as belonging to the samespecies nor to separate species. The taxon of species itself isnot a well-defined concept.
A basic unit ofbiological classification, traditionally interpreted according to thebiological species concept as the members in aggregate of a group ofpopulations of organisms which interbreed or potentially interbreed with each other under natural conditions;[1] a basictaxonomic rank to which individual specimens are assigned and which often but not always corresponds to the definition of a biological species; and a fundamental unit used to interpret and measurebiodiversity in ecological contexts. The concept of species is notoriously complex and oftenproblematic to define precisely; many differentconceptualizations of what is or should be meant by the term have been defined in scientific literature.
A named geographicrace, or a set ofpopulations of the samespecies which share one or more distinctive features and occupy an area that is geographically separate from other subspecies.[1] Not all species are formally divided into subspecies, and the taxon of species itself isnot a well-defined concept.
Abiogeographic approach to species distributions that uses their phylogenetic histories—patterns resulting fromallopatric speciation events in the past.[34]
A phenomenon by which a reduction ofheterozygosity at a particular genetic locus within a population as a whole is observed when two or more subpopulations have differentallele frequencies at that locus, even if the subpopulations themselves are each inHardy–Weinberg equilibrium.
Thephenotype of the typical form of an organism orspecies as it occurs in nature; a product of the standard or "normal"allele at a givenlocus, as opposed to that produced by a non-standardmutant allele.
The degree to which multiple copies of agene,chromosome, orgenome have the same genetic sequence; e.g. in adiploid organism with two complete copies of its genome (one maternal and one paternal), the degree of similarity of thealleles present in each copy. Individuals carrying two different alleles for a particular gene are said to beheterozygous for that gene; individuals carrying two identical alleles are said to behomozygous for that gene. Zygosity may also be considered collectively for a group of genes, or for the entire set of genes and geneticloci comprising the genome.
A type ofeukaryotic cell formed as the direct result of afertilization event between twogametes. In multicellular organisms, the zygote is the earliest developmental stage.
^Anders Ödeen & Ann-Britt Florin (2002), "Sexual selection and peripatric speciation: the Kaneshiro model revisited",Journal of Evolutionary Biology,15 (2):301–306,doi:10.1046/j.1420-9101.2002.00378.x,S2CID82095639
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