Biology is thescientific study oflife and livingorganisms. It is a broadnatural science that encompasses a wide range of fields and unifying principles that explain the structure, function, growth,origin,evolution, and distribution of life. Central to biology are five fundamental themes: thecell as the basic unit of life,genes andheredity as the basis of inheritance, evolution as the driver ofbiological diversity,energy transformation for sustaining life processes, and the maintenance of internal stability (homeostasis).[1][2]
Life onEarth is believed to have originated over 3.7 billion years ago.[3] Today, it includes a vast diversity of organisms—from single-celledarchaea andbacteria to complex multicellularplants,fungi, andanimals.Biologists classify organisms based on shared characteristics and evolutionary relationships, usingtaxonomic andphylogenetic frameworks. These organisms interact with each other and with their environments in ecosystems, where they play roles inenergy flow andnutrient cycling. As a constantly evolving field, biology incorporates new discoveries and technologies that enhance the understanding of life and its processes, while contributing to solutions for challenges such asdisease,climate change, andbiodiversity loss.
The earliest of roots of science, which included medicine, can be traced toancient Egypt andMesopotamia in around 3000 to 1200BCE.[8][9] Their contributions shaped ancient Greeknatural philosophy.[10][8][9][11][12]Ancient Greek philosophers such asAristotle (384–322 BCE) contributed extensively to the development of biological knowledge.[13] He explored biological causation and the diversity of life. His successor,Theophrastus, began the scientific study of plants.[14] Scholars of themedieval Islamic world who wrote on biology includedal-Jahiz (781–869),Al-Dīnawarī (828–896), who wrote on botany,[15] andRhazes (865–925) who wrote onanatomy andphysiology. Medicine was especially well studied byIslamic scholars working in Greek philosopher traditions, while natural history drew heavily on Aristotelian thought.
Biology began to quickly develop withAnton van Leeuwenhoek's dramatic improvement of themicroscope. It was then that scholars discoveredspermatozoa, bacteria,infusoria and the diversity of microscopic life. Investigations byJan Swammerdam led to new interest inentomology and helped to develop techniques of microscopicdissection andstaining.[16] Advances in microscopy had a profound impact on biological thinking. In the early 19th century, biologists pointed to the central importance of thecell. In 1838,Schleiden andSchwann began promoting the now universal ideas that (1) the basic unit of organisms is the cell and (2) that individual cells have all the characteristics of life, although they opposed the idea that (3) all cells come from the division of other cells, continuing to supportspontaneous generation. However,Robert Remak andRudolf Virchow were able to reify the third tenet, and by the 1860s most biologists accepted all three tenets which consolidated intocell theory.[17][18]
Serious evolutionary thinking originated with the works ofJean-Baptiste Lamarck, who presented a coherent theory of evolution.[22] The BritishnaturalistCharles Darwin, combining the biogeographical approach ofHumboldt, the uniformitarian geology ofLyell,Malthus's writings on population growth, and his own morphological expertise and extensive natural observations, forged a more successful evolutionary theory based onnatural selection; similar reasoning and evidence ledAlfred Russel Wallace to independently reach the same conclusions.[23][24]
Model of hydrogen bonds (1) between molecules ofwater
Life arose from the Earth's first ocean, which formed some 3.8 billion years ago.[30] Since then, water continues to be the most abundant molecule in every organism. Water is important to life because it is an effectivesolvent, capable of dissolving solutes such as sodium andchloride ions or other small molecules to form anaqueoussolution. Once dissolved in water, these solutes are more likely to come in contact with one another and therefore take part inchemical reactions that sustain life.[30] In terms of itsmolecular structure, water is a smallpolar molecule with a bent shape formed by the polar covalent bonds of two hydrogen (H) atoms to one oxygen (O) atom (H2O).[30] Because the O–H bonds are polar, the oxygen atom has a slight negative charge and the two hydrogen atoms have a slight positive charge.[30] This polarproperty of water allows it to attract other water molecules via hydrogen bonds, which makes watercohesive.[30]Surface tension results from the cohesive force due to the attraction between molecules at the surface of the liquid.[30] Water is alsoadhesive as it is able to adhere to the surface of any polar or charged non-water molecules.[30] Water isdenser as aliquid than it is as a solid (or ice).[30] This unique property of water allows ice to float above liquid water such as ponds, lakes, and oceans, therebyinsulating the liquid below from the cold air above.[30] Water has the capacity to absorb energy, giving it a higherspecific heat capacity than other solvents such asethanol.[30] Thus, a large amount of energy is needed to break the hydrogen bonds between water molecules to convert liquid water intowater vapor.[30] As a molecule, water is not completely stable as each water molecule continuously dissociates into hydrogen andhydroxyl ions before reforming into a water molecule again.[30] Inpure water, the number of hydrogen ions balances (or equals) the number of hydroxyl ions, resulting in apH that is neutral.
Organic compounds such asglucose are vital to organisms.
Organic compounds are molecules that contain carbon bonded to another element such as hydrogen.[30] With the exception of water, nearly all the molecules that make up each organism contain carbon.[30][31] Carbon can formcovalent bonds with up to four other atoms, enabling it to form diverse, large, and complex molecules.[30][31] For example, a single carbon atom can form four single covalent bonds such as inmethane, twodouble covalent bonds such as incarbon dioxide (CO2), or atriple covalent bond such as incarbon monoxide (CO). Moreover, carbon can form very long chains of interconnectingcarbon–carbon bonds such asoctane or ring-like structures such asglucose.
The simplest form of an organic molecule is thehydrocarbon, which is a large family of organic compounds that are composed of hydrogen atoms bonded to a chain of carbon atoms. A hydrocarbon backbone can be substituted by other elements such as oxygen (O), hydrogen (H), phosphorus (P), and sulfur (S), which can change the chemical behavior of that compound.[30] Groups of atoms that contain these elements (O-, H-, P-, and S-) and are bonded to a central carbon atom or skeleton are calledfunctional groups.[30] There are six prominent functional groups that can be found in organisms:amino group,carboxyl group,carbonyl group,hydroxyl group,phosphate group, andsulfhydryl group.[30]
In 1953, theMiller–Urey experiment showed that organic compounds could be synthesized abiotically within a closed system mimicking the conditions ofearly Earth, thus suggesting that complex organic molecules could have arisen spontaneously in early Earth (seeabiogenesis).[32][30]
The (a) primary, (b) secondary, (c) tertiary, and (d) quaternary structures of ahemoglobin protein
Macromolecules are large molecules made up of smaller subunits ormonomers.[33] Monomers include sugars, amino acids, and nucleotides.[34]Carbohydrates include monomers and polymers of sugars.[35]Lipids are the only class of macromolecules that are not made up of polymers. They includesteroids,phospholipids, and fats,[34] largely nonpolar and hydrophobic (water-repelling) substances.[36]Proteins are the most diverse of the macromolecules. They includeenzymes,transport proteins, largesignaling molecules,antibodies, andstructural proteins. The basic unit (or monomer) of a protein is anamino acid.[33] Twenty amino acids are used in proteins.[33]Nucleic acids are polymers ofnucleotides.[37] Their function is to store, transmit, and express hereditary information.[34]
Cell theory states thatcells are the fundamental units of life, that all living things are composed of one or more cells, and that all cells arise from preexisting cells throughcell division.[38] Most cells are very small, with diameters ranging from 1 to 100 micrometers and are therefore only visible under alight orelectron microscope.[39] There are generally two types of cells:eukaryotic cells, which contain anucleus, andprokaryotic cells, which do not. Prokaryotes aresingle-celled organisms such asbacteria, whereas eukaryotes can be single-celled or multicellular. Inmulticellular organisms, every cell in the organism's body is derived ultimately from asingle cell in a fertilizedegg.
Within the cytoplasm of a cell, there are many biomolecules such asproteins andnucleic acids.[43] In addition to biomolecules, eukaryotic cells have specialized structures calledorganelles that have their own lipid bilayers or are spatially units.[44] These organelles include thecell nucleus, which contains most of the cell's DNA, ormitochondria, which generateadenosine triphosphate (ATP) to power cellular processes. Other organelles such asendoplasmic reticulum andGolgi apparatus play a role in the synthesis and packaging of proteins, respectively. Biomolecules such as proteins can be engulfed bylysosomes, another specialized organelle.Plant cells have additional organelles that distinguish them fromanimal cells such as a cell wall that provides support for the plant cell,chloroplasts that harvest sunlight energy to produce sugar, andvacuoles that provide storage and structural support as well as being involved in reproduction and breakdown of plant seeds.[44] Eukaryotic cells also have cytoskeleton that is made up ofmicrotubules,intermediate filaments, andmicrofilaments, all of which provide support for the cell and are involved in the movement of the cell and its organelles.[44] In terms of their structural composition, the microtubules are made up oftubulin (e.g.,α-tubulin andβ-tubulin) whereas intermediate filaments are made up of fibrous proteins.[44] Microfilaments are made up ofactin molecules that interact with other strands of proteins.[44]
All cells require energy to sustain cellular processes.Metabolism is the set ofchemical reactions in an organism. The three main purposes of metabolism are: the conversion of food to energy to run cellular processes; the conversion of food/fuel to monomer building blocks; and the elimination ofmetabolic wastes. These enzyme-catalyzed reactions allow organisms to grow and reproduce, maintain their structures, and respond to their environments. Metabolic reactions may be categorized ascatabolic—the breaking down of compounds (for example, the breaking down of glucose to pyruvate bycellular respiration); oranabolic—the building up (synthesis) of compounds (such as proteins, carbohydrates, lipids, and nucleic acids). Usually, catabolism releases energy, and anabolism consumes energy. The chemical reactions of metabolism are organized intometabolic pathways, in which one chemical is transformed through a series of steps into another chemical, each step being facilitated by a specific enzyme. Enzymes are crucial to metabolism because they allow organisms to drive desirable reactions that require energy that will not occur by themselves, bycoupling them tospontaneous reactions that release energy. Enzymes act ascatalysts—they allow a reaction to proceed more rapidly without being consumed by it—by reducing the amount ofactivation energy needed to convertreactants intoproducts. Enzymes also allow theregulation of the rate of a metabolic reaction, for example in response to changes in the cell's environment or to signals from other cells.
Cellular respiration is a set of metabolic reactions and processes that take place in cells to convertchemical energy fromnutrients into adenosine triphosphate (ATP), and then release waste products.[45] The reactions involved in respiration arecatabolic reactions, which break large molecules into smaller ones, releasing energy. Respiration is one of the key ways a cell releases chemical energy to fuel cellular activity. The overall reaction occurs in a series of biochemical steps, some of which areredox reactions. Although cellular respiration is technically acombustion reaction, it clearly does not resemble one when it occurs in a cell because of the slow, controlled release of energy from the series of reactions.
Sugar in the form of glucose is the main nutrient used by animal and plant cells in respiration. Cellular respiration involving oxygen is called aerobic respiration, which has four stages:glycolysis,citric acid cycle (or Krebs cycle),electron transport chain, andoxidative phosphorylation.[46] Glycolysis is a metabolic process that occurs in the cytoplasm whereby glucose is converted into twopyruvates, with two net molecules of ATP being produced at the same time.[46] Each pyruvate is then oxidized intoacetyl-CoA by thepyruvate dehydrogenase complex, which also generatesNADH and carbon dioxide. Acetyl-CoA enters the citric acid cycle, which takes places inside the mitochondrial matrix. At the end of the cycle, the total yield from 1 glucose (or 2 pyruvates) is 6 NADH, 2 FADH2, and 2 ATP molecules. Finally, the next stage is oxidative phosphorylation, which in eukaryotes, occurs in themitochondrial cristae. Oxidative phosphorylation comprises the electron transport chain, which is a series of fourprotein complexes that transfer electrons from one complex to another, thereby releasing energy from NADH and FADH2 that is coupled to the pumping of protons (hydrogen ions) across the inner mitochondrial membrane (chemiosmosis), which generates aproton motive force.[46] Energy from the proton motive force drives the enzymeATP synthase to synthesize more ATPs byphosphorylatingADPs. The transfer of electrons terminates with molecular oxygen being the finalelectron acceptor.
If oxygen were not present, pyruvate would not be metabolized by cellular respiration but undergoes a process offermentation. The pyruvate is not transported into the mitochondrion but remains in the cytoplasm, where it is converted towaste products that may be removed from the cell. This serves the purpose of oxidizing the electron carriers so that they can perform glycolysis again and removing the excess pyruvate. Fermentation oxidizes NADH to NAD+ so it can be re-used in glycolysis. In the absence of oxygen, fermentation prevents the buildup of NADH in the cytoplasm and provides NAD+ for glycolysis. This waste product varies depending on the organism. In skeletal muscles, the waste product islactic acid. This type of fermentation is calledlactic acid fermentation. In strenuous exercise, when energy demands exceed energy supply, the respiratory chain cannot process all of the hydrogen atoms joined by NADH. During anaerobic glycolysis, NAD+ regenerates when pairs of hydrogen combine with pyruvate to form lactate. Lactate formation is catalyzed by lactate dehydrogenase in a reversible reaction. Lactate can also be used as an indirect precursor for liver glycogen. During recovery, when oxygen becomes available, NAD+ attaches to hydrogen from lactate to form ATP. In yeast, the waste products are ethanol and carbon dioxide. This type of fermentation is known as alcoholic orethanol fermentation. The ATP generated in this process is made bysubstrate-level phosphorylation, which does not require oxygen.
Photosynthesis changes sunlight into chemical energy, splits water to liberate O2, and fixes CO2 into sugar.
Photosynthesis is a process used by plants and other organisms toconvertlight energy intochemical energy that can later be released to fuel the organism's metabolic activities via cellular respiration. This chemical energy is stored in carbohydrate molecules, such as sugars, which are synthesized from carbon dioxide and water.[47][48][49] In most cases, oxygen is released as a waste product. Most plants,algae, andcyanobacteria perform photosynthesis, which is largely responsible for producing and maintaining theoxygen content of the Earth's atmosphere, and supplies most of the energy necessary for life on Earth.[50]
Photosynthesis has four stages:Light absorption, electron transport, ATP synthesis, andcarbon fixation.[46] Light absorption is the initial step of photosynthesis whereby light energy is absorbed bychlorophyll pigments attached to proteins in thethylakoid membranes. The absorbed light energy is used to remove electrons from a donor (water) to a primary electron acceptor, aquinone designated as Q. In the second stage, electrons move from the quinone primary electron acceptor through a series of electron carriers until they reach a final electron acceptor, which is usually the oxidized form of NADP+, which is reduced to NADPH, a process that takes place in a protein complex calledphotosystem I (PSI). The transport of electrons is coupled to the movement of protons (or hydrogen) from the stroma to the thylakoid membrane, which forms a pH gradient across the membrane as hydrogen becomes more concentrated in the lumen than in the stroma. This is analogous to the proton-motive force generated across the inner mitochondrial membrane in aerobic respiration.[46]
During the third stage of photosynthesis, the movement of protons down theirconcentration gradients from the thylakoid lumen to the stroma through the ATP synthase is coupled to the synthesis of ATP by that same ATP synthase.[46] The NADPH and ATPs generated by thelight-dependent reactions in the second and third stages, respectively, provide the energy and electrons to drive the synthesis of glucose by fixing atmospheric carbon dioxide into existing organic carbon compounds, such asribulose bisphosphate (RuBP) in a sequence of light-independent (or dark) reactions called theCalvin cycle.[51]
Cell signaling (or communication) is the ability ofcells to receive, process, and transmit signals with its environment and with itself.[52][53] Signals can be non-chemical such as light,electrical impulses, and heat, or chemical signals (orligands) that interact withreceptors, which can be foundembedded in thecell membrane of another cell orlocated deep inside a cell.[54][53] There are generally four types of chemical signals:autocrine,paracrine,juxtacrine, andhormones.[54] In autocrine signaling, the ligand affects the same cell that releases it.Tumor cells, for example, can reproduce uncontrollably because they release signals that initiate their own self-division. In paracrine signaling, the ligand diffuses to nearby cells and affects them. For example, brain cells calledneurons release ligands calledneurotransmitters that diffuse across asynaptic cleft to bind with a receptor on an adjacent cell such as another neuron ormuscle cell. In juxtacrine signaling, there is direct contact between the signaling and responding cells. Finally, hormones are ligands that travel through thecirculatory systems of animals orvascular systems of plants to reach their target cells. Once a ligand binds with a receptor, it can influence the behavior of another cell, depending on the type of receptor. For instance, neurotransmitters that bind with aninotropic receptor can alter theexcitability of a target cell. Other types of receptors includeprotein kinase receptors (e.g.,receptor for the hormoneinsulin) andG protein-coupled receptors. Activation of G protein-coupled receptors can initiatesecond messenger cascades. The process by which a chemical or physical signal is transmitted through a cell as aseries of molecular events is calledsignal transduction.
The cell cycle is a series of events that take place in acell that cause it to divide into two daughter cells. These events include theduplication of its DNA and some of itsorganelles, and the subsequent partitioning of its cytoplasm into two daughter cells in a process calledcell division.[55] Ineukaryotes (i.e., animal, plant,fungal, andprotist cells), there are two distinct types of cell division:mitosis andmeiosis.[56] Mitosis is part of the cell cycle, in which replicatedchromosomes are separated into two new nuclei. Cell division gives rise to genetically identical cells in which the total number of chromosomes is maintained. In general, mitosis (division of the nucleus) is preceded by the S stage ofinterphase (during which the DNA is replicated) and is often followed bytelophase andcytokinesis; which divides thecytoplasm,organelles andcell membrane of one cell into two newcells containing roughly equal shares of these cellular components. The different stages of mitosis all together define the mitotic phase of an animal cell cycle—the division of the mother cell into two genetically identical daughter cells.[57] The cell cycle is a vital process by which a single-celledfertilized egg develops into a mature organism, as well as the process by which hair, skin,blood cells, and someinternal organs are renewed. After cell division, each of the daughter cells begin theinterphase of a new cycle. In contrast to mitosis, meiosis results in four haploid daughter cells by undergoing one round of DNA replication followed by two divisions.[58]Homologous chromosomes are separated in the first division (meiosis I), and sister chromatids are separated in the second division (meiosis II). Both of these cell division cycles are used in the process of sexual reproduction at some point in their life cycle. Both are believed to be present in the last eukaryotic common ancestor.
In meiosis, the chromosomes duplicate and thehomologous chromosomes exchange genetic information during meiosis I. The daughter cells divide again in meiosis II to form haploidgametes.
Prokaryotes (i.e.,archaea and bacteria) can also undergo cell division (orbinary fission). Unlike the processes ofmitosis andmeiosis in eukaryotes, binary fission in prokaryotes takes place without the formation of aspindle apparatus on the cell. Before binary fission, DNA in the bacterium is tightly coiled. After it has uncoiled and duplicated, it is pulled to the separate poles of the bacterium as it increases the size to prepare for splitting. Growth of a new cell wall begins to separate the bacterium (triggered byFtsZ polymerization and "Z-ring" formation).[59] The new cell wall (septum) fully develops, resulting in the complete split of the bacterium. The new daughter cells have tightly coiled DNA rods,ribosomes, andplasmids.
Sexual reproduction and meiosis
Meiosis is a central feature of sexual reproduction in eukaryotes, and the most fundamental function ofmeiosis appears to be conservation of the integrity of thegenome that is passed on to progeny by parents.[60][61] Two aspects ofsexual reproduction,meiotic recombination andoutcrossing, are likely maintained respectively by the adaptive advantages of recombinational repair of genomicDNA damage and geneticcomplementation which masks the expression of deleterious recessivemutations.[62]
The beneficial effect of genetic complementation, derived from outcrossing (cross-fertilization) is also referred to as hybrid vigor or heterosis. Charles Darwin in his 1878 bookThe Effects of Cross and Self-Fertilization in the Vegetable Kingdom[63] at the start of chapter XII noted "The first and most important of the conclusions which may be drawn from the observations given in this volume, is that generally cross-fertilisation is beneficial and self-fertilisation often injurious, at least with the plants on which I experimented."Genetic variation, often produced as a byproduct of sexual reproduction, may provide long-term advantages to those sexual lineages that engage inoutcrossing.[62]
Punnett square depicting a cross between two pea plants heterozygous for purple (B) and white (b) blossoms
Genetics is the scientific study of inheritance.[64][65][66]Mendelian inheritance, specifically, is the process by which genes and traits are passed on from parents to offspring.[26] It has several principles. The first is that genetic characteristics,alleles, are discrete and have alternate forms (e.g., purple vs. white or tall vs. dwarf), each inherited from one of two parents. Based on thelaw of dominance and uniformity, which states that some alleles aredominant while others arerecessive; an organism with at least one dominant allele will display thephenotype of that dominant allele. During gamete formation, the alleles for each gene segregate, so that each gamete carries only one allele for each gene.Heterozygotic individuals produce gametes with an equal frequency of two alleles. Finally, thelaw of independent assortment, states that genes of different traits can segregate independently during the formation of gametes, i.e., genes are unlinked. An exception to this rule would include traits that aresex-linked.Test crosses can be performed to experimentally determine the underlyinggenotype of an organism with a dominant phenotype.[67] APunnett square can be used to predict the results of a test cross. Thechromosome theory of inheritance, which states that genes are found on chromosomes, was supported byThomas Morgans's experiments withfruit flies, which established thesex linkage between eye color and sex in these insects.[68]
A gene is a unit ofheredity that corresponds to a region of deoxyribonucleic acid (DNA) that carries genetic information that controls form or function of an organism. DNA is composed of twopolynucleotide chains that coil around each other to form adouble helix.[69] It is found as linearchromosomes ineukaryotes, and circular chromosomes inprokaryotes. The set of chromosomes in a cell is collectively known as itsgenome. In eukaryotes, DNA is mainly in thecell nucleus.[70] In prokaryotes, the DNA is held within thenucleoid.[71] The genetic information is held within genes, and the complete assemblage in an organism is called itsgenotype.[72]DNA replication is asemiconservative process whereby each strand serves as a template for a new strand of DNA.[69] Mutations are heritable changes in DNA.[69] They can arisespontaneously as a result of replication errors that were not corrected by proofreading or can beinduced by an environmentalmutagen such as a chemical (e.g.,nitrous acid,benzopyrene) or radiation (e.g.,x-ray,gamma ray,ultraviolet radiation, particles emitted by unstable isotopes).[69] Mutations can lead to phenotypic effects such as loss-of-function,gain-of-function, and conditional mutations.[69]Some mutations are beneficial, as they are a source ofgenetic variation for evolution.[69] Others are harmful if they were to result in a loss of function of genes needed for survival.[69]
Gene expression is the molecular process by which agenotype encoded in DNA gives rise to an observablephenotype in the proteins of an organism's body. This process is summarized by thecentral dogma of molecular biology, which was formulated byFrancis Crick in 1958.[73][74][75] According to the Central Dogma, genetic information flows from DNA to RNA to protein. There are two gene expression processes:transcription (DNA to RNA) andtranslation (RNA to protein).[76]
The regulation of gene expression by environmental factors and during different stages ofdevelopment can occur at each step of the process such astranscription,RNA splicing,translation, andpost-translational modification of a protein.[77] Gene expression can be influenced by positive or negative regulation, depending on which of the two types of regulatory proteins calledtranscription factors bind to the DNA sequence close to or at a promoter.[77] A cluster of genes that share the same promoter is called anoperon, found mainly in prokaryotes and some lower eukaryotes (e.g.,Caenorhabditis elegans).[77][78] In positive regulation of gene expression, theactivator is the transcription factor that stimulates transcription when it binds to the sequence near or at the promoter. Negative regulation occurs when another transcription factor called arepressor binds to a DNA sequence called anoperator, which is part of an operon, to prevent transcription. Repressors can be inhibited by compounds calledinducers (e.g.,allolactose), thereby allowing transcription to occur.[77] Specific genes that can be activated by inducers are calledinducible genes, in contrast toconstitutive genes that are almost constantly active.[77] In contrast to both,structural genes encode proteins that are not involved in gene regulation.[77] In addition to regulatory events involving the promoter, gene expression can also be regulated byepigenetic changes tochromatin, which is a complex of DNA and protein found in eukaryotic cells.[77]
Development is the process by which amulticellular organism (plant or animal) goes through a series of changes, starting from a single cell, and taking on various forms that are characteristic of its life cycle.[79] There are four key processes that underlie development:Determination,differentiation,morphogenesis, and growth. Determination sets the developmental fate of a cell, which becomes more restrictive during development. Differentiation is the process by which specialized cells arise from less specialized cells such asstem cells.[80][81] Stem cells areundifferentiated or partially differentiatedcells that can differentiate into varioustypes of cells andproliferate indefinitely to produce more of the same stem cell.[82] Cellular differentiation dramatically changes a cell's size, shape,membrane potential,metabolic activity, and responsiveness to signals, which are largely due to highly controlled modifications ingene expression andepigenetics. With a few exceptions, cellular differentiation almost never involves a change in theDNA sequence itself.[83] Thus, different cells can have very different physical characteristics despite having the samegenome. Morphogenesis, or the development of body form, is the result of spatial differences in gene expression.[79] A small fraction of the genes in an organism's genome called thedevelopmental-genetic toolkit control the development of that organism. These toolkit genes are highly conserved amongphyla, meaning that they are ancient and very similar in widely separated groups of animals. Differences in deployment of toolkit genes affect the body plan and the number, identity, and pattern of body parts. Among the most important toolkit genes are theHox genes. Hox genes determine where repeating parts, such as the manyvertebrae of snakes, will grow in a developing embryo or larva.[84]
Evolution is a central organizing concept in biology. It is the change inheritablecharacteristics of populations over successivegenerations.[85][86] Inartificial selection, animals were selectively bred for specific traits.[87] Given that traits are inherited, populations contain a varied mix of traits, and reproduction is able to increase any population, Darwin argued that in the natural world, it was nature that played the role of humans in selecting for specific traits.[87] Darwin inferred that individuals who possessed heritable traits better adapted to their environments are more likely to survive and produce more offspring than other individuals.[87] He further inferred that this would lead to the accumulation of favorable traits over successive generations, thereby increasing the match between the organisms and their environment.[88][89][90][87][91]
A species is a group of organisms that mate with one another and speciation is the process by which one lineage splits into two lineages as a result of having evolved independently from each other.[92] For speciation to occur, there has to bereproductive isolation.[92] Reproductive isolation can result from incompatibilities between genes as described byBateson–Dobzhansky–Muller model. Reproductive isolation also tends to increase withgenetic divergence. Speciation can occur when there are physical barriers that divide an ancestral species, a process known asallopatric speciation.[92]
A phylogeny is an evolutionary history of a specific group of organisms or their genes.[93] It can be represented using aphylogenetic tree, a diagram showing lines of descent among organisms or their genes. Each line drawn on the time axis of a tree represents alineage of descendants of a particular species or population. When a lineage divides into two, it is represented as a fork or split on the phylogenetic tree.[93] Phylogenetic trees are the basis for comparing and grouping different species.[93] Different species that share a feature inherited from a common ancestor are described as havinghomologous features (orsynapomorphy).[94][95][93] Phylogeny provides the basis of biological classification.[93] This classification system is rank-based, with the highest rank being thedomain followed bykingdom,phylum,class,order,family,genus, andspecies.[93] All organisms can be classified as belonging to one ofthree domains: Archaea (originally Archaebacteria), Bacteria (originally eubacteria), or Eukarya (includes the fungi, plant, and animal kingdoms).[96]
The similarities among all known present-dayspecies indicate that they have diverged through the process ofevolution from their common ancestor.[101] Biologists regard the ubiquity of thegenetic code as evidence of universalcommon descent for allbacteria,archaea, andeukaryotes.[102][3][103][104]Microbial mats of coexisting bacteria and archaea were the dominant form of life in the earlyArchean eon and many of the major steps in early evolution are thought to have taken place in this environment.[105] The earliest evidence ofeukaryotes dates from 1.85 billion years ago,[106][107] and while they may have been present earlier, their diversification accelerated when they started using oxygen in theirmetabolism. Later, around 1.7 billion years ago,multicellular organisms began to appear, withdifferentiated cells performing specialised functions.[108]
Algae-like multicellular land plants are dated back to about 1 billion years ago,[109] although evidence suggests thatmicroorganisms formed the earliestterrestrial ecosystems, at least 2.7 billion years ago.[110] Microorganisms are thought to have paved the way for the inception of land plants in theOrdovician period. Land plants were so successful that they are thought to have contributed to theLate Devonian extinction event.[111]
The first observed archaea wereextremophiles, living in extreme environments, such ashot springs andsalt lakes with no other organisms. Improved molecular detection tools led to the discovery of archaea in almost everyhabitat, including soil, oceans, andmarshlands. Archaea are particularly numerous in the oceans, and the archaea inplankton may be one of the most abundant groups of organisms on the planet.
Archaea are a major part ofEarth's life. They are part of themicrobiota of all organisms. In thehuman microbiome, they are important in thegut, mouth, and on the skin.[126] Their morphological, metabolic, and geographical diversity permits them to play multiple ecological roles: carbon fixation; nitrogen cycling; organic compound turnover; and maintaining microbial symbiotic andsyntrophic communities, for example.[127]
Euglena, a single-celled eukaryote that can both move and photosynthesize
Eukaryotes are hypothesized to have split from archaea, which was followed by theirendosymbioses with bacteria (orsymbiogenesis) that gave rise to mitochondria and chloroplasts, both of which are now part of modern-day eukaryotic cells.[128] The major lineages of eukaryotes diversified in thePrecambrian about 1.5 billion years ago and can be classified into eight majorclades:alveolates,excavates,stramenopiles, plants,rhizarians,amoebozoans,fungi, and animals.[128] Five of these clades are collectively known asprotists, which are mostly microscopiceukaryotic organisms that are not plants, fungi, or animals.[128] While it is likely that protists share acommon ancestor (thelast eukaryotic common ancestor),[129] protists by themselves do not constitute a separate clade as some protists may be more closely related to plants, fungi, or animals than they are to other protists. Like groupings such asalgae,invertebrates, orprotozoans, the protist grouping is not a formal taxonomic group but is used for convenience.[128][130] Most protists are unicellular; these are called microbial eukaryotes.[128]
Plants are mainly multicellularorganisms, predominantlyphotosynthetic eukaryotes of thekingdom Plantae, which would exclude fungi and somealgae. Plant cells were derived by endosymbiosis of acyanobacterium into an early eukaryote about one billion years ago, which gave rise to chloroplasts.[131] The first several clades that emerged following primary endosymbiosis were aquatic and most of the aquatic photosynthetic eukaryotic organisms are collectively described as algae, which is a term of convenience as not all algae are closely related.[131] Algae comprise several distinct clades such asglaucophytes, which are microscopic freshwater algae that may have resembled in form to the early unicellular ancestor of Plantae.[131] Unlike glaucophytes, the other algal clades such asred andgreen algae are multicellular. Green algae comprise three major clades:chlorophytes,coleochaetophytes, andstoneworts.[131]
Fungi are eukaryotes that digest foods outside their bodies,[132] secreting digestive enzymes that break down large food molecules before absorbing them through their cell membranes. Many fungi are alsosaprobes, feeding on dead organic matter, making them importantdecomposers in ecological systems.[132]
The origins of viruses in the evolutionaryhistory of life are unclear: some may haveevolved fromplasmids—pieces of DNA that can move between cells—while others may have evolved from bacteria. In evolution, viruses are an important means ofhorizontal gene transfer, which increasesgenetic diversity in a way analogous tosexual reproduction.[140] Because viruses possess some but not all characteristics of life, they have been described as "organisms at the edge of life",[141] and asself-replicators.[142]
Thecommunity of living (biotic) organisms in conjunction with the nonliving (abiotic) components (e.g., water, light, radiation, temperature,humidity,atmosphere,acidity, and soil) of their environment is called anecosystem.[144][145][146] These biotic and abiotic components are linked together throughnutrient cycles and energy flows.[147] Energy from the sun enters the system throughphotosynthesis and is incorporated into plant tissue. By feeding on plants and on one another, animals movematter and energy through the system. They also influence the quantity of plant andmicrobialbiomass present. By breaking down deadorganic matter,decomposers releasecarbon back to the atmosphere and facilitatenutrient cycling by converting nutrients stored in dead biomass back to a form that can be readily used by plants and other microbes.[148]
Reaching carrying capacity through a logistic growth curve
A population is the group oforganisms of the samespecies that occupies anarea andreproduce from generation to generation.[149][150][151][152][153]Population size can be estimated by multiplying population density by the area or volume. Thecarrying capacity of anenvironment is the maximum population size of aspecies that can be sustained by that specific environment, given the food,habitat,water, and otherresources that are available.[154] The carrying capacity of a population can be affected by changing environmental conditions such as changes in the availability of resources and the cost of maintaining them. Inhuman populations, newtechnologies such as theGreen revolution have helped increase the Earth's carrying capacity for humans over time, which has stymied the attempted predictions of impending population decline, the most famous of which was byThomas Malthus in the 18th century.[149]
A (a) trophic pyramid and a (b) simplified food web. The trophic pyramid represents the biomass at each level.[155]
A community is a group of populations of species occupying the same geographical area at the same time.[156] Abiological interaction is the effect that a pair oforganisms living together in a community have on each other. They can be either of the same species (intraspecific interactions), or of different species (interspecific interactions). These effects may be short-term, likepollination andpredation, or long-term; both often strongly influence theevolution of the species involved. A long-term interaction is called asymbiosis. Symbioses range frommutualism, beneficial to both partners, tocompetition, harmful to both partners.[157] Every species participates as a consumer, resource, or both inconsumer–resource interactions, which form the core offood chains orfood webs.[158] There are differenttrophic levels within any food web, with the lowest level being the primary producers (orautotrophs) such as plants and algae that convert energy and inorganic material intoorganic compounds, which can then be used by the rest of the community.[50][159][160] At the next level are theheterotrophs, which are the species that obtain energy by breaking apart organic compounds from other organisms.[158] Heterotrophs that consume plants are primary consumers (orherbivores) whereas heterotrophs that consume herbivores are secondary consumers (orcarnivores). And those that eat secondary consumers are tertiary consumers and so on.Omnivorous heterotrophs are able to consume at multiple levels. Finally, there aredecomposers that feed on the waste products or dead bodies of organisms.[158]On average, the total amount of energy incorporated into thebiomass of a trophic level per unit of time is about one-tenth of the energy of the trophic level that it consumes. Waste and dead material used by decomposers as well as heat lost from metabolism make up the other ninety percent of energy that is not consumed by the next trophic level.[161]
Fast carbon cycle showing the movement of carbon between land, atmosphere, and oceans in billions of tons per year. Yellow numbers are natural fluxes, red are human contributions, white are stored carbon. Effects of theslow carbon cycle, such as volcanic and tectonic activity, are not included.[162]
In the global ecosystem or biosphere, matter exists as different interacting compartments, which can be biotic or abiotic as well as accessible or inaccessible, depending on their forms and locations.[163] For example, matter from terrestrial autotrophs are both biotic and accessible to other organisms whereas the matter in rocks and minerals are abiotic and inaccessible. Abiogeochemical cycle is a pathway by which specificelements of matter are turned over or moved through the biotic (biosphere) and the abiotic (lithosphere,atmosphere, andhydrosphere) compartments of Earth. There are biogeochemical cycles fornitrogen,carbon, andwater.
Conservation biology is the study of the conservation of Earth'sbiodiversity with the aim of protectingspecies, theirhabitats, andecosystems from excessive rates ofextinction and the erosion of biotic interactions.[164][165][166] It is concerned with factors that influence the maintenance, loss, and restoration of biodiversity and the science of sustaining evolutionary processes that engendergenetic, population,species, and ecosystem diversity.[167][168][169][170] The concern stems from estimates suggesting that up to 50% of all species on the planet will disappear within the next 50 years,[171] which has contributed to poverty, starvation, and will reset the course of evolution on this planet.[172][173]Biodiversity affects the functioning of ecosystems, which provide a variety ofservices upon which people depend. Conservation biologists research and educate on the trends ofbiodiversity loss, speciesextinctions, and the negative effect these are having on our capabilities tosustain the well-being of human society. Organizations and citizens are responding to thecurrent biodiversity crisis through conservation action plans that direct research, monitoring, and education programs that engage concerns at local through global scales.[174][167][168][169]
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