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Extinction and diversification – Trilobites and ammonites
If extinction happens more frequently than lineage-splitting, that entire clade will go extinct eventually. For example, trilobites and ammonites had high rates of both diversification and extinction.
Genetic drift example (4 of 4)
In the real world, many, many loci affect the fitness of a population and an adaptive landscape may have multiple peaks and valleys. This graph shows a complex landscape involving…
Genetic drift example (3 of 4)
However, since the population is small, drift can be a powerful force. Just by chance, the frequencies of the B alleles increase in the population over several generations (and the…
Genetic drift example (2 of 4)
Now imagine a small population of all small-beaked individuals (all bb genotypes). They have a high fitness (they are at a local peak), but not as high as a population…
Genetic drift example (1 of 4)
BB individuals have big beaks, Bb individuals have medium-sized beaks, and bb individuals have small beaks. These birds live in a place where large and small seeds are abundant, but…
Pace of evolution hypotheses (4 of 4)
Hypothesis 3: Macromutation — a big mutation produces sudden evolutionary change skipping over transitional forms. The "burst" of evolution is really a burst — there was a lot of evolutionary…
Pace of evolution hypotheses (3 of 4)
Hypothesis 2: Punctuated equilibrium — a large amount of change in a short time, tied to a speciation event. Species 2 and 3 are only 100,000 years younger than ancestor…
Pace of evolution hypotheses (2 of 4)
Hypothesis 1: Phyletic gradualism - slow and steady divergence of lineages. The "burst" of evolution is a geological illusion. It only looks like a burst because a lot of time…
Pace of evolution hypotheses (1 of 4)
In many cases, we seem to observe "bursts" of evolution in the fossil record. In this example, in a lower rock layer, you see ancestor 1. In the next rock…
Neutral theory – Selection and drift on scale
The neutral theory of molecular evolution suggests that most of the genetic variation in populations is the result of mutation and genetic drift, not selection. The theory suggests that if…
Mass extinction
For any one species, extinction may seem catastrophic. But over the grand sweep of life on Earth, extinction is business as usual. Extinctions occur continually, generating a "turnover" of the…
Microevolutionary change can accumulate into macroevolution
Over many generations, evolutionary processes that act at the population level can lead to macroevolutionary change.
Macroevolution
Macroevolution refers to evolution of groups larger than an individual species.
Humans on the tree of life
This tree is based on morphological and genetic data. Chimpanzees and humans form a clade with genes sequences that differ by only 1%. This genetic similarity made it hard to…
Gene expression in different cells
Different cells have different genes expressed.
Chemical signals and cell fate
Chemical signals influence the fate of cells.
Cambrian explosion
The term "explosion" may be a bit of a misnomer. Cambrian life did not evolve in the blink of an eye. The Cambrian was preceded by many millions of years…
Stomata (3 of 3) Indicators of CO2 and temperature
Stomata of fossil plants can be used to directly estimate past carbon dioxide levels, and those carbon dioxide levels can then be used to make an indirect estimate of temperature.…
Stomata (2 of 3) Tradeoff
Levels of carbon dioxide in Earth's atmosphere change over time; so at times when the atmosphere is carbon-dioxide-rich, plants can get away with having fewer stomata since each individual stoma…
Stomata (1 of 3) Function
Carbon dioxide enters, while water and oxygen exit, through a leaf's stomata. Stomata control a tradeoff for the plant: they allow carbon dioxide in, but they also let precious water…
Speciation example (5 of 5)
So we meet again: When another storm reintroduces the island flies to the mainland, they will not readily mate with the mainland flies since they've evolved different courtship behaviors. The…
Speciation example (4 of 5)
The populations diverge: Ecological conditions are slightly different on the island, and the island population evolves under different selective pressures and experiences different random events than the mainland population does.…
Speciation example (3 of 5)
Disaster strikes: A hurricane washes the bananas and the immature fruit flies they contain out to sea. The banana bunch washes up on an island off the coast of the…
Speciation example (2 of 5)
The scene: a population of wild fruit flies is minding its own business on several bunches of rotting bananas, cheerfully laying their eggs in the mushy fruit.
Speciation example (1 of 5)
The branching points on this partial Drosophila phylogeny represent speciation events that happened in the past.
Sickle Cell (3 of 3) Cell
Normal red blood cells (top) and sickle cells (bottom).
Sickle Cell (2 of 3) Protein
Normal hemoglobin (left) and hemoglobin in sickled red blood cells (right) look different; the mutation in the genes changes the shape of the hemoglobin molecule, allowing it to clump together.
Sickle Cell (1 of 3) Genes
Sickle cell anemia is caused by a mutation at the genes level.
Sampling error
Imagine a game in which you have a bag holding 100 marbles, 50 of which are brown and 50 green. You are allowed to draw 10 marbles out of the…
Reproduction (6 of 6) Zygote with recombinant genes
When egg and sperm meet, the baby inherits a combination of genes that is totally unique: it carries versions of genes from all 4 grandparents plus any mutations that occurred…
Reproduction (5 of 6) Meiosis, step two
Meiosis, step two.
Reproduction (4 of 6) Meiosis, step one
Meiosis, step one.
Reproduction (3 of 6) Recombination
Producing eggs and sperm is our first opportunity for mixing and matching genes. When the mother makes an egg, her chromosomes first find their matched partners and exchange some genes…
Reproduction (2 of 6) Chromosome duplication
When eggs and sperm are produced, the parent cell first copies each chromosome, leaving the duplicate pairs attached to one another.
Reproduction (1 of 6) Egg, sperm, and zygote
Eggs and sperm carry only half the usual number of chromosomes just 23 unpaired chromosomes, carrying one version of each gene. When the egg and sperm get together, the baby…
Regulatory genes
Certain genes control where and when other genes are expressed.
Recognizing homologies – Crocodile/mouse limb bones
The same bones (though differently shaped) support the limbs of mice and crocodiles. Homologous bones are colored alike.
Punctuated equilibrium (8 of 8) This process would produce the following pattern in the fossil record
This process would produce the following pattern in the fossil record: Evolution appears to happen in sharp jumps associated with speciation events.
Punctuated equilibrium (7 of 8) Preservation
Preservation: Larger population size and a larger range move us back to step 1: stasis with occasional fossil preservation.
Punctuated equilibrium (6 of 8) Expansion and stasis
Expansion and stasis: The isolated population expands into its past range. Larger population size and a stable environment make evolutionary change less likely. The formerly isolated branch of the mollusk…
Punctuated equilibrium (5 of 8) Reintroduction
Reintroduction: Sea levels rise, reuniting the isolated mollusks with their sister lineage.
Punctuated equilibrium (4 of 8) No preservation
No preservation: No fossils representing transitional forms are preserved because of their relatively small population size, the rapid pace of change, and their isolated location.
Punctuated equilibrium (3 of 8) Strong selection and rapid change
Strong selection and rapid change: The small, isolated population experiences strong selection and rapid change because of the novel environment and small population size: The environment in the newly formed…
Punctuated equilibrium (2 of 8) Isolation
Isolation: A drop in sea level forms a lake and isolates a small number of mollusks from the rest of the population.
Punctuated equilibrium (1 of 8) Stasis
Stasis: A population of mollusks is experiencing stasis, living, dying, and getting fossilized every few hundred thousand years. Little observable evolution seems to be occurring judging from these fossils.
Transitional forms – whale evolution
Transitional forms in whale evolution, highlighting the transition of the walking forelimb to the flipper.
Timeline of Human Evolution
Important events in human history, with approximate dates, which reflect the evidence currently available.
Polytomy (3 of 3) Rapid speciation
A polytomy may mean that multiple speciation events happened at the same time.
Polytomy (2 of 3) Lack of knowledge
A polytomy may mean that we don't have enough data to figure out how the lineages are related. There are six possible solutions to this polytomy. Often, gathering more data…
Polytomy (1 of 3)
Often, one sees phylogenies that include polytomies, nodes with more than two descendent lineages, creating a "pitchfork."
Phylogenies (3 of 3) Order doesn’t matter
For any speciation event on a phylogeny, the choice of which lineage goes to the right and which goes to the left is arbitrary. These phylogenies are equivalent.
Phylogenies (2 of 3) Left to right
Just because we tend to read phylogenies from left to right, there is no correlation with level of "advancement."
Phylogenies (1 of 3) Tree-like not ladder-like
Evolution produces a pattern of relationships A B C D among lineages that is tree-like, not ladder-like.
Peripatric speciation (5 of 5)
After some generations, the island flies become reproductively isolated from the mainland flies. Peripatric speciation has occurred.
Peripatric speciation (4 of 5)
As the island population grows, the unique reproductive features on the island result in a cascade of changes caused by sexual selection. These changes optimize, or at least improve, the…
Peripatric speciation (3 of 5)
These small differences, which are rare on the mainland, drift to fixation in the small population on the island over the course of a few generations (i.e., the entire island…
Peripatric speciation (2 of 5)
These few survivors just by chance carry some genes that are rare in the mainland population. One of these rare genes happens to cause a slight variation in the mating…
Peripatric speciation (1 of 5)
Only a few fruit fly larvae survived the journey from the mainland to colonize the island.
Patterns in macroevolution (5 of 5) Character change
Extinction can be a frequent or rare event within a lineage, or it can occur simultaneously across many lineages (mass extinction). Here, a mass extinction cuts short the lifetimes of…
Patterns in macroevolution (4 of 5) Lineage splitting
Patterns of lineage-splitting (or speciation) can be identified by constructing and examining a phylogeny. The phylogeny might reveal that a particular lineage has undergone unusually frequent lineage-splitting, generating a "bushy"…
Patterns in macroevolution (3 of 5) Trilobite example
Trilobites, animals in the same clade as modern insects and crustaceans, lived over 300 million years ago. Their fossil record clearly suggests that several lineages underwent similar increases in segment…
Patterns in macroevolution (2 of 5) Character change
Lineages can change quickly or slowly. Character change can happen in a single direction, such as evolving additional segments, or it can reverse itself by gaining and then losing segments.…
Patterns in macroevolution (1 of 5) Stasis
Many lineages on the tree of life exhibit stasis, which just means that they don't change much for a long time.
Parsimony (5 of 5) Comparison of two hypotheses
The parsimony principle tells us to choose the simplest scientific explanation that fits the evidence. Hypothesis 1 requires six evolutionary changes and Hypothesis 2 requires seven evolutionary changes, with a…
Parsimony (4 of 5) All clades and phylogeny
If we go through the whole table like this, grouping clades according to shared derived characters (C) we get the following hypothesis (D).
Parsimony (3 of 5) Amniotic egg clade and phylogeny
We focus in on the group of lineages that share the derived form of the egg character, an amniotic egg (A), and hypothesize that they form a clade (B.)
Parsimony (2 of 5) Vertebrate ancestor characters
From studying fossils and lineages closely related to the vertebrate clade, we hypothesize that the ancestor of vertebrates had none of these features.
Parsimony (1 of 5) Vertebrate character matrix
Character data for some major vertebrate lineages. Characters were limited to characters that are likely homologous (note that many vertebrate lineages and many characters were excluded from this example for…
Pace of evolution (3 of 3) Irregular fossil preservation
We expect to see a jump in the fossil record if evolution has occurred as a "quick" jump, but a jump in the fossil record can also be explained by…
Pace of evolution (2 of 3) Quick jumps
If evolution happens in "quick" jumps, we'd expect to see big changes happen quickly in the fossil record, with little transition between ancestor and descendent. Here, the descendent preserved in…
Pace of evolution (1 of 3) Slow and steady
The preservation of many transitional forms, through layers representing a length of time, gives a complete record of slow and steady evolution.
Natural selection in a test tube
How do biologists "evolve" RNA in a test tube? The same way that a population of organisms evolves in the real world: natural selection.
Natural selection – Beetle example
Over the course of many generations, green beetles have been selected against, and brown beetles have flourished.
Natural selection (4 of 4) End result
End result: The more advantageous trait, brown coloration, which allows the beetle to have more offspring, becomes more common in the population. If this process continues, eventually, all individuals in…
Natural selection (3 of 4) Heredity
There is heredity. The surviving brown beetles have brown baby beetles because this trait has a genetic basis.
Natural selection (2 of 4) Differential reproduction
There is differential reproduction. Since the environment can't support unlimited population growth, not all individuals get to reproduce to their full potential. In this example, green beetles tend to get…
Natural selection (1 of 4) Variation
There is variation in traits. For example, some beetles are green and some are brown.
Mutation – DNA
When a cell divides, it makes a copy of its DNA and sometimes the copy is not quite perfect. That small difference from the original DNA sequence is a mutation.
Mutation (4 of 4) Frameshift
Since protein-coding genes is divided into codons three bases long, insertions and deletions can alter a gene so that its message is no longer correctly parsed. These changes are called…
Mutation (3 of 4) Deletion
Deletions are mutations in which a section of genes is lost, or deleted.
Mutation (2 of 4) Insertion
Insertions are mutations in which extra base pairs are inserted into a new place in the genes.
Mutation (1 of 4) Substitution
A substitution is a mutation that exchanges one base for another (i.e., a change in a single "chemical letter" such as switching an A to a G).
Modes of speciation (4 of 4) Sympatric
New species formed from within the range of the ancestral population.
Modes of speciation (3 of 4) Parapatric
New species formed from a continuously distributed population.
Modes of speciation (2 of 4) Peripatric
New species formed from a small population isolated at the edge of a larger population.
Modes of speciation (1 of 4) Allopatric
New species formed from geographically isolated populations.
Mitochondrial genes (2 of 2)
Nuclear DNA is inherited from all ancestors. Mitochondrial DNA is inherited from a single lineage.
Mitochondrial genes (1 of 2)
Mitochondria are passed from mother to child — your mitochondrial genes are a genetic gift from your mother alone. The genes in the nuclei of your cells (your nuclear genes)…
Microevolution – The size of the sparrow
Sparrow populations in the north are larger-bodied than sparrow populations in the south. This divergence in populations is probably at least partly a result of natural selection: larger-bodied birds can…
Microevolution – Change in gene frequency
Microevolution is a change in gene frequency in a population. Suppose you sample a beetle population this year, and determine that 80% of the genes in the population are for…
Microevolution
Microevolution is evolution on a small scale — within a single population. That means narrowing our focus to one branch of the tree of life.
Mechanisms of evolution (4 of 4) Natural selection
Imagine that green beetles are easier for birds to spot (and hence, eat). Brown beetles are a little more likely to survive to produce offspring. They pass their genes for…
Mechanisms of evolution (3 of 4) Genetic drift
Imagine that in one generation, two brown beetles happened to have four offspring survive to reproduce. Several green beetles were killed when someone stepped on them and had no offspring.…
Mechanisms of evolution (2 of 4) Migration
Some individuals from a population of brown beetles might have joined a population of green beetles. That would make the genes for brown beetles more frequent in the green beetle…
Mechanisms of evolution (1 of 4) Mutation
A mutation could cause parents with genes for bright green coloration to have offspring with a gene for brown coloration. That would make the genes for brown beetles more frequent…
Mass extinctions – Five major
Mass extinctions are, by definition, harsh, but they each seem to be disastrous in their own unique way.
Inbreeding depression
The offspring resulting from inbreeding tend to have health problems and lower reproductive success. This is known as inbreeding depression. Inbreeding depression occurs because of a quirk of natural selection…
Horizontal gene transfer
Bacteria can get new gene variants through horizontal gene transfer - they can pass genes back and forth to one another directly.
Homologous tetrapod limbs (6 of 6)
This tree shows how the octopus is related to tetrapods, and the points in their evolutionary histories when their limbs evolved. Tetrapod and octopus limbs evolved independently after their point…
Homologous tetrapod limbs (5 of 6)
Not all similarity is homology. Since the octopus, sea star and grasshopper limbs don't have bones, they are not homologous to tetrapod limbs.
Homologous tetrapod limbs (4 of 6)
This evolutionary tree shows the relationships between different tetrapod lineages, all of which evolved from a single common ancestor. This 350 million year old animal, the first tetrapod, had limbs…
Homologous tetrapod limbs (3 of 6)
Whales, lizards, humans, and birds all have the same basic limb layout. But how did such different animals wind up with the same sort of limb? The answer is that…
Homologous tetrapod limbs (2 of 6)
Notice how these tetrapod limbs are similar to one another: They are all built from many individual bones. They are all spin-offs of the same basic bone layout: one long…
Homologous tetrapod limbs (1 of 6)
These four limbs all belong to tetrapods — animals with four legs.
Greenhouse effect
Just as a glass greenhouse traps heat radiated by the sun, greenhouse gasses in the Earth's atmosphere also trap the sun's heat.
Genetic drift – Small population
The marble-drawing scenario also illustrates why drift affects small populations more. Imagine that your bag is only big enough for 20 marbles (a tiny bag!) and that you can only…
Genetic drift – Large population
Through sampling error, genetic drift can cause populations to lose genetic variation. Imagine that our random draws from the marble bag produced the following pattern: 5:5, 6:4, 7:3, 4:6, 8:2,…
Genetic bottleneck
Population bottlenecks occur when a population's size is reduced for at least one generation. Because genetic drift acts more quickly to reduce genetic variation in small populations, undergoing a bottleneck…
Genes + environment = phenotype
An organism's phenotype is usually the result of both genetic factors, environmental factors, and the interactions between them.
Galapagos finch phylogeny
Genetic sequences show that finches with similar feeding approaches tend to be closely related to one another.
Fitness
Evolution shows that, as much as possible, organisms will evolve to optimize their fitness — the number of descendents they produce for future generations relative to other organisms. So based…
Understanding phylogenies (4 of 4)
Similarly, each lineage has ancestors that are unique to that lineage and ancestors that are shared with other lineages — common ancestors.
Understanding phylogenies (3 of 4)
Phylogenies trace patterns of shared ancestry between lineages. Each lineage has a part of its history that is unique to it alone and parts that are shared with other lineages.
Understanding phylogenies (2 of 4)
When a lineage splits (speciation), it is represented as branching on a phylogeny. When a speciation event occurs, a single ancestral lineage gives rise to two or more daughter lineages.
Understanding phylogenies (1 of 4)
Understanding a phylogeny is a lot like reading a family tree. The root of the tree represents the ancestral lineage, and the tips of the branches represent the descendents of…
Evolution: A progression of scientific thought
This educational infographic retraces the history of evolutionary thought from pre-Darwinian times to the present. It's available in English, French, German, and Spanish.
Evolution of HIV virus
Just as fruit flies on separate islands can evolve into separate species, HIV viruses in separate hosts can evolve into separate lineages.
Vertebrate phylogeny with time
This phylogeny represents vertebrate evolution. The lengths of the branches have been adjusted to show when lineages split and went extinct.
Vertebrate phylogeny with characters
Evolutionary relationships of major vertebrate groups.
DNA, RNA, Proteins (4 of 4) DNA to Proteins
DNA instructions are transmitted via RNA to construct proteins.
DNA, RNA, Proteins (3 of 4) Proteins
Proteins are long chain-like molecules that fold into complicated shapes and perform all sorts of jobs in the cell — from providing raw building materials to running chemical reactions. They…
DNA, RNA, Proteins (2 of 4) RNA
RNA is similar to DNA, but it is shorter and single-stranded. RNA can carry an "imprint" of DNA information and take it to the place in the cell where proteins…
DNA, RNA, Proteins (1 of 4) DNA
DNA is a long, double-stranded molecule twisted into a helix. It is composed of a chemical code (represented by the letters A, T, G, and C) that describes how to…
DNA fingerprinting
In DNA fingerprinting, scientists collect samples of DNA from different sources — for example, from a hair left behind at the crime scene and from the blood of victims and…
Virus evolution and virulence
There is an evolutionary trade off between virus virulence and virus transmission. A virulent virus does a lot of damage to its host, and produces a lot of offspring. However,…
Cospeciation (3 of 3)
When lineages have cospeciated, the parasite phylogeny will "mirror" the host phylogeny. This example is somewhat idealized — rarely do scientists find hosts and parasites with exactly matching phylogenies. However,…
Cospeciation (2 of 3)
When the gopher lineage splits into lineages A and B, lice have few opportunities for gopher-switching, and lice on gopher lineage A don't mate with lice living on gopher lineage…
Cospeciation (1 of 3)
A species of louse lives on a species of gopher. When the gophers get together to mate, the lice get an opportunity to switch gophers and perhaps mate with lice…
DNA structure
DNA is made of a long sequence of smaller units strung together. There are four basic types of unit: A, T, G, and C. These letters represents the type of…
Codon
1 codon = 1 amino acid
Clades (2 of 2) Nested clades
Clades are nested within one another — they form a nested hierarchy. A clade may include many thousands of species or just a few. Some examples of clades at different…
Clades (1 of 2) Definition
A clade is a grouping that includes a common ancestor and all the descendants (living and extinct) of that ancestor. Using a phylogeny, it is easy to tell if a…
Causes of speciation – Reduced gene flow
Speciation can occur even when there is no specific extrinsic barrier to gene flow. Imagine a situation in which a population extends over a broad geographic range, and mating throughout…
Causes of speciation – Geographic isolation
What was once a continuous population is divided into two or more smaller populations. This can occur when rivers change course, mountains rise, continents drift, or organisms migrate. The geographic…
Bottleneck
In genetics, a population bottleneck is an event in which a population's size is greatly reduced. Gene frequencies in the population are likely to change just by random chance and…
Body size over time
Over the history of life, increases in body volume are correlated with sharp increases in the oxygen level of the earth's atmosphere.
Artificial selection of corn
Plants and animals are domesticated through artificial selection, which works like natural selection does, but with humans instead of nature doing the selecting. Here, humans plant only the plump teosinte…
Artificial selection in the lab (2 of 2)
The same experiment was performed in the same pond set-up, but without predators. After fewer than 15 generations of selection, the markings of guppies in different ponds had substantially diverged…
Artificial selection in the lab (1 of 2)
Guppy spots are largely genetically controlled. Spots that help the guppy blend in with its surroundings protect it from predation, but spots that make it stand out help it attract…
Artificial selection (5 of 5) Fish example
This downward trend in body size will continue so long as the largest fish are harvested and there is genetic variation in the population.
Artificial selection (4 of 5) Fish example
In the next generation, the population has evolved: average body size in the population is smaller than it used to be and small-bodied genes are more common than they used…
Artificial selection (3 of 5) Fish example
The population reproduces. Each individual passes their genes on to their offspring; however, since there are more small-bodied parents, there are also more small-bodied offspring.
Artificial selection (2 of 5) Fish example
When the population is fished, many of the largest fish are removed, so more of the small-bodied fish (and their small-bodied genes) remain.
Artificial selection (1 of 5) Fish example
This population of fish exhibits variation in body size. Some have genes for large size and some have genes for small size. This represents genetic variation in the population.
Antibiotic resistance
The evolution of antibiotic resistance occurs through natural selection. Imagine a population of bacteria infecting a patient in a hospital. The patient is treated with an antibiotic. The drug kills…
Analogy (2 of 2) Bird and bat wing phylogeny
Bird and bat wings are analogous — that is, they have separate evolutionary origins, but are superficially similar because they evolved to serve the same function. Analogies are the result…
Analogy (1 of 2) Bird and bat wing diagrams
When we examine bird wings and bat wings closely, we see that there are some major differences. Bat wings consist of flaps of skin stretched between the bones of the…
Analogies (3 of 3) Saberteeth
As they weren't inherited from a common ancestor, the saberteeth in Smilodon and Thylacosmilus evolved independently from one another. That means that one lineage on one part of the tree…
Analogies (2 of 3) Saberteeth
Despite their similarities, the unusual length of these teeth is NOT homologous. One skull belongs to Thylacosmilus, a marsupial mammal. The other belongs to Smilodon, the saber-toothed cat, which is…
Analogies (1 of 3) Saberteeth
These skulls belong to extinct animals, and both of them have saberteeth — long, ferocious canines. Would you guess that these saberteeth are homologous — inherited from a common ancestor…
Adaptive radiation
If a lot of diversification happens in a short amount of time, it is often referred to as an adaptive radiation. Although biologists have different standards for defining an adaptive…