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Adaptive radiation


Allopatric speciation


Analogies (1 of 3) Saberteeth


Analogies (2 of 3) Saberteeth


Analogies (3 of 3) Saberteeth


Analogy (1 of 2) Bird and bat wing diagrams


Analogy (2 of 2) Bird and bat wing phylogeny


Antibiotic resistance


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.


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 (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 (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 to be.


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 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 mates. In this experiment, guppies were raised in ponds that varied in the coarseness of gravel on the bottom. All ponds had predators. After fewer than 15 generations of selection, the markings of guppies in different ponds had substantially diverged as a result of natural selection. In the presence of predators, guppies evolved to blend in with their background.


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 as a result of natural selection. In the presence of predators, guppies evolved to blend in with their background.


Artificial selection of corn


Body size over time




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 of evolution, and many of the animal phyla actually diverged during the Precambrian.


Causes of speciation - Geographic isolation


Causes of speciation - Reduced gene flow


Change through time (1 of 4) Leaves
Leaves on trees change color and fall over several weeks.


Change through time (2 of 4) Mountains
Mountain ranges erode over millions of years.


Change through time (3 of 4) Genealogy


Change through time (4 of 4) Tree of life


Chemical signals and cell fate
Chemical signals influence the fate of cells.


Clades (1 of 2) Definition


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 levels are marked on these phylogenies. Notice how clades are nested within larger clades.


1 codon = 1 amino acid


Cospeciation (1 of 3)


Cospeciation (2 of 3)


Cospeciation (3 of 3)


DNA fingerprinting


DNA structure


DNA, RNA, Proteins (1 of 4) DNA


DNA, RNA, Proteins (2 of 4) RNA


DNA, RNA, Proteins (3 of 4) Proteins


DNA instructions are transmitted via RNA to construct proteins.


Evolution of HIV virus


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.


Extinction and diversification - Trilobites and ammonites




Galapagos finch phylogeny


Gene expression in different cells
Different cells have different genes expressed.


Genes + environment = phenotype


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 can reduce a population's genetic variation by a lot, even if the bottleneck doesn't last for very many generations. This is illustrated by the bags of marbles shown below, where, in generation 2, an unusually small draw creates a bottleneck.


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, 10:0, 10:0, 10:0, 10:0, 10:0… Why did we keep drawing 10:0? Because if the green marbles fail to be represented in just one draw, we can't get them back — we are "stuck" with only brown marbles. The cartoon below illustrates this process, beginning with the fourth draw.


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 draw four marbles to represent gene frequencies in the next generation. Something like this might happen:


Genetic drift example (1 of 4)


Genetic drift example (2 of 4)


Genetic drift example (3 of 4)


Genetic drift example (4 of 4)


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.


Homologous tetrapod limbs (1 of 6)


Homologous tetrapod limbs (2 of 6)


Homologous tetrapod limbs (3 of 6)


Homologous tetrapod limbs (4 of 6)


Homologous tetrapod limbs (5 of 6)


Homologous tetrapod limbs (6 of 6)


Horizontal gene transfer
Bacteria can get new gene variants through horizontal gene transfer - they can pass genes back and forth to one another directly.


Humans on the tree of life


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 and genetics. As natural selection acts on a population, it weeds out genes that have disadvantageous effects, but it can only weed out these genes if they are actually expressed in an individual. For dominant gene versions, that's no problem. Individuals carrying dominant genes with a detrimental effect will be selected against, and eventually, these genes will be purged from the population. For recessive gene versions, however, the story is a bit different. Recessive genes are only expressed when an individual carries two copies of them. Once natural selection has removed most of the detrimental recessive genes from a population, these seldom wind up paired with an identical copy and are effectively hidden from the effects of natural selection. This means that most populations carry many deleterious recessive gene versions that are very rarely expressed — except in cases of inbreeding. Closely related individuals are likely to carry the same deleterious recessive gene versions and pass two copies of that gene on to their offspring.


Macroevolution refers to evolution of groups larger than an individual species.


Mass extinction


Mass extinctions - Five major
Mass extinctions are, by definition, harsh, but they each seem to be disastrous in their own unique way.


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 in the population.


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 population.


Mechanisms of evolution (3 of 4) Genetic drift


Mechanisms of evolution (4 of 4) Natural selection


Mechanisms of microevolution (1 of 4) Mutation


Mechanisms of microevolution (2 of 4) Migration


Mechanisms of microevolution (3 of 4) Genetic drift


Mechanisms of microevolution (4 of 4) Natural selection




Microevolution - Change in gene frequency


Microevolution - The size of the sparrow


Microevolutionary change can accumulate into macroevolution


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) come from both parents.


Mitochondrial genes (2 of 2)
Nuclear DNA is inherited from all ancestors. Mitochondrial DNA is inherited from a single lineage.


Modes of speciation (1 of 4) Allopatric
New species formed from geographically isolated populations.


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 (3 of 4) Parapatric
New species formed from a continuously distributed population.


Modes of speciation (4 of 4) Sympatric
New species formed from within the range of the ancestral population.


Mutation (1 of 4) Substitution


Mutation (2 of 4) Insertion


Mutation (3 of 4) Deletion


Mutation (4 of 4) Frameshift


Mutation - DNA


Natural selection (1 of 4) Variation


Natural selection (2 of 4) Differential reproduction


Natural selection (3 of 4) Heredity


Natural selection (4 of 4) End result


Natural selection - Beetle example


Natural selection in a test tube


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 a population carries several different versions of a gene, odds are that each of those versions is equally good at performing its job — in other words, that variation is neutral: whether you carry gene version A or gene version B does not affect your fitness.


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.


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 a layer directly after the ancestor, showing a big change in a short time, with no transitional forms.


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 irregular fossil preservation. This possibility can make it difficult to conclude that evolution has happened rapidly.


Pace of evolution hypotheses (1 of 4)


Pace of evolution hypotheses (2 of 4)


Pace of evolution hypotheses (3 of 4)


Pace of evolution hypotheses (4 of 4)


Parsimony (1 of 5) Vertebrate character matrix


Parsimony (2 of 5) Vertebrate ancestor characters


Parsimony (3 of 5) Amniotic egg clade and phylogeny


Parsimony (4 of 5) All clades and phylogeny


Parsimony (5 of 5) Comparison of two hypotheses


Patterns in macroevolution (1 of 5) Stasis


Patterns in macroevolution (2 of 5) Character change


Patterns in macroevolution (3 of 5) Trilobite example


Patterns in macroevolution (4 of 5) Lineage splitting


Patterns in macroevolution (5 of 5) Character change


Peripatric speciation (1 of 5)


Peripatric speciation (2 of 5)


Peripatric speciation (3 of 5)


Peripatric speciation (4 of 5)


Peripatric speciation (5 of 5)




Phylogenies (2 of 3) Left to right


Phylogenies (3 of 3) Order doesn't matter


Polytomy (1 of 3)


Polytomy (2 of 3) Lack of knowledge


Polytomy (3 of 3) Rapid speciation


Punctuated equilibrium (1 of 8) Stasis


Punctuated equilibrium (2 of 8) Isolation


Punctuated equilibrium (3 of 8) Strong selection and rapid change


Punctuated equilibrium (4 of 8) No preservation


Punctuated equilibrium (5 of 8) Reintroduction


Punctuated equilibrium (6 of 8) Expansion and stasis


Punctuated equilibrium (7 of 8) Preservation




Recognizing homologies - Crocodile/mouse limb bones
The same bones (though differently shaped) support the limbs of mice and crocodiles. Homologous bones are colored alike.


Regulatory genes
Certain genes control where and when other genes are expressed.


Reproduction (1 of 6) Egg, sperm, and zygote


Reproduction (2 of 6) Chromosome duplication


Reproduction (3 of 6) Recombination


Reproduction (4 of 6) Meiosis, step one
Meiosis, step one


Reproduction (5 of 6) Meiosis, step two
Meiosis, step two


Reproduction (6 of 6) Zygote with recombinant genes


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 bag. Now imagine that the bag is restocked with 100 marbles, with the same proportion of brown and green marbles as you have just drawn out. The ratio of brown to green marbles "drifts" around (5:5, 6:4, 7:3, 4:6 …).


Sickle Cell (1 of 3) Genes


Sickle Cell (2 of 3) Protein


Sickle Cell (3 of 3) Cell


Speciation example (1 of 5)


Speciation example (2 of 5)


Speciation example (3 of 5)


Speciation example (4 of 5)


Speciation example (5 of 5)


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 escape.


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 will be able to bring in more carbon dioxide. During those high-carbon-dioxide times, plants with fewer stomata will have an advantage and will be common. On the other hand, when carbon dioxide levels are low, plants need many stomata in order to scrape together enough carbon dioxide to survive. During low-carbon-dioxide times, plants with more stomata will have an advantage and will be common.


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. Typically (although there are exceptions to the rule), fossils with many stomata (low carbon dioxide) came from times of low global temperature, and fossils with few stomata (high carbon dioxide) came from times of high global temperatures.


Three domains
The three domains: Archaea, Bacteria, and Eukaryota.


Timeline of human evolution
Important events in human history, with approximate dates, which reflect the evidence currently available


Transitional forms - whale evolution
Transitional forms in whale evolution, highlighting the transition of the walking forelimb to the flipper.


Understanding phylogenies (1 of 4)


Understanding phylogenies (2 of 4)


Understanding phylogenies (3 of 4)


Understanding phylogenies (4 of 4)


Vertebrate phylogeny - Amphibia, synapsida, sauropsida
Vertebrate phylogeny


Vertebrate phylogeny with characters
Evolutionary relationships of major vertebrate groups.


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.


Virus evolution and virulence