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.
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…
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…
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…
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…
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…
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…
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…
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…
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…
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…
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…
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 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.…
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…
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…
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…
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.…
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…
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.
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.
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…
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…
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…
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…
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.
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…
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.
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…
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.
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…
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…
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…
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 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"…
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…
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.…
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…
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.)
From studying fossils and lineages closely related to the vertebrate clade, we hypothesize that the ancestor of vertebrates had none of these features.
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…
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…
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…
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…
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.
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…
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)…
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 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…
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…
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.…
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…
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…
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…
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…
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…
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…
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…
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…
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,…
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…
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…
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.
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 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…
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.
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…
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 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…
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…
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,…
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,…
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…
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 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…
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…
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…
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…
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…
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…
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…
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…
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…
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…
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.
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.
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…
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…
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…
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…
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…
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…
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…
In this mode of speciation, something extrinsic to the organisms prevents two or more groups from mating with each other regularly, eventually causing that lineage to speciate. Isolation might occur…
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