Different Patterns of Evolution
Groups of species undergo various kinds of natural selection and, over time, may engage in several patterns of evolution: convergent evolution, divergent evolution, parallel evolution, and coevolution.
Convergent evolution is the process in which species that are not closely related to each other independently evolve similar kinds of traits. For example, dragonflies, hawks, and bats all have wings. None of these organisms owes its wings to genes inherited from any of the others. Each kind of wing evolved independently, suggesting that the trait of flight is a useful one for the purpose of survival and reproduction. These independently evolved wings are called analogous structures.
Divergent evolution is the process in which a trait held by a common ancestor evolves into different variations over time. A common example of divergent evolution is the vertebrate limb. Whale flippers, frog forelimbs, and your own arms most likely evolved from the front flippers of an ancient jawless fish. Because they share a common evolutionary origin, these are examples of homologous structures.
An important consequence of divergent evolution is speciation, the divergence of one species into two or more descendant species. There are four major ways speciation can occur:
- Allopatric speciation occurs when a population becomes separated into two entirely isolated subpopulations. Once the separation occurs, natural selection and genetic drift operate on each subpopulation independently, producing different evolutionary outcomes.
- Peripatric speciation is somewhat similar to allopatric speciation, but specifically occurs when a very small subpopulation becomes isolated from a much larger majority. Because the isolated subpopulation is so small, divergence can happen relatively rapidly due to the founder effect, in which small populations are more sensitive to genetic drift and natural selection acts on a small gene pool.
- Parapatric speciation occurs when a small subpopulation remains within the habitat of an original population but enters a different niche. Effects other than physical separation prevent interbreeding between the two separated populations. Because one of the genetically isolated populations is so small, however, the founder effect can still play a role in speciation.
- Sympatric speciation, the rarest and most controversial form of speciation, occurs with no form of isolation (physical or otherwise) between two populations.
Parallel evolution is sometimes difficult to distinguish from convergent evolution. Parallel evolution occurs when different species start with similar ancestral origins, then evolve similar traits over time. This kind of thing happens because the two different species, though they don't necessarily share a common ancestor, experience similar kinds of environmental pressures and survive only by undergoing similar adaptations. A classic example of parallel evolution is found among plants, in which several similar but distinct forms of leaf evolved in parallel and are evident today.
Coevolution occurs when closely interacting species exert selective pressures on each other, so that they evolve together in a kind of conversation of adaptations. Examples of coevolution are common among predator-prey and host-parasite pairs. More picturesque examples of coevolution occur among hummingbirds and the flowers from which they seek nectar and unwittingly pollinate.