Why does coevolution occur

This system is probably the product of coevolution: the plants would not have evolved hollow thorns or nectar pores unless their evolution had been affected by the ants, and the ants would not have evolved herbivore defense behaviors unless their evolution had been affected by the plants. Learn more about coevolution in context: Biological warfare and the coevolutionary arms race , a case study.

Misconceptions about natural selection. A case study of coevolution: squirrels, birds, and the pinecones they love. Subscribe to our newsletter. Email Facebook Twitter. The most dramatic examples of avian coevolution are probably those involving brood parasites, such as cuckoos and cowbirds, and their hosts. The parasites have often evolved eggs that closely mimic those of the host, and young with characteristics that encourage the hosts to feed them.

In response, some hosts have developed the ability to discriminate between their own and parasitic eggs, and various methods of destroying the latter. As one might expect, Brown-headed Cowbirds have their most serious impact on hosts, such as Kirtland's Warblers, that are thought to have only recently been subjected to cowbird attack and have not yet had time to evolve defensive reactions.

Many examples of coevolution in response to competition between bird species can be inferred from studies of dietary habits and bill structures in various guilds of birds.

Here, as in the other cases mentioned, direct evidence of coevolution is lacking. It is lacking for the same reason that there are very few cases of plain old single-population evolution actually being observed in nature. The process occurs over hundreds or thousands of generations, and extraordinary circumstances are required for it to be "caught in the act.

Ehrlich, David S. Dobkin, and Darryl Wheye. Parasites and hosts : when a parasite invades a host, it will successfully invade those hosts whose defense traits it can circumvent because of the abilities it caries at that time. Thus presence of a parasite on a host does not constitute evidence for coevolution. These criticisms are quite distinct from the opportunity for coevolution once a parasite has established itself on a host. The main point is that any old interaction, symbiosis, mutualism, etc.

In one sense there has definitely been "evolution together" but whether this fits our strict definition of coevolution needs to be determined by careful 1 observation , 2 experimentation and 3 phylogenetic analysis. The classic analogy is the coevolutionary arms race : a plant has chemical defenses, an insect evolves the biochemistry to detoxify these compounds, the plant in turn evolves new defenses that the insect in turn "needs" to further detoxify.

At present the evidence for these types of reciprocal adaptations is limited, but the suggestive evidence of plant animal interactions is widespread. An important point is the relative timing of the evolution of the various traits that appear to be part of the coevolution.

If the presumed reciprocally induced, sequential traits actually evolved in the plant host before the insect parasite became associated with it, we should not call it coevolution. See different example figs. There are a variety of different modes of coevolution. In some cases coevolution is quite specific such as those between two cellular functions.

The endosymbiont theory proposes that current day mitochondria and chloroplasts were once free-living unicellular individuals. These cells entered the cytoplasm of other cells, an example of the general phenomenon of endosymbiosis.

Current-day mitochondrial and chloroplast genomes are much smaller than the genome sizes of their presumed free-living ancestors. Some of this reduction in genome size is due to the transfer of genes from organelle genomes to the nuclear genome.

Thus, being in the cellular environment has influenced the evolution of organelle genomes. There is evidence that the faster rate of evolution of animal mitochondrial DNA has accelerated the rate of evolution of some of the nuclear genes that function in the mitochondria. Thus there is some evidence for reciprocal phenomena. Other modes of coevolution involve competitive interaction between two specific species.

The Plethodon salamander study is a good example: two species are competing: in the Great Smoky mountains the two species compete strongly as evidenced by the fact that each species will increase population size if the other is removed.