Types of evolution

Biological evolution over time can follow several different patterns. Factors such as environment and predation pressures can have different effects on the ways in which species exposed to them evolve. Evolutionary biologists have labelled these differing patterns as divergent, convergent, and parallel evolution.

Divergent evolution
When people hear the word "evolution" they most commonly think of divergent evolution, the evolutionary pattern in which (for example) two species gradually become increasingly different. Divergent evolution occurs when a group from a specific population develops into a new species. In order to adapt to various environmental conditions, the two groups develop into distinct species due to differences in the demands driven by the environmental circumstances. On a large scale, divergent evolution could give rise to create the current diversity of life on earth from the first living cells. On a smaller scale, it could explain the evolution of humans and apes from a common primate ancestor. On a molecular scale, it could account for the evolution of new catalytic functions of enzymes and membrane protein topology.

Divergent evolution and speciation
If different selective pressures act on a particular organism, a wide variety of adaptive traits may result. If only one structure on the organism is considered, these changes can either add to the original function of the structure, or they can change it completely. Divergent evolution leads to speciation, or the development of a new species. Divergence can occur in any group of related organisms. The differences are produced from the different selective pressures. Any genus of plants or animals can show divergent evolution. An example can involve the diversity of floral types in the orchids. The greater the number of differences present, the greater the divergence. Scientists speculate that the more that two similar species diverge indicates a longer length of time over which the divergence has taken place.

Examples of divergent evolution
Nature offers many examples of divergent evolution.


 * If a freely-interbreeding population on an island becomes separated by a barrier, such as a new river, then over time the organisms may start to diverge. If the opposite ends of the island have different pressures acting upon the population, this may result in divergent evolution.
 * If a certain group of birds in a population of other birds of the same species veers from their standard migratory track due to abnormal wind fluctuations, they may end up in new environment. If the food-source in the new surroundings is such that only birds of the population with a variant beak are able to feed, then this trait will evolve by virtue of its selective survival advantage. The same species in the original geographical location and having the original food-source do not require this beak trait and will, therefore, evolve differently.
 * Divergent evolution has also occurred in the case of the red fox and the kit fox. While the kit fox lives in the desert where its coat helps disguise it from its predators, the red fox lives in forests, where its red coat blends into its surroundings. In the desert, the climate makes it difficult for animals to eliminate body heat. The ears of the kit fox have evolved to have a greater surface area so that it can more efficiently remove excess body heat. The different foxes' different evolutionary fates are determined primarily by the different environmental conditions and adaptation requirements, not on genetic differences. If all members of a species live in the same environment, it is likely that they will evolve similarly. Divergent evolution is confirmed by DNA analysis where species that have diverged can be shown to be genetically similar.
 * The human foot evolved to be very different from a monkey's foot, despite their common primate ancestry. It is speculated that a new species (humans) developed because there was no longer was a need for swinging from trees. Upright walking on the ground encouraged alterations in the foot which happened to give better speed and balance. These differing traits soon became characteristics that evolved with the result of facilitating movement on the ground. Although humans and monkeys are genetically similar, their different natural habitats fostered different physical traits to evolve for survival.

Convergent evolution
Convergent evolution causes difficulties in fields of study such as comparative anatomy. Convergent evolution takes place when species of different ancestry begin to share analogous traits because of a shared environment or other selection pressure. Environmental circumstances that require similar developmental or structural alterations for the purposes of adaptation can lead to convergent evolution even though the species differ in descent. These adaptation similarities that arise as a result of the same selective pressures can be misleading to scientists studying the natural evolution of a species. Convergent evolution also creates problems for paleontologists using evolutionary patterns in taxonomy, or the categorization and classification of various organisms based on relatedness. It often leads to incorrect relationships and false evolutionary predictions.

Examples of convergent evolution


One of the best examples of convergent evolution involves how birds, bats, and pterosaurs (all different taxa that evolved along distinct lineages at different times) came to be able to fly. Importantly, each species developed wings independently. These species did not evolve in order to prepare for future circumstances, but rather the development of flight was induced by selective pressure imposed by similar environmental conditions, even though they were at different points in time. The development potential of any species is not limitless, primarily due to inherent constraints in genetic capabilities. Only changes that are useful in terms of adaptation are preserved. Yet, changes in environmental conditions can lead toward less useful functional structures, such as the appendages that might have existed before wings. Another change in environmental conditions might result in alterations of the appendage to make it more useful, given the new conditions.

For example, the wings of all flying animals are very similar because the same laws of aerodynamics apply. These laws determine the specific criteria that govern the shape for a wing, the size of the wing, or the movements required for flight. All these characteristics are irrespective of the animal involved or the physical location. Understanding the reason why each different species developed the ability to fly relies on an understanding of the possible functional adaptations, based on the behavior and environmental conditions to which the species was exposed. Although only theories can be made about extinct species and flight since these behaviors can be predicted using by fossil records, these theories can often be tested using information gathered from their remains. Perhaps the wings of bird or bats were once appendages used for other purposes, such as gliding, sexual display, leaping, protection, or arms to capture prey.

Another example of convergent evolution is the eyes of cephalopods (squid and octopus), which are remarkably similar to those of humans or other mammals. However, mammals and cephalopods have evolved eyes entirely separately, since the evolution of vertebrates and invertebrates diverged approximately 500 million years ago, when all creatures were sightless.

In various species of plants, which share the same pollinators, many structures and methods of attracting the pollinating species to the plant are similar. These particular characteristics enabled the reproductive success of both species due to the environmental aspects governing pollination, rather than similarities derived by being genetically related by descent.

Yet another example of convergent evolution is the case of mantellas and poison-dart frogs. Poison dart frogs live in South America, mantellas in Madagascar. They are totally unrelated, but have identical toxins in their skins, which they get from ants, that also are examples of convergent evolution.

Convergent evolution is supported by the fact that these species come from different ancestors, which has been proven by DNA analysis. However, understanding the mechanisms that brings about these similarities in characteristics of a species, despite the differences in genetics, is more difficult.

Yi qi was a dinosaur that lived around 160 million years ago, found in Hebei, China. It once roamed the wet Jurassic forests of ginkgo and conifer trees, gliding from one tree to another. The unique feature of this dinosaur is that it flew using a thin membrane, much like bats. Yi qi is the only known dinosaur to have this, and is a great example of convergent evolution.

Parallel evolution
Parallel evolution occurs when unrelated organisms develop the same characteristics or adaptive mechanisms due to the nature of their environmental conditions. Or stated differently, parallel evolution occurs when similar environments produce similar adaptations. The morphologies (or structural form) of two or more lineages evolve together in a similar manner in parallel evolution, rather than diverging or converging at a particular point in time.

Examples of parallel evolution
One example is the complex plumage patterns that seem to have evolved independently among many very different bird species.

A molecular example of Parallel Evolution is the ligand specificity of repressors and periplasmic sugar-binding proteins.

Parallel evolution is exemplified in the case of the tympanal and atympanal mouthears in hawkmoths, or Sphingidae species. These insects have developed a tympanum, or eardrum, similar to humans as a means to communicate through sound. Sounds induce vibrations of a membrane that covers the tympanum, known as the tympanic membrane. These vibrations are detected by small proteins at the surface of the tympanic membrane called auditory receptors. Within the Sphingidae species, two differing subgroups acquired hearing capability by developing alterations in their mouthparts by a distinctly independent evolutionary pathway.

Investigating the biomechanics of the auditory system reveals that only one of these subgroups has a tympanum. The other subgroup has developed a different mouthear structure that does not have a typanum, but has a mouthear with functional characteristics essentially the same as the subgroup with the tympanum. The evolutionary significance of how hearing capabilities developed in parallel in two different subgroups of a species reveals that distinct mechanisms can exist leading to similar functional capabilities with differing means for acquiring the same functional attribute. For both subgroups, hearing must have been an important characteristic for the species to survive given the environmental conditions.

Parallel evolution and speciation
Parallel speciation is a type of parallel evolution in which reproductive incompatibility in closely related populations is determined by traits that independently evolve due to adaptation to differing environments. These distinct populations are reproductively incompatible and only populations that live in similar environmental conditions are less likely to become reproductively isolated. In this way, parallel speciation suggests that there is good evidence for natural selective pressures leading to speciation, especially since reproductive incompatibility between to related populations is correlated with differing environmental conditions rather than geographical or genetic distances.

Books

 * Merrell, David J. The Adaptive Seascape: The Mechanism of Evolution. Minneapolis: University of Minnesota Press, 1994.
 * Gould, Stephen Jay. The Structure of Evolutionary Theory. Cambridge, MA: Harvard University Press, 2002.
 * Ridley, Mark. Evolution. Cambridge, MA: Blackwell Scientific Publications, 1993.
 * Good JM, Hayden CA, Wheeler TJ. Adaptive Protein Evolution and Regulatory Divergence in Drosophila. Mol Biol Evol. 2006 Mar 14
 * Yoshikuni Y, Ferrin TE, Keasling JD. Designed divergent evolution of enzyme function. Nature. 2006 Feb 22
 * Rosenblum EB. Convergent evolution and divergent selection: lizards at the white sands ecotone. Am Nat. 2006 Jan;167(1):1-15.
 * Rasmussen, L.E.L., Lee, T.D., Roelofs, W.L., Zhang, A., Doyle Davies Jr, G. (1996). Insect pheromone in elephants. Nature. 379: 684
 * Zhang, J. and Kumar, S. 1997. Detection of convergent and parallel evolution at the amino acid sequence level. Mol. Biol. Evol. 14, 527-36.
 * Dawkins, R. 1986. The Blind Watchmaker. Norton & Company.
 * Mayr. 1997. What is Biology. Harvard University Press
 * Schluter, D., E. A. Clifford, M. Nemethy, and J. S. McKinnon. 2004. Parallel evolution and inheritance of quantitative traits. American Naturalist 163: 809–822.

Periodicals

 * Berger, Joel, and Kaster, "Convergent Evolution." Evolution (1979): 33:511.