The Patterns of Evolution
Divergent Evolution, Convergent Evolution and Coevolution Revealed
Evolutionary biologists have discovered that species evolve in relation to each other over time. These interactive evolutionary relationships (or patterns of evolution as they are often called) occur in three main forms: divergent evolution, convergent evolution, and coevolution.
Divergent Evolution
Divergent evolution occurs when some members of a species take a different evolutionary pathway than others of that species. If different selective pressures are placed 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.
Differences in environmental circumstances and selection pressures between two groups within the same species over a long period of time may result in the formation of a new species splintering off from the original species group.
Divergent Evolution in Action
The apple maggot fly once infested the fruit of a native Australian hawthorn. In the 1860s some maggot flies began to infest apples. They multiplied rapidly because they were able to make use of an abundant food supply. Now there are two distinct species, one that reproduces when the apples are ripe, and another that continues to infest the native hawthorn. They have not only evolved different reproductive timing, but also now have distinctive physical characteristics.
Convergent Evolution
Convergent evolution describes the acquisition of the same basic biological trait in unrelated lineages (species). In other words, species that are genetically unrelated may evolve similar traits and structures as they adapt to similar environments and selection pressures. There are a finite number of effective solutions to some environmental challenges, and some of these solutions emerge independently again and again.
Convergent Evolution in Action
The fish found in frigid polar waters have evolved genes that produce glycoproteins. These proteins act as a natural antifreeze in the blood of the fish by slightly lowering the temperature at which the blood of the fish would freeze with fatal results.
Antifreeze proteins are a clever evolutionary solution to a major problem. But consider this: fish in both Arctic and Antarctic waters are equipped with antifreeze proteins. However, the genetic pathways that produce those proteins are different in Arctic fish than they are in Antarctic fish. This is evidence that quite separate, independent episodes of convergent molecular evolution occurred, with the same functional results.
Other examples are the different species of anteaters, found in Australia and Africa. Though not closely related, they all evolved the structural adaptations necessary to subsist on an ant or termite diet: a long, sticky tongue, few teeth, sturdy claws, and large salivary glands. In each case, evolutionary adaptations allowed them to exploit a food niche of ants and termites, but the developments occurred independently.
Coevolution
The term coevolution is used to describe cases where two (or more) species reciprocally affect each other’s evolution. For example, an evolutionary change in the morphology of a plant, might affect the morphology of an herbivore that eats the plant, which in turn might affect the evolution of the plant, which might affect the evolution of the herbivore...and so on.
Each party in a coevolutionary relationship exerts selective pressures on the other, thereby affecting each others’ evolution. This is known as reciprocal evolutionary change between interacting species and is considered a strict definition of coevolution. Coevolution between species is usually not clear cut and a great deal of observation, experimentation, and genetic analysis is require to establish such a true link.
The following types of species interactions offer the most fertile ground for true coevolution:
- Predator-Prey
- Plant-Herbivore
- Parasite-Host
Species may become so tightly bound to each other through coevolution that one cannot survive without the other. Such a biological relationship is said to be mutualistic.
Coevolution in Action
A classic example of mutualistic coevolution is the termite and the protozoans that inhabit its gut. The termite provides suitable shelter and food for the protozoans while they in turn actually digest the wood eaten by the termite thus providing food for the termite. When the protozoans are removed from the termite and exposed to normal environmental conditions, they perish. Lacking the enzymes necessary to digest wood cellulose, termites starve with a belly full of useless wood fibers when the protozoans are cleared from the termite using antibiotics. The mechanisms of coevolution have bound these two species so tightly together that they have, in essence, become one.
In summary, we see that understanding the various patterns of evolution is crucial to evolutionary biologists as they attempt to decipher the phylogeny that is the Tree of Life.
Darwin's Finches are Convergence in Action
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