Evolution Principles

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This is no place for a broad exposition of evolutionary theory, but a few points should be made so as to be sure everyone is on the same page. Firstly, evolution is something that happens to populations, not individuals. Evolutionary changes occur by such processes as natural selection and genetic drift, among others. Natural selection is often thought of as the most important of the evolutionary causes because it is the only process known to produce adaptation non-randomly. It can be demonstrated that populations have the potentiality to produce many more offspring than can survive over an extended period of time, if for no other reason than eventually resources are insufficient to support a burgeoning population. Thus, on average, a large proportion of every generation dies without reproducing or reproduces less than average for the population. Which individuals depends partly on "luck", but also partly on the genetic makeup of the individuals. Those individuals less genetically fitted for the specific environment they are in tend to survive less often than those more fit. Thus the environment selects against those having less useful genetic variations, leaving the population more fit than it would have been otherwise. Mutations (changes in the DNA) introduces the variations upon which natural selection works.

The evidence is strong that all known life forms are descended from a common ancestor, and thus all living things are literally related to each other. Taxonomy reflects hypotheses concerning the closeness of these relationships. Hypothetically, members of a species are more closely related to one another than to any member of a different species; likewise, the species in a genus are hypothesized to be more closely related to each other than to any species in a different genus, and so forth. Since we normally use some version of the biological species concept in neomammalogy (in paleomammalogy, frequently the morphological concept must be used, as explained in the species module), speciation is involved. Speciation (cladogenesis) is the splitting of a single species into two reproductively isolated populations (species). Such species are more closely related to each other than to any species generated by an earlier speciation event.

Dendrogram to illustrate macroevolution

In the diagram above, each branch point represents a speciation event, the vertical axis represents time, and the horizontal axis morphological distance. The colors are merely to help distinguish various groupings; for example, the magenta and green toward the bottom of the diagram represent two genera, each consisting of two species. Lines not reaching the top represent phylogenetic extinctions. In real life, we would expect many more speciation events than shown here as we go up the diagram. Change of color within a line represents anagenesis (a single line becoming so different through time that a new name must be applied). A phylogenetic line could change greatly over geologic time without ever speciating, but by our taxonomy, different portions of its timeline would have to receive different names.

Speciation usually occurs during geographic isolation, though there are other possibilities. When populations are isolated from each other for extended periods of time, differences in selective pressures, genetic mutations, possible episodes of genetic drift, etc., may result in complete or partial reproductive isolation; in the latter case, if the populations come back into contact (secondary contact), they may interbreed so much that they merge back into interbreeding populations (in which case speciation has not occurred) or hybrids may be selected against, resulting in even greater reproductive isolation (reinforcement).

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Last Update: 18 Jan 2008

Centennial Museum and Department of Biological Sciences, The University of Texas at El Paso