What life is


Mechanisms of Evolution

The first correct account of the mechanism of evolution has been proposed by Charles Darwin. In his theory of evolution, he suggested natural selection to explain the 'descent with modification'. Natural selection of already existing traits best explains how species could change over time by adapting to environmental changes. Darwin suggested that modifications are present in individuals, as can readily be seen in humans and any other animal or plant species. These differences among individuals of the same group (species or population) can be inherited (Gregor Mendel showed how) and individuals of the next generations always have different combinations of traits. Natural selection is thought to edit which traits are best suited to have the most offspring. It will be these traits that will also be found more often in the next generation. Thus the environment (natural selection) affects the obvious variability found in all animal and plant species.

Today we understand natural selection in more details and at the level of genes and mutations. We also understand that many times mutations and corresponding traits can be inherited more frequently or less frequently just by chance, not just because of a physiological advantage. This chance element is a critical part of the modern theory of evolution and explains why this process does not produce designed features as if nature could foresee what is best. Rather, mutations make random adjustments to an individuals physiological capacity and the more offspring an individual has, the more copies of its particular genes it will pass on to the next generation.

The measurement of the number of offspring gives an unbiased account of evolutionary success, or Darwinian fitness. In this theory, you propose that you make changes and then see which ones serve you best. With this in mind, it is easy to understand that evolution is an ongoing process, and that evolution will not be able to produce the perfect organism. The latter is impossible because the environment keeps changing, and natural selection can only favor or disfavor traits that are already there. To understand the inability of perfection, one only has to think about the myriad of diseases that afflict humans and any other species.

Furthermore, many changes do not result in beneficial or detrimental traits. They are said to be neutral and are unaffected by the environment. However, they are still subject to additional random changes. If two populations of individuals reproduce in distinct locations, or because they prefer different mating habits, they can accumulate changes in their genepool independent of each other. After many generations, these changes may restrict successful mating, even if individuals from two isolated populations are given a chance to mate. A natural or biological barrier has evolved that is now reinforcing further independent changes that will likely result in these two populations to be quite different after millions of generations. These reproductive barriers are important steps in forming new species from an ancestral species. If this process of speciation occurs repeatedly, many different derived species can exist. Their common ancestral origin, however, can still be estimated by comparing similarities in anatomical features, behavioral traits, or molecular sequences (genes, proteins).

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 Copyright © 1999-2011 Lukas K. Buehler