Tuesday, March 31, 2009

How Populations Evolve

Evolution is a change in the gene pool of a population over time. A gene is a hereditary unit that can be passed on unaltered for many generations. The gene pool is the set of all genes in a species or population. A population is a group of organisms of the same species usually found in a clearly defined geographical area.

The English moth or the peppered moth, biston betularia, is a frequently cited example of observed evolution. In this moth there are two color morphs, light and dark. Dr. Henry Bernard Davis Kettlewell, a British lepidopterist and medical doctor, is notable for his experiments on the peppered moth, most of which were done in Manchester, England. He found that dark moths constituted less than 2°,o of the population prior to 1848. The frequency of the dark morph increased in the years following. By 1898,95% of the moths in Manchester and other highly industrialized areas were of the dark type. Their frequency was less in rural areas. The moth population changed from mostly light colored moths to mostly dark colored moths. The moths' color was primarily determined by a single gene. So, the change in frequency of dark colored moths represented a change in the gene pool. This change was, by definition, evolution.
The increase in relative abundance of the dark type was due to natural selection. The late eighteenth century was the time of England's industrial revolution. Soot from factories darkened the birch trees the moths landed on. Against a sooty background, birds could see the lighter colored moths better and ate more of them. As a result, more dark moths survived until reproductive age and left offspring. The greater number of offspring left by dark moths is what caused their increase in frequency. This is an example of natural selection.

Populations evolve. In order to understand evolution, it is necessary to view populations as a collection of individuals, each harboring a different set of traits. A single organism is never typical of an entire population unless there is no variation within that population. Individual organisms do not evolve; they retain the same genes throughout their life. When a population is evolving, the ratio of different genetic types is changing-each individual organism within a population does not change. For example, in the previous example, the frequency of black moths increased; the moths did not turn from light gray to dark in concert. The process of evolution can be summarized in three sentences: Genes mutate. Individuals are selected. Populations evolve.

The word evolution has a variety of meanings. The fact that all organisms are linked via descent to a common ancestor is often called evolution. The theory of how the first living organisms appeared is often called evolution. This should be called abiogenesis. And frequently, people use the word evolution when they really mean natural selection-one of the many mechanisms of evolution. Phenotype is the morphological, physiological, biochemical, behavioral, and other properties exhibited by a living organism. Genotype is the genetic make up of an organism.
Evolution can occur without morphological change; and morphological change can occur without evolution. Humans are larger now than in the recent past, a result of better diet and medicine. Phenotypic changes like this, induced solely by changes in environment, do not count as evolution because they are not heritable; in other words, the change is not passed on to the organism's offspring. Most changes due to environment are fairly subtle, for example, size differences. Largescale phenotypic changes are obviously due to genetic changes, and therefore are evolution.

Tags: Bio Technology, Bio Genetics , Evolution

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