NatSelectDriftSexSelec - Mechanisms of Evolution: Selection...

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Mechanisms of Evolution: Selection and Drift Natural Selection Natural selection is one of the mechanisms by which evolutionary change can take place. This mechanism was first proposed by Darwin and Wallace. Three conditions must be met for natural selection to operate: (1) There must be variation within the population in the feature that is changing (2) The feature in question must be heritable - i.e. ‘like breeds like’ (3) Some forms of the feature must survive and reproduce better than others. Organisms that have features that allow them to survive and reproduce better than other organisms are said to have higher fitness than others. Evolutionary fitness is defined as: The reproductive success of an individual relative to other individuals in the same population (includes both survival and reproduction). A fourth condition that is not necessary, but that increases the potential for natural selection, is the production of more offspring than can survive. This ‘excess reproduction’ increases competition between members of the same species. All populations that are not in severe decline have this ‘excess reproduction’. Organisms leaving more surviving offspring will pass on more of their favorable features through the generations. Features that help survival and reproduction persist, those that do not help in this way will die out. Features of organisms that have been produced by natural selection are called adaptations Knowing that a feature has high fitness does not imply ‘good’ or ‘bad’. e.g., nepotism is generally favored by natural selection but we must judge its acceptability in human society by other standards If the three conditions above are met, then change will occur in the population. It is important to keep straight the role of “random genetic variation” in evolution by natural selection. Most organisms (except those produced by asexual cloning) differ genetically from one and other. This genetic diversity arises through the random shuffling of genes during sexual reproduction and from random genetic mutations. Thus, genetic variation is random. Natural selection takes this variation and selects those variants that leave the most surviving offspring – this is a non-random process. So, we have two stages: random genetic variation then non-random selection of the fittest genetic variants. We can classify natural selection into three categories:
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This note was uploaded on 02/02/2012 for the course BIO 203 taught by Professor Davidhaskell during the Spring '09 term at Sewanee.

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NatSelectDriftSexSelec - Mechanisms of Evolution: Selection...

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