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Chapter 14.doc.docx

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Maryann Vaughan

The reproductive contribution of a phenotype to subsequent generations relative to the contributions of other phenotypes is called its fitness.  Changes in absolute numbers of offspring are responsible for increases and decreases in the size of a population, but only changes in the relative success of different phenotypes within a population lead to changes in allele frequencies from one generation to another.  The fitness of a phenotype is determined by the average rates of survival and reproduction of individuals with that phenotype. Natural selection produces variable results  Natural selection can act on characters with quantitative variation in any one several different ways: o Stabilizing selection—favouring average individuals o Directional selection –favouring individuals that vary in one direction from the mean pop. o Disruptive selection—favouring individuals that vary in opposite directions from the mean of the population. Sexual selection influences reproductive success  Sexual selection is a special type of natural selection that acts on characteristics that determine the reproductive success.  Sexual selection may favour traits that enhance the bearer’s chances of reproduction, but reduce its chances of survival. o Such costly traits reliably demonstrate the quality of their possessors as mates b/c they enable the choosing sex (usually females) to distinguish between genuinely fit individuals and exaggerators. How is Genetic Variation Maintained within Populations? Neutral mutations may accumulate within populations  An allele that does not affect the fitness of an organism is called a neutral allele.  Neutral alleles are unaffected by natural selection.  Neutral alleles tend to accumulate in a population over time, providing it with considerable genetic variation. Sexual recombination amplifies the number of possible genotypes  Sexual recombination generates an endless variety of genotypic combinations that increases the
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