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PATTERNS OF VARIATION - Continuously varying traits can be...

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PATTERNS OF VARIATION Evolution by Natural Selection rests on the following principles: 1. there is variation in natural populations 2. the variation is heritable; has a genetic basis 3. more offspring are produced than will survive each generation: struggle for existence 4. if heritable variation affects survival/reproduction, there will be differential reproduction=selection Without genetic variation there will be no evolution. Thus, characterizing the genetic variation in natural populations is fundamental to the study of evolution. (see The Genetic Basis of Evolutionary Change by Lewontin , 1974) What kinds of variation are there? Discrete polymorphisms (e.g., Biston betularia ) easily noticed, but not frequent or representative of the variation in natural populations (eye color in humans also quasi discrete).
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Unformatted text preview: Continuously varying traits can be described by the mean x = ( X i )/n and variance V = 1/n S (X i-x) 2 . Examples: the carrots in the Burpee Catologue; human height. Continuously varying traits will have both a genetic and environmental components. How much genetic variation is there? Historical debate: Classical school held that there was very little genetic variation, most individuals were homozygous for a "wild-type" allele. Rare heterozygous loci due to recurrent mutation; natural selection purges populations of their "load" of mutations. Balance school held that many loci will be heterozygous in natural populations and heterozygotes maintained by "balancing selection" (heterozygote advantage). Selection thus plays a role in maintaining variation....
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