An important consequence of this is that as selection proceeds

An important consequence of this is that as selection proceeds

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An important consequence of this is that as selection proceeds, the additive genetic variation will be reduced ("low" alleles removed). As the V A decreases, the heritability decreases (see equation for heritability above). Will selection come to a halt?? Probably not because mutation is constantly introducing a trickle of new alleles each with different additive effects. Under this view the gradual changes in phenotype seen over long evolutionary times might be explained by a continual mutation- selection balance . (see box 9.2 pg. 248). What are the heritabilities of various traits in nature? They vary a lot (see figure 9.13, pg. 245). One trend is that fitness-related traits tend to have lower heritabilities than other traits. Why? In natural populations fitnesses determine our "selection differentials" so selection should remove genetic variation for fitness traits and heritabilities will drop. Why then are not the heritabilities of fitness related traits
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Unformatted text preview: zero???? One answer: genetic correlations. Due to the linkage of genes along chromosomes (or epistatic interactions among genes) selection of one trait can lead to selection for another trait. If a viability gene is linked between genes for bristle number, selection for high bristle number could lead to low viability if the low viability allele became associated with the high oil content alleles. If this were the case, there would b a negative genetic correlation between bristle number and viability. As it turns out, many fitness related traits have negative genetic correlations (e.g., size of eggs negatively correlated with number of eggs). Thus if fitness determines selection differential, selection in natural populations could not remove all the additive genetic variation for two fitness traits that are negatively correlated....
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