Chapter17_SSM - 65781_CH17_324_346.qxd 8/1/08 1:23 PM Page...

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Chapter 17: Molecular Evolution and Population Genetics Chapter Summary The sequences of protein and nucleic acid molecules change through evolutionary time as new mutations occur and some eventually become fixed. The accumulation of sequence differences is the basis for estimating a gene tree depicting the ancestral history of a group of molecules. Proteins and different classes of DNA sequences evolve at very different rates. Among the fastest evolving DNA sequences are those of pseudogenes, introns, and fourfold degenerate sites. New genes usually evolve from existing genes through the acquisition of novel or specialized functions among duplicate copies (paralogs). In most natural popula- tions, many genes are polymorphic in that they have two or more common alleles. The relationship between the relative proportions of particular alleles (allele frequencies) and genotypes (genotype frequencies) is determined in part by the frequencies with which particular genotypes form mating pairs. In random mating, mating pairs are independent of genotype. When a population undergoes random mating for an autosomal gene with two alleles, the frequencies of the genotypes are given by the Hardy–Weinberg principle. If the alleles of the gene are A and a , and their allele frequencies are p and q , respectively, then the Hardy–Weinberg principle states that the genotype frequencies with random mating are AA , p 2 ; Aa, 2 pq ; and aa , q 2 . These are often good approximations for genotype frequencies within subpopulations. An important implication of the Hardy–Weinberg principle is that rare alleles are found much more frequently in heterozygotes than in homozygotes (2 pq versus q 2 ). Inbreeding means mating between relatives, and the extent of inbreeding is measured by the inbreeding coefficient, F. The main consequence of inbreeding is that an allele present in a common ancestor may be transmitted to both parents of an inbred individual in a later generation and become homozygous in the inbred offspring. The replicas of an ancestral allele are said to be identical by descent. The inbreeding coef- ficient of an inbred organism can be deduced directly from the pedigree of inbreeding by calculating the probability of identity by descent along every possible path of descent through each common ancestor. Among inbred individuals, the frequency of heterozygous genotypes is smaller, and that of homozygous genotypes greater, than it would be with random mating. 324
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325 Evolution is cumulative genetic change in a population through time. Adaptation is a progressive increase in the degree to which a species becomes genetically well suited to its environment. A principal mechanism of adaptation is natural selection, in which individuals superior in survival or reproductive ability in the prevailing environment contribute a disproportionate share of genes to future generations, thereby gradually increasing the frequency of the favorable alleles in the whole population. At least three other processes also
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Chapter17_SSM - 65781_CH17_324_346.qxd 8/1/08 1:23 PM Page...

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