Gtacattgctgc

...GTACATTGCTGC... —*■ ...GTACATaGCTGC... —► ... cTACATaGCgGC...

Time

Figure 8.4 The process of divergence for two DNA sequences that descended as identical copies of an ancestral sequence. Each sequence experiences neutral mutations, some of which are eventually fixed by genetic drift. These fixed mutations replace all other alleles and are therefore substitutions (indicated by blue lower-case letters). As substitutions accumulate, the two sequences diverge from the ancestral sequence as well as from each other. In this example, the two sequences are eventually divergent at five of 12 nucleotide sites due to substitutions. The dashed line indicates complete isolation of the two populations containing the derived sequences.

sequences and thereafter be reproductively isolated from each other. DNA sequences compared between the two species would each experience DNA sequence divergence due to mutations occurring over time.

Substitution The complete replacement of one allele previously most frequent in the population with another allele that originally arose by mutation.

The neutral theory predicts the rate at which allelic substitutions occur and thereby the rate at which divergence occurs. Predicting the substitution rate for neutral alleles requires knowing the probability that an allele becomes fixed in a population and the number of new mutations that occur each generation. A new mutation in a population of diploid individuals is initially present as just a single copy out of a total of 2N copies of the locus. Therefore, the initial frequency of a new mutation is Under genetic drift, the chance of fixation of any neutral allele is simply its initial frequency (see Chapter 3). Each generation, the chance that an allele copy mutates is || and there are a total of 2N allele copies. Therefore, the expected number of new mutations in a population each generation is 2N|. Multiplying the probability of fixation by the expected number of mutations per generation, eration, symbolized by the substitution rate k. Notice that this equation simplifies to

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