23.1 – 1 What did Mendel’s findings about genetics add to Darwin’s theory of evolution by natural selection?

23.1 – 1 Mendel showed that inheritance is particulate, and subsequently it was shown that this type of inheritance can preserve the variation on which natural selection acts.

23.1 – 2 Suppose a population of organisms with 500 gene loci is fixed at half of these loci, and has two alleles at each of the other loci. How many alleles are found in the gene pool? Explain.

23.1 – Half of the loci (250) are fixed, meaning only one allele exists for each locus: 250 x 1 = 250. There are two alleles each for the other loci: 250 x 2 = = 750.

23.1 – 3 Which parts of the Hardy-Weinberg equation (p 2 + 2pq + q 2 = 1) correspond to the frequency of individuals that have at least one PKU allele?

23.1 – 3 2pq + q 2 2pq represents heterozygotes with one PKU allele and q 2 represents homozygotes with two PKU alleles.

23.2 – 1 Of all the mutations that occur, why do only a small fraction become widespread in a gene pool?

23.2 – 1 Most mutations occur in somatic cells that do not produce gametes and so are lost when the organism dies. Of mutations that do occur in cell lines that produce gametes, many do not have a phenotypic effect on which natural selection can act. Others have a harmful effect and are thus unlikely to spread in a population from generation to generation because they decrease the reproductive success of their bearers.

23.2 – 2 How does sexual recombination produce variation?

23.2 – 2 A population contains a vast number of possible mating combinations, and fertilization brings together the gametes of individuals with different genetic backgrounds. Sexual reproduction reshuffles alleles into fresh combinations every generation.

23.3 – 1 In what sense is evolution more “predictable” than genetic drift?

23.3 – 1 Natural selection is more “predictable” in that it tends to increase or decrease the frequency of alleles that correspond to variations that increase or decrease an organism’s reproductive success in its environment. Alleles subject to genetic drift all have the same likelihood of increasing or decreasing frequency.

23.3 – 2 Distinguish genetic drift and gene flow in terms of (a) how they occur and (b) their implications for future genetic variation in a population.

23.3 – 2 Genetic drift results from chance fluctuations of allele frequencies from generation to generation; it tends to decrease variation over time. Gene flow is the exchange of alleles between populations; it tends to increase variation within a population but decrease allele frequency differences between populations.

23.4 – 1 Does nucleotide variability in a population always correspond to the phenotypic polymorphism? Why or why not?

23.4 – 1 No. Many nucleotides are in noncoding portions of DNA or in pseudogenes that have been inactivated by mutations. A change in a nucleotide may not even change the amino acid encoded because of the redundancy of the genetic code.

23.4 – 2 What is the relative fitness of a sterile mule? Explain.

23.4 – 2 Zero, because fitness includes reproductive contribution to the next generation, and a sterile mule cannot produce offspring.

23.4 – 3 Explain what is meant by the “reproductive handicap” of sex.

23.4 – 3 Only half of the members (the females) of a sexual population actually produce offspring, while all the members of an asexual population can produce offspring.