Question

1) If a population stopped reproducing sexually (but still reproduced asexually), how would its genetic variation be affected over time? Explain. 2) A locus that affects susceptibility to a degenerative brain disease has two alleles, A and a. In a population, 16 people have genotype AA, 92 have genotype Aa, and 12 have genotype aa. Is this population evolving? Explain. 3) Explain why natural selection is the only evolutionary mechanism that consistently leads to adaptive evolution.

Answer 1

1) Genetic variation would most likely drop over time. In sexual reproduction is where alleles are shuffled and create variations. With only asexually reproduction there are not as many combinations that can be formed.

The genetic variation would be dramatically reduced to a single cloned population which is equivalent to one individual or possibly several lineages. That number of lineages would probably reduce overtime if they weren't isolated. There would only be the changes that happened from mutations to individuals. They would still be passed down so variation would slowly increase over time but it would be difficult for bad genes not to build up in the population.
2) there are 120 individuals in the population, so there are 240 alleles, of these, there are 124 "A" alleles (32 from the 16 "AA" individuals and 92 from the 92 "Aa" individuals) thus, the frequency of the "A" allele is p=124/240=.52; hence the frequency of the 'a' allele is q=.48. Based on the hardy-Weinberg equation, if the population were not evolving, the frequency of genotype "AA" should be p2=.52x.25=.27; the frequency of genotype "aa" should be q2=.48x.48=.23. In a population of 120 individuals, these expected genotype frequencies lead s to predict that there would be 32 "AA" individuals (.27x.120), 60 "Aa" individuals (.51x120), and 28 "aa" individuals ( .23x120). The actual numbers for this population deviate from these expectations (fewer homozygotes and more heterozygotes than expected. This indicates that the population is not in hardy Weinberg equilibrium and hence may be evolving at this locus.

3) Natural selection is the only mechanism of adaptive evolution; it is defined as differential reproductive success of pre- existing classes of genetic variants in the gene pool.

The most common action of natural selection is to remove unfit variants as they arise via mutation. [natural selection: differential reproductive success of genotypes] In other words, natural selection usually prevents new alleles from increasing in frequency. This led a famous evolutionist, George Williams, to say "Evolution proceeds in spite of natural selection."

Natural selection can maintain or deplete genetic variation depending on how it acts. When selection acts to weed out deleterious alleles, or causes an allele to sweep to fixation, it depletes genetic variation. When heterozygotes are more fit than either of the homozygotes, however, selection causes genetic variation to be maintained. [heterozygote: an organism that has two different alleles at a locus. | homozygote: an organism that has two identical alleles at a locus] This is called balancing selection.

Balancing selection is rare in natural populations. [balancing selection: selection favoring heterozygotes] Only a handful of other cases beside the sickle-cell example have been found. At one time population geneticists thought balancing selection could be a general explanation for the levels of genetic variation found in natural populations. That is no longer the case. Balancing selection is only rarely found in natural populations.