recap

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22.2 recap

When two parts of a population become isolated from each other by some barrier to gene flow, they begin to diverge genetically. The Dobzhansky–Muller model describes how new alleles or chromosomal arrangements that arise in the two descendent lineages can lead to genetic incompatibility, and hence reproductive isolation, of the two lineages.

learning outcomes

You should be able to:

  • Make inferences and draw conclusions based on the Dobzhansky–Muller model.

  • Evaluate data from two populations to determine if they are reproductively isolated.

Question 1

The Dobzhansky–Muller model suggests that divergence among alleles at different gene loci leads to genetic incompatibility between species. Why is genetic incompatibility between two alleles at the same locus considered less likely?

If the only difference between the diverging lineages is at a single locus, then both of the new alleles must be functional when they interact with the products of other gene loci (in both lineages). Any genetic incompatibility produced by these new alleles would be expected to affect the parental lineages as well. There are much greater numbers of possible incompatibilities across different gene loci. Rather than two deleterious changes at the same locus (one in each lineage), the Dobzhansky–Muller model allows neutral changes at any pair of loci whose products interact. It is the negative interaction of these products in the hybrid between the two lineages that results in genetic incompatibility.

Question 2

Why do some combinations of chromosomal centric fusions cause problems in meiosis? Can you diagram what would happen at meiosis in a hybrid of the divergent lineages shown in Figure 22.4?

Segregation doesn’t occur normally, because chromosome arms are linked differently on the different species, resulting in duplications and deletions in meiosis. If two different fusions of chromosomes occur in two different lineages, then the resulting chromosomes cannot pair normally in meiosis in the hybrids. If you attempt to diagram meiosis in the hybrid that would result from a cross of the divergent lineages in Figure 22.4 (see below), you will see that homologous pairings require parts of different chromosomes to align with one another. These chromosomes will then be pulled in two different directions as the cell divides in meiosis I, resulting either in a likely failure of the cell to divide or in an uneven distribution of the chromosome arms in the two daughter cells. Production of normal cells with an even distribution of the various chromosomes’ arms is impossible, so the hybrid organisms will not produce functional gametes.

image

Question 3

Assume that the reproductive isolation seen in Phlox strains results from lethal combinations of incompatible alleles at several loci among the various strains. Given this assumption, why might the reproductive isolation seen among these strains be partial rather than complete?

A likely possibility is that the incompatible alleles have not yet become fixed in the various strains, so only some combinations of crosses result in genetic incompatibility.

We have now seen how the splitting of an ancestral population leads to genetic divergence and reproductive incompatibility in the two descendant lineages. Next we will consider ways in which the descendant lineages could have become separated in the first place.