Evolution__3rd_Edition

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Diploid genotype
Gene
duplication
Ancestral species
Population
1
Population
2
Gene loss
Gene loss
Figure 14.6
Gene duplications can provide an example of the
Dobzhansky–Muller process (which was illustrated in
Figure 14.5). A gene duplicates in an ancestral species.
The species then splits into two populations, and different
. . . parasite–host relations ...
. . . and gene duplication and loss
...
. . . are four (of many) examples
that illustrate the Dobzhansky–
Muller set-up
CHAPTER 14 / Speciation 393
Hybrid
Hybrid offspring
(some lack
both genes)
copies of the duplicated gene are lost in each population.
If members of the two populations later interbreed, the
hybrids will have reduced fertility because some of their
offspring (one-sixteenth of them in a simple case)
lack both copies of the gene.
Half the sperm–egg encounters among hybrids will be incompatible and something
may go wrong in them.
Thirdly, consider parasite–host coevolution. In Section 12.2.3 (p. 323) we looked at
the way in which hosts may evolve specific resistance mechanisms against locally abundant
parasites. Population 1 may evolve a set of resistance genes (R 1 R 2 ) that work
against parasites in its environment; population 2 may evolve another set of resistance
genes (R 3 R 4 ) that work against its parasites. But hybrids may contain combinations of
resistance genes (R 1 R 3 or R 2 R 4 ) that do not work against any parasites.
Fourthly, consider gene duplication and loss (Figure 14.6). (Sections 2.5, p. 30,
10.7.2, p. 275, and 19.3, p. 559, provide background on gene duplications.) A gene may
be duplicated, or a species may split into two and the two new populations may lose different
copies of the gene. If members of the two populations then meet, hybrids may
initially be viable because they contain copies of the gene from both parental populations.
However, recombination within hybrids can produce offspring with no copies of
the gene, as Figure 14.6 illustrates. This is the phenomenon of “hybrid breakdown,”
which is a form of postzygotic isolation (Table 13.1, p. 356). The hybrids themselves are
healthy but subsequent generations have reduced fitness.
These are only four of the many ways in which simple evolutionary change in separate
populations may fit the Dobzhansky–Muller process. Almost any genetic changes in
the DNA have the potential to prove incompatible with genetic changes elsewhere in
the DNA. There are extensive interactions between gene loci within a body, and these
interactions will not proceed smoothly by chance. The genes in a human body interact
well because natural selection has been acting, over millions of years, to favor versions
of our genes that interact well. No such force constrains our genes to interact well with
the genes of other species, such as chimpanzees. Human–chimpanzee hybrids would be
likely to show a great genetic snarl-up, and be inviable. The Dobzhansky–Muller theory
has good biological plausibility as well as being theoretically coherent and empirically
supported.