Evolution__3rd_Edition

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Could postzygotic isolation evolve
by changes at one locus?
rarely arranged in a ring. But several examples do exist (Irwin et al. 2001b). The herring
gull (Larus argentatus) and lesser black-backed gull (L. fuscus) of northern Europe are a
ring species, connected by a ring of populations around the North Pole. In central Asia,
the warbler Phylloscopus trochiloides is distributed in a ring around the treeless Tibetan
Plateau. The species appears to have originated in the south, in India. It then colonized
northwards, either side of the Tibetan Plateau. The colonizing populations evolved
slight genetic differences, and the songs diverged between the eastern and western
populations. Eventually, these two lines of populations meet up again in central
Siberia, but here the songs are so different that the members of the east population do
not interbreed with members of the western population (Irwin et al. 2001a). And on
the tiny Pacific island of Moorea, certain species of snail in the genus Partula had
evolved ring species around individual small mountains (Murray & Clarke 1980). Alas,
the Partula of Moorea are extinct. They all succumbed in the 1980s and 1990s, in yet
another unnecessary manmade ecological disaster (Tudge 1992).
14.3.4 Speciation as a by-product of divergence is well documented
In summary, we have abundant evidence that reproductive isolation evolves as a byproduct
of divergence between geographically separate populations. If populations are
experimentally kept in different conditions in the lab, those populations evolve adaptations
to their conditions a and reproductive isolation also evolves as a consequence. If
separate populations adapt to different local conditions in nature, they also evolve
reproductive isolation as a consequence. For prezygotic isolation, we saw that the genetic
basis is probably pleiotropy or hitch-hiking. However, that explanation is mainly hypothetical
at present, because little genetic research has been done on prezygotic isolation.
We can turn now to the genetics of postzygotic isolation. It has been the subject of
much genetic research, and is well understood both empirically and theoretically.
14.4 The Dobzhansky–Muller theory of postzygotic isolation
14.4.1 The Dobzhansky–Muller theory is a genetic theory of postzygotic
isolation, explaining it by interactions among many gene loci
What kind of genetic changes give rise to postzygotic isolation? Postzygotic isolation
means that hybrid offspring are produced but they either die before breeding or live
and are sterile. In this section, we shall see that postzygotic isolation is likely to be
caused, genetically, by an interaction between the genotypes at multiple loci, rather
than by genotypes at a single locus. The simplest hypothetical genetic control would
have one genetic locus, and the fitnesses of the genotypes would be as follows:
Species 1 Hybrid Species 2
One-locus genotype AA Aa aa
Fitness High Zero High
CHAPTER 14 / Speciation 389