Evolution__3rd_Edition

418 PART 4 / Evolution and Diversity
later update of his views.) Fourthly, new species often arise by hybridization, particularly
in plants. Fifthly, Darwin’s idea that isolation evolves as a by-product was
expanded and explained genetically in the Dobzhansky–Muller theory.
Now, 50 or more years later, the allopatric theory of speciation still stands up. Many
biologists would allow some contribution from sympatric speciation, but most accept
that allopatric speciation is the main process. In this respect, biologists now agree with
the modern synthesis rather than Darwin. The second claim, that speciation is often
powered by genetic drift now has few supporters. It is the least important of the five
claims listed above, and may not have been strongly believed in even during the period
from the 1930s to the 1950s. In the 1920s, biologists often suggested that the characters
that differ between species are non-adaptive. This partly inspired “non-adaptive”
theories of speciation, but few biologists now argue that species differences are nonadaptive.
The experimental evidence and theory of speciation suggest the genetic drift
is not all that important in speciation. Speciation is probably more often a by-product
of normal adaptive divergence between populations.
The theory of reinforcement has had its ups and downs. Reinforcement continues to
tantalize biologists, but a compelling case for its importance has yet to be made. The
theory of hybrid speciation in plants, by contrast, has held up well. New genetic techniques
have enabled biologists to trace the ancestry of modern species, providing a
detailed description of hybrid speciation.
Finally, the genetics of postzygotic isolation has become a major field of research.
Darwin seems to have been right that postzygotic isolation evolves as an incidental byproduct
of divergence. The Dobzhansky–Muller theory improved our understanding
of the genetic events by which postzygotic isolation can incidentally drop out of normal
evolutionary change. Evidence for the theory has accumulated, not least as the evidence
for Haldane’s rule has been incorporated (if partly) in the general theoretical scheme.
The Dobzhansky–Muller theory looks as if it may continue to inspire research as the
techniques of modern genomics are imported into the study of speciation.
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