Evolution__3rd_Edition

..
Other factors are probably at work
too
Competition is strongest within a species. Each individual will, in many cases,
encounter more members of its own species than of other species. Also, the members of
its own species are more similar to it, exploiting more similar resources. One way to
avoid competition is to become different from the competitors; hence there will be a
force pushing similar competing types apart in evolution. Competition between similar
individuals will make for the evolution of new adaptations in each that reduce the
intensity of competition; divergence will thus result. Character displacement (Section
13.6, p. 366) provides evidence that competition can cause divergence between closely
related species. Evolutionary divergence is not inevitable, however. It depends on the
contingencies of competition in particular cases. Provided that, on average, more similar
individuals compete more closely, divergence is likely to result.
Other factors probably also contribute to causing divergence. For instance, speciation
is often allopatric and each pair of sister species becomes increasingly isolated
over time by the Dobzhansky–Muller process (Section 14.4, p. 389). While the members
of two populations interbreed, natural selection favors genetic changes that are
advantageous in both populations. The two populations are kept relatively similar.
Once the two populations have evolved apart and no longer interbreed, no force
constrains the genetic changes that occur in one population also to be favorable in
members of the other population. Incompatible genetic changes accumulate in the
two populations (or species, as they are by this stage). The two gene pools have
evolutionarily “escaped” from each another and are free to diverge further. Our
modern genetic understanding of speciation has added to Darwin’s explanation for
divergence.
16.9 Conclusion
CHAPTER 16 / Classification and Evolution 489
Most biologists now accept that cladism is theoretically the best justified system of
classification. It has a deep justification that phenetic and partly phenetic systems lack.
Cladism is objective, and objective classifications are preferable to subjective ones. But
despite cladism’s theoretical advantages, it can run into practical problems. The uncertainties
of phylogenetic inference make cladistic classifications liable to frequent revision.
True cladists are not very worried about that, and remark that all healthy theories
are modified as new facts come in.
Cladistic classification has become popular only recently. Evolutionary classification
was the orthodox school from the “modern synthesis” of the 1930s (or even from
Darwin’s time in the 1860s) until about 20 years ago. Numerical phenetics enjoyed a
surge of support from the late 1950s to the early 1970s. Now, however, both schools
have succumbed to cladistic criticism and have relatively few supporters. At all events,
the most important point is to understand the arguments that have been used for and
against each school: these arguments are of permanent importance in evolutionary
biology.