untitled

the other northeast into China and Siberia. The two arms differ progressively in

morphology, ecology, and song characteristics. The arms meet again in central

Siberia; in the overlap zone the two no longer recognize each other’s song and do

not interbreed. These physical differences are due to genetic differences, an example

of genetic differentiation-by-distance that we discussed above (Irwin et al. 2005). Irwin

et al. (2001b) review other cases of ring species.

Populations isolated by geographical barriers are called allopatric. In isolation they

can become genetically different through adaptation to their different areas. If the

populations then meet and overlap in range (become sympatric) they may not

interbreed if they have diverged too far, in which case they have become separate

species; or they may interbreed and form a zone of hybrids, an area of higher genetic

variability with many intermediate forms. The parent types would then be called races

or subspecies. An example is seen in waterbuck in Africa which has a northern form

with a white rump patch and a southern form with a thin white ring on the rump.

They overlap in a narrow zone in Kenya where they interbreed, producing various

rump patterns.

So far we have considered genetic variability within and between populations. There

is another form of variability which we call a polymorphism. The formal definition

of this by E.B. Ford (1940) is “the occurrence together in the same habitat of two

or more discontinuous forms of a species in such proportions that the rarest of them

cannot merely be maintained by recurrent mutation or immigration.” This means that

the different morphs, often quite visibly different, live together in the same habitat

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Fig. 3.6The distribution ofCervus elaphusshows a cline from the small red deer of Scotland to the large elk of North
America. The numbers and their associated shaded areas indicate different races in the cline. (After Whitehead 1972.)

3.7.3Individual
variability within
geographic regions