Evolution__3rd_Edition

360 PART 4 / Evolution and Diversity
Figure 13.5
Size of male house sparrows
in North America. Size is
measured as a “principal
component” score, derived
from 15 skeletal measurements.
The score of 8 is for the largest
birds, the score of 1 is for the
smallest. The study described
in Section 3.2 (pp. 46–7) is a
precursor of this research.
Redrawn, by permission, from
Gould & Johnston (1972),
corrected from Johnston &
Selander (1971). © 1972 Annual
Reviews Inc.
Sparrows illustrate Bergman’s rule
San
Francisco
Minneapolis
St Paul
Atlanta
Miami
1 2 3 4 5 6 7
powerfully in his book Animal Species and Evolution (1963, chapter 11), has collected
more evidence about geographic variation than anybody else and he concludes that
“every population of a species differs from all others,” and “the degree of difference
between different populations of a species ranges from almost complete identity to distinctness
almost of species level.”
The second point to notice in Figure 13.5 is that the form of the geographic variation
is explicable. House sparrows are generally larger in the north, in Canada, than in the
center of America. The generalization is imperfect (compare, for instance, the sparrows
of San Francisco and Miami); but in so far as it applies, it illustrates Bergman’s rule.
Animals tend to be larger in colder regions, presumably for reasons of thermoregulation.
Geographic variation in these two species is therefore adaptive: the form of the
sparrows differs between regions because natural selection favors slightly differing
shapes in different regions.
13.4.2 Geographic variation may also be caused by genetic drift
House mice (Mus musculus) have a standard diploid chromosomal set of 40 chromosomes
(2N = 40). The centromeres of all 20 chromosomes are near the chromosomes
ends and, perhaps for this reason, chromosomal fusions often take place in this
species. In a chromosomal fusion, two chromosomes join together at their terminal
8
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