Evolution__3rd_Edition

..
We have good evidence of
geographic variation
CHAPTER 13 / Species Concepts and Intraspecific Variation 359
In conclusion, over evolutionary time the amount of isolation between two species
will increase and the species will eventually be isolated by most of the barriers listed
in Table 13.1. (Think of how humans are isolated from a distant species, such as a
baboon a we are probably isolated from them by everything in the list except habitat
and breeding season.) Experiments can be done to reveal what the particular isolating
barriers are between closely related species. These experiments can reveal what isolating
barriers are at work in the early stages of speciation. We return to this topic in
Chapter 14.
13.4 Geographic variation within a species can be
understood in terms of population genetic and
ecological processes
Intraspecific variation exists both at any one place and between different places. If
we sample a number of individuals belonging to one species at any one locality, those
individuals may differ a variation within a population a often showing a normal distribution
(Section 9.2, p. 226). Also, if we sample individuals belonging to one species,
from different places, they may differ a variation between populations, or geographic
variation.
We need to examine intraspecific variation both in order to understand the nature of
species and also to understand how new species evolve. As Chapter 14 will discuss, the
evolution of new species consists of the conversion of variation within one species into
differences between species. Chapters 5–9 looked at the factors that control variation
within a population: variation may be maintained by natural selection, or a balance
of selection and mutation, or a balance of drift and mutation. Here we shall look at
variation between populations (geographic variation), and its relation with variation
within each population. (The theory in Section 5.14, p. 129 is related to the topic here.)
13.4.1 Geographic variation exists in all species and can be caused by
adaptation to local conditions
Johnston & Selander (1971)measured 15 morphological variables in 1,752 house sparrows
(Passer domesticus) sampled from 33 sites in North America. The 15 characters
can be reduced to a single abstract character of “body size” (to be statistically exact, this
character was the first principal component). In Figure 13.5 the average body size of
house sparrows is plotted on a map and two things are immediately important.
First, and more important for our purposes, is simply that the characters vary in
space: house sparrows from one part of the continent differ from those in other parts.
Almost every species that has been studied in different places has been found to vary
in some respect. Not all characters vary (for instance, humans have two eyes everywhere),
but populations always differ in some characters. Different populations have
been found to differ in morphology, in the amino acid sequences of their proteins,
and the base sequence of their DNA. Geographic variation is ubiquitous. Mayr, most