Geography Reference
In-Depth Information
Chance events also evolve populations by changing the relative occur-
rence of variants—what geneticists call “alleles”—through random processes
rather than through adaptive adjustment to natural selection. This is called
“genetic drift.” As a population shrinks, those pairs that survive may be
thought of as a random sample of all possible pairs. This random sample will
likely include a dif erent frequency of alleles that control important traits than
was found in the whole population. This change is evolution caused by genetic
drift. As forest was cleared for streets and yards in the subdivisions I studied,
some wren territories were removed while others were untouched. Clearing
was unplanned with respect to the genetic composition of the wrens; some
just happened to be where planners sited streets. The drift in genetic compo-
sition of the wrens that remained versus those that were originally present in
the larger forest counteracted the population's ability to adapt to local condi-
tions. Perhaps the most innovative wrens or those with beaks or plumage best
suited to urban life were removed. The full potential of the wren population
was not allowed to confront urbanization. As small random samples of the
whole, small populations are less diverse and more under the control of luck
than of natural selection. Extinction is often the outcome.
Species that should have an evolutionary advantage in urban environ-
ments because of large population size include those that directly benei t from
our subsidies, actions, and structures. Species that nest in our buildings, eat
our food, and use our planted landscapes should be able to build large popu-
lations that are responsive to novel challenges such as exotic predators, new
diseases, extreme temperatures, variable food supplies, trai c hazards, pollu-
tion, and localized persecution.
High reproductive rates, even in small populations, may also reduce the
risk of extinction by increasing the amount of random phenotypic variation
within a generation. Species with large broods or that nest frequently each
year may increase the production of extreme phenotypes and thereby the
