Examples of microevolution
Microevolution is defined as a change in gene frequency in a population. Because of the short timescale of this sort of evolutionary change, we can often directly observe it happening. We have observed numerous cases of natural selection in the wild, as exemplified by the three shown here.
The size of the sparrow
House sparrows were introduced to North
America in 1852. Since that time the sparrows have evolved different
characteristics in different locations. Sparrow populations in the north
are larger-bodied than sparrow populations in the south. This divergence
in populations is probably at least partly a result of natural selection:
larger-bodied birds can often survive lower temperatures than smaller-bodied
birds can. Colder weather in the north may select for larger-bodied
As this map shows, sparrows in colder places are now generally larger than sparrows
in warmer locales. Since these differences are
probably genetically based, they almost certainly
descended from the same ancestral population have different gene frequencies.
Coping with global warming
We observe natural selection following
many human-induced changes in the environment. For example, global warming
has caused slightly higher temperatures and longer summers.
What are the evolutionary effects of this environmental change? We are
just beginning to figure out the answers to this question as new data
Consider the potential effect of global warming on organisms that are
dormant during the winter. These organisms stop growth and reproduction
during the winter. They would probably be more "fit" if
they could spend more of their time reproducing and gathering resources
for reproduction, but the low temperatures don't allow it. However,
global warming would allow them to do just that: spend more time growing
and reproducing but taking advantage of this opportunity is likely
to require evolutionary change.
The mosquito species Wyeomyia smithii, shown here in a pitcher plant,
has evolved in response to global warming. Mosquitoes use day length (not temperature) as a cue to tell them what
time of year it is and when to overwinter this "cuing" is
genetically controlled. In a warmer climate with shorter winters, we'd
expect mosquitoes that waited a little longer to go dormant to have higher
fitness and be selected for. And in fact, researchers who have been collecting
data on these mosquitoes for almost 30 years have observed exactly this
sort of change. Mosquito populations have evolved so that slightly shorter
days are required as a cue for going dormant.
|This graph illustrates changes in global temperature from 1880 to 2000.
Between 1972 and 1996 mosquito populations at 50 N latitude evolved
to wait 9 days later to go dormant.
Pesticide resistance, herbicide resistance,
and antibiotic resistance are all examples of microevolution by natural
selection. The enterococci bacteria, shown here, have evolved a resistance to several
kinds of antibiotics.