Microevolution
House sparrows have adapted to the climate of North America, mosquitoes have evolved
in response to global warming, and insects have evolved resistance to our pesticides.
These are all examples of microevolution—evolution on a small scale.
Here, you can explore the topic of microevolution through several case studies in which
we’ve directly observed its action.
We can begin with an exact definition.
Defining Microevolution
Microevolution is evolution on a small scale—within a single population. That means
narrowing our focus to one branch of the tree of life.
If you could zoom in on one branch of the tree of life scale—the insects, for example—
you would see another phylogeny relating all the different insect lineages. If you continue
to zoom in, selecting the branch representing beetles, you would see another phylogeny
relating different beetle species. You could continue zooming in until you saw the
relationships between beetle populations. Click on the button below to see this in action!
But how do you know when you’ve gotten to the population level?
Defining populations
For animals, it’s fairly easy to decide what a population is. It is a group of organisms that
interbreed with each other—that is, they all share a gene pool. So for our species of beetle,
that might be a group of individuals that all live on a particular mountaintop and are
potential mates for one another.
The potential to interbreed in nature defines the
boundaries of a population.
Biologists who study evolution at this level define evolution as a change in gene
frequency within a population.
Detecting Microevolutionary Change
We’ve defined microevolution as a change in gene frequency in a population and a
population as a group of organisms that share a common gene pool—like all the
individuals of one beetle species living on a particular mountaintop.
Imagine that you go to the mountaintop this year, sample these beetles, and determine
that 80% of the genes in the population are for green coloration and 20% of them are for
brown coloration. You go back the next year, repeat the procedure, and find a new ratio:
60% green genes to 40% brown genes.
You have detected a microevolutionary pattern: a change in gene frequency. A change in
gene frequency over time means that the population has evolved.
The big question is: How did it happen?
Mechanisms of Microevolution
There are a few basic ways in which microevolutionary change happens. Mutation,
migration, genetic drift, and natural selection are all processes that can directly affect
gene frequencies in a population.
Imagine that you observe an increase in the frequency of brown coloration genes and a
decrease in the frequency of green coloration genes in a beetle population. Any
combination of the mechanisms of microevolution might be responsible for the pattern,
and part of the scientist’s job is to figure out which of these mechanisms caused the
change:
Mutation: Some “green
genes” randomly mutated
to “brown genes”
(although since any
particular mutation is
rare, this process alone
cannot account for a big
change in allele
frequency over one
generation).
Migration (or gene
flow): Some beetles with
brown genes immigrated
from another population,
or some beetles carrying
green genes emigrated.
Genetic drift: When the
beetles reproduced, just
by random luck more
brown genes than green
genes ended up in the
offspring. In the diagram
at right, brown genes
occur slightly more
frequently in the
offspring (29%) than in
the parent generation
(25%).
Natural selection:
Beetles with brown genes
escaped predation and
survived to reproduce
more frequently than
beetles with green genes,
so that more brown genes
got into the next
generation.
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.
1. 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 smallerbodied birds can. Colder weather in the north may select for larger-bodied birds.
As this map1 shows, sparrows in colder
places are now generally larger than
sparrows in warmer locales. Since these
differences are probably genetically based,
they almost certainly represent
microevolutionary change: populations
descended from the same ancestral
population have different gene frequencies.
2. 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 are collected.
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.2
This graph illustrates changes in
global temperature from 1880 to
2000.3 Between 1972 and 1996
mosquito populations at 50 N
latitude evolved to wait 9 days later
to go dormant.
3. Building Resistance
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.
1
Map adapted from Gould, S.J & Johnston, R.F. (1972) Geographic Variation. Annual
Review of Ecology and Systematics. 3:457-498
2
Bradshaw & Holzapfel, 2001
3
Global climatic data from the National Climatic Data Center
Artificial Selection in the Lab
For thousands of years, humans have been influencing evolution, through changes we
have caused in the environment—and through artificial selection in the domestication of
plants and animals. In many cases, scientists have carefully documented evolution
through artificial selection in the lab.
The spots on guppies can be manipulated
through artificial selection.
John Endler performed experiments in microevolution, allowing artificial selection to
manipulate the spots on guppies1. Guppy spots are largely genetically controlled. Spots
that help the guppy blend in with its surroundings protect it from predation—but spots
that make it stand out help it attract mates. Endler set up similar populations of guppies in
artificial ponds in the laboratory. Ponds varied in the coarseness of gravel on the bottom
and all ponds had predators. Below is a simplified representation of Endler’s experiment.
After fewer than 15 generations of selection, the markings of guppies in different ponds
had substantially diverged as a result of natural selection. In the presence of predators,
guppies evolved to blend in with their background.
Endler then performed another experiment, with the same pond set-ups but without
predators.
Without predators, there was sexual selection for male guppies that stood out from their
background and attracted the attention of the females.