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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.
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. Take a look at the image on the right.
But how do you know when you’ve gotten to the population level?
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.
Biologists who study evolution at this level define evolution as a change in gene frequency within a population.
Detecting micro-evolutionary 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:
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.
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%).
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.
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 birds.
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 represent microevolutionary change: populations 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 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.
he 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.
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.
Source: Berkeley University
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