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Descent with modification
We've defined evolution as descent with modification from a common
ancestor, but exactly what has been modified? Evolution only occurs when there is a
change in gene
frequency within a population over
time. These genetic differences are heritable and can be passed on
to the next generation — which is what really matters in evolution: long term change.
Compare these two examples of change in beetle populations. Which one is an example of evolution?
1. Beetles on a diet
Imagine a year or two of drought in which there are few plants that these beetles can eat. |
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| All the beetles have the same chances of survival and reproduction, but because of food restrictions, the beetles in the population are a little smaller than the preceding generation of beetles. |
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2. Beetles of a different color
Most of the beetles in the population (say 90%) have the
genes for bright green
coloration and a few of them (10%) have a gene that makes them more brown. |
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| Some number of generations later, things have changed: brown beetles are more
common than they used to be and make up 70% of the population.
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Which example illustrates descent with modification — a change in gene frequency over time?
The difference in weight in example 1 came about because of environmental influences — the low food
supply — not because of a change in the frequency of genes. Therefore, example 1 is not
evolution. Because the small body size in this population was not genetically determined, this generation of small-bodied beetles will produce beetles that will grow to normal size if they have a normal food supply.
The changing color in example 2 is definitely evolution: these two generations of the
same population are genetically different. But how did it happen?
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