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?
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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