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. First generation of starving beetles
    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. Second generation of starving beetles

  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. First Generation
    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. Second Generation

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?

Explore further
Genotype versus phenotype

Teach this!
•  Lesson plans for teaching about gene frequency
•  Lesson plans for teaching descent with modification

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