Mechanisms of Evolution
Mechanisms: the processes of evolution
Evolution is the process by which modern organisms have descended from ancient ancestors. Evolution is responsible for both the remarkable similarities we see across all life and the amazing diversity of that life — but exactly how does it work?
Fundamental to the process is genetic variation upon which selective forces can act in order for evolution to occur. This section examines the mechanisms of evolution focusing on:
Descent and the genetic differences that are heritable and passed on to the next generation;
Mutation, migration (gene flow), genetic drift, and natural selection as mechanisms of change;
The importance of genetic variation;
The random nature of genetic drift and the effects of a reduction in genetic variation;
How variation, differential reproduction, and heredity result in evolution by natural selection; and
How different species can affect each other’s evolution through coevolution.
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.
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.
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.
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.
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?
Mechanisms of change
Each of these four processes is a basic mechanism of evolutionary change.
Mutation
A mutation could cause parents with genes for bright green coloration to have offspring with a gene for brown coloration. That would make genes for brown c