Which comes first the mutant chicken or the selective agent

Here's a major stumbling block many people have in understanding how mutations are involved in the process of evolution. Many people think that fctABE*


selectively advantageous mutations occur in response to environmental factors that make them advantageous. The thinking goes this way: All these short-necked creatures are running around on the plains in Africa; their main food supply is the vegetation on the ground, but they eat so much, not enough ground vegetation is left to feed the population. What do they do? They evolve to have longer necks so that they can eat the leaves off the trees. Voilà! Giraffes evolve, and the problem is solved. But this scenario is absolutely not how evolution works.

Mutations don't occur in response to environmental factors; they already exist in the population. In the presence of some new environmental factor that makes the mutation beneficial, the organism with that mutation is more likely to survive. In the example of the short-necked creatures in Africa, suppose that a few random mutations result in a few long-necked individuals. The long-necked ones and the short-necked ones still eat the rapidly diminishing grass supply, but the long-necked ones have an advantage because, in addition to grass, they can eat the leaves off trees. Being better fed, they are more likely to survive and reproduce.

This principle — that the mutation already exists in the population — is crystal clear when you witness evolution in progress, which is possible with organisms that have short generational time spans. Take a flask of bacteria, for example; put in some penicillin; and you're left with a flask that contains only a few surviving bacteria, all of which are penicillin resistant. What happened? Some of the bacteria in the flask already had slightly different DNA sequences that made them resistant to penicillin. Examples illustrating the evolution of antibiotic resistance come up several times in this book because they so beautifully illustrates how organisms — both simple and complex — evolve.

Variation needs to exist in a population before the evolutionary forces can select for it. Once the new selective force (climate change, new disease, and so on) appears, it's too late for any new mutations to be beneficial. Only individuals that already have characteristics that are beneficial in the presence of the new selective force will have an advantage.

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