The process of evolution (via natural selection, drift, or both) eliminates heritable variation. If not all the alleles make it into the next generation — because some are selected against or disappear due to random forces — heritable variation eventually goes away. That scenario leads to the subject of mutation, which is the ultimate source of genetic variation.
Mutations are changes in an organism's DNA. If all the deer in the forest have only genes to produce normal noses and one deer is born with a blinking red nose (a heritable trait), a mutation must have occurred in one of the gametes (either the sperm or the egg) that led to that red-nosed baby deer.
DNA is an extremely stable molecule; that's what makes it so good for storing your genetic information. But mutations still occur:
1 DNA is tough, but it's not indestructible. It can be damaged, and although your cells try to repair the damage, they don't always get it right, resulting in mutations. Agents that have this damaging effect are referred to as mutagens. Examples of mutagens include the ionizing radiation associated with nuclear material, ultraviolet radiation from the sun, and many chemicals.
1 The process of DNA replication that occurs in all cell division — be it in the gametes (egg and sperm) or in somatic tissue (everything else) — is another source of mutations. DNA replication is an error-prone process — sometimes the copying isn't exact, and that also results in mutations. (To find out more about DNA replication, refer to Chapter 3.)
When you hear the term mutation,the first thing that may spring to mind are mutations that cause diseases such as cancer. Stay out in the sun too long, and ultraviolet radiation bombards your skin cells. This radiation causes changes in your DNA that affect the regulation of cell division, and all of a sudden, one of your cells doesn't play nice with the rest of your body. These types of mutations certainly are important medically, but they don't affect the genetic composition of your offspring.
Cancer and other such mutations can be important in evolution if there are differences in the mutation rate between individuals, for example, with some individuals being much more likely to die of cancer than others and, therefore, less likely to have descendants in the next generation. In such a scenario, the characteristic of having a higher mutation rate would be the character under selection, not the cancer itself.
Sometimes, mutations are selectively advantageous. (Rudolph the Red-Nosed Reindeer did save Christmas, after all.) But most of the time, they're not. Mutations are either selectively neutral (they have no effect on fitness) or deleterious (they decrease fitness). Regardless, without mutation, there'd be no variation on which natural selection can act. Yet a number of forces can reduce variation from generation to generation. Variations that are selectively disadvantageous would be eliminated from the population over time, and the random forces of genetic drift (see Chapter 6) eventually would purge diversity from the population.
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