Survival is not hard to understand. It means living longer. Darwin’s contemporary Herbert Spencer coined the phrase “survival of the fittest” to describe natural selection.
Animals compete against each other to survive, as in who can get the best food or who can best escape being eaten by larger animals. In a group of zebras, for example, the ones who run the slowest are most likely to be eaten by lions, so the ones born to be fast are more likely to live long enough to pass along their genes.
Survival depends in part on the circumstances in your environment. Consider the coloring of fish. Almost all fish have a relatively light colored belly and a relatively dark colored top or back. Why? Th at coloring is adapted for survival in the water. Most fish live until a bigger fish eats them, making the ability to hide from bigger fish an important trait for survival. Some big fish swim near the surface and look downward for food. Th e lower (deeper) you go in the water, the darker it gets. When a big fish looks downward, therefore, it can’t see dark-colored fish very well, so fish who are dark on the top side are harder to see (and therefore safer). Meanwhile, some big fish lurk in the depths and look upward for their food. Looking upward is looking into the light, so the best way for a fi sh to blend in is to have a light coloring on its underside. Over millions of years, the fish who were dark on top and light on the bottom survived longest because they were the hardest for the bigger fish to see, so they were less likely to be eaten and, therefore, more likely to make more baby fish with the same coloring. As a result of this selection process, most fish have this coloring today.
Gradually, biologists have shifted their emphasis from survival to reproduction as the single most important factor in natural selection. Survival is important mainly as a means to achieve reproduction. Reproduction means producing babies—though the babies also have to survive long enough to reproduce. Reproductive success consists of creating many off spring who will in turn create many off - spring. Put another way, nature judges you by how many grandchildren you produce.
For example, suppose there were a mutation (that is, a new gene or combination of genes) today that doubled the expected life span of a woman, from about 75 years to about 150. Th at is, one particular woman was born with the biological makeup to enable her to live 150 years. Would subsequent generations have more and more of this trait, and thus be more and more like this woman (and hence able to live longer?). Possibly not. If the woman was still done having babies around the age of 40, and if her longer life did not improve her quantity or quality of children and grandchildren, then her genetic traits would not spread. Now imagine another woman born with a mutation that doubled the number of children she produced, even though she would die at age 75 just like the others. Subsequent generations would contain more and more people like her.
Much of the recent work in evolutionary theory has focused on gender differences (Buss, 1994; Symons, 1979; Trivers, 1972). For example, evolution would likely select men to want more sex partners than women want. A woman can only have one baby a year no matter how many men she has sex with, but a man can father dozens of children each year if he has sex with many women. Moreover, a woman’s children would be most likely to survive to adulthood if they were cared for by two parents rather than just their mother. Hence men today are probably descended from men who desired multiple partners, whereas today’s women probably descended from female ancestors who preferred long-lasting monogamous relationships. Current research suggests that this pattern is found all over the world, in many different cultures: Men desire more sex partners than women (Schmitt, 2003).
How, exactly, does biological evolution produce changes? Th e causal processes depend entirely on random changes to physical entities, such as genes. Th e person (or other creature) is programmed to respond a certain way. Crucially, nothing has to be thought, understood, or spoken in order for these changes to occur. Th at is, meaning has nothing to do with it. Molecules, chemicals, electrical impulses in the body, and other physical mechanisms produce the results. Behavior changes because the physical makeup of the newborn individual is different. Th is is quite different from how culture works, as we shall see.
