Evolution  (Page 4/9)

The second thing that Darwin observed, and that was a huge factor in his synthesis of these observations into his theory, is that not all of the individuals in a given generation will survive and reproduce to the same degree. Simple mathematics corroborates that. If all of the fruit flies from a single pair of fruit flies survived and produced a maximal number of offspring, after a mere 25 generations (which can take just a single year in this species) that population of flies would fill a ball 96 million miles in diameter, or more than the distance from the earth to the sun. Fruit flies have been around for lots longer than a year, and you can still see the sun, so obviously fruit flies do not all survive and reproduce.

Finally, the third condition necessitates that these heritable variations can result in differences in survival or reproductive success. Again, there is abundant evidence for that. Inherited human conditions that result in mental retardation, or physical deformation, often mean that the affected individual will not survive or reproduce. Medical intervention has, in some cases, been able to counteract those disabilities and allow individuals with some inherited conditions to survive and reproduce, but in previous generations, or in populations of organisms that do not have access to medical care, many heritable variations were not represented in the next generation because the individuals with those variations simply did not reproduce.

So the model Darwin proposed is quite simple. If all of those conditions were true, organisms with heritable variations that enhanced their chances for survival and reproduction would be more likely to be among the parents of the next generation, and the frequency of those organisms with those particular heritable variations would increase in the next generation. This is a simple idea, but it has many ramifications for the study of biology.

It seems clear, just from observations we all have made, that these three conditions do pertain in the natural world. If that is the case, then the process of natural selection could operate, and variations that resulted in reproductive success would become more common in the population. It is important to understand that this process is the result of an interaction between the organisms and their environment. Over time, organisms that fit better into that environment will become more abundant in the population, and may eventually be the only organisms in the population. The term fitness , in this context, simply is a measure of how well individuals with certain traits survive and reproduce in a particular environment. The environment is an incredibly important aspect of this process. If the environment changes, organisms which were fit for the previous environment may suddenly find themselves less well-adapted, and rare organisms that were ill-adapted in the previous environment may suddenly become more fit to that new environment. Fitness is relative, and the environment is a major player in the determination of fitness.