Michael H. Hart
Section 1 – Natural selection
The theory of evolution is the central, unifying principle of modern biology. Although entire books have been devoted to explaining the theory in detail, its funda- mental ideas can be summarized briefly.
1) The individual members of any given species vary considerably from each other, and some of that variation is due to genetic factors. Furthermore, new heritable traits are constantly arising because of random mutations.
2) Since, in each species, there are more offspring than the environment can support, many individuals die without reproducing.
3) Those members of the species who have inherited traits that make them less likely to survive will, on average, have fewer offspring. Hence, in the next generation, there will be fewer individuals with those traits.
4) The result is the selective elimination of less useful traits and, in comparison, the natural selection of more useful traits.
5) This leads to a series of small changes within a species; and the gradual accu- mulation of many such small changes eventually results in the formation of a new species, related to the original one.1
Note that two separate processes are involved: (a) random mutations (which occur entirely by chance); and (b) selection of the more useful traits (a process that is far from random). The notion that evolution is governed entirely by chance is therefore false.
At no stage in either process have the resulting organisms been deliberately designed. However, since natural selection results in organisms that are very well adapted to their environments, they usually give the appearance of having been designed.
Most frequently, selection is for traits that make it more likely that the individual organism will survive (such as greater size, speed, strength, intelligence, or resistance to disease). This might be called “survival selection.” However, in sexually repro- ducing species, an individual cannot pass on its genes unless it mates with a member of the opposite sex. Consequently, traits that make an individual a more attractive mate will also be selected for, whether or not they aid in survival. This is called “sexual selection,” and is an important evolutionary mechanism.