On his return to England in 1836 and for the next 17 years, Darwin worked diligently on the notes and specimens he had collected and made new observations. He was particularly interested in the obvious success of breeders in developing desired variations in plant and animal stocks. He wondered if this artificial selection of traits could have a parallel in the natural world.
Initially, Darwin was unable to find a natural process similar to artificial selection. However, in 1838, he read an essay by Thomas Malthus (1766-1834) entitled Essay on the Principle of Population. Malthus believed that the human population has the potential to increase geometrically. However, because resources cannot keep pace with the increased demands of a burgeoning population, population-restraining factors, such as poverty, wars, plagues, and famine, begin to have an influence. Darwin realized that a similar struggle to survive occurs in nature. This struggle, when viewed over generations, could be a means of natural selection. Traits that were detrimental for an animal would be eliminated by the failure of the animal containing them to reproduce.
By 1844 Darwin had formulated, but not yet published, his ideas cxi natural selection. The essence of his theory is as follows:
1. All organisms have a far greater reproductive potential than is ever realized. For example, a female oyster releases about 100,OCX) eggs with each spawning, a female sea star releases about 1 million eggs each season, and a female robin may lay four fertile eggs each season. What if all of these eggs were fertilized and developed to reproductive adults by the
following year? A half million female sea stars (half of the million eggs would produce females and half would produce males), each producing another million eggs, repeated over just a few generations would soon fill up the oceans! Even the adult female robins, each producing four more robins, would result in unimaginable resource problems in just a
2. Inherited variations arise by random mutation. Seldom are any two individuals exactly alike. Some of these genetic variations may confer an advantage to the individual possessing them. In other instances, variations may be harmful to an individual. In still other instances, particular variations may be neither helpful nor harmful. (These are said to be neutral.) These variations can be passed on to offspring.
3. Because resources are limited, existence is a constant struggle. Many more offspring are produced than resources can support; therefore, many individuals die. Darwin reasoned that the individuals that die are those with the traits (variations) that make survival and successful reproduction less likely. Traits that promote successful reproduction are said to be adaptive.
4. Adaptive traits are perpetuated in subsequent generations.
Because organisms with maladaptive traits are less likely to reproduce, the maladaptive traits become less frequent in a population and eventually are eliminated.
With these ideas, Darn-in formulated a theory that explained how the tortoises and finches of the Galdpagos Islands changed over time. In addition, Darwin’s theory explained how some animals, such as the ancient South American horses, could become extinct. What if a group of animals is faced with a new environment to which it is ill-adapted? Climatic changes, food shortages, and other environmental stressors could lead to extinction.