Darwin's Theory of Evolution
Darwin's Theory of Evolution
Theory of Evolution
he recognized that, even within species, individuals showed variation in traits and that the variations could be passed to offspring.
He predicted that individuals with traits which suited the environment would survive and reproduce to pass their favorable traits to offspring
Those whose traits were less suited to the environment would die. Just as humans select for breeding those cattle which produce more milk, he reasoned that nature (the environment) selects individuals which use resources most efficiently. Thus, he called his explanation of how species naturally change over time natural selection.
Darwin defined natural selection as the "principle by which each slight variation [of a trait], if useful, is preserved," and he later regretted that he had not named it “natural preservation.” Today it is often defined as the process by which a certain trait becomes more common within a population. Let’s look once more at the parts of this process and then we will consider its consequences.
Everyone is Different
Darwin did not know that genes made of DNA determine traits. Much later, scientists learned that mutations in DNA can change genes and produce variations in traits.
However, his observations of animal breeding and his detailed studies of both barnacles and other species convinced him that small, heritable variations in traits were common among individuals within a species and that these variations occur by chance.
Darwin probably recognized that sexual reproduction increased variety in offspring. He expressed considerable concern that his own health problems might be heritable, especially when his beloved daughter Annie grew ill and died. He believed that his marriage to his cousin may have contributed to his children's weaknesses.
Overproduction of Offspring
Malthus argued that human populations grow exponentially if unchecked, but that disease, starvation, or war will limit population growth eventually. High birth rates and high death rates were characteristic of human history.
Darwin himself had ten children; three died before maturity. Darwin reasoned that populations of all species have the capacity to grow. Simply put, species produce more offspring than can survive
However, his observations showed that most populations remained stable due to environmental limits. He concluded that for population sizes to remain stable, many offspring must die. The phrases "overproduction of offspring" and "struggle for existence" summarize this idea.
Survival of the Offspring
Although heritable variations appeared to be random, Darwin reasoned that death was not random.
Offspring which, by chance, had variations which “fit” or adapted them to their environment would have a greater chance to survive to maturity and a greater chance to reproduce. Offspring without such beneficial adaptations were more likely to die and either not reproduce, or not reproduce as much as others.
Thus, well-adapted individuals produce more offspring. "Differential survival and reproduction" is a cornerstone of natural selection. The phrase "survival of the fittest" summarizes this idea.
Gradually, Species Change
Can an individual organism evolve? No. The accommodation of an individual organism to its environment is not evolution.
Though an individual organism can be better adapted to its environment, a sexually reproducing organism still must mate with others of its species, so by definition, it is not a new species. It is just an individual with a better chance of survival in its environment.
It is the gradual accumulation of many adaptations that, over many generations within one lineage of organisms, results in a new species. These adaptations occur through genetic change.
Through chance variation, overproduction of offspring, and differential survival and reproduction, the proportion of individuals with a favorable trait (or favorable phenotype) will increase.