Genetics and Evolution - Natural Selection

Natural selection is a mechanism of evolution. The principles of organic evolution by means of natural selection were described by the British naturalist Charles Darwin. Much of his early research focused on geology, and he developed theories about the origin of different land formations when he went on a five-year expedition around the world aboard the HMS Beagle. During his travels he developed an interest in population diversity.

When Darwin identified twelve different species of finches in the islands of the Galapagos chain off the coast of Ecuador, he speculated that the birds must have descended from a common ancestor even though they differed in terms of beak shape and overall size. The birds became known as "Darwin's finches" and are examples of a process called adaptive radiation, in which species from a common ancestor successfully adapt to their environment via natural selection. Darwin suspected that the finches had become geographically isolated from one another and after years of adapting to their distinctive environments had developed and gradually evolved into separate species incapable of interbreeding.

To explain this occurrence, Darwin relied on his own observations of the existence of variation in and between species, his knowledge of animal breeding, and the results of zoological research conducted by the French Jean-Baptiste Chevalier de Lamarck (1744–1829). Lamarck suggested four laws to explain how animal life might change:

• The life force tends to increase the volume of the body and to enlarge its parts.
• New organs can be produced in a body to satisfy a new need.
• Organs develop in proportion to their use.
• Changes that occur in the organs of an animal are transmitted to that animal's progeny.

Darwin famously took issue with this last point, Lamarck's theory of acquired traits, particularly his suggestion that giraffes who, by stretching to reach the uppermost leaves on tall trees, make their necks longer would then pass on longer necks to their offspring. But while Darwin discredited the specifics of Lamarck's theories concerning evolution, he agreed with Lamarck's idea that species changed over time, and he acknowledged Lamarck as an important forerunner and influence on his own work.

Darwin's ideas were also influenced by a 1798 pamphlet written by Thomas Malthus (1766–1834), titled An Essay on the Principle of Population. In this work Malthus put forth his hypothesis that unchecked population growth always exceeds the growth of the means of subsistence (the food supply needed to sustain it). In other words, if there were no outside factors stopping population growth, there would inevitably be more people than food. According to Malthus, actual population growth is kept in line with food supply growth by "positive checks" such as starvation and disease, which increase the death rate, and "preventive checks" such as postponement of marriage, which reduce the birthrate. Malthus's hypothesis suggested that actual population always tended to rise above the food supply, but that historically overpopulation had been prevented by wars, famine, and epidemics of disease.

Darwin also knew that farmers had been able to modify species of domestic animals for hundreds of years. Cattle breeders produced breeds that yielded exceptional milk production by mating their best milkers. Superior egg-laying hens had been bred using the same technique. Since it was possible for farmers to modify a species by artificially selecting those members permitted to reproduce, Darwin hypothesized that nature might have a comparable mechanism for determining which characteristics might be passed on to future generations.

He also realized that while individual organisms in every species had the potential to produce many offspring, the natural population of any species remains relatively constant over time. Darwin concluded that the natural environment acts as a natural selector, determining over long periods of time which variations are best suited to survive and, by virtue of their survival, reproduce and pass on traits and adaptations that improve health and longevity.

Applying the principles of natural selection to the question of giraffes' neck lengths provides an explanation that is different from the one proposed by Lamarck. Less able to obtain food, short-necked giraffes faced starvation. The genes linked to the potential to develop long necks were more likely to be passed to the next generation than the genes for short necks. Over time, the process of natural selection resulted in a population of giraffes with long necks.

Laboratory research and observation also refuted the theory of "inheritance of acquired characteristics." When the tails were cut off of white mice and they were permitted to reproduce, each new generation was born with tails. Children of parents who had suffered amputations or disfiguring accidents did not share their parents' disabilities. Darwin's belief in evolution by natural selection was based on four premises:

• Individuals within a species are variable.
• Some of these variations are passed on to offspring.
• In every generation more offspring are produced than can survive.
• The survival and reproduction of individuals are not random. The individuals who survive and reproduce or reproduce the most are those with the most favorable variations. They are naturally selected.

As support for the theory of acquired inheritance diminished, appreciation of the underlying assumptions for the role of natural selection in evolution grew.

Attacks on Darwin's Theories

Darwin's theories were met with criticism from scientists and members of the clergy. Even some scientists who subscribed to evolutionary theory took issue with the concept of natural selection. Followers of Lamarck, known as "Lamarckians," were among the most outspoken opponents of Darwin's theories. This was especially ironic since it was Lamarck's work that had inspired Darwin.

Other objections raised by scientists were related to how poorly inheritance was understood at that time. The notion of "blending inheritance" was popular. This is the idea that an organism blends together the traits it inherits from its parents. Those who endorsed it observed that, according to Darwin's assumptions, any new variation would mix with existing traits and would no longer exist after several generations. Although Gregor Mendel published a paper in 1866 proposing particulate as opposed to blended inheritance, his theory was not widely accepted until 1900, when it was revisited and confirmed by three different scientists.

The other objection to Darwin's theory was the argument that variation within species was limited and that, once the existing variation was exhausted, natural selection would cease abruptly. In 1907 Thomas Morgan and his colleagues effectively dispelled this objection. Their experiments with fruit flies demonstrated that new hereditary variation occurs in every generation and in every trait of an organism.

The clergy were even more vociferous adversaries. Darwin's major works, On the Origin of Species by Means of Natural Selection (1859) and The Descent of Man (1871), were published during a period of heightened religious fervor in England. Many religious leaders were aghast at Darwin's assertion that all life had not been created by God in one fell swoop. Moral outrage and opposition to Darwinian theory persisted into the twentieth century. Although society grew more tolerant, and many religions accepted and incorporated evolutionary theory into their beliefs, in the early twenty-first century the debate was revived, as many fundamentalist Christian denominations in the United States became more vocal about their creationist beliefs.

In some instances opponents protested the teaching of evolution in schools and continued to defend creationist theory. In July 1925 a science teacher named John Scopes was tried in a Tennessee court for teaching his high school class Darwin's theory of evolution. Scopes had violated the Butler Act, which prohibited teaching evolution theory in all public schools in Tennessee. Dubbed the "Monkey Trial" because of the simplified interpretation of Darwin's idea that humans evolved from apes, the courtroom drama pitted defense attorney Clarence Darrow against prosecutor William Jennings Bryan in a debate that began over the teaching of evolution but became a conflict of deeply held social, intellectual, and religious values. In his acerbic account of the trial proceedings, American social critic H. L. Mencken wrote:

The Scopes trial, from the start, has been carried on in a manner exactly fitted to the anti-evolution law and the simian imbecility under it. There hasn't been the slightest pretense to decorum. The rustic judge, a candidate for re-election, has postured the yokels like a clown in a ten-cent sideshow, and almost every word he has uttered has been an undisguised appeal to their prejudices and superstitions.… Darrow has lost this case. It was lost long before he came to Dayton. But it seems to me that he has nevertheless performed a great public service by fighting it to a finish and in a perfectly serious way. Let no one mistake it for comedy, farcical though it may be in all its details. It serves notice on the country that Neanderthal man is organizing in these forlorn backwaters of the land, led by a fanatic, rid of sense and devoid of conscience. Tennessee, challenging him too timorously and too late, now sees its courts converted into camp meetings and its Bill of Rights made a mock of by its sworn officers of the law. There are other States that had better look to their arsenals before the Hun is at their gates.

At the end of deliberations, Darrow requested a guilty verdict so that the case could be heard before the Tennessee Supreme Court on appeal. The jury complied, and the presiding judge fined Scopes $100. A year later, however, the Tennessee Supreme Court overturned the verdict on a technicality. Wanting to close the case once and for all, the Court dismissed it altogether.

Misuse of Darwin's Theories

After Darwin's theories became well known, some made use of his terminology and concepts to argue that certain groups of human beings were naturally superior to others. The term "Social Darwinism" is used to refer to these ideas, but it is important to note that Darwin himself did not believe in Social Darwinism.

One example of Social Darwinist thinking would be arguing that the rich and successful members of society are fitter, superior, or in some way more highly evolved than the poor. Social Darwinism has also been used to justify racism and colonialism, as in the nineteenth and early twentieth century when many white Europeans and Americans asserted that they were naturally superior to Africans and Asians and as such had every right to take control of their land and resources. Some argued that it was the "White Man's Burden" for Europe and America to rule over and civilize the supposedly lesser peoples of the world who in their view were unfit to govern themselves. The belief of Adolf Hitler and the Nazis that Aryans (Germans) are the "master race" was another form of Social Darwinism.

None of these Social Darwinist theories are scientific in nature, and all of them are false. Modern genetics has shown that there is no group of human beings that is more evolved or otherwise better than the rest of humanity. Despite having been discredited as science Social Darwinism continues to be used in attempts to justify various prejudices and inequalities.