When Charles Darwin published his groundbreaking book Origin of Species in 1859, the concept of evolution was not new, but was widely accepted. What Darwin actually described was not the idea of evolution, but rather the mechanism that made evolution possible. This mechanism is natural selection.
The idea of natural selection is based on the observation that within a population there is variation among individuals. That is, individuals are not totally alike but differ in various features. While Darwin and other nineteenth-century scientists, saw only phenotypic (that is, how things looked) variation, today’s scientists can describe the genetic variation within a population. This genetic variation, caused by factors such as mutation and segregation, results in phenotypic variation. In his textbook, Biological Anthropology, Michael Park reports:
“Much of a species’ phenotypic variation may make very little difference to the fitness and reproductive success of individuals.”
In some situations, however, such as environmental change, some of these features may provide individuals with an increased chance of survival and longevity.
In addition, there is variation in reproduction: some individuals have lots of offspring while others have few or none. With regard to Charles Darwin, Brian Fagan, in his book Men of the Earth: An Introduction to World Prehistory, writes:
“He concluded that individuals endowed with the most favorable variations would have the best chance at survival and passing on their favorable characteristics to their descendants.”
According to Darwin, over a long period of time, new species would emerge out of old species and some species would become extinct. In other words, natural selection does not ensure that a species will survive. Earlier theories of evolution, such as those of Jean Baptiste Lamarck (1744-1829), held that extinction was not possible.
The accumulation of favorable characteristics, that is characteristics which enabled individuals to survive and reproduce, allowed for speciation. Brian Fagan writes:
“Natural selection serves both as a conservative force pruning out aberrant forms and as a positive force that allows the fit to produce, the obvious and fundamental condition to survival.”
At the time Darwin was writing, very little was known about genetics. While Gregor Mendel, working in his monastery garden, had discovered some of the basic concepts of genetics, his findings would remain unknown to the scientific world until the twentieth century. Brian Fagan writes:
“Unfortunately, Darwin was unaware of the publications of an Austrian monk, Gregor Johann Mendel. In 1866 Mendel demonstrated the fundamental laws of inheritance, using the common garden pea as the basis for comprehensive breeding experiments under controlled conditions.”
With regard to the lack of understanding regarding genetics, Daniel Dennett, in his book Darwin’s Dangerous Idea: Evolution and the Meaning of Life, writes:
“Darwin never hit upon the central concept, without which the theory of evolution is hopeless: the concept of the gene. Darwin had no proper unit of heredity, and so his account of the process of natural selection was plagued with entirely reasonable doubts about whether it would work.”
In his entry on Darwin in The New Encyclopedia of Unbelief, Richard Dawkins writes:
“Knowing nothing of genes beyond the folk wisdom that like begets like, Darwin treated natural selection at the level of organisms. He took the Malthusian point that in every generation there is overproduction.”
In his 1798 Essay on the Principle of Population, the English cleric and mathematician Thomas Malthus had argued that population explosion and famine were inevitable. Regarding the influence of Malthus on Charles Darwin, Matt Ridley, in his book Nature via Nurture: Genes, Experience, and What Makes Us Human, reports:
“…while reading Malthus’s essay on population, he had his sudden insight into what we now know as natural selection.”
The population overproduction which Malthus wrote about, creates competition and the struggle for existence. Richard Dawkins writes:
“The individuals best fitted to survive the struggle in each generation contribute disproportionally to the next. Future generations inherit those attributes that fitted their parents to survive and pass them on.”
Genetics
During the twentieth century, the science of genetics was developed and added a greater understanding to how natural selection works. The works of Gregor Mendel were rediscovered in 1900 and the role of mutation in genetic inheritance was discovered in the 1920s. Toward the end of the twentieth century, the studies of DNA significantly increased understanding of the processes of evolution. Eugene Harris writes:
“The genome is like a giant puzzle with its pieces consisting of many thousands of recombining genomic segments. Today, the field of evolution has moved well beyond studies of single genes to analyzing the many thousands of unlinked genomic segments making up our genome.”
In the twenty-first century, natural selection continues to be one of the major areas for understanding evolution. In his book The Greatest Show on Earth: The Evidence for Evolution, Richard Dawkins writes:
“All reputable biologists go on to agree that natural selection is one of its most important driving forces, although—as some biologists insist more than others—not the only one.”
In their textbook Human Evolution and Culture: Highlights of Anthropology, Carol Ember, Melvin Ember, and Peter Peregrine summarize the importance of natural selection this way:
“Natural selection is a process we can see at work in the world today. It is a process that has been studied in the laboratory and in nature, and it is a process that most scientists would argue is very well understood. However, as in all scientific endeavors, understanding grows and changes as new information is obtained. The understanding of natural selection we have today is quite different from that originally put forward by Darwin.”
Mutation
Mutation—a change in the genetic code—is an important force for evolution because mutation provides for variation within a population. In his book The Dragons of Eden: Speculations on the Evolution of Human Intelligence, Carl Sagan writes:
“The raw materials of evolution are mutations, inheritable changes in the particular nucleotide sequences that make up the hereditary instructions in the DNA molecule. Mutations are caused by radioactivity in the environment, by cosmic rays from space, or, as often happens, randomly—by spontaneous rearrangements of the nucleotides which statistically occur by the organism itself.”
In their book Man’s Evolution: An Introduction to Physical Anthropology, C.L. Brace and Ashley Montagu write:
“Darwin, with the insight built upon a lifetime of careful observation, simply took it on faith that the necessary variation arose by natural means. Long after his death and after the rebirth of the field of genetics, the source for this variation was finally located in those minute modifications of the gene, namely, mutation.”
Mutations are random, unpredictable, most are not inherited, and most do not provide any survival or reproductive benefit. Brain Fagan puts it this way:
“Most gene mutations are changes for the worse, which is hardly surprising since normal genes represent the most favorable mutations accumulated over long periods of time.”
The majority of these mutations are negative: they often result in death or lack of descendants, causing the elimination of negative mutations from the population. On the other hand, there are a few mutations which may provide survival value and increased reproductive success. In his book The Making of Mankind, Richard Leakey writes of mutations:
“Their effect on the organism is random: most often they are harmful and the mutant individual does not survive, sometimes they are neutral and make no difference to their bearers, but very occasionally they are beneficial, in which case the origin of a new species is possible.”
New traits which prove to be beneficial thus increase in a population and may help provide the basis for speciation. One of the factors that influences whether or not a mutation has survival benefit is ecological change. Brain Fagan writes:
“Whether or not a given mutation has a selective advantage depends on the environment or the ecological niche with which the population has to contend. Environments are constantly changing, which in turn means that the selective effort on the gene pools of all the populations inhabiting these environments also alters. Environmental change is just as important a part of evolution as genetic change.”
In his essay in The Epic of Evolution: Science and Religion in Dialogue, John Haught writes:
“The better adapted obviously have a higher probability of surviving and producing offspring, and so nature ‘selects’ the reproductively fit and eliminates all the others. Over a long period of time the natural selection of variations favorable to survival and reproduction brings a wide diversity of species, including primates among whom the human species finds its biological classification.”
In his book Ancestors in Our Genome: The New Science of Human Evolution, Eugene Harris writes:
“Molecular theory suggests that any DNA differences between species that we see today must have arisen through the process of mutation in the past, as species branched from each other.”
With regard to the importance of mutation in evolution, Michael Park, in his textbook Biological Anthropology, writes:
“Without mutation—if the first life forms reproduced themselves absolutely without error—nothing would have changed, and the first living things would be the only living things. Mutations are the ‘natural resources’ of evolution.”
Michael Park also writes:
“Mutation and natural selection are the major processes of evolution. Mutation provides new genetic variation. Natural selection selects phenotypes for reproductive success based on their adaptive relationship with its environment.”
There is no master plan or natural law that predicts when mutations occur or whether or not these mutations will be beneficial or destructive. In addition, mutations do not occur because they are needed: in order to provide survival and reproductive advantages, mutations must occur before they are needed. The genetic mutations that led to bipedalism, for example, occurred before our ancestors adopted a savannah lifestyle.
Basic Concepts
In general, the scientific theory of natural selection as an evolutionary mechanism has led to three related concepts. First, evolution is random and not, as envisioned by some nineteenth century scientists, progressing in any particular direction. Natural selection does not result in a progression from simple to complex.
Second, as a random process, evolution doesn’t always work: that is, some species become extinct. Extinction may occur when the environment changes and none of the variations within the species are adaptive to the new environment. In his textbook Biological Anthropology, Michael Park writes:
“Extinction is, in fact, the norm. Nine-tenths of all species that have ever lived are now extinct.”
And finally, new species arise from natural selection. Michael Park writes:
“If populations within a species become environmentally separated, these populations will be under different selective pressures—different traits will differently adapt to each environment.”
C.L. Brace and Ashley Montagu sum up natural selection this way:
“In summary, then, we may say that natural selection is the process of diversification of adaptations in a population, and the product of evolution is a diverse but well-adapted population. Evolution, in short, consists of the changes which occur in genotypes, and having a great store of such diverse potentials makes for evolutionary adaptiveness.”
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