Philosophy of Genetics: Neo-Darwinism and the Modern Synthesis
Neo-Darwinism is an attempt to reconcile gradual evolution by means of natural selection ("survival of the fittest") as proposed by Robert Charles Darwin (1809-1882), with developments in the field of genetics.
Darwinism asserted that evolution is a gradual process in which minute changes in organisms accumulate over time and eventually result in the appearance of new species. This transmutation was the result of breeding that favored the replication of specific traits that aided a species' overall adaptability, or "fitness".
However, Darwin only speculated about the means by which traits were passed on from generation to generation. Like many of his naturalist contemporaries, Darwin thought that paternal characteristics were somehow transmitted through the blood. This reliance on what was termed "blood theory" led Darwin to further conclude that unexpressed traits and variants could be diluted out of a population. This wrongly conceived explanation for the mechanisms of heredity became an obstacle to the continued acceptance of Darwinian evolution as it was originally proposed.
Seven years after the publication of Darwin's work on evolution, On the Origin of Species, German monk and geneticist, Gregor Mendel (1822-1844), proposed a new theory of biological inheritance, or heredity. Mendel's theory states that genetic material is transmitted as units. Each parent passes to their offspring a set of genes (units that represents each inheritable characteristic) that do not mix or blend, but remain separate and whole. Some genes are expressed, while others are dormant. Genes are thus unable to be diluted. Mendel's research remained in obscurity for three decades, but upon its rediscovery it challenged the prevailing interpretation and acceptance of Darwinism.
The scientific community in the late eighteenth century did not seek to actively discredit the notion of organic evolution that Darwin proposed. Rather, they criticized the mechanisms by which evolution was accomplished. Advocates of Mendelian genetics argued that the continual and infinitesimal transmutations central to Darwinism were antithetical to more broad changes observed by Mendel. Furthermore, geneticists were concerned with the seemingly infinite possibilities of variation that would result from constant, infinitesimal changes.
Also competing with Darwinism in he late 1800s was another theory of organic evolution known as neo-Lamarkism. The theory borrowed the concept of use and disuse, championed by French naturalist Jean-Baptiste Lamarck (1744-1829), to explain an organism's development of habits and adaptation to the environment. Neo-Lamarckism denied the role of natural selection as a means of electing traits of maximum adaptiveness, and favored the view that the environment itself directly acted on organisms, resulting in acquired traits that were also inheritable. Through his pioneering germ-plasm research German biologist August Weismann (1834-1914) proved that such traits of habit were not able to be passed on to offspring. Neo-Lamarckism largely fell out of favor, but the need for some sort of reconciliation between genetics and evolution theory was made clear. Again, scientists turned their attention to Darwinism. In light of his own research, Weismann became a champion of natural selection -- but replaced Darwin's concepts of heredity with those of Mendel. Weismann and his followers became known as neo-Darwinists.
The actual synthesis of Darwinian evolution and genetic change was cemented by the work of Russian-born American naturalist and experimental geneticist Theodosius Dobzhansky. His book, Genetics and the Origin of the Species, published in 1937, presented a comprehensive explanation of evolution through genetic mechanisms. By 1960, more than a century after Darwin first introduced his theories, natural selection by genetic mechanisms was the most widely accepted theory of evolution in the scientific community.
Despite its general acceptance, many questions have arisen over the past 40 years regarding the comprehensiveness of neo-Darwinism. Advances in molecular biology have yielded conclusive observations of the microevolutionary processes, changes in a gene pool over successive generations. However, macroevolution, or evolutionary trends on a large scale, is more problematic to research. DNA and enzyme analysis allows scientists to quantitatively study species divergence, the extent of genetic variation in populations, and the degree of genetic change in species formation. In 1968, Japanese geneticist Motoo Kimura (1924 -) advanced the neutrality theory, which states that at the foundation levels of life, such as DNA and protein sequences, some changes are "neutral" and do not affect the overall adaptiveness or function of an organism. He further theorized that the degree of difference between various amino acids sequences could estimate the time at which species diverged. This is known as the "molecular clock".
During the 1970s and '80s physical anthropologists made numerous discoveries of hominid fossils and had increasing success charting the progress of the evolution of man. Such discoveries in the natural sciences have made it gradually became that the molecular clock is not exact; nevertheless, it has continued to provide a reliable source of evidence for reconstructing a rough history of evolution.
Present-day the techniques of DNA cloning and sequencing have provided a new and more powerful means of investigating evolution at the molecular level and further assessing challenges levied against neo-Darwinism, but the exact relationship between microevolution and macroevolution remains unknown.
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