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The Modern Synthesis

When Darwin developed his theory of natural selection, modern ideas of genetics did not yet exist, and Austrian monk Gregor Mendel's work in the field still languished undiscovered. Darwin himself, along with most other prominent theorists of his day, subscribed to the "blending" theory of inheritance, which suggested that traits of each parent would mix together to form the traits of the child. (See Introduction to Genetics and Mendelian Inheritance.) Many also suggested variants of Lamarckish versions of inheritance as important to evolution: use and disuse of parts, shaping of the organism by its environment, and inheritance of acquired characteristics were all common beliefs.

However, these beliefs began to come under close scrutiny around the turn of the century. Though Mendel's work with genetics had yet to be rediscovered, some scientists began to call into question the possibility of inheritance of acquired characteristics and associated neo-Lamarkcian ideas. Later, Mendel's theories were rediscovered, and genetics was incorporated into the theory of natural selection.

Weismann and Neo-Lamarckism

In 1883, German biologist August Weismann developed the germ-plasm theory of heredity, which was in opposition to the beliefs of the neo-Lamarckians. According to Weismann's theory, the body's material, or soma, was entirely separate from the hereditary material, which he called the germ-plasm. By insisting on a strict distinction between some and germ-plasm, Weismann eliminated the idea of inheritance of acquired characteristics from the realm of possibility. In addition, Weismann insisted that natural selection was the only force of evolutionary change, thus denying the neo-Lamarckian's other arguments as well. (Weismann's ideas are known as neo-Darwinism, though the term is often applied to all of 20th-centruy evolutionary biology.)

Weismann's theory had strong experimental support. Weismann conducted numerous experiments in which he sought even one bit of evidence that offspring ever inherited characteristics that the parents had acquired. Weismann cut the tails off generations of mice, but not one was ever born without a tail. In light of this evidence, he concluded that inheritance of acquired characteristics was simply contrary to fact.

Naturally enough, Weismann's germ-plasm theory generated some controversy. Heated debates took place, and evolutionists separated into neo-Lamarckian and neo-Darwinian camps. The debate continued, though it would change form several times, until the advent of the modern synthesis.

Weismann's Other Theories

In addition to neo-Darwinism, Weismann also made other important contributions to the future of evolutionary biology. He discovered the phenomenon of crossing over during meiotic division of gametes, and recognized that the process provides for the genetic variability that is the basis of natural selection. Weismann was also the first to argue that sexual reproduction provided the raw variation needed for natural selection to operate.

The Rediscovery of Genetics

Around 1900, Gregor Mendel's ideas began to resurface, partly because other scientists had discovered them through their own research. However, many early geneticists thought that Mendelian ideas of particulate inheritance equaled a death-blow to Darwinism. They misunderstood the implications of discrete genes and upheld saltationism, or the belief in large spontaneous mutations as a major evolutionary force.

In opposition to the explicit saltationism of the Mendelians were another alternative school of thought called the naturalists, who studied the dynamics of natural populations and supported the theory of natural selection, which was violently opposed by the Mendelians. The former Lamarckian/Darwinian debate shifted into a new form, naturalists vs. Mendelians.

The Modern Synthesis

What is now known as the modern synthesis (or synthetic theory or evolutionary synthesis) is the eventual marriage of neo-Darwinism, with its support of natural selection and rejection of the inheritance of acquired characteristics, to Mendelian genetics, with its particulate inheritance. The modern synthesis is the fundamental basis for all current work in evolutionary biology.

The main conclusions of the modern synthesis were that natural selection is the driving force of evolution and that evolution is understandable in terms of mutations in and recombinations of particulate genes. The modern synthesis also emphasized the definition of a species as a reproductively isolated group of organisms that share a common gene pool. The synthesis reinforced the idea that saltationism is impossible (the sole "exception" to this rule is the phenomenon of ploidy in plants) and that "mutation pressure", a result of the early Mendelians' attempt to find a force of evolution, was false (mutations may be beneficial, harmful, or neutral).

Some scientists who were extremely important in the shaping of the modern synthesis were Theodosius Dobzhansky, George Simpson, Ernst Mayr, and R. A. Fisher, whose book The Genetical Theory of Natural Selection is now a considered a classic of evolutionary biology.

Looking Further: Links and References

The following links and references are useful in the study of the events and discoveries that led up to the modern synthesis.

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