From Mendel To The Modern Synthesis

Darwin was not able to fully realize his theory of natural selection because at the time no one understood the mechanism of inheritance, the force behind "descent with modification." Darwin laid down the foundation for evolution by hypothesizing that traits are passed on to offspring and if a trait is advantageous it will increase in frequency in a population because the offspring with the trait will survive and reproduce better and will pass that trait onto their offspring and so on. Accumulation of many traits like this will change a population from their original state to a different one in successive generations. However, no one could explain how those traits could be carried over in future generations.

Enter the Austrian monk Gregor Mendel (1822-1884). Using pea plants in breeding experiments, he worked out the missing piece, the unknown mode of inheritance, which allowed evolutionary theory to flourish and grow to where it stands today. Mendel remained unknown to evolutionary biologists for thirty-five years after he published his results.

Before genes were discovered, concepts of heredity included the medieval idea of "preformation" in which the gametes contained a ho-munculus, or tiny, complete human. Then, the notion of blending inheritance, which was supported by Lamarck and Darwin, among others, postulated that the heritable material from the parents is blended together in the offspring. Problematically, this idea contradicts the maintenance of variation that is clearly occurring in populations. It predicts that traits would be muddied or diluted out. For example, a dark-skinned mother and a light-skinned father would always blend to have medium-pigmented children and they, in turn would only offer medium pigmentation to their children, eventually leading to a uniform skin color in future generations. In reality, those parents can produce a spectrum of pigmentation in their children and they in turn can produce children that look like themselves, like their mate, like their own parents, or like their mate's parents.

These and other early ideas, like Darwin's pangenesis, were dropped once Mendel's experiments were brought to light. Although what Mendel discovered is an oversimplified view, heredity is perhaps easiest to understand through his work. Mendel's experiments were scientific. He kept track of the numbers of offspring and their phenotypes which resulted from different plants bred together. He also performed a large number of crosses to lessen the effects of chance and this enabled him to replicate experiments as well. He chose to observe contrasting, binary traits (e.g., yellow/green and smooth/wrinkled), which are easy to track through generations, because they are discrete traits, as opposed to continuous ones like height or human skin color. According to the rules of good science his hypotheses were falsifiable and then useful for predicting further observations. For instance he found that when he bred true strains of yellow and green pea plants the ratio of offspring was nearly always three yellow to one green. These types of experiments helped him realize a few simple rules of simple inheritance put here into modern terms based on current understanding (e.g., the term "gene" was not introduced until 1909).

1. Two gene variants, or alleles, one from each parent, determine the phe-notype. The phenotype will be produced by the dominant allele, not the recessive one. Alleles are not blended. They are inherited and expressed separately.

2. Each parent has two alleles for each trait, and the chances are equally likely that offspring will receive either allele from the parent. This notion is what is known as Mendel's "Law of Segregation." At conception, when the zygote is made from the union of sperm and egg, alleles from the mother and the father segregate randomly into daughter cells so there is a predictable 50 percent chance of getting one allele or the other. This process contributes to the maintenance of variation in a population for natural selection to act upon, since no two offspring will have the same combination of alleles.

3. Traits are inherited independently. That is, pea color was not affected by whether or not the pea was smooth or wrinkled. This is known as

Mendel's "Law of Independent Assortment" and is another means of perpetuating variation in populations.

Although Mendel's explanations of inheritance (based on carefully chosen traits) proved to be oversimplified, Mendel was a pioneer who laid the foundation for the whole of modern genetics. It is widely and wildly apparent that Mendelian-style inheritance is the exception rather than the rule. Today it is well known that the majority of traits are complex and expressed by multiple genes that are also often linked in inheritance. A deep understanding of modern developmental, cell, and molecular biology is required to comprehend the complex modes of inheritance of most traits.

Mendel published his findings in 1866, just six years after Darwin's treatise. It went unnoticed by the scientific community until 1900, when finally, a mechanism of inheritance could be married to natural selection and shortly thereafter a theoretical revolution known as the "Modern Synthesis" was born. Then, once James Watson, Francis Crick, and Rosalind Franklin rendered the first accurate model of the DNA molecule in 1953, the fundamental genetic component of evolution was revealed.

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