Evolutionary biologists are often concerned with genetic variation, a term which in modern times has come to refer to differences in DNA sequences among individuals. However, quantifying and understanding genetic variation has been a central aim of those interested in understanding the varied life on earth since long before the sequencing of the first full genome, and even before the discovery of DNA as the molecule responsible for heredity. While today's definition of genetic variation relies on contemporary molecular genetics, the idea of heritable variation was of central importance to those interested in the substance and development of life even before the writings of Charles Darwin. The concept of heritable variation—the presence of innate differences between life forms that are passed from parents to offspring, especially within categories such as species—does not rely on modern ideas of genetics, which were unavailable to 18th- and 19th-century minds.
Pre-Darwinian concepts of heritable variation In the mid-1700s,
Pierre Louis Maupertuis, a French scholar now known primarily for his work in mathematics and physics, posited that while species have a true, original form, accidents during the development of nascent offspring could introduce variations that could accumulate over time. In his 1750
Essaie de Cosmologie, he proposed that the species we see today are only a small fraction of the many variations produced by "a blind destiny", and that many of these variations did not "conform" to their needs, thus did not survive. In fact, some historians even suggest that his ideas anticipated the laws of inheritance further developed by
Gregor Mendel. Simultaneously, French philosopher
Denis Diderot proposed a different framework for the generation of heritable variation. Diderot borrowed Maupertuis' idea that variation could be introduced during reproduction and the subsequent growth of offspring, and thought that production of a "normal" organism was no more probable than production of a "monstrous" one. However, Diderot also believed that matter itself had lifelike properties and could self-assemble into structures with the potential for life. Both Maupertuis and Diderot built on the ideas of Roman poet and philosopher
Lucretius, who wrote in
De rerum natura that all the universe was created by random chance, and only the beings that were not self-contradictory survived. Maupertuis' work is distinguished from the work of both Lucretius and Diderot in his use of the concept of conformity in explaining differential survival of beings, a new idea among those who believed that life changed over time. Similarly, Lamarck's theory of the variability among living things was rooted in patterns of use and disuse, which he believed led to heritable physiological changes.
Darwin's concept of heritable variation Charles Darwin's ideas of heritable variation were shaped by both his own scientific work and the ideas of his contemporaries and predecessors. Darwin ascribed heritable variation to many factors, but particularly emphasized environmental forces acting on the body. His theory of inheritance was rooted in the (now disproven) idea of
gemmules - small, hypothetical particles, which capture the essence of an organism and travel from all over the body to the reproductive organs, from which they are passed to offspring. Darwin believed that the causal relationship between the environment and the body was so complex that the variation this relationship produced was inherently unpredictable. However, like Lamarck, he acknowledged that variability could also be introduced by patterns of use and disuse of organs. Darwin was fascinated by variation in both natural and domesticated populations, and his realization that individuals in a population exhibited seemingly purposeless variation was largely driven by his experiences working with animal breeders. Darwin believed that species changed gradually, through the accumulation of small, continuous variations, a concept that would remain hotly contested into the 20th century.
Post-Darwinian concepts of heritable variation In the 20th century, a field that came to be known as
population genetics developed. This field seeks to understand and quantify genetic variation. If the heterozygosity of a population is zero, every individual is homozygous; that is, every individual has two copies of the same allele at the locus of interest and no genetic variation exists. •
1918 -
Variance: In a seminal paper entitled "The correlation between relatives on the supposition of Mendelian inheritance",
R.A. Fisher introduced the statistical concept of
variance; the average of squared deviations of a collection of observations from their mean (\sigma^2=\frac{1}{I}\sum_{i=1}^I(x_i-\mu)^2), where \sigma^2 is the variance and \mu is the mean of the population from which the observations x_i are drawn). R.A. Fisher's work in population genetics was not just important to population genetics; these ideas would also form the foundations of modern statistics. •
1918, 1921 -
Additive and dominant genetic variance: R.A. Fisher subsequently subdivided his general definition of variance into two components relevant to population genetics: additive and dominant genetic variance. An additive genetic model assumes that genes do not interact if the number of the genes affecting the phenotype is small and that a trait value can be estimated simply by summing the effect of each gene on the trait. Under Fisher's model, the total genetic variance is the sum of the additive genetic variance (the variance in a trait due to these additive effects) and the dominant genetic variance (which accounts for interactions between genes). • '
1951 -
F-statistics':
F-statistics, also known as fixation indices, were developed by population geneticist
Sewall Wright to quantify differences in genetic variation within and between populations. The most common of these statistics, FST, considers in its simplest definition two different versions of a gene, or alleles, and two populations that contain one or both of these two alleles. FST quantifies the genetic variability among these two populations by computing the average frequency of heterozygotes across the two populations relative to the frequency of heterozygotes if the two populations were pooled. F-statistics introduced the idea of quantifying hierarchical concepts of variance and would become the foundation of many important population genetic methods, including a set of methods that tests for evidence of natural selection in the genome. == See also ==