Ecological genetics

Ecological genetics is the study of genetics in natural populations. It combines ecology, evolution, and genetics to understand the processes behind adaptation.

This contrasts with classical genetics, which works mostly on crosses between laboratory strains, and DNA sequence analysis, which studies genes at the molecular level.

Research in this field is on traits of ecological significance—traits that affect an organism's fitness, or its ability to survive and reproduce. Examples of such traits include flowering time, drought tolerance, polymorphism, mimicry, and avoidance of attacks by predators.

Research usually involves a mixture of field and laboratory studies. Samples of natural populations may be taken back to the laboratory for their genetic variation to be analyzed. Changes in the populations at different times and places will be noted, and the pattern of mortality in these populations will be studied. Research is often done on organisms that have short generation times, such as insects and microbial communities.

History
Although work on natural populations had been done previously, it is acknowledged that the field was founded by the English biologist E.B. Ford (1901–1988) in the early 20th century. Ford started research on the genetics of natural populations in 1924 and worked extensively to develop his formal definition of genetic polymorphism. Ford's magnum opus was Ecological Genetics, which ran to four editions and was widely influential.

Other notable ecological geneticists include R. A. Fisher and Theodosius Dobzhansky. Fisher helped form what is known as the modern synthesis of ecology, by mathematically merging the ideas of Darwin and Mendel. Dobzhansky worked on chromosome polymorphism in fruit flies. He and his colleagues carried out studies on natural populations of Drosophila species in western USA and Mexico over many years.

Philip Sheppard, Cyril Clarke, Bernard Kettlewell and A.J. Cain were all strongly influenced by Ford; their careers date from the post World War II era. Collectively, their work on lepidoptera and on human blood groups established the field and threw light on selection in natural populations, where its role had been once doubted.

Inheritance and natural selection
Ecological genetics is closely tied to the concept of natural selection. Many classical ecology works have employed aspects of ecological genetics, investigating how inheritance and the environment affect individuals.

Industrial melanism in peppered moths
Industrial melanism in the peppered moth Biston betularia is a well-known example of the process of natural selection. The typical wing color phenotype of B. betularia is black and white flecks, but variant 'melanic' phenotypes with increased amounts of black also occur. In the nineteenth century, the frequency of these melanic variants increased rapidly. Many biologists proposed explanations for this phenomenon. It was demonstrated in the early 1910s, and again in many later studies, that the melanic variants were a result of dominant alleles at a single locus in the B. betularia genome. The proposed explanations, then, centered around various environmental factors that could contribute to natural selection. In particular, it was proposed that bird predation was selecting for the melanic moth forms, which were more cryptic in industrialized areas. H. B. D. Kettlewell investigated this hypothesis extensively in the early 1950s.

Uncertainty surrounding whether birds preyed on moths at all posed an initial challenge, leading Kettlewell to perform a series of experiments with captive birds. These experiments, while inititally unsuccessful, found that when a variety of insects are provided, the birds did preferentially prey on the most conspicuous moths: those with coloration unmatched to their surroundings. Kettlewell then performed field experiments using mark-recapture techniques to investigate the selective predation of moths in their natural habitat. These experiments found that, in woods near industrialized areas, melanic moth forms were recaptured at much higher rates than the traditional lighter-colored forms, while in non-industrialized woods, the reverse held true.

More recent research has further emphasized the role of genetics in the case of industrialized melanism in B. betularia. While research had already emphasized the role of alleles in determining wing-color phenotype, it was still unknown whether the melanic alleles had a single origin or had arisen multiple times independently. The use of molecular marking and chromosomal mapping in conjunction with population surveys demonstrated in the early 2010s that the melanic B. betularia variants have one single ancestral origin. Additionally, the melanic variants appear to have arisen by mutation from a typical wing-color phenotype.

Polygenic selection
Research on ecologically important traits often focuses on single alleles. However, it has been found that in many cases, phenotypes have a polygenic basis - they are controlled by many different alleles. Complex traits in particular are more likely to have a polygenic basis. Advances in genetic technology have allowed scientists to more closely investigate the genetic basis of complex traits, leading to an accumulation of evidence supporting the importance of polygenic control in understanding the evolution of these traits.

A major line of evidence can be drawn from what we about artificial selection and its influence on traits. Many experiments that have utilized artificial selection have found traits to respond quickly and steadily. If only a small amount of genes have a large influence on a particular trait, this would not be seen. The way that complex traits with continuous variation change in response to natural selection can most reasonably be explained by many alleles having a small effect on the phenotype of interest.

The prevalance of traits with a polygenic basis poses some issues when researching traits and adaptation in natural populations. Separating the effects of genes, environmental factors, and random genetic drift on traits can be difficult with complex traits.

Limitations
Work of this kind needs long-term funding, as well as grounding in both ecology and genetics. These are both difficult requirements. Research projects can last longer than a researcher's career; for instance, research into mimicry started 150 years ago and is still going strongly. Funding of this type of research is still rather erratic, but at least the value of working with natural populations in the field cannot now be doubted.