Why is genetics important for evolution research

I will show that the central early controversy over the roles of random genetic drift and natural selection in evolution has continued to this day, not withstanding the apparent technological refinements afforded by the availability of biochemical and DNA sequence data.

That is, finer scale or more reductionistic genetic data has not yet led to a resolution of the original conceptual issues that lie at the foundation of ecological genetics. Historically, the starting point of ecological genetic research has been the discovery of variation within a natural population, i. The subsequent goal is three-fold: 1 determination of whether or not the polymorphism has a genetic component; 2 determination of the frequency of each of the polymorphic types; and, 3 determination of how natural selection maintains the polymorphism, either alone or in combination with other evolutionary forces.

Ford p. Although recurrent mutation in conjunction with mutation can maintain a polymorphism indefinitely at mutation-selection balance, here Ford is clearly interested in a more active role for natural selection in the maintenance of polymorphism. The reasoning stems from the theoretical findings that, in large populations, it is unlikely that the positive and negative effects of an allele or chromosomal inversion on fitness will be exactly balanced and that the number of individuals with a rare neutral mutation is proportional to the number of generations since its origin.

In addition, recurrent mutation as a cause of persistent polymorphism was considered most unlikely and, in fact, this evolutionary cause is explicitly excluded from the definition of genetic polymorphism by Ford see above. Hence, neutral genetic polymorphism was considered an exceptionally rare event by the founders of ecological genetics and, consequently, such polymorphisms were the hallmark of strong, active natural selection. Ford further distinguished two types of selective polymorphism, transient polymorphism and balanced polymorphism.

This and statements like it reflect the viewpoint that organisms in nature are exquisitely adapted to their environments by the long-acting process of Fisherian gradualism.

It is a prelude to the more explicitly adaptationist views found in the current behavioral literature see review in Shuster and Wade This view of the evolutionary process as primarily one of refinement of existing organismal adaptation is an essential part of the Fisherian theory of evolutionary genetics Wade and Goodnight Thus, the primary goal of the ecological geneticist is to discern exactly how natural selection is acting to maintain a balanced polymorphism by the relative strength of opposing fitness effects acting on the different sexes or at different stages in the life history of the organism.

Fisher first argued that, because every individual has a mother and a father, the mean fitness of males must be equal to the mean fitness of females multiplied by the sex ratio, expressed as the number of females to males i.

As a result fitness increases with rarity, and, in this circumstance, whenever the population sex ratio deviates from unity, a gene that increases the numbers of the minority sex at birth will have a selective advantage. In general, the fitnesses of the different types constituting a phenotypic polymorphism must be equal to be maintained within a population by natural selection at a non-zero equilibrium frequency a point recognized by Darwin , p.

However, the balance of selective forces for non-sex related or even sex-linked polymorphisms is very different from that required to maintain an equal sex ratio, namely, the necessity that each offspring inherit equally from each sex parent. Using the existence of the separate sexes as an example of a balanced polymorphism is misleading or, at least unrepresentative, of the selective forces necessary to sustain balanced polymorphisms in general. The founding ecological geneticists dismissed any significant role for random genetic drift in evolution.

The theoretical interaction of random genetic drift and natural selection for single genes with constant effects can be seen in Figure 1. Fisher in his evolutionary theory assumed that natural populations achieved or sustained the very large sizes as seen in his in correspondence with S. Similarly, according to his intellectual biographer W. Ewens , p. With very large N e , the domain of random genetic drift is greatly restricted even as that of natural selection is expanded see Fig.

However, ecological geneticists did not dismiss random genetic drift as a significant evolutionary force for the same reasons that Fisher did. Field observations conducted with the mark-recapture methods developed by ecological geneticists documented generation-to-generation fluctuations in population size up to or exceeding an order of magnitude in most natural populations studied long term.

Thus, small local population sizes were not seen as unusual by ecological geneticists. Despite the not infrequent occurrence of small population sizes where drift would be expected to be most efficacious, random genetic drift was considered an irrelevant evolutionary force in ecological genetics because natural selection was viewed as being particularly strong during periods of population decline.

The smallest populations showed little phenotypic variation, which was seen as evidence that they were the most fit or most finely adapted populations. The stressful environmental conditions responsible for the decline in numbers also were seen as causing particularly strong natural selection.

Thus, the lack of phenotypic variation in small populations was owing to it having been eliminated by natural selection during the immediately prior period of decline. Conversely, under periods of population increase, natural selection was seen as weaker and more permissive of variation. This concept of relaxed selection provided Ford with a cause for the increase in observations of rare phenotypic variants in large and growing natural populations.

If selection pressure increases inversely to population size, then the role of random genetic drift in evolution must be greatly restricted. In addition, Ford , p. Interestingly, Ford and his colleagues believed that genetic subdivision of the sort postulated by Wright would promote rapid evolution but for very different genetic reasons and by different genetic mechanisms natural selection instead of random genetic drift, local selection, and interdemic selection.

Here, he proposes a trade-off between specialized adaptation to local conditions in the absence of migration and generalized adaptation to global conditions in the presence of migration. In modern terms, this is called genotype-by-environment interaction, where the selective effect, s , of a gene changes with change in the environment.

A gene might be adaptive in one environmental context i. Migration between local environments mixes the adaptive and maladaptive responses to selection and reduces the average magnitude of gene frequency change. In this sense, genotype-by-environment interaction is viewed as an evolutionary constraint because is limits the rate of gene frequency change.

The restraint can be removed simply by stopping gene flow or the mixing of genes across different local environments. Thus, the fixed selective effect illustrated in Figure 1, must be considered an average selective effect across environments.

Clearly, large local effects of opposite sign must be averaged when there is gene flow among habitats and the averaging tends to reduce the gene's selective effect. That is, he claims interactions among genes, or epistasis, contribute to local adaptation. Thus, Ford invokes genotype-by-environment interactions for fitness as well as gene-gene interactions for fitness in his cases of rapid evolution.

Both of these kinds of interactions change the depiction of the threshold separating natural selection from random genetic drift Figure 1 in important ways see below. Before turning to interaction effects, I will examine a representative discussion of ecological genetics of random genetic drift using data from a natural population.

Several wing coloration variants segregating in a small natural population of the moth, Panaxia dominula Fisher and Ford , were investigated using mark-recapture in one of the longest continuous studies of a single population in evolutionary research. The goal of Fisher and Ford was to determine whether year-to-year fluctuations in the frequency of the variants medionigra , a heterozygote, and bimaculata , a homozygote were better explained by natural selection or by random genetic drift.

They inferred from their analysis. With this paper, Fisher and Ford moved the long-standing debate between Wright and Fisher over the relative roles of natural selection and random genetic drift in evolution from theory to nature. It is remarkable that, in the first such study with only eight years of observations on a single locus with alternative alleles, they are confident in rejecting Wright's theory and random genetic drift in its entirety. In his response Wright , Wright pointed out, first, that his theory of evolution explicitly involved the simultaneous action of several forces selection, drift, mutation, and migration and he emphatically rejected the paradigm of Fisher and Ford that either selection or drift alone had to be responsible for all of the observed fluctuation in gene frequencies.

Wright noted that, in order to reach their statistical conclusion, Ford and Fisher had to include gene frequency data from a decade before the more careful study, notably a period without any estimates of population size.

Without this earlier data point, the average fluctuations were much smaller and not significant. He pointed out that, like the mark-recapture estimates of population numbers, the gene frequencies themselves were estimates whose variation, based on the reported sample sizes, accounted for more than half He then showed that, if one assumed only the unitary explanation of natural selection, then the observed gene frequency fluctuations were so large even without the sampling variance that the temporal variations in the allelic selection coefficients must range from near lethality or sterility to tremendous advantage i.

However, Fisher and Ford provided no indication of comparable levels of temporal variation in any environmental factor acting as a selective agent. In an unyielding reply, Fisher and Ford labeled chance or random fluctuations in gene frequency, the Sewall Wright Effect , a term which has endured to the present day as a synonym for random genetic drift.

With a larger data set covering several more years, Ford , p. Ford also showed that the selective advantage for the rarer of the genes varied widely, from He did not find, however, the expected negative correlation between strength of selection and population size in these data. In the intervening decades, data from a variety of other organisms and natural population had become available and its review led Ford , p.

Later laboratory research has shown that the expression of the color patterns is sensitive to the thermal environment during development and thus the gene frequency estimates may be subject to significant measurement error, owing to the misclassification of genotypes.

This is yet another source of variation, not accounted for in the Ford analyses. In addition, empirical evidence has found, as Wright expected, that temporal fluctuations in population size, large variance among females in fecundity, and sexual selection reduce the effective number to less than half the Fisher-Ford estimate.

In addition, more careful studies have reduced Ford's estimates of the magnitude of the average genic selection coefficient by about two thirds [cf. Cook and Jones ]. Ecological geneticists like Ford postulated interactions of the sort that could change the sign of genic selection coefficients with changes in the environment including density or in the genetic background. As the two environments fluctuate in frequency, spatially or temporally, the selective effect of an A allele changes in both magnitude and sign see Figure 2.

The smaller the amount of migration between the environments, the greater is the degree of local adaptation to each as Ford suggested see above. However, the average selective effect of the gene in the sense of Fisher's theory must be smaller than the average observation in a particular locality at a particular time because the long-term average contains both positive and negative values of s.

Furthermore, to the extent that the local value of s changes sign owing to continuous fluctuations in local environmental conditions, the A allele will also move from the domain of selection to the domain of drift as Wright suggested.

Thus, the very kind of population subdivision imagined by Ford, with selection acting in every locality albeit in different directions, creates, rather than eliminates, the opportunity for random genetic drift. The central problem with using conspicuous polymorphisms for investigating the relative roles of the variety of different evolutionary forces is that it is not an unbiased sample of genetic diversity with respect to either degree of adaptive function or amount of genetic variation.

Topics that will be examined include:. Enter your keywords. Claude Flamand : predicting epidemics with satellite images Dr Amy Kristine Bei to head a new 4-year research group on malaria in Dakar Dr. The Insitut Pasteur is addressing the major scientific and health issues facing the world today The Institut Pasteur in A legacy of excellence The Research Journal : the most read articles in ! The Research Journal: 's most read articles! News "COVID, cancers and antibiotic resistance: meeting the researchers of the Institut Pasteur": programme available for replay "Mental anchorpoints" used by musicians to identify pitch "N-terminomics" reveals how Listeria bacteria detect and react to stress "Pasteur, the experimenter" — the must-see exhibition!

How does hearing work? Why does bronchiolitis only affect infants? You are here. Home The research journal Reports Human evolutionary genetics: the benefits of genetic diversity. The research journal. All reports. Print Share. Subscribe to the Institut Pasteur Newsletter. By clicking OK, you are agreeing to receive the Institut Pasteur newsletter each month.

You can easily unsubscribe at any moment by using the unsubscribe link that appears in each email. Your personal data will be used exclusively to send you this newsletter. The effects of drift are particularly important in small populations; genetic migration , the transfer of genes from one population to another because of the migration of individuals.

Genetics : the science of heredity. Gene : a piece of DNA that constitutes a unit of information. Epigenetics : changes to gene expression that do not affect the DNA sequence. Human genetics in numbers. Almost 20, genes. More than 3 million. Almost , years.

A demographic history, from our origins to the present day The genetic diversity of the recent human species was particularly shaped by the effects of migration. Question marks had always remained as to the migratory route taken by these peoples: a first theory claimed that the Bantu population immediately split into two movements after leaving their homeland, one heading east and one south; and a second theory suggested that they crossed the equatorial forest today part of Gabon before dividing into two waves of migration, one continuing south and the other heading to East Africa.

Video copyright: Institut Pasteur Learn more about the inaugural lesson on February 6, , 6 p. Topics that will be examined include: an introduction to population genetics , genetic and phenotypic diversity in humans, a genetic reconstruction of the demographic history of our species , natural selection and adaptive phenotypes, genetic diversity and cultural forces , human adaptation to pathogens : immunity and infectious diseases. Related press documents.

Press release. Cocaine addiction : impact of genetic mutations elucidated. Cocaine addiction is a chronic disorder with a high rate of relapse for which no effective treatment is currently available. Scientists from the En savoir plus. Institut Pasteur isolates strains of coronavirus nCoV detected in France. As well as sequencing the whole genome of coronavirus nCoV, the Institut Pasteur continued to work on the samples taken from the first confirmed Zika: silent long-term circulation in Thailand.

The circulation of the dengue virus for the past sixty years in South-East Asia is relatively well known. For Zika, the situation is much less clear An imaging and modeling approach to characterize the structure of DNA in human cells. The structure of DNA and chromosomes is a major focus of research, but still holds a number of mysteries. Scientists from the Institut Pasteur and Mitochondrial diseases: a new screening leads to insight on the effect of various genes on DOA.

How Important is Genetics for an Understanding of Evolution? Museum of Comparative Zoology, Harvard University. Oxford Academic. Google Scholar. Cite Cite R. Select Format Select format. Permissions Icon Permissions. Issue Section:. Download all slides. View Metrics. Email alerts Article activity alert.