Population genetics (ecology)

In population genetics a sexual population is a set of organisms in which any pair of members can breed together. This means that they can regularly exchange gametes to produce normally-fertile offspring, and such a breeding group is also known therefore as a gamodeme. This also implies that all members belong to the same of species, such as humans. If the gamodeme is very large (theoretically, approaching infinity), and all gene alleles are uniformly distributed by the gametes within it, the gamodeme is said to be panmictic. Under this state, allele (gamete) frequencies can be converted to genotype (zygote) frequencies by expanding an appropriate quadratic equation, as shown by Sir Ronald Fisher in his establishment of quantitative genetics.

This seldom occurs in nature: localisation of gamete exchange – through dispersal limitations, or preferential mating, or cataclysm, or other cause – may lead to small actual gamodemes which exchange gametes reasonably uniformly within themselves, but are virtually separated from their neighbouring gamodemes. However, there may be low frequencies of exchange with these neighbours. This may be viewed as the breaking up of a large sexual population (panmictic) into smaller overlapping sexual populations. This failure of panmixia leads to two important changes in overall population structure: (1) the component gamodemes vary (through gamete sampling) in their allele frequencies when compared with each other and with the theoretical panmictic original (this is known as dispersion, and its details can be estimated using expansion of an appropriate binomial equation); and (2) the level of homozygosity rises in the entire collection of gamodemes. The overall rise in homozygosity is quantified by the inbreeding coefficient (f or φ). Note that all homozygotes are increased in frequency – both the deleterious and the desirable. The mean phenotype of the gamodemes collection is lower than that of the panmictic "original" – which is known as inbreeding depression. It is most important to note, however, that some dispersion lines will be superior to the panmictic original, while some will be about the same, and some will be inferior. The probabilities of each can be estimated from those binomial equations. In plant and animal breeding, procedures have been developed which deliberately utilise the effects of dispersion (such as line breeding, pure-line breeding, back-crossing). It can be shown that dispersion-assisted selection leads to the greatest genetic advance (ΔG = change in the phenotypic mean), and is much more powerful than selection acting without attendant dispersion. This is so for both allogamous (random fertilization) and autogamous (self-fertilization) gamodemes

2. Population ecology or autecology is a sub-field of ecology that deals with the dynamics of species populations and how these populations interact with the environment.^[1] It is the study of how the population sizes of species change over time and space. The term population ecology is often used interchangably with population biology or population dynamics.

The development of population ecology owes much to demography and actuarial life tables. Population ecology is important in conservation biology, especially in the development of population viability analysis (PVA) which makes it possible to predict the long-term probability of a species persisting in a given habitat patch. Although population ecology is a subfield of biology, it provides interesting problems for mathematicians and statisticians who work in population dynamics.

The human population is growing at anexponential rate and is affecting the populations of other species in return. Chemical pollution,deforestation, and irrigation are examples of means by which humans may influence the population ecology of other species. As the human population increases, its effect on the populations of other species may also increase.

Populations cannot grow indefinitely. Population ecology involves studying factors that affect population growth and survival. Mass extinctions are examples of factors that have radically reduced populations' sizes and populations' survivability. The survivability of populations is critical to maintaining high levels ofbiodiversity on Earth.

3. Population fluctuations are undoubtedly one of the most fascinating phenomena in ecology. Some of the earliest writings known to man describe outbreaks of pests, such as the fabled locust plagues in Egypt. Some species, such as the snowshoe hare or larch budmoth, cycle through changes in abundance as regular as clockwork. Many other species exhibit more irregular patterns of oscillation, and some have even been shown to fluctuate in truly chaotic fashion. Making sense of this bewildering array of dynamical patterns has been a central theme in population ecology, involving some of the leading scientists in both the experimental and theoretical realms. Key elements in this search for ecological understanding relate to measuring and characterizing different patterns of population fluctuation, theoretical modeling of hypothetical processes and mechanisms that can cause cycles or other patterns of population fluctuation, and developing ways to test these hypotheses and/or models using observational and experimental data.

General Overviews

By definition, population dynamics requires a quantitative point of view. Hastings 1997 presents an excellent introduction to the theoretical basis for population dynamics, developing many of the key concepts employed by contemporary ecologists. Lande, et al. 2003 and Ranta, et al. 2006 provide more sophisticated reviews of contemporary issues in population modeling, including effects of age and stage structure, environmental and demographic stochasticity, and the interplay between evolution and ecology. From the outset there was simmering debate among ecologists about the importance of density-dependent processes in regulating animal populations. Early lab experiments had clearly demonstrated how increases in population density were associated with diminished per capita rates of population growth. Nonetheless, many field ecologists were skeptical, arguing that climatic forcing made density-dependent processes essentially irrelevant.Andrewartha and Birch 1954 nicely summarizes many of these arguments but also introduces novel topics such as spatial heterogeneity and movement processes that have emerged in their own right as important research topics for contemporary ecologists. Sinclair 1989 provides an excellent review of the historical debate about the importance of population regulation. While most ecologists agree that the issue has now been largely resolved, Bjǿrnstad and Grenfell 2002 andCoulson, et al. 2004 point out some of the considerable remaining challenges in clarifying the relative importance of population processes versus climatic forcing in causing population fluctuations. The highly relevant monograph Turchin 2003 provides the most comprehensive review of both the theoretical basis and empirical evidence for complex population dynamics with heavy emphasis on population cycles.