Population biology is the study of the ecological and evolutionary aspects of the "distribution and abundance of animals"(Andrewartha and Birch 1954). The ecological aspect focuses on living organisms as individuals, groups, species, and interacting assemblages of species. The evolutionary aspect focuses on genetic and environmental changes that shape observed characteristics–the phenotypes–of organisms past and present. Like demography, population biology is a discipline with fuzzy boundaries that shade into specialized subjects: ecology per se, not directly concerned with evolution; population genetics, focused on genetic change and variation, and evolution; epidemiology, concerned with host–parasite associations; paleobiology, focused on the historical record; behavioral ecology; physiological ecology; and many others. This entry is a selective account of population biology at the start of the twenty-first century.
From Individual to Population
A basic unit of analysis in population biology is the individual, with a focus on the individual as ontogeny–a process of development and change from birth to death. Demographers and ecologists often use the terms life cycle or life history for this process. The transitions that occur during development can be complex in different species, involving distinct developmental stages that can differ in terms of actual habitat (e.g., water, land, or air), growth and form, feeding, and reproduction. A staggering variety of life cycles is observed in nature, to the delight of natural historians since before Charles Darwin, but this variety challenges scientific analysis. As just one example of the scale of variation, population biologists deal with species whose characteristic life spans range from an hour (or less, for bacterial division) to centuries (long-lived trees such as redwoods).
The study of life cycles in population biology focuses on the qualitative and quantitative analysis of life cycle transitions–their sequencing, rates, inter-action, proximate determinants, and evolutionary determinants. An organizing principle here, enunciated by the statistician Ronald Fisher (1890–1962) in the 1930s and implicit even in the work of Darwin, is the allocation of resources–in the course of life, individuals gather resources such as energy and materials that are allocated among processes such as metabolism, growth of reproductive and other body parts, foraging, repair of internal systems, mating, and reproduction. It is commonly accepted that life cycles in nature are adapted to the environments in which they are found–it would be surprising if they were not–and hence that resources must be allocated over the life cycle in an optimal way. The claim of optimality in population biology is loosely supported by the notion that Darwinian natural selection results in life cycles that have a high fitness in their environmental context relative to other possible life cycles. It is difficult to say how fitness is to be measured in a given context, or how an optimum is defined, but in practice optimality conditions can be defined by the context and nature of life cycles. Foraging birds or large predators, for example, are likely to be efficient in terms of how they use their time or focus their effort. Many population biologists employ such criteria, plus the tools of optimality theory, to gain useful insight into life cycles.
The environment in which organisms live is critically important, as it is in all demographic studies. A major component of the environment is biotic: the set of other individuals with which any given individual interacts. In the simplest case, the density of other individuals in space and time and how this density affects life cycle transitions is of primary importance. Thus, death rates can be regulated by density-dependent interactions between individuals of the same species (such as competition or cannibalism), or interactions between individuals of different species (a familiar example is predation; less familiar may be symbiosis, a mutually beneficial relationship exemplified by rhizobia bacteria that live in structures called nodules on the roots of plants and fix atmospheric nitrogen for the plants in return for energy and a habitat). In a finer-grained view, interactions can depend on the characteristics and behavior of the individuals involved. Competition for food, for example, may depend on body size; success in attracting a mate can depend on phenotypes such as plumage or antler size; and so on. The abiotic environment also plays a major role by setting resource levels, environmental conditions (such as light or moisture or temperature), and the predictability or unpredictability of such factors.
The analysis of interactions between individuals requires aggregation from individuals to populations, a process affected by scale in space and time. The simplest case is an isolated population of a single species, such as a bacterial culture in a laboratory Petri dish. Spatially localized populations in nature are often studied as isolated populations if migration is not significant. More generally, populations have patchy distributions over space, and a population may really be an aggregation over many spatial patches connected by migration. Depending on the species, a population may occupy some spatial locations only some of the time, as with migrating birds or butterflies that trace a roughly regular spatial migration route in the course of a year. In other cases, local patches can be ephemeral, supporting a small population of a species for a small part of an occasional year. Spatial distributions may be weakly or strongly determined by underlying physical and biological features in space and time.
For a population of a single species, the key questions concern dynamics: How and why do population numbers and composition change over time and space; what is the short-run and long-run variability in population; what is the viability (i.e., ability to persist at reasonable numbers) of a population? These questions are also central to the sciences of conservation biology and population management. Conservation biologists are typically interested in viability, in the likelihood of extinction, and in spatial distributions; exploited populations (e.g., for fishing, hunting, recreation) are, in a surprising number of cases, actively managed for abundance and persistence.
From Populations to Ecosystems
As noted, interactions between individuals of different species can be important in the life cycles of individuals and in the dynamics of populations. Some interactions between pairs of species are studied using common concepts and methods. Examples include interactions between predator and prey, host and parasite, or species competing for similar resources. Interactions play a central role in population biology, underlying pattern and process in ways that were first highlighted by the British ecologist Charles Elton (1900–1991) and the American ecologist G. Evelyn Hutchinson (1903–1991); current paradigms in the field build on the work of the American ecologists Robert MacArthur and Edward O. Wilson, among others.
Predator–prey interactions are often analyzed in terms of the response of predator behavior to changes in the distribution and abundance of prey–a general theory for this functional response of the predator has been developed and successfully applied in several cases.
Host–parasite interactions are more complex because parasites come in many varieties, from parasitic organisms that spend part or all of their life cycle within one or more hosts, to insect parasitoids that spend their adult lives outside a host but need to find, and lay their eggs inside or on, a host. There are useful general methods for analyzing different classes of host–parasite interaction. A significant part of the science of host–parasite interactions originated in the study of human malaria, with the work of the great British epidemiologist Ronald Ross (1857–1932), and from subsequent epidemiological work on human and non-human disease. Since about 1980 population biologists and epidemiologists have paid increasing attention to the transmission and control of viral infections ranging from influenza to HIV.
Competitive interactions vary depending on what competition exists (for space, food, mates, and so on) and the type of competitive interaction (which may involve quite distinct abilities, e.g., efficiency at finding a resource, or ability to displace other individuals from a resource). Here too, a number of general principles of analysis have been developed and tested. Individual behavior clearly plays a key role in interactions, and behavioral ecology is the study of behaviors and their evolution.
Moving up in scale from pair-wise interactions, there are communities of species that interact in many different ways over some spatial region. The complexity of communities depends on their diversity (measured in terms of the numbers and relative abundance of different species), their trophic structure (if all species in the community are arranged into a who-eats-whom pyramid there is a hierarchy of levels; the subset of species at each level is called a trophic level), and the network of interactions between each species and the rest. Communities range from a few dozen bacterial species in a square foot of soil to assemblages of hundreds of species in a large national preserve or park. Still further up in scale, there are ecosystems that may contain numerous communities that interact weakly with each other, from complete islands to areas of subcontinental scale. As spatial scale changes, so does the time scale over which communities display significant change. For communities and ecosystems, ecologists are concerned with their dynamics, viability, resilience (ability to withstand perturbations such as changing environmental conditions or invasion by new species), biodiversity, and biogeography. A subject of increasing interest is the dynamic interaction between humans and ecosystems.
In population biology, evolution can make causality run from individual characteristics to interactions (e.g., the outcome of competition is determined by the characteristics of competing individuals), or from interactions to characteristics (e.g., characteristics such as plumage displays can evolve in response to their impact on relative mating success). Time scale is important in determining which effects are studied: In the conventional paradigm, evolutionary changes are usually much slower than ecological changes. Even so, the evolutionary changes people can observe and document are those that take place over a period shorter than the human life span. The classic example is the change in the moth Biston betularia from mostly light to mostly dark individuals in the half-century beginning in about 1848 as the industrial revolution increasingly polluted Britain. Many population biologists are concerned with historical evolutionary processes that generated the behaviors, interactions, and life cycles observable in the present; others, with how evolution is changing the patterns observed in the early twenty-first century.
Much of evolutionary ecology generalizes Darwin's insights: Elton did so in 1930, with an emphasis on the evolution of individual characteristics in ecological contexts. But many new insights have emerged in studying phenomena that Darwin did not consider or found too challenging. The study of social evolution–how collective behavior evolves–was given a solid basis by the British evolutionary biologist William Hamilton (1936–2000) in 1964, by the introduction of the concept of inclusive fitness, which extends fitness to include the effects of an individual's behavior on the fitness of kin. Work on social traits in behavioral ecology and evolution has also provided insights in subjects such as anthropology and human ecology. Evolutionary questions that remain topics of active research include aspects of the evolution of sex and sex ratios, the evolution of senescence (the British biologist J. B. S. Haldane (1892–1964) and the British immunologist Peter Medawar (1915–1987) were pioneers in this area), and the evolution of larger-scale patterns in ecology.
Evolutionary ecologists tend to think largely in terms of natural selection as the main force for adaptation and evolution. Population genetics takes a more careful look at the evolutionary process. The American population biologist Richard Lewontin (born 1929) emphasized that evolution is a multi-level affair: An individual's genes make up its genotype; genotype plus environment interact to determine the individual's phenotype. The crux is the presence of heritable genetic variation between individuals–variation is modified by differential selection on phenotypes, by random events (especially drift, the unavoidable randomness in choosing a finite sample from a finite population), and by the introduction of naturally occurring random mutations into genotypes. Evolution is the consequence of a dynamic process of change in the genetic variation within populations under the forces of selection, drift, and mutation. Many basic features of evolution were worked out by Fisher, Haldane, and the American population geneticist Sewall Wright (1889–1988) in the 1930s and 1940s.
In subsequent decades population geneticists have brought increasing experimental power to bear on the measurement and characterization of genetic and phenotypic variability within populations, from the early studies of the American population geneticist Theodosius Dobzhansky (1900–1975) on chromosome structures in fruit flies (the genus Drosophila) to current studies of genetic sequences and gene microarrays. A striking overall finding is that there is a huge amount of genetic variability within populations, and a key question in population genetics is why so much variability exists. In broad terms, there are two answers. One, originating with Dobzhansky, holds that selection is responsible for much of the variation; the other, originating with the Japanese population geneticist Motoo Kimura (born 1925), holds that much variation is simply neutral, a consequence of mutation followed by random drift, and has little selective significance. It is known that each answer applies in some domain of nature, but precise formulation and testing of these and more sophisticated hypotheses remains an important subject for research.
Since about the 1980s, population geneticists have increasingly turned to the analysis of phylogeny, the historical relationships among lineages of organisms or their parts (including their genes). Given a sample of genes from different humans in the year 2000, for example, one can ask whether these genes exist as the result of evolution from a single ancestral population, and estimate the time it took for the current genetic variation to materialize. Phylogenetics also provides powerful insights into the evolutionary trees that have resulted in variation between species as found at the beginning of the twenty-first century.
To someone concerned with human demography, population genetics is key to understanding how genetics and evolution have shaped aspects of human traits and behavior. Population genetics has been strikingly effective in the analysis of genetic variants that have major effect on health (such as the Apo E locus, implicated in Alzheimer's disease and atherosclerosis). Biodemographers look to population genetics as a way of identifying segments of the genome that have major effects on quantitative traits such as components of mortality. Demographers are becoming interested in the genetic contribution to traits as diverse as fertility and health, partly stimulated by the availability of modern technologies for extracting genetic information from small samples of human cells collected in surveys. However, it is far from obvious that genetic analyses can sensibly inform an understanding of complex traits that have major social and cultural components. As demographers' earlier experience with eugenics demonstrates, there are sound historical reasons to be cautious and precise in such work.
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Crow, James F. 1986. Basic Concepts in Population, Quantitative, and Evolutionary Genetics. New York: W. H. Freeman.
Elton, Charles S. 2001. Animal Ecology (1927), with new introductory material by Mathew A. Leibold and J. Timothy Wootton. Chicago: University of Chicago Press.
Fisher, Ronald Aylmer. 1958. The Genetical Theory of Natural Selection, rev. 2nd edition. New York: Dover Publications.
Futuyma, Douglas J. 1998. Evolutionary Biology, 3rd edition. Sunderland, MA: Sinauer Associates.
Krebs, John R., and Nicholas B. Davies, ed. 1997. Behavioural Ecology: An Evolutionary Approach. Cambridge, MA: Blackwell Science.
Lewontin, Richard C. 1974. The Genetic Basis of Evolutionary Change. New York: Columbia University Press.
Maynard Smith, John. 1998. Evolutionary Genetics, 2nd edition. New York: Oxford University Press.
Ricklefs, Robert E., and Gary L. Miller. 2000. Ecology, 4th edition. New York: W. H. Freeman.
Shripad D. Tuljapurkar