Selection In Heterogeneous Environments

Premise 3 (Chapter 1) states that phenotypes emerge out of a genotype-by-environment interaction. In Chapter 11 we saw that natural selection arises from this premise when there is genetic variation in the population to produce heritable variation in the phenotype of fitness. There is often variation not only in genotypes but also in environments. We have already seen this before. For example, in Chapter 8, Fisher's basic quantitative genetic model has an environmental deviation that is modeled as a random variable assigned independently to each individual. Thus, there is both genetic and environmental variation in Fisher's model. We have also seen examples of environmental variation that is not random for each individual. For example, we discussed how the environment changed in wet, tropical Africa after the introduction of the Malaysian agricultural complex and how this environmental change altered the phenotype of fitness associated with genetic variation at the human ß -globin locus. This chapter will focus on how natural selection operates when there is both genetic and environmental heterogeneity influencing the interaction of genotypes and environments in producing the phenotype of fitness.

Just as we modeled genotypic variation to examine natural selection, we will now need to model environmental heterogeneity in order to study how populations adapt to changing environments. In particular, we now consider two dimensions of environmental heterogeneity. One dimension refers to the physical source of environmental heterogeneity: spatial versus temporal. Environments can vary over space and over time. For example, some regions of the world have environmental conditions conducive to the existence of the malarial parasite whereas others do not. Therefore, there is spatial heterogeneity at any given time for humans living in malarial and nonmalarial geographical areas. However, as seen with the introduction of the Malaysian agricultural complex in wet, tropical Africa, there is also temporal heterogeneity in malarial versus nonmalarial environments. The same region in space can be a nonmalarial environment during some times and a malarial environment at other times.

Population Genetics and Microevolutionary Theory, By Alan R. Templeton Copyright © 2006 John Wiley & Sons, Inc.

The other dimension is environmental grain, how organisms perceive environmental heterogeneity. Species differ in size, their ability to move, and generation times. This creates differences in scale in the perception of environmental heterogeneity, both spatially and temporally. For example, a grizzly bear can move over large areas throughout its life and thereby experience a variety of different environments that vary spatially. In contrast, a tree cannot move through space and hence must deal with the particular spatial environment in which it originally germinated. Thus, spatial variation is experienced differently by grizzly bears and by trees. As another example, humans are a long-lived species, and as such we experience seasonal variation within our lives. In contrast, the zebra swallowtail butterfly (Eurytides marcellus) has two generations a year (spring and summer) in Missouri. The environmental conditions during these two seasons are quite distinct and interact with the swallowtail's basic developmental program to yield two distinct forms. The spring form is smaller and less melanic and has proportionally shorter swallowtails (extensions from the hind wings) than the summer form. In this case seasonal variation is not experienced by individual butterflies, but is only experienced across the generations in this species. Thus, the temporal variation of the seasons is experienced very differently by humans and by zebra swallowtails.

The grain of the environment can take on two extreme values. A fine-grained environment occurs when the individual experiences environmental heterogeneity within its own lifetime, and a coarse-grained environment occurs when an individual remains in a single environment throughout its lifetime, but the environment varies between demes occupying different spatial locations or across generations. The individual perceives the coarse-grained environment as a constant, but from the gamete's perspective, the gametes of the individual may experience a different environment either spatially (via gene flow) or temporally (across the generations). Since the gamete's perspective is the one that counts with respect to natural selection and adaptation, coarse-grained heterogeneity can have a great evolutionary impact even though no individual experiences this heterogeneity. Fine and coarse grains are extremes in a continuum. In many cases, individuals experience environmental variation, but the average environment experienced by all individuals in a deme also varies spatially among demes and temporally across generations. However, it is convenient to organize our exploration of environmental grain through these two extremes. Coupling fine and coarse grain with spatial and temporal variation leads to a total of four combinations of environmental heterogeneity. However, because an individual can only be in one place at one time, the individual always experiences fine-grained spatial heterogeneity as a sequence of temporal changes. For example, we can describe the environmental heterogeneity of a grizzly bear wandering through a series of heterogeneous spatial patches as a temporal sequence within that bear's lifetime. Therefore, we need only consider three types of environmental heterogeneity: coarse-grained spatial, coarse-grained temporal, and fine grained.

There is one other type of environmental heterogeneity that deserves special consideration. An important aspect of the environment of any species is the other species with which it coexists and interacts. Once again, we have already seen this in previous chapters in our discussion of the impact of the malarial parasite upon the selective environment that induces in its human host. There is nothing unusual about one species constituting an important part of the environment of another. When two or more species are interacting with one another, there is the possibility that all species constitute part of each other's environment. Thus, when one species evolves in response to its interspecific interactions, that evolutionary change constitutes a changed environment for the other species. The other species can then evolve in response to its altered biological environment, which in turn constitutes a changed environment for the species of interest. Coevolution occurs when two or more species mutually adapt to one another through interspecific interactions. This is a special form of environmental heterogeneity because the "environment" is changing because it is capable of evolving.

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