All populations show variation in genetic composition (frequencies of various alleles) among individuals and through time. The degree of genetic variability and the frequencies of various alleles depend on a number of factors, including mutation rate, environmental heterogeneity, and population size and mobility (Hedrick and Gilpin 1997, Mopper 1996, Mopper and Strauss 1998). Genetic variation may be partitioned among isolated demes or affected by patterns of habitat use (Hirai et al. 1994). Genetic structure, in turn, affects various other population parameters, including population viability (Hedrick and Gilpin 1997).
Populations vary in the frequency and distribution of various alleles. Widespread species might be expected to show greater variation across their geographic range than would more restricted species. Roberds et al. (1987) measured genetic variation from local to regional scales for the southern pine beetle, Dendroctonus frontalis, in the southeastern United States. They reported that allelic frequencies were somewhat differentiated among populations from Arkansas, Mississippi, and North Carolina but that a population in Texas was distinct. They found little or no variation among demes within each state and evidence of considerable inbreeding among beetles at the individual tree level.
Roberds et al. (1987) also reported that only 1 allele of the 7 analyzed showed significant variation between demes that were growing and colonizing new trees and demes not growing or colonizing new trees. The genetic variation of the founders of a new deme is relatively low, simply because of the small number of colonists and the limited proportion of the gene pool that they represent. Colonists from a population with low genetic variability start a population with even lower genetic variability (Hedrick and Gilpin 1997). Therefore, the size and genetic variability of the source populations, as well as the number of colonists, determine genetic variability in founding populations. Genetic variability remains low during population growth unless augmented by new colonists. This is especially true for parthenogenetic species, such as aphids, for which an entire population could represent clones derived from a founding female. Differential dispersal ability among genotypes affects heterozygosity of colonists. Florence et al. (1982) reported that the frequencies of 4 alleles for an esterase (esB) converged in southern pine beetles collected along a 150-m transect extending from an active infestation in east Texas. As a result, heterozygosity increased significantly with distance, approaching the theoretical maximum of 0.75 for a gene locus with 4 alleles. These data suggested a system that compensates for loss of genetic variability as a result of inbreeding by small founding populations and maximizes genetic variability in new populations coping with different selection regimens (Florence et al. 1982). Nevertheless, dispersal among local populations is critical to maintaining genetic variability (Hedrick and Gilpin 1997). If isolation restricts dispersal and infusion of new genetic material into local demes, inbreeding may reduce population ability to adapt to changing conditions, and recolonization following local extinction will be more difficult.
Polymorphism occurs commonly among insects and may underlie their rapid adaptation to environmental change or other selective pressures, such as predation (A. Brower 1996, Sheppard et al. 1985). Among the best-known examples of population response to environmental change is the industrial melanism that developed in the peppered moth, Biston betularia, in England following the industrial revolution (Kettlewell 1956). Selective predation by insectivorous birds was the key to the rapid shift in dominance from the white form, which is cryptic on light surfaces provided by lichens on tree bark, to the black form, which is more cryptic on trees blackened by industrial effluents. Birds preying on the more conspicuous morph maintained low frequencies of the black form in preindus-trial England, but later they greatly reduced frequencies of the white form. Other examples of polymorphism also appear to be maintained by selective predation. In some cases, predators focusing on inferior Müllerian mimics of multiple sym-patric models may select for morphs or demes that mimic different models (e.g., A. Brower 1996, Sheppard et al. 1985).
Genetic polymorphism can develop in populations that use multiple habitat units or resources (Mopper 1996, Mopper and Strauss 1998, Via 1990). Sturgeon and Mitton (1986) compared allelic frequencies among mountain pine beetles, Dendroctonus ponderosae, collected from three pine hosts [ponderosa (Pinus ponderosa), lodgepole (P. contorta), and limber (P. flexilis)] at each of five sites in Colorado. Significant variation occurred in morphological traits and allelic fre quencies at five polymorphic enzyme loci among the five populations and among the three host species, suggesting that the host species is an important contributor to genetic structure of polyphagous insect populations.
Via (1991a) compared the fitnesses (longevity, fecundity, and capacity for population increase) of pea aphid, Acyrthosiphon pisum, clones from two host plants (alfalfa and red clover) on their source host or the alternate host. She reported that aphid clones had higher fitnesses on their source host, compared to the host to which they were transplanted, indicating local adaptation to factors associated with host conditions. Furthermore, significant negative correlations for fitness between source host and alternate host indicated increasing divergence between aphid genotypes associated with different hosts. In a subsequent study, Via (1991b) evaluated the relative importance of genetics and experience on aphid longevity and fecundity on source and alternate hosts. She maintained replicate lineages of the two clones (from alfalfa versus clover) on both host plants for three generations, then tested performance of each lineage on both hosts. If genetics is the more important factor affecting aphid performance on source and alternate host, then aphids should have highest fitness on the host to which they were adapted, regardless of subsequent rearing on the alternate host. However, if experience is the more important factor, then aphids should have highest fitness on the host from which they were reared. Via found that three generations of experience on the alternate host did not significantly improve fitness on that host. Rather, fitness was highest on the plant from which the clone was derived originally, supporting the hypothesis that genetics is the more important factor. These data indicated that continued genetic divergence of the two subpopulations is likely, given that individuals dispersing between alternate hosts cannot improve their performance through time as a result of experience.
Biological factors that determine mate selection or mating success also affect gene frequencies, perhaps in concert with environmental conditions. In a laboratory experiment with sex-linked mutant genes in Drosophila melanogaster (Peterson and Merrell 1983), mutant and wild-male phenotypes exhibited about the same viability, but mutant males showed a significant mating disadvantage, leading to rapid elimination (i.e., within a few generations) of the mutant allele. In addition, whereas the wild-male phenotype tended to show a rare male advantage in mating (i.e., a higher proportion of males mating at low relative abundance), mutant males showed a rare male disadvantage (i.e., a lower proportion of males mating at low relative abundance), increasing their rate of elimination. Malausa et al. (2005) used a combination of genetic and stable isotope (13C) techniques to identify host plant sources of 396 male and 393 female European corn borer, Ostrinia nubilalis, collected at multiple sites, and of 535 spermatophores carried by these females, over a 2-year period (2002-2003). Moths could be differentiated unambiguously on the basis of larval host, either C3 or C4 plants. All but 5 females (3 in 2002 and 2 in 2003) had mated with a male from the same host race, indicating >95 assortative mating. These data indicate that nonrandom mating patterns can lead to rapid changes in gene frequencies among diverging races from different hosts.
Insect populations can adapt to environmental change more rapidly than can longer-lived, more slowly reproducing, organisms (Mopper 1996, Mopper and Strauss 1998). Heterogeneous environmental conditions tend to mitigate directional selection: any strong directional selection by any environmental factor during one generation can be modified in subsequent generations by a different prevailing factor. However, changes in genetic composition occur quickly in insects when environmental change does impose directional selective pressure, such as in the change from preindustrial to postindustrial morphotypes in the polymorphic peppered moth (Kettlewell 1956).
The shift from pesticide-susceptible to pesticide-resistant genotypes may be particularly instructive. Selective pressure imposed by insecticides caused rapid development of insecticide-resistant populations for many species. Resistance development is facilitated by the widespread occurrence in insects, especially herbivores, of genes that encode for enzymes that detoxify plant defenses because ingested insecticides also are susceptible to detoxification by these enzymes. Although avoidance of directional selection for resistance to any single tactic is a major objective of integrated pest management (IPM), pest management in practice still involves widespread use of the most effective tactic. Following the appearance of transgenic insect-resistant crop species in the late 1980s, genetically engineered, Bt toxin-producing corn, cotton, soybeans, and potatoes have replaced nontransgenic varieties over large areas, raising concern that these crops might quickly select for resistance in target species (Alstad and Andow 1995, Tabashnik 1994,Tabashnik et al. 1996).
Laboratory studies have shown that at least 16 species of Lepidoptera, Coleoptera, and Diptera are capable of developing resistance to the Bt gene as a result of strong selection (Tabashnik 1994). However, few species have shown resistance in the field. The diamondback moth, Plutella xylostella, has shown resistance to Bt in field populations from the United States, Philippines, Malaysia, and Thailand. Resistance in some species has been attributed to reduced binding of the toxin to membranes of the midgut epithelium. A single gene confers resistance to four Bt toxins in the diamondback moth (Tabashnik et al. 1997), and >5000-fold resistance can be achieved in a few generations (Tabashnik et al. 1996). Resistance can be reversed when exposure to Bt toxin is eliminated for several generations, probably because of fitness costs of resistance (Tabashnik et al. 1994), but some strains can maintain resistance in the absence of Bt for more than 20 generations (Tabashnik et al. 1996).
Resistance development in the field can be minimized by alternating control strategies to prevent strong directional selection in exposed populations. In particular, a strategy of high Bt concentration in transgenic crops, together with nontransgenic refuges, has been successful both in reducing use of conventional insecticides and in preventing resistance development (Alstad and Andow 1995, Carrière et al. 2001b, 2003). High concentration of Bt minimizes survivorship on the transgenic crop, and greater survivorship in the nontrans-genic crop prevents fixation of resistance genes in the population (see Chapter 16).
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