Darwin and Mendel complemented each other well and their twin visions of the natural world revolutionized biology. The synthesis of the theory of natural selection and genetics gave rise to modern evolutionary ecology. Here we examine five major concepts within the area of population genetics and natural selection.Phenotypic variation among individuals in a population results from the combined effects of genes and environment. The first biologists to conduct thorough studies of phenotypic and genotypic variation and to incorporate experiments in their studies, focused on plants. Clausen, Keck, and Hiesey explored the extent and sources of morphological variation in plant populations, including both the influences of environment and genetics. Case determined that the best predictor of chuckwalla, Sauromalus, body length was average winter rainfall. Tracy's laboratory growth experiments indicated that variation in body size among chuckwalla populations is at least partly determined by genetic differences among populations.The Hardy-Weinberg equilibrium model helps identify evolutionary forces that can change gene frequencies in populations. Because evolution involves changes in gene frequencies in a population, a thorough understanding of evolution must include the area of genetics known as population genetics. One of the most fundamental concepts in population genetics, the Hardy-Weinberg principle, states that in a population mating at random in the absence of evolutionary forces, allele frequencies will remain constant. For a population in Hardy-Weinberg equilibrium in a situation where there are only two alleles at a particular locus, p + q = 1.0. The frequency of genotypes in a population in Hardy-Weinberg equilibrium can be calculated as (p + q)2 = (p + q) x (p + q) = p2 + 2pq + q2 = 1.0. The conditions necessary to maintain constant allele frequencies in a population are: (1) random mating, (2) no mutations, (3) large population size, (4) no immigration, and (5) equal survival and reproductive rates for all genotypes. When a population is not in Hardy-Weinberg equilibrium, the Hardy-Weinberg principle helps us to identify the evolutionary forces that may be in play.Natural selection is the result of differences in survival and reproduction among phenotypes. Natural selection can lead to change in populations but it can also serve as a conservative force, impeding change in a population. Stabilizing selection acts against extreme phenotypes and as a consequence, favors the average phenotype. By favoring the average phenotype, stabilizing selection decreases phenotypic diversity in populations. Directional selection favors an extreme phenotype over other phenotypes in the population. Under directional selection, the average of the trait under selection can change over time. Disruptive selection favors two or more extreme phenotypes over the average phenotype in a population, leading to a increase in phenotypic diversity in the population.The extent to which phenotypic variation is due to genetic variation determines the potential for evolution by natural selection. The most general postulate of the theory of natural selection is that the environment determines the evolution of the anatomy, physiology, and behavior of organisms. Some of the clearest demonstrations of natural selection have resulted from studies of populations of Galápagos finches. Losos, Warheit, and Schoener used replicated field experiments to study natural selection for changes in morphology in Anolis lizard populations. Their results indicate that colonizing populations can adapt rapidly to new environmental conditions. Studies by Carroll and several colleagues show that soapberry bug populations living on native and introduced host plants have undergone natural selection for traits that favor their survival and reproduction on particular host plant species. Hundreds of other examples of natural selection have been brought to light during the nearly one and a half century since Darwin published his theory. Still, evolutionary ecology remains a vigorous field of inquiry with plenty of debate, self-criticism, and significant work yet to be done.
The earlier research on adaptation of populations to local environmental conditions used transplant experiments to detect genetic differences among populations. More recent research on genetic variation within and among populations has applied techniques developed in molecular biology. Ecologists now have many powerful tools, ranging from classical techniques to modern technologically sophisticated approaches, for assessing the extent of genetic variation within and among populations and meeting the challenge of documenting and conserving biodiversity.Random processes, such as genetic drift, can change gene frequencies in populations, especially in small populations. Genetic drift is theoretically most effective at changing gene frequencies in small populations such as those that inhabit islands. One of the greatest concerns associated with fragmentation of natural ecosystems due to human land use is that reducing habitat availability will decrease the size of animal and plant populations to the point where genetic drift will reduce the genetic diversity within natural populations. Ledig and his colleagues found a significant positive correlation between population size and genetic diversity in populations of Chihuahua spruce, a naturally fragmented population of trees living on mountain islands. Frankham showed that compared to mainland populations, island populations generally include less genetic variation. Saccheri and his colleagues found that higher heterozygosity (genetic diversity) was associated with lower rates of population extinction through the effects of heterozygosity on larval survival, adult longevity, and egg hatching in populations of the Glanville fritillary butterfly, Melitaea cinxia.
