Natural selection and evolution

Abstract Evolutionary change in organisms is affected by the interaction of several major forces, including mutation (broadly conceived), recombination, inbreeding, migration, natural selection, and genetic drift, operating at present on past evolutionary constraints, of individuals and higher levels of organization. However, the relative importance of these evolutionary forces in the processes of adaptation and speciation of natural populations remains as enigmatic now as ever (Nevo, 1978, 1988a, 1990a, 1998b; Nevo et al., 1984a). The following discussion will focus on natural selection which seems to me the predominant evolutionary force underlying the twin evolutionary processes of adaptation and speciation. In 1859, Darwin and Wallace suggested that the major cause of evolutionary change is natural selection. Despite the passage of 140 years, the exact nature and relative importance of diverse mechanisms of natural selection (e.g. stabilizing, diversifying, balancing, frequency-dependent cyclical, etc.) in evolutionary change at single loci and multilocus structures, and its applicability to both the genotypic and phenotypic levels, are still the topic of many debates.