Chapter 4: Principles of Population Biology

Principles of Population Biology defines microevolution as the alteration of gene frequencies within a population, driven by five primary agents: mutation pressure, segregation distortion, gene flow, natural selection, and the random sampling errors known as genetic drift. The text explores how phenotypic variation is partitioned into genetic and environmental components, using the concept of heritability to predict evolutionary plasticity. Significant attention is given to the maintenance of genetic variation through mechanisms such as balanced polymorphism, heterozygote superiority, and frequency-dependent selection, while also addressing the theoretical challenges of genetic load. The author provides a detailed analysis of inbreeding and kinship, utilizing Sewall Wright’s coefficients to quantify genetic relationships and inbreeding depression, which are critical for understanding the evolution of social cohesion and altruism. The narrative then shifts to population dynamics, contrasting exponential growth with logistic growth to explain how populations are regulated relative to the carrying capacity of their environment. It categorizes density-dependent controls—including emigration, endocrine exhaustion, reproductive inhibition, competition, and predation—and examines their role in driving population cycles. Demographic principles are elucidated through life tables, survivorship curves, and fertility schedules, which allow for the calculation of the net reproductive rate and the intrinsic rate of increase. The concept of reproductive value is introduced to analyze optimal life history strategies, distinguishing between semelparity (single reproductive episodes) and iteroparity (repeated breeding) based on physiological costs and benefits. Finally, the chapter culminates in a discussion of the r-selection and K-selection continuum, explaining how environmental stability dictates evolutionary strategies regarding growth rates, dispersal patterns, and the development of complex social behaviors.