Final answer:
Option (a), Effective population size is influenced by factors such as population size, demographic characteristics, and reproductive success, affecting allele frequencies and population genetics.
Step-by-step explanation:
The factor that is influenced by a variety of demographic characteristics, including fluctuations in population size, age structure, sex ratio, and variance in individuals' reproductive success is effective population size. Effective population size is a key concept in population genetics and helps in understanding how selective forces change the allele frequencies in a population over time. Inherited traits, the genetic makeup, and the frequency of alleles are impacted by reproductive success and demographic characteristics, and this can affect the evolution of the population.
Demographic factors such as birth rates exceeding death rates lead to an increase in population size, while the reverse leads to a decrease. Life expectancy also plays a crucial role, affecting resources, reproduction, and overall health. Demography, which studies these population changes, incorporates various factors affecting populations' birth rates, such as population size, population density, and sex ratio.
Further, populations evolve over time, displaying life-history characteristics that may change due to density-dependent and density-independent factors. Understanding this evolution requires analyzing the genetic structure, which includes the distribution of genotypes within the population, and how traits are passed on and persist in populations through natural selection, gene flow, and genetic drift.