Final answer:
The presence and proportions of alleles in a population's next generation depend on allele frequencies and natural selection. Genetic drift and environmental changes influence these frequencies, while the Hardy-Weinberg principle allows for genotype frequency predictions.
Step-by-step explanation:
The alleles that occur in a population in the next generation and their proportions are influenced mainly by allele frequencies and genotype frequencies, which are governed by two primary factors: genetic drift and natural selection. Genetic drift refers to random changes in allele frequencies, which are especially prominent in small populations. Natural selection is the process through which advantageous traits become more common in a population because individuals with these traits are more likely to survive and reproduce. Changes in the environment can shift the balance towards certain alleles that confer a survival advantage, increasing their frequency in the population. For example, if a given allele results in a phenotype that better suits the current environment, it facilitates higher reproductive success. Over time, this leads to an increase in the frequency of that allele within the gene pool.
According to the Hardy-Weinberg principle, if allele frequencies (p and q) are known, and assuming ideal conditions with no other forces acting on the population, the genotype frequencies can be predicted by the equations p² (frequency of homozygous dominant), 2pq (frequency of heterozygous), and q² (frequency of homozygous recessive). This principle allows population geneticists to estimate genotype frequencies and thus infer phenotype distribution in a population.