Final answer:
The fixation of loci within a population leads to decreased nucleotide variability and average heterozygosity, but it does not affect chromosome number. These decreases are due to the lack of allelic variation at fixed loci, resulting in greater homozygosity and lower DNA sequence variation.
Step-by-step explanation:
The higher the proportion of loci that are "fixed" in a population, the lower the population's nucleotide variability and average heterozygosity. When loci are fixed, it implies that all individuals in the population are homozygous for the same allele at these loci. Consequently, nucleotide variability is reduced because there is a lack of different nucleotides that might otherwise be present in a more diverse gene pool. Similarly, average heterozygosity decreases as more loci become fixed since heterozygosity measures the frequency of heterozygotes, which diminishes when alleles are fixed.
Chromosome number, however, is not directly affected by the fixation of loci. Chromosome number is a species-specific characteristic and doesn't change about the fixation of loci within a population. Hence, the correct answer that corresponds to the effects of loci fixation is nucleotide variability and average heterozygosity, but not chromosome number.