Which of the following contributes to genetic variation within populations?

Crossing over during meiosis is a critical process that contributes significantly to genetic variation within populations. During meiosis, homologous chromosomes come together and exchange segments of genetic material in a process called recombination or crossing over. This exchange results in new combinations of alleles that are different from those found in the parents. As a result, the offspring produced through sexual reproduction have unique genetic makeups, which enhances genetic diversity within a population.

This genetic variation is essential for the process of natural selection, as it provides a wider pool of traits that may allow some individuals to adapt better to changing environments or new challenges such as diseases. As a result, populations with greater genetic variation are generally more resilient and have better survival odds over time.

The other options do not contribute to genetic variation. Stable allele frequencies indicate a lack of change in the genetic makeup over generations. Uniform gene expression suggests that all individuals express the same genes in the same way, which would lead to homogeneity rather than variation. Identical chromosomes would mean a lack of genetic diversity, as all individuals would carry the same genetic information.