Final answer:
A cline is a gradual change in gene frequencies between neighbouring populations, which can lead to divergent adaptations and even speciation due to environmental differences and restricted gene flow.
Step-by-step explanation:
A cline represents a gradual shift in gene frequencies between neighboring populations. In a geographically continuous population, gene flow allows for a homogeneous gene pool because individuals can move and mate with others in new locations, thus maintaining similarity in allele frequency across the species' range. However, when populations become geographically discontinuous, this free flow of alleles is hindered, and separation over time enables the populations to evolve along different trajectories, diverging in their allele frequencies due to independent mutations, natural selection, and genetic drift.
Environmental conditions, such as climate, resources, predators, and competitors, typically differ between two separated populations, leading natural selection to favor divergent adaptations in each group. When there is a restriction in gene flow, populations may show abrupt differences in phenotype, which can even lead to speciation.