The ancestors of modern Lemurs arrived in Madagascar and spread across the island. At some point after this, the once contiguous population became fragmented into three separate populations. Each of these populations inhabits a different part of the island, each representing a different habitat with unique selection pressures. Each population contains genetic variation and certain individuals are favoured due to their genetic variants being useful in the new habitat. These individuals are better able to survive and reproduce, increasing the frequency of these beneficial genes in the population. This process proceeds in each of the isolated populations, with populations becoming more and more genetically dissimilar to each other until they can no longer successfully interbreed