The Hawaiian archipelago includes 19 islands and more than 100 atolls stretching 1,500 miles in the Pacific Ocean. These islands arose from hot spots on the ocean floor (Section 16.5). Because they were the tops of volcanoes, we can assume that their fiery surfaces were initially barren and inhospitable to life. Later, winds and currents carried a few individuals of mainland species to them. The individuals reproduced, and their descendants established populations. The lack of gene flow with mainland populations allowed the island populations to diverge. Today, thousands of species are unique to this island chain. Consider Hawaiian honeycreepers, birds that are descendants of a mainland finch species that arrived on the islands about 3.5 million years ago. A buffet of fruits, seeds, nectars, tasty insects, and the near absence of competitors and predators allowed the finch’s descendants to thrive. In the absence of gene flow, the island finch population diverged from the ancestral mainland species. Further divergences occurred because habitats on the landmasses of the Hawaiian archipelago vary dramatically—from lava beds, rain forests, and grasslands to dry woodlands and snow-capped peaks. Selection pressures differ within and between these habitats. The cumulative result of all these divergences is a spectacular array of honeycreeper species (Figure 17.18).