8.19: Geographic patterns of species distributions reflect species’ evolutionary histories.

When it comes to species distributions, history matters. Species were not designed from scratch to fill a particular niche. Rather, whatever arrived in a geographic region first—usually a nearby species—took up numerous different lifestyles in numerous different habitats, and the populations ultimately adapted to and evolved in each environment. In Hawaii, it seems that a finch-like descendant of the honeycreepers arrived 4–5 million years ago and rapidly evolved into a large number of diverse species. The same process occurred and continues to occur in all locales, not just on islands.

The study of the distribution patterns of living organisms around the world is called biogeography. This is the second line of evidence that helps us to see that evolution takes place and to better understand the process. The patterns of biogeography that Darwin and many subsequent researchers noticed provide strong evidence that evolutionary forces are responsible for these patterns. Species often more closely resemble other species that live less than a hundred miles away but in radically different habitats than they resemble species living thousands of miles away in nearly identical habitats. In Hawaii, for example, nearly every bird is some sort of modified honeycreeper, from seed-eating honeycreepers to curved-bill nectar-feeding honeycreepers (FIGURE 8-36).

Large, isolated habitats also have interesting biogeographic patterns. Australia and Madagascar are filled with unique organisms that are clearly not closely related to organisms elsewhere. In Australia, for example, marsupial species, rather than placental mammals, fill all of the usual roles. There are marsupial “wolves,” marsupial “mice,” marsupial “squirrels,” and marsupial “anteaters” (FIGURE 8-37).

The marsupials of Australia physically resemble their placental counterparts for most traits, but molecular analysis shows that they are actually more closely related to one another, sharing a common marsupial ancestor. Their relatedness to each other is also revealed by similarities in their reproduction: females give birth to offspring at a relatively early stage of development, and the offspring finish their development in a pouch. The presence of marsupials in Australia does not simply mean that marsupials are better adapted than placentals to Australian habitats. When placental organisms are transplanted to Australia they do just fine, often thriving to the point of endangering the native species. Instead, it appears that the terrestrial placental mammals in Australia disappeared about 55 million years ago—although the reasons are not clear—giving the marsupials the chance to flourish and diversify.

Biogeographic patterns such as those seen in honeycreepers and in the marsupials of Australia illustrate that evolution doesn’t necessarily lead to the same “solutions” each time a particular set of environmental conditions occurs. Rather, the traits in populations that happen to be in a particular location gradually change, and those species become better adapted to the habitats they occupy.