With their isolation and unchanging borders, islands can serve as useful laboratories for the study of evolution. One of the reasons for this is that when some species arrives on an island, individuals will take advantage of different ecological niches—some may make their living in treetops, while others may settle near a riverbank.
Over time, these different lifestyles can result in new adaptations, and over the course of many generations, the single species that arrived can evolve into numerous distinct animals occupying different niches—a process known as “adaptive radiation."
Found only on the island of Madagascar, lemurs are thought to be a prime example of adaptive radiation. But a study by Museum researcher James Herrera, published this week in the journal Royal Society Open Science, suggests that these primates don’t follow all the rules laid down by this phenomenon.
Typically, adaptive radiation occurs in an “early burst” of branching evolution, as new species arise quickly to stake out their spots in an island’s ecosystem. This burst is generally followed by a tapering off—once existing ecological niches are filled, it’s less likely that new species will evolve.
In Madagascar, though, lemur diversification stayed high, but did not appear to slow down over time. Instead, niches were taken advantage of by multiple species of lemur which evolved over time. A single niche—say, high in the canopy of a tree—could be occupied by large lemurs and small lemurs, some of which are active during the day and others that are nocturnal.
The paper also found key differences in how lemurs evolved depending on what niche they occupied. Nocturnal lemurs with omnivorous diets tend to evolve smaller bodies, while larger lemurs that are active during the day rely on tree leaves for their meals.
This continuing growth in lemur diversity suggests that lemurs may not be the textbook case of adaptive radiation they were once considered.
Luckily, these primates were far from the only example of this phenomenon. It’s a common thread among many island species, such as anole lizards. You can learn more about the adaptive radiation of these reptiles in the exhibition ¡Cuba!, which is open now and free for Members.