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An adaptive radiation is the adaptation of a species to a new environment. When new environments open up — for example, when a land bridge connects two previously separated continents or islands — life forms normally migrate into these new environments. If they survive, the colonists adapt to the new ecological conditions and, over evolutionary time, become adapted to those conditions. When the colonists are so different from their ancestral population (the ones who didn’t cross the land bridge, for example) that they can no longer interbreed with those ancestral forms, speciation has occurred.
Going ape (and prosimian): Primate subgroups
All the primates have the characteristics I mention in the preceding section, but even a quick look at the primates reveals some clear divisions. The following sections describe the four main kinds of primates.
Squirrel-cats: The prosimians
One of the major divisions in the Primate order is that between the Anthropoidea (the people-like apes and monkeys) and the Prosimii (or prosimians, which are pretty different from people even though they’re clearly primates). Baboons, chimpanzees, and gorillas — all in the Anthropoidea — are very obviously similar to humans, but connecting to, say, the ring-tailed lemur (a cat-like prosimian of Madagascar that has a long, striped tail) or the tiny, bug-eyed, shrew-like tarsier that can fit in the palm of your hand is a little more difficult. Still, these animals are primates — even though they can look like a cross between a squirrel and a cat — and they typically have the following distinctive traits:
Relatively long snouts in some species (long for primates, anyway), although they may also have very large eyes
A dental formula of 2.1.3.3
Small body size compared to other primates; they range from mouse-size to cat-size, averaging about 5 kilograms or 10 pounds
Some are nocturnal and have a diet that favors insects but includes tree saps, grubs, fruit, flowers, and leaves
Probably the strangest primate is the aye-aye of Madagascar. About the size of a cat with enormous, hairless ears, the aye-aye climbs through trees by moonlight listening for larvae beneath tree bark. When it hears a squirming treat, it uses a thin, elongated finger to scoop the meal out of the bark. Even the driest textbooks of primatology can’t help but marvel over this creature, which one author called the most “improbable” primate; another said that the aye-aye, though clearly a primate, displayed the most extreme specialization of anatomy in the order. This means that although most primates are somewhat general in their diet (many have a varied, omnivorous diet), the aye-aye is quite specialized and inflexible in its diet. Unfortunately, such specialization can prove disastrous if the prey species itself becomes extinct or somehow declines.
So if the prosimians are so strange, why are they considered primates? Well, they generally have nails rather than claws, focus on vision rather than smell for their sensory specialty, have relatively mobile wrists and ankles, and live mostly in the trees. For all these reasons (as well as connections shown to the rest of the primates in the genetic data), the prosimians are, in fact, relations (albeit some pretty strange ones; of course, they could say the same about us). Because the prosimians are very much like the earliest primates, understanding them and what they can reveal about primate origins is important; unfortunately, they’re endangered.
www.wildmadagascar.org.
The Old World monkeys
The monkeys of the Old World, members of the parvorder (a major division in the order) Catarrhini (meaning narrow-nosed) are distinct from the New World monkeys because they live on a different continent, distinct from the apes because the apes are generally larger, and different from the prosimians because they’re generally larger and have evolved more ecological adaptations than the prosimians. They also have the following distinctive anatomical characteristics:
Narrow nose with nostrils facing down (as opposed to wide-nosed, out-facing nostrils in New World monkeys)
A dental formula of 2.1.2.3 (one premolar fewer than the New World monkeys) with some species having molars shaped like knives for shearing vegetation
Lack of a prehensile tail (see the next section for more on prehensile tails)
Both arboreal and terrestrial lifestyles
The Old World monkeys are themselves split into at least two main groups: the subfamilies Cercopithecinae (including the terrestrial, brilliantly colored mandrill baboons) and Colobinae, which include the large-nosed proboscis monkey and the leaf-devouring colobus monkey, with its large, complex, leaf-digesting stomach. Old World monkeys live in diverse habitats, from dry African savanna to the snowy mountains of Japan. Africa’s patas monkey, distributed south of the Sahara, is a consummate survivor, consuming fruit, bird eggs, roots, and leaves; it can also sprint at up to 34 miles per hour, making it the fastest primate. Japanese snow monkeys spend winter hours soaking in natural hot springs.
The New World monkeys
The New World (South America) is home to primates as well; they’re members of the parvorder Platyrrhini, meaning “broad-nosed,” as compared to their Old World counterparts discussed in the last section. Shortly after the origin of the primates around 40 million years ago, South America was already sliding away from its previous link with Africa, and riding on it (or perhaps drifting to it on natural rafts of vegetation, purely by accident) were the ancestors of the New World monkeys. They survive into the present and have the following distinctive characteristics:
Wide nose (compared to the Old World monkeys)
Dental formula of 2.1.3.3 (an extra premolar)
Most have a prehensile tail used to grasp tree limbs
A completely arboreal lifestyle
The New World monkeys include the very loud howler monkey (which scares tourists because the howl sounds like a Hollywood jaguar), the fruit-eating spider monkey (which has a very handy prehensile tail), and the strange little marmosets, which live high in the trees on a diverse diet of insects, fruits, and leaves. Generally speaking, the New World monkeys are somewhat smaller than those of the Old World, with most species
