Adaptive Radiation

To be fruitful and multiply, all living things have to acquire energy (through photosynthesis or by consuming other living things), avoid predation and illness, and reproduce. As is clear from the study of natural history, there are many different ways that organisms manage to perform these tasks, which reflects both the variety of environments on Earth and the variety of living things. Any environment—marine, terrestrial, arboreal, aerial, subterranean—contains many ecological niches that provide means that living things use to make a living. The principle of adaptive radiation helps to explain how niches get filled.

The geological record reveals many examples of the opening of a new environment and its subsequent occupation by living things. Island environments such as the Hawaiian Islands, the Galapagos Islands, Madagascar, and Australia show this especially well. The Hawaiian archipelago was formed as lava erupted from undersea volcanoes, and what we see as islands actually are the tips of volcanic mountains. Erosion produced soils and land plants—their seeds or spores blown or washed in— subsequently colonized the islands. Eventually land animals reached the islands as well. Birds, insects, and a species of bat were blown to Hawaii or rafted there from other Pacific islands on chunks of land torn off by huge storms.

The Hawaiian honeycreepers are a group of approximately twenty-three species of brightly colored birds that range from four to eight inches long. ornithologists have studied them extensively and have shown them to be very closely related. Even though they are closely related, honeycreeper species vary quite a bit from one another and occupy many different ecological niches. Some are insectivorous, some suck nectar from flowers, others are adapted to eating different kinds of seeds—one variety has even evolved to exploit a woodpecker-like niche. The best explanation for the similarity of honeycreepers in Hawaii is that they are all descended from a common ancestor. The best explanation for the diversity of these birds is that the descendants of this common ancestor diverged into many subgroups over time as they became adapted to new, open ecological niches. Honeycreepers are, in fact, a good example of the principle of adaptive radiation, by which one or a few individual animals arrive in a new environment that has empty ecological niches, and their descendants are selected to quickly evolve the characteristics needed to exploit these niches. Lemurs on Madagascar, finches on the Galapagos Islands, and the variety of marsupial mammals in Australia and prehistoric South America are other examples of adaptive radiation.

A major adaptive radiation occurred in the Ordovician period (about 430 million years ago), when plants developed protections against drying out and against ultraviolet radiation, vascular tissue to support erect stems, and other adaptations allowing for life out of water (Richardson 1992). It was then that plants could colonize the dry land. The number of free niches enabled plants to radiate into a huge number of ways of life. The movement of plants from aquatic environments onto land was truly an Earth-changing event. Another major adaptive radiation occurred about 400 million years ago in the Devonian, when vertebrates evolved adaptations (lungs and legs) that permitted their movement onto land. One branch of these early tetrapods radiated into the various amphibians and another branch into reptiles and mammals. A major difference between the reptile and mammal branch and amphibians was the amniotic egg: an adaptation that allowed reproduction to take place independent of a watery environment.

During the late Cretaceous and early Cenozoic, about 65 million years ago, mammals began adaptively radiating after the demise of the dinosaurs opened up new ecological niches for them. Mammals moved into gnawing niches (rodents), a variety of grazing and browsing niches (hoofed quadrupeds, the artiodactyls and perissodactyls), insect-eating niches (insectivores and primates), and meat-eating niches (carnivores). Over time, subniches were occupied: some carnivores stalk their prey (lions, saber-toothed cats), and others run it down (cheetahs, wolves); some (lions, wolves, hyenas) hunt large-bodied prey, and some (foxes, bobcats) hunt small prey.

If a particular adaptive shift requires extensive changes, such as greatly increasing or reducing the size or number of parts of the body, the tendency is for that change to occur early in the evolution of the lineage rather than later. Although not a hard-and-fast rule, it follows logically from natural selection that the greatest potential for evolutionary change will occur before specializations of size or shape take place. Early in evolutionary history, the morphology of a major group tends to be more generalized, but as adaptive radiation takes place, structures are selected to enable the organisms to adapt better to their environments. In most cases, these adaptations constrain, or limit, future evolution in some ways. The forelimbs of perch are committed to propelling them through the water and are specialized for this purpose; they will not become grasping hands.

We and all other land vertebrates have four limbs. Why? We tend to think of four limbs as being "normal," yet there are other ways to move bodies around on land. Insects have six legs and spiders have eight, and these groups of animals have been very successful in diversifying into many varieties and are represented in great numbers all over the world. So, there is nothing especially superior about having four limbs, although apparently, because no organism has evolved wheels for locomotion, two or more limbs apparently work better. But all land vertebrates have four limbs rather than six or eight because reptiles, birds, and mammals are descended from early four-legged creatures. These first land vertebrates had four legs because the swimming vertebrates that gave rise to them had two fins in front and two in back. The number of legs in land vertebrates was constrained because of the number of legs of their aquatic ancestors. Imagine what life on Earth would have looked like if the first aquatic vertebrates had had six fins! Might there have been more ecological niches for land creatures to move into? It certainly would have made sports more interesting if human beings had four feet to kick balls with—or four hands to swing bats or rackets.

We see many examples of constraints on evolution; mammalian evolution provides another example. After the demise of the dinosaurs, mammals began to radiate into niches that had previously been occupied by the varieties of dinosaurs. As suggested by the shape of their teeth, mammals of the late Cretaceous and early Paleocene were small, mostly undifferentiated creatures that occupied a variety of insectivorous, gnawing, and seed-eating niches that dinosaurs were not exploiting. As new niches became available, these stem mammals quickly diverged into basic mammalian body plans: the two kinds of hoofed mammals, the carnivores, bats, insectivores, primates, rodents, sloths, and so on. Once a lineage developed (for example, carnivores), it radiated within the basic pattern to produce a variety of different forms (for example,

Figure 2.5

Vertebrate forelimbs all contain the same bones, although these bones have evolved over time for different locomotor purposes, such as running, swimming, flying, and grasping. Courtesy of Janet Dreyer.

cats, dogs, bears, raccoons) in many sizes and shapes, all of which inherited basic dental and skeletal traits from the early carnivore ancestor. Once a lineage is "committed" to a basic way of life, it is rare indeed for a major adaptive shift of the same degree to take place. Although both horses and bats are descended from generalized quadrupedal early mammalian ancestors, the bones in a horse's forelimbs have been modified for swift running: some bones have been greatly elongated, others have been lost completely, and others have been reshaped. A bat has the same basic bones in its forelimb, but they have been greatly modified in other ways: some bones have been elongated, others have been lost, and yet others have been reshaped for flight (Figure 2.5).

Humans belong in the primate group of mammals, and primates are characterized by relatively fewer skeletal changes than have occurred in other mammal lineages. A primate doesn't have the extensive remodeling of the forelimb and hand that resulted in a bat's wing or a horse's hoof. We primates have a relatively basic "four on the floor" quadruped limb pattern of one bone close to the body (the femur in the leg and the humerus in the arm), two bones next to that one (the tibia and fibula in the leg and the radius and ulna in the arm), a group of small bones after this (tarsal or ankle bones in the leg and carpal or wrist bones in the arm), and a fanlike spray of small bones at the end of the limb (metatarsals and toe bones in the leg and metacarpals and finger bones in the arm) (Figure 2.5). Most primates locomote using four limbs; we human primates have taken this quadrupedal pattern and tipped it back so that our hind limbs bear all our weight (and not too successfully, as witnessed by hernias and the knee and lower-back problems that plague our species). Being bipedal, though, meant that we did not have to use our hands for locomotion, and they were thus freed for other purposes, like carrying things and making tools. Fortunately for human beings, dependent on tools and brains to survive, our early primate ancestors did not evolve to have specialized appendages like those of horses or bats.

Which is better, to be generalized or to be specialized? It's impossible to say without knowing more about the environment or niche in which a species lives. Specialized

Figure 2.5

Vertebrate forelimbs all contain the same bones, although these bones have evolved over time for different locomotor purposes, such as running, swimming, flying, and grasping. Courtesy of Janet Dreyer.

organisms may do very well by being better able to exploit a resource than are their possible competitors, yet generalized organisms may have an advantage in being able to adjust to a new environmental challenge.

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