Evolution

D

Coevolution

Often two or more organisms in an ecosystem fall into evolutionary step with one another, each adapting to changes in the other, a pattern known as coevolution. Coevolution is often apparent in flowers and their pollinators. Hummingbirds, for example, have long, narrow beaks and a relatively poor sense of smell, and they are attracted to the color red. Fuchsias, flowering plants that rely on hummingbirds for pollination, usually have long, slender flowers in various shades of red, and they have little or no fragrance. What at first appears to be a remarkable coincidence is, in fact, the product of thousands of generations of evolutionary fine-tuning. More likely to attract hummingbirds than fuchsias with different coloration, red-flowered individuals had greater reproductive success. And hummingbirds tended to spend more time extracting nectar from the flowers of fuchsias with shapes that matched the size of their slender beaks, thus increasing the likelihood of successful pollination. By the same token, those hummingbirds with long, slender beaks were best able to collect nectar from the long-necked flower. Over many generations, long-beaked hummingbirds became the rule, rather than the exception, in hummingbird populations.

VII

How Scientists Study Evolution

Species do not change overnight, or even in the course of one lifetime. Rather, evolutionary change usually occurs in tiny, almost imperceptible increments over the course of thousands of generations—periods that range from decades to millions of years. To study the evolutionary relationships among organisms, scientists must perform complex detective work, deriving indirect clues from the fossil record, patterns of animal distribution, comparative anatomy, molecular biology, and finally, direct observation in laboratories and the natural environment.

A

Fossils

One way biologists learn about the evolutionary relationships between species is by examining fossils. These ancient remains of living things are created when a dead plant or animal is buried under layers of mud or sand that gradually turn into stone. Over time, the organism remains themselves may turn to stone, becoming preserved within the rock layer in which they came to rest. By measuring radioactivity in the rock in which a fossil is embedded, paleontologists (scientists who study the fossil record) can determine the age of a fossil (see Dating Methods).

Fossils present a vivid record of the earliest life on Earth, and of a progression over time from simple to more-complex life forms. The earliest fossils, for example, are those of primitive bacteria some 3.5 billion years old. In more recent layers of rock, the first animal fossils appear—primitive jellyfish that date from 680 million years ago. Still more-complex forms, such as the first vertebrates (animals with backbones), are documented by fossils some 570 million years old. Fossils also indicate that the first mammals appeared roughly 200 million years ago.

---

---

Although these ancient forms of life have not existed on Earth for millions of years, scientists have been able, in many instances, to show a clear evolutionary line between extinct animals and their modern descendants. The horse’s lineage, for example, can be traced back about 50 million years to a four-toed animal about the size of a dog. Fossils provide evidence of several different transitional forms between this ancient horselike animal and the modern species. In another example, the extinct, winged creature Archaeopteryx lived about 145 million years ago. Its fossil shows the skeleton of a dinosaur and the feathers of a bird. Many paleontologists consider this creature an intermediate step in the evolution of reptilian dinosaurs into modern birds. Fossils show clear evidence that the earliest human species had many apelike features. These features included large, strong jaws and teeth; short stature, long, curved fingers; and faces that protruded outward from the forehead. Later species evolved progressively more humanlike features.

B

Distribution of Species

Scientists also learn about evolution by studying how different species of plants and animals are geographically distributed in nature, and how they relate to their environment and to each other. In particular, populations that exist on islands provide living clues of patterns of evolution. The study of these evolutionary relationships, known as island biogeography, has its roots in Darwin’s observations of the adaptive radiation of the Galapagos finches. The Hawaiian Islands provide similar examples, particularly in the species of birds known as honeycreepers. Like the Galapagos finches, the honeycreepers of Hawaii evolved from a common ancestor and branched into several species, showing a striking variety of beak shapes adapted for obtaining different food sources in their various niches.

C

Anatomical Similarities

Detailed study of the internal and external features of different living things, a discipline known as comparative anatomy, also provides a wealth of information about evolution. The arm of a human, the flipper of a whale, the foreleg of a horse, and the wing of a bird have different forms and are adapted to different functions. Yet they correspond in some way, and this correspondence extends to many details. In the case of the arm, flipper, foreleg, and wing, for example, each appendage shows a similar bone structure. The study of comparative anatomy has revealed many instances of correspondence within various groups of organisms and these bodily structures are said to be homologous. Evolutionary biologists suggest that such homologous structures originated in a common ancestor. The differences arose as each group diverged from the common ancestor and adapted to different ways of life. The more recent the common ancestor, the more similar the species.

The skeletons of humans, for instance, retain evidence of a tail-like structure that is probably a relic from previous mammalian ancestors. This feature, called the coccyx, or more commonly, the tailbone, has little apparent function in modern humans. Relic features such as the coccyx are called vestigial organs. Another vestigial organ in humans is the appendix, a narrow tube attached to the large intestine. In some plant-eating mammals, the appendix is a functioning organ that helps to digest plant material. In humans, however, the organ lacks this purpose and is considerably reduced in size, serving only as a minor source of certain white blood cells that guard against infection.

The field of embryology, the study of how organisms develop from a fertilized egg until they are ready for birth or hatching, also provides evolutionary clues. Scientists have noted that in the earliest stages of development, the embryos of organisms that share a recent common ancestor are very similar in appearance. As the embryos develop, they grow less similar. For example, the embryos of dogs and cats, both members of the mammal order Carnivora, are more similar in the early stages of development than just before birth. The same is true of human and ape embryos.

	More from Encarta
	•  Important Notice: MSN Encarta to be discontinued. Learn more. •  Presidential Myths Quiz •  Coffee break: Recharge your brain