Cracking the AP Biology Exam

12

Evolution

All of the organisms we see today arose from earlier organisms. This process, known as evolution, can be described as a change in a population over time. Interestingly, however, the driving force of evolution, natural selection, operates on the level of the individual. In other words, evolution is defined in terms of populations but occurs in terms of individuals.

NATURAL SELECTION

What is the basis of our knowledge of evolution? Much of what we now know about evolution is based on the work of Charles Darwin. Darwin was a nineteenth-century British naturalist who sailed the world in a ship named the HMS Beagle.

Darwin developed his theory of evolution based on natural selection after studying animals in the Galápagos Islands and other places.

Darwin concluded that it was impossible for the finches and tortoises of the Galápagos simply to “grow” longer beaks or necks. Rather, the driving force of evolution must have been natural selection. Quite simply put, this means that nature would “choose” which organisms survive on the basis of their fitness. For example, on the first island Darwin studied, there must once have been short-necked tortoises. Unable to reach the higher vegetation, these tortoises eventually died off, leaving only those tortoises with longer necks. Consequently, evolution has come to be thought of as “the survival of the fittest”: Only those organisms most fit to survive will survive.

Darwin elaborated his theory in a book entitled On the Origin of Species. In a nutshell, here’s what Darwin observed:

  • Each species produces more offspring than can survive.
  • These offspring compete with one another for the limited resources available to them.
  • Organisms in every population vary.
  • The offspring with the most favorable traits or variations are the most likely to survive and therefore produce more offspring.

LAMARCK AND THE LONG NECKS

Darwin was not the first to propose a theory explaining the variety of life on earth. One of the most widely accepted theories of evolution in Darwin’s day was that proposed by Jean-Baptiste de Lamarck.

In the eighteenth century, Lamarck had proposed that acquired traits were inherited and passed on to offspring. For example, in the case of our tortoises, Lamarck’s theory said that the tortoises had long necks because they were constantly reaching for higher leaves while feeding. This theory is referred to as the “law of use and disuse,” or, as we might say now, “use it or lose it.” According to Lamarck, tortoises have long necks because they constantly use them.

We know now that Lamarck’s theory was wrong: Acquired changes—that is, changes at a “macro” level in somatic (body) cells—cannot be passed on to germ cells. For example, if you were to lose one of your fingers, your children would not inherit this trait.

EVIDENCE FOR EVOLUTION

In essence, nature “selects” which living things survive and reproduce. Today, we find support for the theory of evolution in several areas:

  • Paleontology, or the study of fossils. Paleontology has revealed to us both the great variety of organisms (most of which, including trilobites, dinosaurs, and the woolly mammoth, have died off) and the major lines of evolution.
  • Biogeography, or the study of the distribution of flora (plants) and fauna (animals) in the environment. Scientists have found related species in widely separated regions of the world. For example, Darwin observed that animals in the Galápagos have traits similar to those of animals on the mainland of South America. One possible explanation for these similarities is a common ancestor. As we’ll see below, there are other explanations for similar traits. However, when organisms share multiple traits, it’s pretty safe to say that they also shared a common ancestor.
  • Embryology, or the study of the development of an organism. If you look at the early stages in vertebrate development, all the embryos look alike! All vertebrates—including fish, amphibians, birds, and even humans—show fishlike features called gill slits.
  • Comparative anatomy, or the study of the anatomy of various animals. Scientists have discovered that some animals have similar structures that serve different functions. For example, a human’s arm, a dog’s leg, a bird’s wing, and a whale’s fin are all the same appendages, though they have evolved to serve different purposes. These structures, called homologous structures, also point to a common ancestor.

In contrast, sometimes animals have features with the same function but that are structurally different. A bat’s wing and an insect’s wing, for example, are both used to fly. They therefore have the same function, but have evolved totally independently of one another. These are calledanalogous structures. Another classic example of an analogous structure is the eye. Though scallops, insects, and humans all have eyes, these three different types of eyes are thought to have evolved entirely independently of one another. They are therefore analogous structures.

  • Molecular biology. Perhaps the most compelling proof of all is the similarity at the molecular level. Today, scientists can examine the nucleotide and amino acid sequences of different organisms. From these analyses, we’ve discovered that organisms that are closely related have a greater proportion of sequences in common than distantly related species. For example, most of us don’t look much like chimpanzees. However, by some estimates, as much as 99% of our genetic code is identical to that of a chimp.