Concept 1.4: Evolution Explains the Diversity as Well as the Unity of Life

Evolution—change in the genetic makeup of biological populations through time—is a major unifying principle of biology. Any process that can lead to changes in the frequencies of genes in a population from generation to generation is an evolutionary process. A common set of evolutionary processes is at work in populations of all organisms. The constant change that occurs in these populations gives rise to all the diversity we see in life. These two themes—unity and diversity—provide a framework for organizing and thinking about the evolution of life. The similarities of life allow us to make comparisons and predictions from one species to another, as we have discussed. The differences are what make biology such a rich and exciting field for investigation and discovery.

Natural selection is an important process of evolution

Charles Darwin compiled factual evidence for evolution in his 1859 book On the Origin of Species. Since then, biologists have gathered massive amounts of data supporting Darwin’s idea that all living organisms descended from a common ancestor. Darwin also proposed one of the most important processes that produce evolutionary change. He argued that the differing survival and reproduction among individuals in a population, which he termed natural selection, could account for much of the evolution of life.

When Darwin proposed that living organisms descended from a common ancestor and are therefore related to one another, he did not have the advantage we have today of understanding the processes of genetic inheritance. Those processes, which we will cover in depth in Chapters 7–9, were not widely understood until the early 1900s. But he knew that offspring differed from their parents, even though they showed strong similarities. And he knew that any population of a plant or animal species displays variation.

Darwin himself bred pigeons, and he knew that if you select breeding pairs on the basis of some particular trait, then that trait is more likely to be present in their offspring than in the general population. He was well aware of how pigeon fanciers selected breeding pairs to produce offspring with unusual feather patterns, beak shapes, or body sizes. He realized that if humans could select for specific traits in organisms such as pigeons, a similar process could operate in nature. Darwin emphasized that human-imposed selection, which he called “artificial selection,” has been practiced on crop plants and domesticated animals since the dawn of human civilization. In coining the term “natural selection,” he argued that a similar process occurs in nature. But in nature, the “selection” occurs not by human choice but by the fact that some individuals contribute more offspring to future generations than others.

How does natural selection work? Darwin thought that differing probabilities of survival and reproductive success could account for evolutionary change. He reasoned that the reproductive capacity of plants and animals, if unchecked, would result in unlimited growth of populations, but we do not observe such growth in nature. In most species, only a small percentage of offspring survive to reproduce. For this reason, any trait will spread in the population if that trait gives an individual organism even a small increase in the probability that the individual will survive and reproduce.

Because organisms with certain traits survive and reproduce best under specific sets of conditions, natural selection leads to adaptations: structural, physiological, or behavioral traits that increase an organism’s chances of surviving and reproducing in its environment. For example, remember the frog in the opening photograph of this chapter? Look at the frog’s feet and notice that the frog’s toes appear to be greatly expanded. These expanded toes would be especially obvious if you could compare them with the toes of frog species that do not live in trees. Expanded toes increase the ability of tree frogs to climb trees, which allows them to hunt insects for food in the treetops and to escape predators on the ground. For this reason the expanded toe pads of tree frogs are an adaptation to life in trees. FIGURE 1.10 shows other adaptations in the limbs of frogs to different environments.

Figure 1.10: Adaptations to the Environment The limbs of frogs show adaptations to the different environments of each species.

Biologists often think about two different kinds of explanations for adaptations. On the one hand, we can consider the immediate genetic, physiological, neurological, and developmental processes that explain how an adaptation works. We call these proximate explanations. For example, a proximate explanation for the toes of tree frogs might examine the physical structure of the toe pads and explain how expansion of the toe leads to greater adhesion to a substrate. Such an explanation tells us how the adaptation works, but it does not explain how tree frogs came to possess such toe pads. An ultimate explanation, on the other hand, concerns the processes that led to the evolution of toe pads in various groups of climbing frogs. Ultimate explanations involve comparison of variation within and among species and describe how a given trait affects an organism’s chances for survival and reproduction.

Natural selection has been demonstrated in countless biological investigations, but it is not the only process that results in evolution, as we will explore in Chapters 15–18. An example of another evolutionary process is genetic drift, which refers to random changes in gene frequencies in a population because of chance events. As a result of the various evolutionary processes, all biological populations evolve through time. All the evolutionary processes operating over the long history of Earth have led to the remarkable diversity of life on our planet.

Evolution is a fact, as well as the basis for broader theory

The famous biologist Theodosius Dobzhansky once wrote that “Nothing in biology makes sense except in the light of evolution.” Dobzhansky was emphasizing the need to include an evolutionary perspective and approach in all aspects of biological study. Everything in biology is a product of evolution, and biologists need a perspective of change and adaptation to fully understand biological systems.

You may have heard someone say that evolution is “just a theory,” implying that there is some question about whether or not biological populations evolve. This is a common misunderstanding that originates in part from the different meanings of the word “theory” in everyday language and in science. In everyday speech, some people use the word “theory” to mean “hypothesis” or even—disparagingly—“a guess.” In science, however, a theory is a body of scientific work in which rigorously tested and well-established facts and principles are used to make predictions about the natural world. In short, evolutionary theory is both (1) a body of knowledge supported by facts and (2) the resulting understanding of the various processes by which biological populations have changed and diversified over time, and by which Earth’s populations continue to evolve.

We can observe and measure evolution directly, and many biologists conduct experiments on evolving populations. We constantly observe changes in the genetic makeup of populations over relatively short time periods. For example, every year health agencies need to produce new flu vaccines, because populations of influenza viruses evolve so quickly that last year’s vaccines may not be effective against this year’s populations of viruses. In addition, we can directly observe a record of the history of evolution in the fossil record over the almost unimaginably long periods of geological time. Exactly how biological populations change through time is something that is subject to testing and experimentation. The fact that biological populations evolve, however, is not disputed among biologists.