CONCEPTS IN BIOLOGY
PART IV. EVOLUTION AND ECOLOGY
13. Evolution and Natural Selection
Study Reveals Information on Human Diversity and Evolution
To understand the population genetics of any human population, it is necessary to understand Africa first.
Africa is the birthplace of modern humans and, as a result, Africans have had more time to accumulate changes in their DNA. According to one researcher,... genetically, Africans have been the most neglected and under-represented of any continental group, because the most diverse groups are often remote ... and don’t usually get to clinics.”
That is until recently. An international team has analyzed nuclear DNA collected over a decade from 113 populations of Africans from across the continent. The team has found that Africans are descended from 14 ancestral populations, which often correlate with language and cultural groups. They found that all hunter-gatherers and pygmies in Africa today shared ancestors 35,000 years ago and that East Africa was the source of the great migration that populated the rest of the world. They also learned that African-American individuals, on average, have mixed ancestry from all over western Africa. This makes it difficult for African Americans to trace their roots to specific ethnic groups in Africa. The data also support research indicating that the source population for the out-of-Africa migration of modern humans came from east Africa near the Red Sea. These data give us raw material for understanding human evolution that has not been available until now.
• How does separating groups of the same species into different geographic areas affect their diversity?
• What is the ultimate source of genetic variation among different populations?
• What environmental factors might have played roles in generating the genetic differences identified by this research?
ü Background Check
Concepts you should already know to get the most out of this chapter:
• Traits that make something alive (chapter 1)
• How an allele is involved in protein synthesis (chapter 8)
• The reasons why genetically different populations exist (chapter 12)
• How genetic diversity comes about (chapter 12)
13.1. The Scientific Concept of Evolution
People use the term evolution in many ways. We talk about the evolution of economies, fashion, and musical tastes. From a biological perspective, the word has a more specific meaning. Evolution is a change in the frequency of genetically determined characteristics within a population over time. Evolution can be looked at from two points of view. Microevolution occurs when there are minor differences in allele frequency between populations of the same species, as when scientists examine genetic differences between subspecies. Macroevolution occurs when there are major differences that have occurred over long periods that have resulted in so much genetic change that new kinds of species are produced (figure 13.1).
FIGURE 13.1. Microevolution and Macroevolution
(a) Microevolution occurs when gene frequencies change within the gene pool of a species. Different populations of peppered moths show different gene frequencies based on the color of the bark of the trees they rest on. Black moths are conspicuous on light-colored trunks and light moths are conspicuous on dark colored trunks. Predation by birds removes more of the conspicuous moths and leads to the different gene frequencies in the two populations. These are relatively minor changes, compared with macroevolution changes, which result in new species from common ancestors. (b) The macroevolutionary pattern shown here may require tens of millions of years to occur and results in the formation of organisms that are so different that they are unable to interbreed.
Regardless of the perspective, the ways these differences are brought about are basically the same. The focus of this chapter is on the processes that result in microevolutionary change. Chapter 14 focuses on processes that lead to macroevolutionary change—that is, the development of new species. Evolution, from both perspectives, involves changes in characteristics and the genetic information that produces these characteristics over many generations (Outlooks 13.1).
Common Misconceptions About the Theory of Evolution
1. Evolution happened only in the past and is not occurring today. In fact, there is much evidence that changes in the frequency of alleles are occurring in the populations of current species (e.g., antibiotic resistance, pesticide resistance).
2. Evolution has a predetermined goal, or "it was meant to be." Natural selection selects the organisms that best fit the current environment. As the environment changes, so do the characteristics that have value. Random events, such as changes in sea level, major changes in climate, volcanic eruptions, earthquakes, and collisions with asteroids, have had major influences on the subsequent natural selection and evolution.
3. Changes in the environment cause the mutations that are needed to survive under the new environmental conditions. Mutations are random events and are not necessarily adaptive. However, mutations that were originally detrimental or neutral may have greater value after the environment changes. The genetic information does not change, but the environmental conditions do. In some cases, the mutation rate may increase, or there may be more frequent genetic exchanges between individuals when the environment changes, but the mutations are still random. They are not directed toward a particular goal.
4. Individual organisms evolve. Individuals are stuck with the genes they have inherited from their parents. Although individuals may adapt by changing their behavior or physiology, they cannot evolve; only populations can change gene frequencies.
5. Many of the current species can be shown to be derived from other present-day species (e.g., apes gave rise to humans). There are few examples in which it can be demonstrated that one current species gave rise to another. Apes did not become humans, but apes and humans had a common ancestor several million years ago.
6. Alleles that are valuable to an organism's survival become dominant. An allele that is valuable may be either dominant or recessive. However, if it has a high value for survival, it will become common (more frequent). Commonness has nothing to do with dominance and recessiveness.
13.1. CONCEPT REVIEW
1. Describe the biological meaning of the word evolution.