4: Natural Selection and Evolution
- Page ID
- 103324
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Chapter Summary
All species of living organisms—from the bacteria on our skin, to the trees in our yards, to the birds outside—evolved at some point from a different species. Although it may seem that living things today stay much the same from generation to generation, that is not the case: evolution is ongoing. Evolution is the process through which the characteristics of species change and through which new species arise.
The theory of evolution is the unifying theory of biology, meaning it is the framework within which biologists ask questions about the living world. Its power is that it provides direction for predictions about living things that are borne out in experiment after experiment. The Ukrainian-born American geneticist Theodosius Dobzhansky famously wrote that “nothing makes sense in biology except in the light of evolution.” (Dobzhnsky, 1964).He meant that the principle that all life has evolved and diversified from a common ancestor is the foundation from which we understand all other questions in biology. This chapter will explain some of the mechanisms for evolutionary change and the kinds of questions that biologists can and have answered using evolutionary theory.
References
Theodosius Dobzhansky. “Biology, Molecular and Organismic.” American Zoologist 4, no. 4 (1964): 449.
After reading this chapter you should be able to:
- Describe the observations and evidence that lead Darwin and Wallace to form the theory of Natural Selection.
- Outline the process of Natural Selection emphasizing the importance of variation and heritability.
- Explain why natural selection depends on the environment.
- Diagram and describe the 3 different types of natural selection (directional, stabilizing, and disruptive).
- Provide examples of convergent and divergent evolution.
- Define species and describe how scientists identify species as different.
- Describe genetic variables that lead to speciation.
- Identify prezygotic and postzygotic reproductive barriers.
- Explain allopatric and sympatric speciation.
- Describe adaptive radiation.
- Explain sources of evidence for evolution
- Identify common misconceptions about evolution
- 4.1: Darwin, Wallace, and the Theory of Natural Selection
- When Charles Darwin embarked on his around the world voyage on the HMS Beagle, he was 22 years old. As the ship's naturalist, he was responsible for exploring and collecting biological and geological specimens wherever they went ashore. This allowed him to compare global patterns in living and fossil organisms, leading eventually to the development of the theory of evolution through natural selection. Although he published first, others, including Alfred Wallace, were developing similar ideas.
- 4.2: Evolution
- Evolution describes the cumulative inherited change in a population of organisms through time leading to change in a lineage and/or the appearance of new forms or species. Small-scale changes (microevolution) drive huge events such as speciation (macroevolution). Microevolution is the change in allele frequencies that occurs over time within a population and may or may not be adaptive. This change is due to four different processes: natural selection, mutation, genetic drift, gene flow, and sexu
- 4.3: Macroevolution and Speciation
- Speciation is an event that splits one one ancestral species into two or more descendant species. A number of mechanisms for speciation have been proposed and studied, and all of them include reproductive isolation. Isolating mechanisms are those that prevent individuals from the two new populations from interbreed, or reduce the fitness of those who do. Biologists have broadly classified speciation into two categories; allopatric describes a speciation event driven by geographical separation of
- 4.4: Evidence of Evolution
- The evidence for evolution is found at all levels of organization in living things and in the extinct species we know about through fossils. Fossils provide evidence for the evolutionary change through now extinct forms that led to modern species. For example, there is a rich fossil record that shows the evolutionary transitions from horse ancestors to modern horses that document intermediate forms and a gradual adaptation t changing ecosystems.
- 4.5: Common Misconceptions about Evolution
- Although the theory of evolution initially generated some controversy, by 20 years after the publication of On the Origin of Species it was almost universally accepted by biologists, particularly younger biologists. Nevertheless, the theory of evolution is a difficult concept and misconceptions about how it works abound. In addition, there are those that reject it as an explanation for the diversity of life.