Chapter 15: Mechanisms of SpeciationOverview |
Speciation is among the most fundamental events in the history of life. It has occurred millions, if not billions, of times since life originated some 3.8 billion years ago. In addition to its intrinsic importance, though, studying speciation has interesting practical applications.
Understanding what species are and how they form is central to efforts to preserve biodiversity. Refer to Chapter 15 to explore the field’s most fundamental question: What is a species?
15.1 Species Concepts All human cultures recognize different types of organisms in nature and name them. These taxonomic systems are based on judgments about the degrees of similarity among organisms. People intuitively group like with like. The challenge to biologists has been to move beyond these informal judgments to a definition of species that is mechanistic and testable and to a system for naming and classifying the diversity of life that accurately reflects the evolutionary history of organisms.
Turn to section 15.1 of your textbook to learn more about mechanistic systems of classifying species.
15.2 Mechanisms of Genetic Isolation Given that several tools are available for identifying species, we can turn our attention now to the question of how species form. Classically, speciation has been hypothesized to be a three-stage process: an initial step that isolates populations, a second step that results in traits such as mating system or habitat use to diverge, and a final step that produces reproductive isolation. According to this model, the isolation and divergence steps were thought to take place over time and to occur while populations were located in different geographic areas.
Refer to section 15.2 of your textbook to learn more about how diverged populations came back into physical contact in an event known as secondary contact.
15.3 Mechanisms of Divergence Polyploidization, dispersal, and vicariance only create the conditions for speciation. For the event to continue, genetic drift and natural selection have to act on mutations in a way that creates divergence in the isolated populations.
Turn to section 15.3 of your textbook and explore how drift and selection act on closely related populations once gene flow between them has been reduced or eliminated.
15.4 Secondary Contact If speciation begins with the geographic isolation of two populations and a corresponding reduction in gene flow, and then continues as selection, mutation, and drift cause genetic divergence between the two groups, an important question arises. What happens if the recently diverged populations come back into contact and have the opportunity to interbreed?
Turn to section 15.4 to learn the answer to this question.
15.5 The Genetics of Speciation What degree of genetic differentiation is required to isolate populations and produce new species? The traditional view was that some sort of radical reorganization of the genome, called a genetic revolution, was necessary (Mayr 1963). This hypothesis was inspired by a strict interpretation of the biological species concept.
Refer to section 15.5 to explore the logic behind this hypothesis and learn more about genetic incompatibility.