Speciation and Extinction
16.1 What Is a Species?
1. Linnaeus's species designations helped scientists communicate. Darwin added evolutionary meaning and Mayr added the requirement for reproductive isolation to define biological species.
2. Diagrams of branching lines represent evolutionary relationships among species. Such evolutionary trees are called phylogenies.Speciation is the formation of a new species.
3. Comparing sequences of mtDNA is increasingly being used to clarify and supplement species relationships based on visible characteristics.
16.2 How Do Reproductive Differences That Distinguish Species Arise?
4. Prezygotic reproductive isolation occurs before or during fertilization. It includes obstacles to mating in space, time, and behavior; molecular mismatches between gametes; and incompatible chromosomes.
5. Postzygotic reproductive isolation results in weak or infertile offspring, or may act more gradually by disrupting beneficial gene linkages.
16.3 Speciation and Space
6. Allopatric speciation occurs when a geographic barrier separates populations, which then diverge genetically to the point that members of the two populations can no longer produce fertile offspring together.
7. Parapatric speciation occurs when two populations live in neighboring areas but share a border zone. Genetic divergence between the two groups exceeds gene flow, driving speciation.
8. Sympatric speciation enables populations that occupy the same area to diverge, via drastic genetic change such as polyploidy or the use of different resources.
16.4 Speciation and Time
9. Evolutionary change occurs at many rates, from slow and steady gradualism, to the "leaps and starts" of punctuated equilibrium.
10. In adaptive radiation, an ancestral species rapidly branches into several new species, reflecting sudden availability of new habitats or resources or a particularly beneficial adaptation. A group of species related by recent common descent is a clade.
16.5 How Do Species Become Extinct?
11. Extinction is the disappearance of species.
12. Decreased genetic diversity may lead to extinction of populations and possibly species.
13. Background extinctions are ongoing. Mass extinctions result from global changes.