Biology | Answers to Review Questions

Author Dr. George B. Johnson, Washington University
Author Dr. Peter H. Raven, Missouri Botanical Gardens & Washington University
Contributor Dr. Susan Singer, Carleton College
Contributor Dr. Jonathan Losos, Washington University

Answers to Review Questions

Chapter 22 (p. 476)

1. Sympatry is the occurrence of several species together in one place. Sympatric speciation is improbable because species tend not to hybridize in nature; those that do tend to produce infertile offspring. Furthermore, species sharing genes expand the species gene pool, which tends to bring them together, not separate them.

2. The biological species concept holds that a species is all individuals that can interbreed and produce fertile offspring.

3. Prezygotic mechanisms prevent zygote formation. Postzygotic mechanisms prevent zygotes that do form from functioning properly.

4. Geographical, ecological, temporal, behavioral, and mechanical isolations exist to the formation of hybrids. Prezygotic mechanisms prevent gamete fusion; postzygotic mechanisms involve embryo developmental failure or abnormalities. The term "isolating mechanism" is misleading because the traits were likely acquired for other reasons than reproductive isolation.

5. Selective forces that are strong produce rapid divergence. If forces are similar, there is slow divergence. Reducing dispersal can increase divergence.

6. A definitive number of genes involved in speciation cannot be given. However, the number can be very small.

7. Hybrids are at a fitness disadvantage when the hybrids are sterile, nearly sterile, or are less well adapted to survive in the environment. The disadvantage to the hybrid is that they often do not survive, and if they do, they do not pass their genes on to following generations.

8. Polyploidy is an amount of genetic material greater than 2N.

9. Adaptive radiation refers to groups of closely related species, which evolved from a common ancestor. An example would be Darwin's finches or Hawaiian Drosophila. Isolated discontinuous habitats (such as islands or lakes) encourage adaptive radiation, because they are subject to a unique combination of selective pressures in a small place, often leading to rapid divergence among small subpopulations, leading to the development of new species.

10. Gradualism contends that changes occur gradually over a long period of time. The concept of punctuated equilibrium suggests that changes occur in spurts punctuated by periods of little change.

11. Hybridization is found to be far more common than previously believed. While hybridization is not universal, it is too common to support the biological species concept.