The Origin of Species

(a) Which species concept(s) could you apply to both asexual and sexual species? (b) Which would be most useful for identifying species in the field? Explain.

Suppose two bird species live in a forest and are not known to interbreed. One species feeds and mates in the treetops and the other on the ground. But in captivity, the birds can interbreed and produce viable, fertile offspring. What type of reproductive barrier most likely keeps these species separate in nature? Explain.

Interpret your graph by (a) explaining in words any pattern indicating a possible relationship between the variables and (b) hypothesizing the possible cause of such a relationship.

State the researchers’ hypothesis, and identify the independent and dependent variables in this study. Explain why the researchers used four mating combinations for each pair of populations. 

Calculate the value of the reproductive isolation index if (a) all of the matings within a population were successful, but none of the matings between populations was successful; (b) salamanders are equally successful in mating with members of their own population and members of another population.

Make a scatter plot to help you visualize any patterns that might indicate a relationship between the variables. Plot the independent variable on the x-axis and the dependent variable on the y-axis. (For additional information about graphs, see the Scientific Skills Review in Appendix F and the Study Area of MasteringBiology.) 

Summarize key differences between allopatric and sympatric speciation. Which type of speciation is more common, and why?

Describe two mechanisms that can decrease gene flow in sympatric populations, thereby making sympatric speciation more likely to occur.

Is allopatric speciation more likely to occur on an island close to the mainland or on a more isolated island of the same size? Explain your prediction.

Review the process of meiosis in Figure 13.8. Describe how an error during meiosis could lead to polyploidy.

What are hybrid zones, and why can they be viewed as “natural laboratories” in which to study speciation?

Consider two species that diverged while geographically separated but resumed contact before reproductive isolation was complete. Predict the outcome over time if the two species mated indiscriminately. 

(a) hybrid offspring survived and reproduced more poorly than offspring from intraspecific matings or 

(b) hybrid offspring survived and reproduced as well as offspring from intraspecific matings.

Speciation can occur rapidly between diverging populations, yet the time between speciation events is often more than a million years. Explain this apparent contradiction.

Summarize evidence that the yup locus acts as a prezygotic barrier to reproduction in two species of monkeyflowers. Do these results demonstrate that the yup locus alone controls barriers to reproduction between these species? Explain

Compare Figure 13.12 with Figure 24.18. What cellular process could cause the hybrid chromosomes in Figure 24.18 to contain DNA from both parent species? Explain.

The largest unit within which gene flow can readily occur is a 

(A) population.

(B) species.

(C) genus.

(D) hybrid.

Males of different species of the fruit fly Drosophila that live in the same parts of the Hawaiian Islands have different elaborate courtship rituals. These rituals involve fighting other males and making stylized movements that attract females. What type of reproductive isolation does this represent? 

(A) habitat isolation 

(B) temporal isolation 

(C) behavioral isolation 

(D) gametic isolation.

According to the punctuated equilibria model, 

(A) given enough time, most existing species will branch gradually into new species. 

(B) most new species accumulate their unique features relatively rapidly as they come into existence, then change little for the rest of their duration as a species. 

(C) most evolution occurs in sympatric populations. 

(D) speciation is usually due to a single mutation.

Bird guides once listed the myrtle warbler and Audubon's warbler as distinct species. Recently, these birds have been classified as eastern and western forms of a single species, the yellow-rumped warbler. Which of the following pieces of evidence, if true, would be cause for this reclassification? 

(A) The two forms interbreed often in nature, and their offspring survive and reproduce well. 

(B) The two forms live in similar habitats and have similar food requirements. 

(C) The two forms have many genes in common. 

(D) The two forms are very similar in appearance.

Which of the following factors would not contribute to allopatric speciation? 

(A) The separated population is small, and genetic drift occurs. 

(B) The isolated population is exposed to different selection pressures than the ancestral population. 

(C) Different mutations begin to distinguish the gene pools of the separated populations. 

(D) Gene flow between the two populations is extensive.

Plant species A has a diploid chromosome number of 12. Plant species B has a diploid number of 16. A new species, C, arises as an allopolyploid from A and B. The diploid number for species C would probably be 

(A) 14.

(B) 16. 

(C) 28. 

(D) 56.

Explain the biological basis for assigning all human populations to a single species. Can you think of a scenario by which a second human species could originate in the future?

In this chapter, you read that bread wheat (Triticum aestivum) is an allohexaploid, containing two sets of chromosomes from each of three different parent species. Genetic analysis suggests that the three species pictured following this question each contributed chromosome sets to T. aestivum. (The capital letters here represent sets of chromosomes rather than individual genes, and the diploid chromosome number for each species is shown in parentheses.) Evidence also indicates that the first polyploidy event was a spontaneous hybridization of the early cultivated wheat species T. monococcum and a wild Triticum grass species. Based on this information, draw a diagram of one possible chain of events that could have produced the allohexaploid T. aestivum.

In sexually reproducing species, each individual inherits DNA from both parent organisms. In a short essay (100–150 words), apply this idea to what occurs when organisms of two species that have homologous chromosomes mate and produce (F1) hybrid offspring. What percentage of the DNA in the F1 hybrids’ chromosomes comes from each parent species? As the hybrids mate and produce F2 and later-generation hybrid offspring, describe how recombination and natural selection may affect whether the DNA in hybrid chromosomes is derived from one parent species or the other. 

Suppose that females of one population of strawberry poison dart frogs (Dendrobates pumilio) prefer to mate with males that are orange-red in color. In a different population, females prefer males with yellow skin. Explain how such differences could arise and how they could affect the evolution of reproductive isolation in allopatric versus sympatric populations.