Cultivated American cotton plants have a total of 52 chromosomes (2n = 52). In each

The origin of species is the source of biological diversity. Speciation, the process by which one species splits into two or more species, accounts for both the unity and diversity of life.

There are several ways to define a species. The biological species concept holds that a species is a group of populations whose members can interbreed and produce fertile offspring with each other but not with members of other species. This concept emphasizes reproductive isolation. Most organisms are classified based on observable traitsthe morphological species concept.

Reproductive barriers keep species separate. Such barriers isolate a species gene pool and prevent interbreeding.

In allopatric speciation, geographic isolation leads to speciation. Geographically separated from other populations, a small population may become genetically unique as its gene pool is changed by natural selection, mutation, or genetic drift.

Reproductive barriers can evolve as populations diverge. Researchers have documented the beginning of reproductive isolation in fruit fly populations adapting to different food sources and have identified a gene for flower color involved in the pollinator choice that helps separate monkey flower species.

Sympatric speciation takes place without geographic isolation. Many plant species have evolved by polyploidy, duplication of the chromosome number due to errors in cell division. Habitat differentiation and sexual selection, usually involving mate choice, can lead to sympatric (and allopatric) speciation.

Most plant species trace their origin to polyploid speciation. Many plants, including food plants such as bread wheat, are the result of hybridization and polyploidy.

Isolated islands are often showcases of speciation. Repeated isolation, speciation, and recolonization events on isolated island chains have led to adaptive radiations of species, many of which are found nowhere else in the world.

A long-term field study documents evolution in Darwins finches. Peter and Rosemary Grant have documented changes in beak size resulting from natural selection due to drought and competition, as well as interbreeding between some Galpagos finch species.

Hybrid zones provide opportunities to study reproductive isolation. Hybrid zones are regions in which populations of different species overlap and produce at least some hybrid offspring. Prezygotic barriers Habitat isolation Temporal isolation Behavioral isolation Mechanical isolation Gametic isolation Postzygotic barriers Reduced hybrid viability Reduced hybrid fertility Hybrid breakdown Gametes Zygote Viable, fertile offspring Over time, reinforcement may strengthen barriers to reproduction, or fusion may reverse the speciation process as gene flow between species increases. In stable hybrid zones, a limited number of hybrid offspring continue to be produced.

Speciation can occur rapidly or slowly. The punctuated equilibria model draws on the fossil record, where species change most as they arise from an ancestral species and then change relatively little for the rest of their existence. Other species appear to have evolved more gradually. The time interval between speciation events varies from a few thousand years to tens of millions of years.

Name the two types of speciation represented by this diagram. For each type, describe how reproductive barriers may develop between the new species.

Fill in the blanks in the following concept map.

Which concept of species would be most useful to a field biologist identifying new plant species in a tropical forest? a. biological b. ecological c. morphological d. phylogenetic e. a and b combined

The largest unit within which gene flow can readily occur is a a. population. d. phylum. b. species. e. hybrid offspring. c. genus.

Bird guides once listed the myrtle warbler and Audubons warbler as distinct species that lived side by side in parts of their ranges. However, recent books show them as eastern and western forms of a single species, the yellow-rumped warbler. Most likely, it has been found that these two kinds of warblers a. live in similar habitats and eat similar foods. b. interbreed often in nature, and the offspring are viable and fertile. c. are almost identical in appearance. d. have many genes in common. e. sing similar songs.

Which of the following is an example of a postzygotic reproductive barrier? a. One Ceanothus shrub lives on acid soil, another on alkaline soil. b. Mallard and pintail ducks mate at different times of year. c. Two species of leopard frogs have different mating calls. d. Hybrid offspring of two species of jimsonweeds always die before reproducing. e. Pollen of one species of tobacco cannot fertilize a different species of tobacco.

Biologists have found more than 500 species of fruit flies on the various Hawaiian Islands, all apparently descended from a single ancestor species. This example illustrates a. polyploidy. d. sympatric speciation. b. temporal isolation. e. postzygotic barriers. c. adaptive radiation.

A new plant species C, which formed from hybridization of species A (2n = 16) with species B (2n = 12), would probably produce gametes with a chromosome number of a. 12. d. 28. b. 14. e. 56. c. 1

A horse (2n = 64) and a donkey (2n = 62) can mate and produce a mule. How many chromosomes would there be in a mules body cells? a. 31 d. 126 b. 62 e. 252 c. 63 1

What prevents horses and donkeys from hybridizing to form a new species? a. limited hybrid fertility b. limited hybrid viability c. hybrid breakdown d. gametic isolation e. prezygotic barrier 1

When hybrids produced in a hybrid zone can breed with each other and with both parent species, and they survive and reproduce as well as members of the parent species, one would predict that a. the hybrid zone would be stable. b. sympatric speciation would occur. c. reinforcement of reproductive barriers would keep the parent species separate. d. reproductive barriers would lessen and the two parent species would fuse. e. a new hybrid species would form through allopatric speciation. 1

Which of the following factors would not contribute to allopatric speciation? a. A population becomes geographically isolated from the parent population. b. The separated population is small, and genetic drift occurs. c. The isolated population is exposed to different selection pressures than the parent population. d. Different mutations begin to distinguish the gene pools of the separated populations. e. Gene flow between the two populations continues to occur. 1

Explain how each of the following makes it difficult to clearly define a species: variation within a species, geographically isolated populations, asexual species, fossil organisms. 1

Explain why allopatric speciation would be less likely on an island close to a mainland than on a more isolated island. 1

What does the term punctuated equilibria describe? 1

Can factors that cause sympatric speciation also cause allopatric speciation? Explain. 1

Cultivated American cotton plants have a total of 52 chromosomes (2n = 52). In each cell, there are 13 pairs of large chromosomes and 13 pairs of smaller chromosomes. Old World cotton plants have 26 chromosomes (2n = 26), all large. Wild American cotton plants have 26 chromosomes, all small. Propose a testable hypothesis to explain how cultivated American cotton probably originated. 1

Explain how the murky waters of Lake Victoria may be contributing to the decline in cichlid species. How might these polluted waters affect the formation of new species? How might the loss of cichlid species be slowed? 1

The red wolf, Canis rufus, which was once widespread in the southeastern and southcentral United States, was declared extinct in the wild by 1980. Saved by a captive breeding program, the red wolf has been reintroduced in areas of eastern North Carolina. The current wild population estimate is about 100 individuals. It is presently being threatened with extinction due to hybridization with coyotes, Canis latrans, which have become more numerous in the area. Red wolves and coyotes differ in terms of morphology, DNA, and behavior, although these differences may disappear if interbreeding continues. Although designated as an endangered species under the Endangered Species Act, some people think that the endangered status of red wolves should be withdrawn and resources should not be spent to protect what is not a pure species. Do you agree? Why or why not?