L#42%2004-23-10%20ch24%20Origin%20of%20species - 1Biology...

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1 Biology 1107 Lecture #42 Friday April 23, 2010 (04-23-10) Lecture based on CHAPTER 24 THE ORIGIN OF SPECIES A. What Is a Species? In 1942 Ernst Mayr enunciated the biological species concept to address biological diversity. A species is a population or group of populations whose members have the potential to interbreed with each other in nature to produce viable, fertile offspring, but who cannot produce viable, fertile offspring with members of other species. A biological species is the largest set of populations in which genetic exchange is possible and that is genetically isolated from other populations. For example, the eastern and western meadowlarks may have similar shapes and coloration, but differences in song help prevent interbreeding between the two species. In contrast, humans have considerable diversity, but we all belong to the same species because of our capacity to interbreed. 1. The biological species concept emphasizes reproductive isolation. 2. Prezygotic and postzygotic barriers isolate the gene pools of biological species Reproductive barriers can be categorized as prezygotic or postzygotic, depending on whether they function before or after the formation of zygotes. Prezygotic barriers impede mating between species or hinder fertilization of ova if members of different species attempt to mate. These barriers include habitat isolation, behavioral isolation, temporal isolation, mechanical isolation, and gametic isolation. Habitat isolation . Two organisms that use different habitats even in the same geographic area are unlikely to encounter each other to even attempt mating. This is exemplified by the two species of garter snakes, in the genus Thamnophis , that occur in the same areas but because one lives mainly in water and the other is primarily terrestrial, they rarely encounter each other. Behavioral isolation . Many species use elaborate behaviors unique to a species to attract mates. For example, female fireflies only flash back and attract males who first signaled to them with a species- specific rhythm of light signals. In many species, elaborate courtship displays identify potential mates of the correct species and synchronize gonadal maturation. Temporal isolation . Two species that breed during different times of day, different seasons, or different years cannot mix gametes. For example, while the geographic ranges of the western spotted skunk and the eastern spotted skunk overlap, they do not interbreed because the former mates in late summer and the latter in late winter. Mechanical isolation. Closely related species may attempt to mate but fail because they are anatomically incompatible and transfer of sperm is not possible. To illustrate, mechanical barriers contribute to the reproductive isolation of flowering plants that are
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This note was uploaded on 10/12/2010 for the course BIO 3453 taught by Professor Dervartarian during the Spring '09 term at UGA.

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L#42%2004-23-10%20ch24%20Origin%20of%20species - 1Biology...

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