This preview has intentionally blurred sections. Sign up to view the full version.View Full Document
Unformatted text preview: Chapter 20: Speciation and Macroevolution 209 20 Speciation and Macroevolution Lecture Outline I. There is more than one concept of species. A. Morphological species are based on structural differences. B. Biological species are based on reproductive isolation. 1. Problems a) It only applies to sexually reproducing organisms. b) Different species do sometimes interbreed. C. Evolutionary (phylogenetic) species II. Species are reproductively isolated in various ways. A. According to the biological species concept, a species can be defined as a group of organisms that are reproductively isolated from other groups of organisms. 1. Members of the same species can reproduce with each other and produce fertile offspring. 2. Reproductive isolating mechanisms prevent successful mating between members of two closely related, but different, species. a) Mechanisms may be prezygotic or postzygotic. B. Prezygotic barriers interfere with fertilization. 1. Prezygotic barriers act to prevent fertilization. 2. Temporal isolation is due to different mating times. 3. Habitat isolation occurs when organisms live and breed in different habitats in the same geographic area. 4. Behavioral, or sexual, isolation is based on differences in courtship behaviors or other reproductive signals. 5. Mechanical isolation is due to incompatibility of reproductive structures. 6. Plants may have different floral morphologies that prevent them from being cross ‐ pollinated by different insects. 7. Gametic isolation is incompatibility between eggs and sperm of different species. a) Sperm of aquatic species that are externally fertilized may not be able to bind to eggs of different species. b) Plant pollen may not germinate if it lands on the stigma of a different plant species. C. Postzygotic barriers prevent gene flow when fertilization occurs. 1. Often, the embryo of an interspecific hybrid dies in the embryonic stage and spontaneously aborts, known as hybrid inviability. 2. Hybrids may not be able to reproduce due to differences in courtship behaviors, or hybrid sterility may result because the chromosomes of the hybrid may be incompatible with meiosis. Instructor’s Manual for Solomon, Berg, and Martin’s Biology, 9 th Edition 210 3. On occasion, F 2 hybrids may exhibit hybrid breakdown, the inability to reproduce due to some genetic defect. D. The genetic basis of isolating mechanisms is being elucidated. 1. Some mechanisms are based on molecular differences, for example binding proteins between sperm and egg. III. Reproductive isolation is the key to speciation. A. Speciation is the formation of a new species. B. Long physical isolation and different selective pressures result in allopatric speciation....
View Full Document
This note was uploaded on 11/17/2011 for the course BIO 101 taught by Professor Martin during the Spring '08 term at Rutgers.
- Spring '08