The Origin of Species Chapter 24

Macroevolution vs. Microevolution When changes in allele frequency in a population occur over time Can be supported through direct observation Macroevolution A broad pattern of evolution over long time spans. Involves the formation of new species from old and how all of life evolved from a single celled organism Supported by the fossil record

Speciation The process by which one species splits into 2 or more new species Members of a species often resemble each other because their populations are connected by gene flow Biological species are defined in terms of reproductive compatibility, the formation of a new species must involve reproductive isolation (no gene flow) Relies on the existence of barriers to prevent 2 species from producing fertile offspring by preventing gene flow

Reproductive Isolation Barriers I Prezygotic Barriers Blocks fertilization from happening; occurs before the zygote Act by… Impeding members of different species from attempting to mate Preventing an attempted mating from being completed successfully Hindering fertilization if mating is completed Postzygotic Barriers Contribute to reproductive isolation after the hybrid zygote is formed Embryo may not survive due to developmental errors or may be infertile preventing gene flow

Reproductive Isolation Barriers II Prezygotic Barriers Habitat isolation Temporal isolation Behavioral isolation Mechanical isolation Gametic isolation Postzygotic Barriers Reduced hybrid viability Reduced hybrid fertility Hybrid breakdown

24.2 Allopatric vs. Sympatric Speciation When gene flow is interrupted because a population is divided into geographically isolated subpopulations Once separated, rely on mutations, natural selection, and genetic drift to change allele frequencies Sympatric When speciation occurs in populations that live in the same geographic area. Gene flow is reduced by factors such as polyploidy, habitat differentiation, and sexual selection Polyploidy occurs when there is an error during cell division such as a cell not dividing after it has replicated its chromosomes; 2n = 4n

Allopatric vs. Sympatric Speciation II

24.3 Hybrid Zones A region in which members of different species mate, producing at least some offspring of mixed ancestry

24.4 The Time Course of Speciation Speciation can occur rapidly or gradually The fossil record includes many episodes in which new species appear suddenly in geologic stratum, persist relatively unchanged through several strata, and then disappear Stephen Jay Gould and Niles Eldredge coined the term punctuated equilibria to describe this pattern Other species do not show a punctuated pattern; instead, they change more gradually over long periods of time

Studying the Genetics of Speciation By identifying the genes that control the traits that cause reproductive isolation, scientists can explore how many genes actually change when a new species forms Sometimes it is just the mutation of one gene Ex) the gene that controls the direction in which a snail shell spirals induces a mechanical barrier to reproduction