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Chapter 26

BIO Chapter 26

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McMaster University
Jon Stone

Chapter 26: Speciation If mutation, natural selection and genetic drift cause isolated population to diverge sufficiently, distinct types of species, form. This process is called speciation.  Speciation is the evolutionary process in which a new biological species arise. o Once speciation is complete, a new branch is added to the tree of life. o Begins when gene flow between populations is reduced or eliminated.  Speciation results from genetic isolation and genetic divergence.  Genetic isolation results from lack of gene flow.  Divergence occurs because selection, genetic drift and mutation process independently in the isolated population. 26.1: How Are Species Defined and Identified?  Species are distinct types of organisms and represent evolutionarily independent population or group of populations.  Species are distinct from one another in terms of: o Behaviour o Appearance o Habitat use o Other traits  Species differ in genetic characteristics. Genetic distinction is due to mutation, natural selection and genetic drift.  What makes one species “evolutionarily independent” of other species? o Answer: Lack of gene flow  If gene flow between populations stops, then mutation, natural selection, and drift begins to act on the populations independently.  If a new mutation creates an allele that changes the phenotype of individuals in one population, there is no longer a way for that allele to appear in the other population.  When allele frequencies change sufficiently over time, populations become distinct species.  3 criteria in identifying species are: 1. Biological species concept 2. The morphospecies concept 3. The phylogenetic species concept The Biological Species Concept (pg. 552)  According to biological species concept, to identify a species, you use the reproductive isolation. o No gene flow occurs in a population with reproductive isolation.  If two different populations do not interbreed or produce a viable offspring then they are considered distinct species.  To organize specific populations, biologist distinguish: REFER TO FIGURE 26.1 1. Prezyogtic means before-zygote 2. Isolation which prevents individuals of different species from mating 3. Postzygotic means after-zygote 1 Chapter 6: Speciation Prezygotic Isolation Type Description Example Populations are isolated Bishop Pines and Monterey Temporal because they breed at pines release their pollen at different times. different times of the year. Populations are isolated Parasites that begin to Habitat because they breed in exploit new host species different habitats. are isolated from their original population. Populations do not To attract male fireflies, Behavioural interbreed because their female fireflies gives a wooing ability differs from species-specific sequence other species. of flashes Mating fail because eggs Closely related population and sperm are unsuited for cannot mate with each Gametic barrier each other. other. Only population of the same kind can mate. Ex. A goldfish can’t mate with a shark fish Mating fails because male Species of the same kind; if and female genitalia are a change in either Mechanical unsuited for each other. reproductive organ can influence reproductive isolation between them. Postzygotic Isolation Type Description Example Hybrid viability Hybrid offspring do not When ring-necked doves develop normally and die as mate with rock doves, less embryos. than 6% of eggs hatch. Hybrid sterility Hybrid offspring mature but Eastern meadowlarks and are sterile as adults. western meadowlarks are almost identical morphologically, but hybrid offspring are largely infertile. Chapter 6: Speciation 2 The Morphospecies Concept (pg. 552)  Morphos
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