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Unformatted text preview: _____________________________________________________________________________________ Lectures 24-26 and Discussion 7: SPECIES & SPECIATION _____________________________________________________________________________________ I. DEFINITIONS OF SPECIES 1. The Biological Species Concept : A species consists of groups of actually, or potentially, interbreeding natural populations of organisms that are reproductively isolated from other such groups . a) Breeding units: 1) Populations/Gene Pools 2) Populations vs . Species b) In other words species are breeding units that are evolutionarily independent from other breeding units . c) In still other words, SPECIES ARE FUNDAMENTAL EVOLUTIONARY UNITS. 1) Individuals do NOT evolve (their genetic composition barring mutation doesnt change). 2) Populations and species DO evolve. 3) Species represent separate evolutionary units that can no longer exchange genes, hence evolve independently of each other. d) Problems with the Biological Species Concept 1) Actual vs. potential mating opportunities in allopatric (geographically separated) populations. See issues raised in next point below. 2) Whats natural? Mating in the lab vs . in nature. Just because they mate in the lab doesnt necessarily mean they will mate in the wild, if theres behavioral isolation. So scientists need to take into account not just if they mate when given no other choices , but also if they have a preference for mating with their own type (i.e. behavioral isolation). If they have a preference, is it absolute, so no interbreeding would occur in sympatry? If not, then how much interbreeding is too much to consider them true species? No easy answers here. 3) What about extinct organisms? 4) What about asexual organisms? II. TYPES OF GENETIC BARRIERS OR REPRODUCTIVE ISOLATING MECHANISMS The evolution of a new species is equivalent to the evolution of genetic barriers to gene flow between populations. (Doug Futuyma) 1. Reproductive isolation is the critical component of what makes a species a species, and thus it is a critical component to the process of speciation, the formation of 2 new daughter species from one original parental species. Speciation is typically gradual, so it can often be difficult to determine exactly when speciation has occurred. That leads to a lot of gray areas...situations where it is difficult to say whether a population is a new species or not (sometimes in practice, we call these populations subspecies to acknowledge that they are differentiated, but not yet enough to be a good species; or we call them incipient species to indicate that its an ongoing process that isnt quite finished)....
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- Spring '10