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Lecture 16

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Biology 1001A
Tom Haffie

Biology Lecture 16: Species and Speciation  microevolution: changes of allele frequencies within a population (genetic drift, natural selection, sexual selection, mutations, etc.)  macroevolution: concerned with a generation of new species  Morphological species concept (MSC): o Species are identified as organisms that look the same to one another  Problems with MSC: o When a lot of phenotypic variation occurs in a single population, it becomes problematic to classify species based on their phenotype  Biological species concept BSC: o Defines species as a group of actually or potentially interbreeding organisms, reproductively isolated from other such groups o Not based on how you look, based on who you have sex with o It has more explanatory power than the MSC; species are common because they share a common gene pool o It is also more observably testable than the MSC o If they would not breed under normal conditions, or if their offspring are fertile; they are not considered the same species (Example: lions + tigers = ligers)  Prezygotic Isolating Mechanisms (See Table 18.1): o For various reasons, individuals of two different species never reach the point where they from a zygote o Temporal isolation: differences in time of breeding o Habitat isolation: differences in habitat preferences o Behavioural isolation: certain traits of a male are not preferred by a female o Mechanical isolation: differences in body that does not allow reproduction  Postzygotic Isolating Mechanisms: o For various reasons, individuals of two different species can form a zygote, but the zygote has various issues o Hybrid inviability: zygotes are formed, but either never makes through embryonic development or never makes it to adulthood o Hybrid Sterility: offspring are viable, but not fertile (sterile) o Hybrid Breakdown: zygote makes it to adulthood and is somewhat sterile, thus it’s offspring have less fitness  Clicker Question: Which of the isolating mechanism is most costly in terms of wasted reproductive effort? A. Behavioural Isolation B. Temporal Isolation C. Hybrid Inviability D. Hybrid sterility E. None of these are  Postzygotic isolating mechanisms tend to be more costly due to the reproductive efforts put in  The sterile hybrid cannot create new generations, thus it is wasted reproductive effort to create hybrid offspring  Problems with BSC: o It can be difficult to know if two populations would interbreed, or if their offspring would be fertile if they do not live in the same area o Asexual species don't interbreed so it becomes impossible to apply the BSC o For organisms no longer living we cannot observe there mating behaviour o Ring Species: special case in which a population or a group of populations form a circular distribution of habitat  RIng species or two? o Adjacent populations that can exchange genetic material directly, but gene flow between distant populations occurs only through intermediary populations  Phylogenetic specie
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