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Evolution Individuals of different species cannot interbreed under natural conditions Described as being reproductively isolated from one another Populations of different species do NOT exchange genetic information They have different gene pools Two types of isolating mechanisms Pre-zygotic ▪ Impede mating between species or prevent fertilization of eggs if individuals from different species try to mate Post-zygotic ▪ Occurs after fertilization occurs ▪ Mechanisms that prevent hybrid zygotes from developing into viable, fertile individuals 1. Behavioural Isolating Mechanisms Different species use different mating signals Prevents wasted effort with a partner that will not produce fertile offspring with you 2. Ecological Isolation Two species may live in the same general region but in different habitats Eg. Frogs – some breed in fast moving streams while other frogs breed in ponds 3. Temporal Isolating Mechanisms Flowering or mating occurs at different times 4. Mechanical Isolating Mechanisms Structural differences in reproductive organs prevent copulation Genital anatomy can be used to classify species based on morphology 5. Gamete Isolating Mechanisms Failure of gametes from different species to fuse Prevents fertilization at the molecular level Ex. In plants, pollen grains of one species fail to germinate on the stigma of another species 1. Zygote Mortality Fertilized egg (zygote) fails to divide 2. Hybrid Inviability Genetic incompatibility of the interbred species stops development of the zygote 3. Hybrid Sterility (Infertility) Offspring lives but gametes cannot undergo meiosis (i.e. mule) The process of species requires populations of organisms to become, and largely to remain, genetically isolated from one another Two types of Speciation – based on how gene flow is disrupted within a population Sympatric Speciation Allopatric Speciation Varieties of the same species living in the same area at the same time Each variation adapts to reduce competition (as a result of disruptive selection) When a population is split into two or more isolated groups by a geographic barrier Split populations become so distinct that the two groups are unable to interbreed even if they are brought back together Situations where population size is reduced dramatically is a called Genetic Bottleneck Results in a loss of genetic diversity Small populations that become isolated from the original population are more likely to change enough to become new species This is known as the founder effect