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Speciation Professor Andrea Garrison Biology 11 Illustrations ©2010 Pearson Education, Inc. , unless otherwise noted Speciation • Natural selection not disputed – Natural selection = microevolution (small genetic changes) • Special cases of selection due to chance changes – Genetic drift • Founder Effect – subgroup leaves population and stays together • Bottleneck– population decimated by disaster Speciation 2 Genetic Drift • Chance loss of alleles – Found in small populations Speciation 3 Genetic Drift • Founder Effect – Establishment of small population whose gene pool differs from parent population Speciation 4 Genetic Drift • Bottleneck Effect – Natural disasters decrease population to extent some alleles underrepresented by chance – Difficult to maintain healthy population Speciation 5 Speciation • Natural selection drives evolution by favoring genetic variations that enable organisms to survive and reproduce • Works on level of gene pool – Gene pool = all alleles in population – Affects entire population • Agents of change – Mutation (= chance changes in DNA) – Sexual reproduction (speeds up opportunities for selection of alleles) Speciation 6 Speciation • What is a species? Speciation 7 Speciation • What is a species? – Group of similar organisms which can interbreed and produce fertile offspring Speciation 8 Speciation • What is a species? – Group of similar organisms which can interbreed and produce fertile offspring • Thus, different species are organisms that differ in one or more characteristics and do not interbreed extensively in nature, even if they occur together Speciation 9 Speciation • Each species has its own niche • What is a niche? Speciation 10 Speciation • Each species has its own niche • What is a niche? – The resources used by a species and the manner in which those resources are used – Encompasses all aspects of a species’ environment Speciation 11 Speciation • Each species has its own niche • What is a niche? – The resources used by a species and the manner in which those resources are used – Encompasses all aspects of a species’ environment • Each species is biologically suited (physically, metabolically, personality-wise) to occupation of its own niche Speciation 12 Speciation • How does one species evolve from another? A → A1 non-branching evolution A1 A divergence (branching evolution) A2 Speciation 13 Speciation • Key to speciation is presence or absence of reproduction – Local subgroups may have chance mutations – Not new species until so different they cannot interbreed Speciation 14 Speciation • Population = all members of a species within given geographic area (often taken to mean individuals interbreeding with one another) • Non-branching evolution A → A1 – changes over time, A1 cannot interbreed with A Speciation 15 Speciation • Population = all members of a species within given geographic area (often taken to mean individuals interbreeding with one another) • Divergence – A1 and A2 must be kept from interbreeding until they are so different they cannot interbreed A1 A A2 Speciation 16 Speciation • Divergence occurs because local populations breed only with their neighbors, and not those members of the same species on “the other side of the hill” – If members of two subgroups interbreed, no divergence • They share any chance mutations – If members of two subgroups do not interbreed, divergence can occur • No sharing of chance mutations – Chance mutations are different – Two different environments select differently for different mutations Speciation 17 Speciation • Even some organisms recognized as separate species today are actually capable of interbreeding, and would converge back to one species if the opportunity existed • REPRODUCTIVE ISOLATING MECHANISMS – Prevent reproduction between two populations – Maintain separate species Speciation 18 Speciation 19 Reproductive Isolating Mechanisms • Prezygotic mechanisms – Geographical isolation • Geographical ranges do not coincide • Species never encounter each other – Example: English Oak in Europe and Valley Oak in California • Never close enough to interbreed, although they could Speciation 20 Reproductive Isolating Mechanisms • Prezygotic mechanisms – Ecological (habitat) isolation • Two species live in same area, but different habitats Speciation 21 Reproductive Isolating Mechanisms • Prezygotic mechanisms – Temporal isolation • Closely related species mate at different times of the year Speciation 22 Reproductive Isolating Mechanisms • Prezygotic mechanisms – Behavioral isolation • Closely related species with different mating rituals Speciation 23 Reproductive Isolating Mechanisms • Prezygotic mechanisms – Mechanical isolation • Structural differences between species (copulatory organs, pollen grains, etc. Speciation 24 Reproductive Isolating Mechanisms • Prezygotic mechanisms – Gametic isolation • Chemical differences between gametes (e.g., proteins on surface of gametes) Speciation 25 Speciation 26 Reproductive Isolating Mechanisms • Postzygotic mechanisms – Reduced hybrid viability • Hybrid is weak or dies Speciation 27 Reproductive Isolating Mechanisms • Postzygotic mechanisms – Reduced hybrid fertility • Hybrid is sterile Speciation 28 Reproductive Isolating Mechanisms • Postzygotic mechanisms – Hybrid breakdown • Hybrid reproduction produces weak offspring Speciation 29