Chapter 24 1 Biological Species Concept Individuals can or could interbreed under natural circumstances a Population or group of populations b c Produce viable and fertile offspring d Are reproductively isolated from other such groups 2 Population group of individuals in the same species that live in a given area 3 Natural conditions not in a lab zoo would occur in nature 4 Potential no genetic reason why they could not mate 5 Anagenesis entire species undergoes change until transformed into recognizably different species a Cladogenesis single species diverges into two new species Occurs when populations become reproductively isolated from each other and thereafter evolve independently 6 In the BSC species are based on infertility rather than physical similarity a Easter and western meadowlarks are similar in morphology b Difference in courtship song separates them c Prevents interbreeding 7 Reproductive Isolating Mechanisms a Prevents 2 species from producing viable fertile hybrids b Not geographic barriers c Looking for intrinsic barriers to reproduction hardwired in the DNA barriers that prevent interbreeding even when the populations overlap d Mechanisms may be prezygotic or postzygotic before or after the zygote is formed 8 Prezygotic Mechanisms before zygote is formed prevents fertilization a Habitat isolation geographic isolation i Evolved to live in different habitats terrestrial aquatic grassland forest i Mate during different times day night or as in the spotted skunk species during Example water snake vs land snake b Temporal isolation different seasons c Behavioral Isolation i Courtship rituals ii Fire flies flashing patterns d Mechanical Isolation anatomical differences i Flowers pollinators snails with different way shells e Gametic Isolation i Gametes do not fuse ii Important in marine inverts that spawn gametes into the water recognitions of chemicals on membrane surface 9 Postzygotic Isolating Mechanisms after zygote is formed a Reduced hybrid viability death or weakness b Reduced hybrid fertility sterility c Hybrid breakdown i First generation is viable and fertile second and third is not d All of these may serve as selective pressures natural selection for the development of reproductive isolating mechanisms did not produce hybrids i Parents of a weak hybrid how high is their reproductive success vs parents that ii Example mules a hybrid between horses and donkeys iii Horses and donkeys are separated by a postzygotic isolating mechanism 10 Limitations of Biological Species Concept hybrid sterility a Difficult to classify fossils b Asexual organisms c Exceptions exist i Dogs and wolves produce fertile hybrids 11 Other species concepts a Other concepts have been developed that allow some exchange of genes i Morphological species concept each species has a unique set of the way we describe most species ii Ecological species concept based on niche a parasite may be defined in part by its adaptations to a specific organism iii Pluralistic species concept factors that determine a species as a discrete unit vary Reproductive isolation ecological niche iv Genealogical species concept unique lines of evolution a Speciation occurs after a reproductive isolating mechanism evolves i Allopatric speciation reproductive isolation evolves in non overlapping populations ii Sympatric speciation reproductive isolation evolves in overlapping populations 12 Speciation 13 Allopatric Speciation not separated geographically a Geographic separation of populations restricts gene flow b Isolating mechanism may evolve via i Natural selection ii Mutations iii Genetic drift c Barriers vary between species bird populations animals i A fence may limit dispersal of a small mammal would be unlikely to separate ii Mountain ranges gorges rivers glaciers may all serve as barriers for terrestrial iii Aquatic organisms may be separated by a land bridge d PHYSICAL BARRIER IS NOT A REPRODUCTIVE ISOLATING MECHANISM BUT IT MAY ALLOW ONE TO EVOLVE e Colonization of island habitats may lead to allopatric speciation i Founder effect genetic drift and natural selection ii May lead to adaptive radiation on an island chain Evolution of reproductive isolation in allopatric species f g Geographic barrier does not equal reproductive isolation h Intrinsic mechanism evolves by natural selection genetic drift mutation or a combination of all 14 Can adaptation to different environments lead to reproductive isolation a Flies separated into vial with different growth media b After several generations flies brought together to see if they would mate i Flies preferentially mated with their own population starch or maltose c Pleiotropy multiple effects to a single gene i Gene conferring advantage for carbohydrate metabolism may all affect mate 15 Sympatric speciation recognition 16 Polyploidy in plants a Speciation occurs in overlapping populations there is no physical barrier to gene flow b Reproductive isolation often evolves quickly a Oats cotton potatoes tobacco wheat b Accidents during cell division may produce a plant with multiple sets of chromosomes i Autopolyploid more than two chromosome sets derived from a single species ii Allopolyploid different species contribute to a polyploid hybrid 17 Sympatric speciation in animals a Cichlid fishes in Lake Victoria Africa i No geographic barriers ii Specialization on different food resources and non random mating sexual selection likely led to reproductive isolating mechanisms iii Prezygotic mechanism mate selection separates them 18 How many genes must change for speciation a One or more b One gene changed shell spiraling in Euhadra 19 Hybrid zones and timing of isolation a Fire bellied toad and yellow bellied toad b Hybrids have lower fitness malformations and higher mortality This is preventing the two gene pools from mixing beyond the hybrid zone Reproductive barriers is reinforced may be stabile Fusion Reproductive barriers are being weakened c 20 The pace of evolution a Darwin favored gradualism Slow accumulation of changes over long periods produced large changes i Fossil record sometimes show jumps b Punctuated Equilibrium Model i Long periods of stasis followed by periods of rapid change ii Punctuated Equilibrium model and descent with modification 21 Descent with modification a Macroevolution major changes in morphology observed in the fossil record b Complex structure observed now evolved from simple forms present in ancestors c Development of complex eyes