Chapter 24 Wednesday February 3 2016 2 19 PM Chapter 24 The Origin of Species o Defining Species Originally Latin for kind Morphological species Smallest set of organisms that look alike Looks may deceive Ex Eastern Meadowlark and Western different species Ex Variations within the same species Ex Army ants different castes but sisters in the same colony Ecological species A set of organisms adapted to a specific set of resources Ex Utilize different organic food sources Phylogenetic species Smallest distinct set of organisms that share a common ancestor Ex North and South American mountain lions Biological species Sets of populations whose members potentially interbreed in nature to produce fertile viable young and do not successfully interbreed with other such groups Stay separate from other groups that interbreed with one another Ex Meadowlarks different species Ex Humans same species Biological Species Concept Members of same biological species Share the same gene pool There is gene flow between populations Are reproductively isolated from other species cannot have gene flow By natural biological barriers Prezygotic or postzygotic Reproduction has to produce fertile viable offspring Prezygotic barriers Prevent fertilization that actually produces a zygote Habitat ecological isolation Temporal isolation How a species is using the resources in the environment they live Ex Field country mice never meet because of their different habitats no chance of mating Behavioral isolation Mating at different times Ex Frogs Females can distinguish the males even though they are mating at the same time Courtship cues Mechanical isolation Insects have to mate end to end Have to stay linked together Gametic isolation Gametes being released and gametes coming together o Postzygotic barriers Act after a hybrid zygote is formed Reduced hybrid viability Embryo fails to develop or is weak Embryo may never develop very far born but weak feeble Reduced hybrid fertility Hybrid breakdown o Not able to reproduce Hybrid survives but is sterile or almost so Infertile Ex Mule 2nd generation hybrids are feeble or sterile Ex Wheat 24 3 Problems with the Biological Species Concept o Fossil species Have to use morphological o Asexual species o Sometimes hybrids happen How many genes are responsible for reproductive isolation o May be many genes o But as few as one Ex Variation in one gene keeps 2 snail species from mating gene changes direction of spiral preventing mating mechanical isolation How Can New Species Form Speciation and Evolution o Speciation increases diversity of life formation of new species Microevolution alone does not make more species Microevolution no increase in diversity of life Microevolution with speciation greater diversity o Fossil record shows increased of species Gradual pattern of change Start off with one species and slowly diverges ultimately forming two species Slow divergence as new species form Ex Butterflies Punctuated equilibrium Brief period of rapid change as new species form long period of stability Ex Butterflies o Speciation may occur with or without geographic separation Allopatric speciation geographic barrier Can account for vast majority of speciation common Sympatric speciation doesn t require geographic barrier Reference is to geography Allopatric Speciation o How geographic separation makes a new species Keeps one population genetically isolated from the rest of the species Physical barrier isolates one population Ex Birds isolated on the island and barrier is the ocean Ex Small geographic distance fish isolated in different pool Isolated population diverges genetically due to natural selection and or genetic drift Subject to different environments meaning different natural selection During allopatry biological reproductive barriers evolve creating separate species No reproduction Original and daughter species new diversity increase in diversity o Hybrid zones may exist during allopatric speciation Areas of contact may allow some mating Look back at the figures for examples of three possible outcomes No prezygotic barriers or weak barriers Three possible outcomes Reinforcement hybrids are weak sterile not successful Better and better fewer hybrids in future Fusion hybrids are healthy 100 viable Gene pools come back together two species are fusing together Stable hybridization zone Hybrids continue to be formed but other factors keep gene pools separated o Evidence for this process Shrimp speciation and the Isthmus Shrimp in Panama area land bridge formed millions years ago 15 species living on both sides of Panama Closest related sister species live on opposite sides of land bridge Divergence of species A and B from north and south Barrier allowed divergence and speciation into 30 species Mosquitofish in Bahamas Isolated populations under different predator pressures also form reproductive barriers Difference in morphology Under high predation pond with predators had wider tails for length for burst of speed to escape predators Pond without predators have sleek body Sleek bodied preferred sleek Wide bodied preferred wide Separation in natural selection yielded less cross mating reproductive barrier Fruit fly populations Flies isolated and raised on different foods showed mate preference divergence starch medium and maltose medium Mating preference based on their food type No mating based on origin of population Sympatric Speciation o speciation in same geographical area Gene pools become isolated in other ways Very rare process esp in animals o Isolated behaviorally by sexual selection Female chooses same species of male under normal light Female chooses male at random under monochromatic light female choice maintains gene flow barrier Animals only Males will have characteristics different from the females May have led to two different species Ex Females pick mate on the basis of visual cues Used monochromatic light and chose male species at random Females actually use visual cues for mates Outcome female choice maintains the two species o Isolated by habitat differentiation Plant or animal populations isolated by use of different resources Ex Apple maggot fly Hawthorn mistaken with apple fruit o Isolated by polyploidy complete nondisjunction cell division errors duplicate entire genome Instead of 32 46 pairs Gametes of polyploid offspring not compatible with parent species Spindle pulls or cytokinesis doesn t happen for some reason tetraploid 4 Tetraploid goes