Chapter 13 The Origin Of Species - Saint Xavier University

4/25/2019Chapter 13:The origin of speciesSpecies are independently evolvinglineages General lineage species concept:species are metapopulations thatexchange alleles frequently enough tocomprise the same gene poolWays to identify speciesSpeciation involves barriers toreproduction Biological species concept: species aregroups of interbreeding populations thatare reproductively isolated from other suchgroups Phylogenetic species concept: smallestpossible group descending from acommon ancestor and recognizable byunique, derived traitsIsolating barriersElk and red deer: two species? Geographic: extrinsic properties oflandscape that prevent gene flow– Allopatry Reproductive: features of organisms thatprevent interbreeding– Barriers effective even in sympatry1

4/25/2019Premating barriers: timing ofreproductionPremating barriers: pollinatorisolationPostmating-prezygotic barriers:Postzygotic barriers Gametic incompatibility: sperm or pollenfrom one species fails to penetrate andfertilize the egg of another species Hybrids are produced but have low fitnessGenetics basis of postzygotic isolationExample of B-D-M incompatibility– Hybrid inviability– Hybrid sterility– Ecological inviability– Behavioral sterility Bateson-Dobzhansky-Mullerincopatibilties: arise from epistaticinteractions at two or more loci2

4/25/2019Key Concepts Geographic barriers to gene flowphysically separate populations– Important for all species concepts Reproductive barriers are intrinsic featuresof organisms that reduce likelihood ofinterbreeding Separation of populations in time canreduce gene flowKey ConceptsAllopatric speciation Divergent floral traits can cause pollinatorisolation and reduce interbreeding Molecular incompatibilities betweengametes can prevent interbreeding Genetic incompatibilities that reducehybrid fitness can be important barriers togene flowReinforcement increasesprezygotic reproductive isolation Reinforcement: natural selection favorsprezygotic isolation mechanisms thatprevent the formation of hybrids withreduced fitnessSympatric speciation Reproductive isolation evolves withoutgeographic isolation– Requires nonrandom mating based ongenetic or phenotypic factors– Postzygotic isolation must evolve in allopatry3

4/25/2019Parapatric speciation Geographic barrier only partially separatespopulations– Some gene flow possibleEcological speciation Selection for different ecological traitscreates reproductive barriers– Can lead to pre- and postzygotic isolationPhylogenetic evidence for allopatric speciationIsolation by distance Populations tend to breed with those inclose proximity– Causes variation across the rangeKey Concepts New species can form when populationsbecome geographically isolated Speciation can even occur in sympatrywhen populations rarely interbreed Isolation by distance occurs becauseindividuals tend to mate with individualsfrom the same or nearby populations Speciation can arise as a by-product ofdivergent ecological adaptationSympatric speciation in RhagoletisEcological speciation:adaptation to different hostplants drives reproductiveisolation4

4/25/2019Islands provide opportunity forallopatric speciationColonization leads to speciationSpecies often differ in malesong and femalepreferences for the songsEcological speciation andinterspecies gene flow Genes under strong selectionin polar bears linked toecological niche– APOB proteins– Heart function– PigmentationKey Concepts Speciation is a complex process Scientists have been able to testhypotheses about the speciation processwith diverse lines of evidence Interbreeding has occurredsince divergence– Gene flow from polar bears tobrown bearsThe speed of speciationAllopolyploidy can lead toextremely rapid speciation5

4/25/2019Key ConceptsCryptic diversity in skipper butterflies The speed of speciation variesenormously among taxa– Can be rapid in plants due to allopolyploidy– Reproductive isolation can take millions ofyears in birds and mammalsKey ConceptsHorizontal gene transfer in E. coli Cryptic species may diverge fromancestral populations without easilydistinguishable morphologies Identifying cryptic species is important formeasures of biodiversityHorizontal gene transfer makesclassification difficultHorizontal gene transfer is common6

4/25/2019Bacterial species often defined byspecific adaptationsTaking horizontal gene transfer into accountSome scientists criticize this methodKey Concepts Microbial organisms present challenges tospecies concepts developed foreukaryotes New species concepts in microbiologymay provide valuable insight into theevolutionary history of disease-causingorganisms7

Chapter 13:The origin of species Species are independently evolving lineages General lineage species concept: species are metapopulations that exchange alleles frequently enough to comprise the same gene pool Ways to identify species Biological species concept: species are