CHAPTER 24 SPECIATION

  • step function graph: fitness increases in sudden bursts over generations (punctuated equilibrium theory)

  • requires genetic isolation and divergence
    i. isolation: gene flow barrier isolating two populations of same species
    ii. divergece: two populations evolve independently

biological species concept

  • reproductive isolation: don’t interbreed, offspring not viable/fertile

  • prezygotic isolation: barrier to successful mating

  • postzygotic isolation: hybrid offspring of successful matings not viable/fertile

  • disadvantages
    i. can’t use for asexual reprod. organisms
    ii. can’t use for populations that don’t come into contact
    iii. reprod. isolation is a spectrum

morphospecies concept

  • morphological differences

  • advantage: widely applicable

  • disadvantages
    i. can’t use for polymorphic species (diverse phenotypes) or cryptic species (differ in traits other than morphology)
    ii. subjective (how different do they have to be?)

phylogenetic species concept

  • evolutionary history

  • synapomorphy: trait in certain groups of organisms that’s missing in more distant ancestors
    i. used to identify monophyletic groups (clades/lineages): ancestor and all its descendants
        a. species are smallest monophyletic groups

  • advantage: widely applicable

  • disadvantages
    i. data only available for small subset of organisms

  • recognizes more species than morphospecies/bio concept (more complex naming)

  • anagenesis: if a species lives long enough, it will evolve into another species

  • allopatry: gene flow barrier due to geographical isolation
    i. allopatric speciation: speciation due to allopatry
    ii. dispersal: individuals moving
    iii. vicariance: habitat splitting

  • sympatry: geographically-proximal populations
    i. sympatric speciation CAN occur
        a. disruptive selection favors extreme phenotypes → may become different enough that speciation occurs

  • niche: range of ecological resources/conditions a given species can use/tolerate
    i. prezygotic behavioral isolation → reproductive isolation resulting from adaptations to different niches
        a. evolve because individuals with lower gamete wastage have higher fitness (produce more viable offspring)

  • polyploidy: more than two complete sets of chromosomes due to mitosis/meiosis errors (sympatric speciation caused by chromosome-level mutations)
    i. autopolyploid: chromosome number doubles, parent same species
        a. less common
    ii. allopolyploid: two species mate + mitosis error → viable and nonsterile offspring with two full chromosome sets
        a. higher heterozygosity levels than diploid organisms (beneficial)
        b. can tolerate higher levels of self-fertilization (not as affected by inbreeding depression)
        c. genes on duplicated chromosomes diverge independently → increased genetic variation → rapid speciation
    iii. polyploids reproductively isolated from normal ploidy organisms
        a. can self-fertilize or mate with other polyploids

  • divergence occurs and prezygotic isolation → inter-population mating rare, minimal gene flow, populations still diverge

  • species not usually in contact haven’t developed prezygotic isolation → mate
    i. fusion: two populations fuse and gene flow erases distinctions (one species necessarily goes extinct)
    ii. reinforcement: natural selection for traits promoting prezygotic isolation
        a. traits evolved while populations diverged
        b. sympatric species exhibit prezygotic isolation while allopatric don’t
    iii. hybrid zones: geographic areas where interbreeding produces viable, fertile hybrid offspring
    iv. speciation by hybridization: hybrid genomes have beneficial allele combinations that parents don’t have → thrive in environments parents can’t → geographic isolation enforces speciation by limiting gene flow