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Sexual Selection in the harlequin beetle riding pseudoscorpion 1 Multiple Paternity in the HB-riding Pseudoscorpion • Males A & B captured with mother (M) on same harlequin beetle • Numbered lanes are DNAsamples from offspring of the mother 2 Female pseudoscorpions discriminate against previous mates 3 Why Polyandry in Cordylochernes? (34,770 nymphs counted!) • The average lifetime reproductive success of females mated to two males is 32% higher than that of females mated twice to a single male 4 Percent of Females Achieving Zero Reproductive Success 5 Jeanne Zeh’s Genetic Incompatibility Avoidance Hypothesis Polyandry enables females to exploit postcopulatory mechanisms for reducing the risk and/or cost of fertilization by genetically incompatible sperm • Sperm competition • Female choice of sperm • Re-allocation of maternal resources in live-bearing species In 33% of “infertile” couples, the female’s cervical mucus agglutinated partner’s sperm but not donor sperm Dondero et al. 1978 6 Human Leukocyte Antigen Matching and Fetal Loss (Ober et al. 1998) • 10-year study of HLA matching and pregnancy in 111 Hutterite couples • Matching at entire 16-locus haplotype and/or allele sharing at HLA-B-linked locus confers significant risk for fetal loss New Topic: Speciation • Speciation is the process by which one genetically-cohesive population splits into two or more reproductively-isolated populations. 7 Ernst Mayr (1942) Biological Species Concept • Species are groups of actually or potentially interbreeding natural populations which are reproductively isolated from other such groups; i.e., they have distinct gene pools Allopatric Speciation • Speciation in a different place. Argued to be the most common (least controversial) form of speciation. • Evolution of reproductive isolating mechanisms between populations that are geographically separate 8 Allopatric Distributions • Vicariance: Widespread population becomes divided: – New river or mountain range (barriers); emergence of the Isthmus of Panama – Extinction of intermediate populations due to habitat fragmentation • Rare dispersal and colonization – Founder Effect (or Peripatric) speciation (e.g., Galapagos Finches) Vicariant Speciation in Antelope Ground Squirrels 9 Sexual Selection and Allopatric Speciation • Sexual selection can cause rapid evolution of epigamic traits and mating preferences • It can cause rapid evolution of premating isolating mechanisms in allopatry • Evidence? – Of relatively sexually monomorphic and dimorphic sister clades, the latter is usually more speciose Allopatric Speciation - Summary I • No gene flow between gene pools due to geographical separation • Independent evolution --> divergence • Divergence includes traits associated with reproduction, which causes evolution of reproductive isolating mechanisms 10 Allopatric Speciation - Summary II • Mutation + drift --> slow allopatric speciation • Natural selection and sexual selection can lead to faster allopatric speciation • Extrinsic barriers to hybridization permit independent evolution until speciation is complete. Mechanisms of Non-Allopatric Speciation: I. Parapatric Speciation • Speciation along side; development of reproductive isolation among members of a population in the absence of an obvious geographic barrier • Looks much like secondary contact • May occur when a sharp environmental discontinuity develops within the range of a species. • Plants growing on mine tailings flower at different times than plants growing in surrounding pastures in Wales 11 Mechanisms of Non-Allopatric Speciation: II. Sympatric Speciation • Speciation in the same place; development of reproductive isolation without geographic barriers • FAIRLY COMMON IN PLANTS VIA POLYPLOIDY • 47% of flowering plant species are polyploid Polyploidy • Multiplication of the number of chromosomes – autopolyploidy - duplication of the chromosomes of a single species – allopolyploidy - duplication of a combination of chromosomes from different species 12 Polyploidy & Speciation • Allopolyploidy can result in nearly instantaneous speciation. • Polyploid individuals cannot interbreed successfully with members of either parental population because polyploid individuals have twice as many chromosomes as their parents. Therefore the chromosomes cannot pair properly during the first metaphase of meiosis. Sympatric Speciation: Plants 13 Sympatric Speciation: Animals • Sympatric speciation is a controversial mechanism of speciation in animals. • How can reproductive isolation be achieved in the absence of barriers to gene flow? • Again, there must be selection for assortative mating - i.e., like genotypes mate with like • Many entomologists argue that sympatric speciation is common in phytophagous insects via host plant shifts Sympatric Speciation: Rhagoletis pomonella • Tephritid fly which is a pest species on apples and hawthorn • Females infrequently oviposit in new host plants and then both males and females show an affinity for the new host plant 14