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Sex and Selection: Costs and Benefits

This text explores the advantages and disadvantages of asexual and sexual reproduction, including maximizing gene transfer and survivorship, genetic variation, and adaptation to changing environments. It also discusses the Red Queen Hypothesis and the impact of parasites on reproductive strategies.

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Sex and Selection: Costs and Benefits

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  1. III. Sex and Selection • Costs and Benefits • 1. Asexual • - major benefit of asexual: maximizes transfer of genes to next generation, and maximizes survivorship in a constant environment “Vegetative” Reproduction: Growth and fragmentation

  2. Vegetative reproduction in animals

  3. Parthenogenesis: Production of gametes that develop without fertilization Cnemidophorus nemexicanus in middle, flanked by diploid parent species. Aphid ‘stem mother’ producing offspring parthenogenically Rare but observed in zoo populations of Komodo dragons. May be as high as 22% in Copperheads. Bees and wasps – males are haploid offspring Mulga (Australian Acacia)

  4. Parthenogenesis: Production of gametes that develop without fertilization A a Direct development from oogonium to egg, and subsequent mitotic divisions to produce embryo – GENETICALLY IDENTICAL TO MOTHER B b AABB AB AAbb A a Ab Meiosis I B b aaBB Separation of chromatids without cytokinesis results in diploid eggs that are not identical to mother or each other aB aabb ab

  5. III. Sex and Selection • Costs and Benefits • 1. Asexual • - major benefit of asexual: maximizes transfer of genes to next generation, and maximizes survivorship in a constant environment • - major costs of asexual: “all or none” survivorship in changing environment, and “Muller’s Ratchet” Mutations can only accumulate in a lineage; all genes passed to all offspring.

  6. III. Sex and Selection • Costs and Benefits • 1. Asexual • - major benefit of asexual: maximizes transfer of genes to next generation, and maximizes survivorship in a constant environment • - major costs of asexual: “all or none” survivorship in changing environment, and “Muller’s Ratchet” • 2. Sexual: • - major cost: only ½ genes to each offspring • (not necessarily complementary sets, either. Only 1 in ~8 million chance of producing two gametes that have receive opposite homologs from each homologous set in humans).

  7. III. Sex and Selection • Costs and Benefits • 1. Asexual • - major benefit of asexual: maximizes transfer of genes to next generation, and maximizes survivorship in a constant environment • - major costs of asexual: “all or none” survivorship in changing environment, and “Muller’s Ratchet” • 2. Sexual: • - major cost: only ½ genes to each offspring • - major benefit: extraordinary production of variation; sexual lineages are more likely to survive over time in changing environments, and adapt to rapidly evolving parasites.

  8. The “Red Queen” Hypothesis Parasites have shorter life spans than their host, and so have more opportunities for selection to adapt them to their hosts. Host must continually ‘reshuffle the genetic deck” to stay ahead of their parasites…

  9. Moran et al., (2011) Running with the Red Queen…. Science 333:216-218. C. Elegans are either males or hermaphroditic females that can reproduce by self fertilization (not parthenogenesis). They are parasitized by bacteria, that can kill them in 24 hrs. Experiment 1: rates of sexual reproduction in wild-type populations of C. elegans In the absence of parasites, selfing-fertilization is preferred (80%:20%). Repeated exposure to the same strain of parasite selects for outcrossing (80%), until an adapted genotype is produced that is resistant. Then reproduction ‘relaxes’ to selfing. Continued exposure to an evolving parasitic population selects for increasing rates of outcrossing reproduction. Wildtype outcrossing rates over time. Outcrossing rates in wildtype populations were not manipulated and free to evolve during the experiment. The wildtype populations were exposed to three different treatments: control (no S. marcescens; dotted line), evolution (fixed strain of S. marcescens; dashed line), and coevolution (coevolving S. marcescens; solid line) for thirty generations. Error bars represent two standard errors of the mean (SE).

  10. Moran et al., (2011) Running with the Red Queen…. Science 333:216-218. Experiment 2: Efficacy of parasites against populations of C. elegans with different fixed reproductive strategies • The ‘coevolved’ bacteria were more lethal to all ancestral populations of C. elegans, suggesting they had evolved to avoid the defenses of the host.

  11. Moran et al., (2011) Running with the Red Queen…. Science 333:216-218. Experiment 2: Efficacy of parasites against populations of C. elegans with different fixed reproductive strategies • The ‘coevolved’ bacteria were more lethal to all ancestral populations of C. elegans, suggesting they had evolved to avoid the defenses of the host. • Populations of obligate selfers went extinct before 30 generations occurred, when confronted by an evolving parasite. (consistent with Red Queen)

  12. Moran et al., (2011) Running with the Red Queen…. Science 333:216-218. Experiment 2: Efficacy of parasites against populations of C. elegans with different fixed reproductive strategies • The ‘coevolved’ bacteria were more lethal to all ancestral populations of C. elegans, suggesting they had evolved to avoid the defenses of the host. • Populations of obligate selfers went extinct before 30 generations occurred, when confronted by an evolving parasite. (consistent with Red Queen) • Populations capable of mixed mating, or forced to outcross, evolved lower mortality in 30 generations, even when evolving with a progressively more infectious pathogen. (consistent with Red Queen)

  13. III. Sex and Selection • Costs and Benefits • 1. Asexual • 2. Sexual • 3. The Distribution of Sexes Hermaphrodites: flowers have both sexes - individual animals are both sexes Dioecious plants: individuals have either male or female flowers Monoecious: individuals have both male flowers and female flowers.

  14. III. Sex and Selection • Costs and Benefits • 1. Asexual • 2. Sexual • 3. The Distribution of Sexes Here, selection favors hermaphrodite over being one sex or another. ‘Adding’ the second sex has a low cost and big benefit. Hermaphrodites: flowers have both sexes - individual animals are both sexes Dioecious plants: individuals have either male or female flowers Monoecious: individuals have both male flowers and female flowers. When is hermaphrodism favored? For plants that already have invested in most flower structures (sepals and petals and nectar), adding stamens or ovaries is a relatively small costs, relative to dramatically increasing reproductive output. OR, for animals that just shed gametes into water (no genitalia) Hermaphrodism often favored.

  15. III. Sex and Selection • Costs and Benefits • 1. Asexual • 2. Sexual • 3. The Distribution of Sexes If ‘adding’ a second sex is costly, benefits may not outweigh it. Dioecy selected for. Hermaphrodites: flowers have both sexes - individual animals are both sexes Dioecious plants: individuals have either male or female flowers Monoecious: individuals have both male flowers and female flowers. When is hermaphrodism favored? For many animals, adding new reproductive structures is very costly, energetically. Dioecy favored.

  16. III. Sex and Selection • Costs and Benefits • 1. Asexual • 2. Sexual • 3. The Distribution of Sexes • 4. Sex Determination • - chromosomal (XX = female, XY = male; in fowl it is ZZ = male, ZW = female)

  17. III. Sex and Selection • Costs and Benefits • 1. Asexual • 2. Sexual • 3. The Distribution of Sexes • 4. Sex Determination • - chromosomal (XX = female, XY = male; in fowl it is ZZ = male, ZW = female) • - environmental

  18. - environmental temperature Adaptive? Warm = male (bank) Cool = female (shade) Warm = female (shallow) Cool = male (deep) MT FT

  19. - environmental temperature nutrients, energy availability Arisaema triphyllum “Jack-in-the-Pulpit” Small plants - male Large plants - female

  20. - environmental temperature nutrients, energy availability social environment Sexually mature female (Inhibits development of males) Sexually mature male Immature males Wouldn’t the species do better if there were more females/group? Yes, but selection favors individual reproductive success.

  21. - environmental temperature nutrients, energy availability social environment Midas cichlid Brood

  22. - environmental temperature nutrients, energy availability social environment Midas cichlid Add Larger juveniles Brood female

  23. - environmental temperature nutrients, energy availability social environment Midas cichlid Add smaller juveniles Brood male

  24. III. Sex and Selection • Costs and Benefits • 1. Asexual • 2. Sexual • 3. The Distribution of Sexes • 4. Sex Determination • 5. Sex Ratio Benefit of being male – quantity of offspring Benefit of being female – regulate quality of offspring Cervus elaphus Red deer Small does and starving does selectively abort male embryos. Small daughters may still mate; small sons will not acquire a harem and will not mate. Selection has favored females who save their energy, abort male embryos when starving, and maybe live to reproduce next year.

  25. III. Sex and Selection • Costs and Benefits • Sexual Selection • Mating Systems: • - polygamy or monogamy? The polygamy threshold: If resources are limiting and patchy, a resident on a high quality territory may provide more resources to TWO mates than a resident on a low quality territory could provide to ONE mate. Selection may favor polygamy (unless both parents are needed to raise offspring).

  26. III. Sex and Selection • Costs and Benefits • Sexual Selection • Mating Systems: • - polygamy or monogamy? Monogamy: When two parents needed to rear offspring and can contribute equally (birds) Polygamy: When one sex controls resources or access to mates (usually males), or profits from sperm competition (females).

  27. III. Sex and Selection • Costs and Benefits (notes) • Sexual Selection • - selective pressures may differ between the sexes, resulting in selection for different physiologies, morphologies, and behaviors. • Mating Systems: • - Polygyny or polyandry? Which sex has multiple mates? The sex that invests more energy in reproduction should be choosy (monogamous) male NEWTS Also, the sex with lower reproductive potential (lower maximum fecundity) should be choosier. # offspring female # mates Jones (2002)

  28. III. Sex and Selection • Costs and Benefits (notes) • Sexual Selection • - selective pressures may differ between the sexes, resulting in selection for different physiologies, morphologies, and behaviors. • Mating Systems: • - Polygyny or polyandry? Which sex has multiple mates? female PIPEFISH Also, the sex with lower reproductive potential (lower maximum fecundity) should be choosier. # offspring male # mates Jones (2002)

  29. III. Sex and Selection • Costs and Benefits (notes) • Sexual Selection • - selective pressures may differ between the sexes, resulting in selection for different physiologies, morphologies, and behaviors. • Mating Systems: • 2. Intrasexual Selection – where competition within a sex occurs

  30. Sexual Selection 2. INTRAsexual Competition - competition for harems, territories

  31. Sexual Selection 2. INTRAsexual Competition - competition for harems, territories - sperm competition

  32. Sexual Selection 2. INTRAsexual Competition - competition for harems, territories - sperm competition - infanticide

  33. Sexual Selection 2. INTRAsexual Competition - competition for harems, territories - sperm competition - infanticide - female mimicry in male subadults and "stealth matings"

  34. "sneaker males" FEMALE "Sneaker" male

  35. Sexual Selection 3. INTERsexual Selection (Mate Preference) - A behavior or morphology that is only performed during the reproductive season, which DECREASES SURVIVORSHIP. Only adaptive because it INCREASES THE PROBABILITY OF REPRODUCTION disproportionally.

  36. Sexual Selection 3. INTERsexual Selection (Mate Preference) - A behavior or morphology that is only performed during the reproductive season, which DECREASES SURVIVORSHIP. Only adaptive because it INCREASES THE PROBABILITY OF REPRODUCTION disproportionally. showy breeding plumage calling displays

  37. 3. INTERsexual Selection (Mate Preference) - WHY is it ADAPTIVE for the FEMALE to choose a given male?? 1. Direct Benefit - parental care

  38. 3. INTERsexual Selection (Mate Preference) - WHY is it ADAPTIVE for the FEMALE to choose a given male?? 1. Direct Benefit - nuptual gifts (energy for egg making) Dance flies Scorpionflies

  39. 3. INTERsexual Selection (Mate Preference) - WHY is it ADAPTIVE for the FEMALE to choose a given male?? 1. Direct Benefit - nuptual gifts (energy for egg making)

  40. 3. INTERsexual Selection (Mate Preference) - WHY is it ADAPTIVE for the FEMALE to choose a given male?? 1. Direct Benefit 2. Indirect Benefit: Good Genes - ‘Handicap Theory’

  41. 3. INTERsexual Selection (Mate Preference) - WHY is it ADAPTIVE for the FEMALE to choose a given male?? 1. Direct Benefit 2. Indirect Benefit: Good Genes - handicap theory and runaway selection

  42. 3. INTERsexual Selection (Mate Preference) - WHY is it ADAPTIVE for the FEMALE to choose a given male?? 1. Direct Benefit 2. Indirect Benefit: Good Genes - ‘Parasite Load Theory’

  43. 3. INTERsexual Selection (Mate Preference) - WHY is it ADAPTIVE for the FEMALE to choose a given male?? 1. Direct Benefit 2. Indirect Benefit: Good Genes - ‘Symmetry Theory’ Some studies show a preference for mates that are more symmetrical. Post-hoc explanations suggest this might be adaptive because symmetry arises as a consequence of integrated development on both sides of the body, perhaps as a consequence of a well-integrated genome. Hmmm....

  44. 3. INTERsexual Selection (Mate Preference) - WHY is it ADAPTIVE for the FEMALE to choose a given male?? 1. Direct Benefit 2. Indirect Benefit: Good Genes - ‘Symmetry Theory’ Hmmm....

  45. 3. INTERsexual Selection (Mate Preference) - WHY is it ADAPTIVE for the FEMALE to choose a given male?? 1. Direct Benefit 2. Indirect Benefit: Good Genes - ‘Symmetry theory’ Hmmm....

  46. 3. INTERsexual Selection (Mate Preference) - WHY is it ADAPTIVE for the FEMALE to choose a given male?? 1. Direct Benefit 2. Indirect Benefit: Good Genes - ‘Symmetry Theory’ Hmmm.... Hey, It Works!!

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