Transcript Slide 1
Evolution of Sex I. Asexual v. Sexual Reproduction Theories on the evolution of Sex A. B. 1) 2) 3) C. Unpredictable environment – Red Queen Deleterious mutation – Muller’s ratchet Side effect of a cell survival strategy Evolution of two sexes Sexual Selection & Dimorphism II. A. B. C. Why do females prefer certain phenotypic traits? Natural Selection v. Sexual Selection Males more affected A. Asexual v. Sexual Reproduction 1) Asexual Reproduction Binary fission Parthenogenesis Adventitious embryony Vegetative reproduction 2) Sexual Reproduction Production of haploid gametes a. In many species, haploid (n) gametes are descended from germ cells that are originally diploid (2n) haploid – Gametes then combine in fertilization to reconstitute the diploid complement found in parental cells Meiosis Two successive nuclear divisions, MI & MII, which produces haploid gametes that differ genetically b. Cost of Sex Advantage of asexual females Generation Sexuals Asexuals 1 FxM F 2 FxM FxM FFFF 3 FxM FxM FxM FxM FFFF FFFF FFFF FFFF Fraction of individuals that are asexual B. Theories on the evolution of Sex 1) 2) 3) 4) Unpredictable environment – Red Queen sex results in more genetically varied offspring, thus sexual reproduction can increase fitness by producing an offspring that will survive in an unpredictable and changing environment. Deleterious mutations – Muller’s ratchet In a strain of asexual species the number of deleterious mutations accumulate with time. The only way to purge the mutation is by death. When the genotype with the smallest number of mutations nx dies , the genotype with the smallest number of mutations in now nx+1 and the "ratchet" has clicked up one Sex breaks the ratchet, Sex allows advantageous alleles in different individuals to be combined together into the same individual. Parasite coevolution theory – Side effect of Cell Survival Strategy The two alleles are recombined rapidly in (a), a sexual population, but in (b), an asexual population, the two alleles must independently arise. Kondrashov argues that the slightly deleterious nature of mutations mean that the population will tend to be composed of individuals with a small number of mutations. C. Evolution of two sexes Essential difference: size of gametes 50/50 sex ratio common in nearly all obligatory sexually reproducing species. Different gamete size requires II. Sexual Dimorphism & Sexual Selection Sexual dimorphism = Sex “complicates life by adding a third major challenge: finding a member of the opposite sex and persuading him/her to cooperate” Sexual Selection Directional selection that acts on genetically variable phenotypic traits that affect the reproductive success of the individuals of a particular sex Darwin suggested two avenues for the evolution of these traits: POSSIBLE MECHANISMS: Pre-mating sexual selection: selection for species recognition (females mating w/own species) could possibly lead to elaborate secondary sexual characteristics females could detect differences in the number of deleterious alleles in an male (i.e. athletic ability required for courtship dance confers fitness level) Post-mating sexual selection: sperm competition polyandrous species have larger sperm, faster swimming sperm, more aggressive sperm A. Why do females prefer certain phenotypic traits? 1) Direct Benefit: Certain male characters (nuptial gifts of food or defensive compounds, care provided to offspring) 2) Good genes: Male characters are "indicators" of "good genes", i.e., that males possess genes that will provide high fitness for the offspring. 3) Sensory bias or sensory drive: some aspect of the sensory world biases females to B. Natural Selection v. Sexual Selection In some situations, the two forces are actually opposed – Example: C. Males more affected by sexual selection than females Females – spend energy on egg production ability Males – Magnitude of sperm production favors fertilization of eggs from many females Therefore traits that increase their ability to acquire many mates affected, enhance their competitive ability Males, do they have it easy or what? Males do not need to find quality mates, rather quantity mates… Females choose by: Sex v. Gender Sex – Gender -