What is sex? What about asexual reproduction? What is sexual reproduction?

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1 BIOL2007 EVOLUTION OF SEX AND SEXUAL SELECTION Kanchon Dasmahapatra EVOLUTIONARY QUESTIONS ABOUT SEX Today: A) The evolution of sex 1) What is the advantage of sex? B) Ev. of sexual dimorphism - sexual selection 2) Why are there "secondary sexual characteristics" What is sex? What is sexual reproduction? Union of two genomes. In eukaryotes: via the union of gametes. Involves meiosis. Causes RECOMBINATION of genomes Even bacterial have recombination bacterial conjugation bacterial transformation What about asexual reproduction? Asexual species trapped on twigs in phylogenies Only one parent. No gamete fusion. NO RECOMBINATION. Offspring identical to parent. but often have some sexual reproduction too S S S S A S S S S S S S S S S A aphid yeast strawberry Asexual species sporadically distributed Usually of recent origin 1

2 Bdelloid rotifers Costs of sexual reproduction (relative to asexual reproduction) Asexual lineage of ~300 species. Lineage ~100 million years old 25 Feb Dr Tim Barraclough (Imperial) Speciation and natural selection in anciently asexual bdelloid rotifers AV Hill LT at 5pm The two-fold cost of sex Suppose each female produces 2.4 progeny Advantages of sex a) recombination increases evolutionary rate Recombination allows advantageous mutations to combine in the same individuals. and so on.. A sexual population should be very vulnerable to invasion by asexual forms This is a simplification, and the asexual advantage may not reach two-fold HUGE cost of sexual over asexual reproduction If other species evolving/adapting faster, competition may cause extinction of asexuals one of the few cases where group selection may actually be operating. 2

3 b) Individual selection Survival in a coevolutionary "arms race Why are there secondary sexual characters? Why is there sexual dimorphism? Environment of offspring may be very different from that of their parent, parent s adaptations may not be sufficient (e.g. aphids). Sex produces recombined variable offspring. The RED QUEEN hypothesis Resistance to rapidly evolving pathogens may be one such changing environment "It takes all the running you can do, to keep in the same place." said the Red Queen Sexual selection In every species, sexual dimorphism has evolved, so is a product of natural selection. What makes a male male, and a female female? MALES FEMALES The natural selection responsible for differences between sexes is called sexual selection. Sexual selection results from intraspecific interactions 3

4 Consequences of being male or female Males: compete for access to females. more indiscriminate, less to lose. More matings mean more offspring. Females: plenty of willing males around worthwhile to be choosy, because number of matings doesn t limit offspring number. Elephant seals: > 90% males father no offspring; fittest fathered 93. But > 50% females have 1 offspring. Males: higher variance (reproductive skew) in offspring number. Sexual selection results from: male-male competition AND female choice (for males). Male-male competition Higher reproductive skew results in greater sexual dimorphism Selection for greater size or weaponry countered by ecological constraints (Cordoba-Aguilar et al J. Zoology) 4

5 Sperm competition in primates Female choice Elongated Control I Control II Shortened Before manipulation Monogamous primates have smaller testes than polygamous primates Widowbird, Euplectes progne After manipulation Elongated Control I Control II Shortened Average no. mates Andersson 1982 Nature Direct benefits of female choice Why should females choose males with exaggerated and costly traits? Male may provide nutrition, superior territory, or parental care Dance flies mating Two major theories: a) Sensory bias b) Indirect benefits i) sexy son s hypothesis ii) good-genes hypothesis nuptial gift Inca terns 5

6 Sensory bias Males exploiting a pre-existing sensory bias/preference in females P -, T - P +, T - P +, T + Túngara frogs (Physalaemus pustulosus) Female frog like calls with chucks whine Male trait evolves whine + chuck Indirect benefits: runaway selection/sexy son hypothesis Proposed by Fisher Linkage disequilibrium between: a) genes for male trait b) genes for female preference for male trait Both sexes carry both genes: females express preference genes males express the trait genes P -, T - Female preference/ sensory bias evolves Stalk-eyed flies Wilkinson & Reillo (1994) Proc R Soc Indirect benefits: handicap/good genes hypothesis Proposed by Zahavi Male trait is an honest signal of his genetic quality Male signal must be costly Offspring will inherit male s good genes. 6

7 Barn swallows Sexual selection in humans Desmond Morris, Jared Diamond, evolutionary psychologists : Møller 1988 Nature Møller 1990 Evolution Peculiarly human features such as: bare skin, hairlessness lips hair colour, skin colour enlarged breasts in females beard in males copulation when not in oestrus weapons, war? ornaments, jewellery? music? art?... etc. Reading FUTUYMA, Evolution. Ch 14: , Ch 17: Krebs & Davies, (3 rd edition) An Introduction to Behavioural Ecology. Chapter 8. Krebs & Davies, (4th edition) Behavioural Ecology. Chapter 8. Take home points Evolution of sex: The basis of sex is recombination Sex is usually costly Separate sexes may exist because of: faster evolution in sexual lineages offspring variability Sexual selection/sexual dimorphism: Male-male competition fighting and/or sperm competition Female choice sensory bias direct benefit to self/offspring indirect (inherited benefits) sexy sons and/or good genes 7

8 OTHER interesting questions could be asked: 1) What is the advantage of sex? 2) Why are there secondary sexual characteristics? 3) Why meiosis? 4) Why only two sexes? 5) What is the optimal sex ratio? (Kevin) 6) Why does the Y chromosome become degenerate (noncoding) 7) How does sex determination evolve? 8) Sexual conflict why and how? See: BIOL2011 (Behavioural Ecology & Sociobiology) and BIOL3012 (Sex, Genes and Evolution). 8