Sex

Outline • What is sex?

• Cost/Benefits of Sex • What defines the sexes?

• Sexual Origins • Sexual Conflict – Male-male Competition – Female Choice – Cryptic Female Choice – *Sperm Competition • Where is this leading?

What is Sex?

• Sex: Meiosis and Syngamy – Meiosis - Reduction – Syngamy - Fusion

Syngamy • Fusion of gametes • 2 kinds: – Isogamy- protists, fungi, algae • costs – Anisogamy • beneficial

Benefits of Having Sex • Traditional: increase the rate of adaptive evolution --> decreases extinction risk • Creates genetic variation • spread of advantageous traits and elimination of deleterious genes • Resistance to parasites (Red Queen hypothesis) • Repair of damaged DNA via recombination

Costs of Sex • Only one sex can bear young – Asexual population: both “sexes” can bear young, so population increases more rapidly • Males and females must find each other to mate • Predation risk

What defines the sexes? • Gamete size!

– Larger gamete = Female • Increase zygote size – Zygote fitness – Smaller gamete = Male • Get more for less, just like Walmart!

• This defines Anisogamy – There is variation in gamete size though…

Giant Sperm

Drosophila bifurca

Sexual Origin •

Giardia lamblia

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Sexual Conflict • Male-Male Competition • Female Choice • Cryptic Female Choice • Sperm Competition

Male-Male Competition • Can lead to extravagant characters and sexual dimorphism • Territory size and quality • Ornaments vs. Armaments • Mate Gaurding

Female Choice • Females preferentially mate with males that have larger or more intense exaggerated characters • Morphological characteristics, display behaviors, territory size, nuptial gifts • Honest indicators, sensory bias

Indirect Benefits of Mate Choice • Sexy sons: female preferences are inherited • Runaway selection • Good Genes • Handicap Principle

Cryptic Female Choice • Female chooses which sperm will fertilize her eggs after mating with several males • Denial of deeper genetalic access • Discharge or digestion of sperm • Breaking copulation

Cryptic Female Choice • If females do these things non-randomly then it is cryptic female choice

Sperm Competition • Sperm from two or more males compete inside the female –

Calopteryx

• Number of Sperm • Sperm Quality • Mating Order

Sperm Competition • Sperm Displacement – Producing more sperm – Copulatory organ modifications • Humans – Genital plugging and removal

Where is this leading us?

• Mating Systems – Monogamy – Polyandry – Polygyny – Promiscuity • Life History Evolution