Evolution of Sex I - Easy

• Evolutionary Biology: Evolution of Sex I (Lecture 17) focuses on the two-fold cost of sex, Muller's ratchet, the Red Queen Effect, Darwin's sexual selection theory, and reproductive strategies.

Two-Fold Cost of Sex

• Sexual species reproduce at half the rate of asexual species, daughters produce more daughters in asexual reproduction
• Sexual reproduction requires mating with a male, leading to half the offspring being male (unable to directly reproduce)
• Asexual species should be more abundant than sexual species if offspring production and fitness are equal.
• The significant disadvantage of sexual reproduction is the two-fold cost.

Persistence of Sexual Reproduction

• Sexual reproduction's benefits must outweigh the costs, given its prevalence.
• Sexual reproduction enables the purging of deleterious alleles, preventing their irreversible accumulation, unlike in asexual populations (Muller's Ratchet).
• Sex is advantageous amidst evolving parasites, as it generates new genetic combinations for host resistance (Red Queen Effect).

Muller's Ratchet

• Muller's ratchet causes the accumulation of deleterious mutations in asexual populations
• Asexual populations lack genetic recombination through sex, analogous to a ratchet that can only turn in one direction.
• Harmful mutations arise over time in different individuals.
• Without sex, subsequent generations inherit existing deleterious mutations, potentially gaining new ones.
• Unlike sexual reproduction, asexual populations cannot easily eliminate harmful genes, lacking recombination.
• The gradual accumulation of irreversible deleterious mutations builds a genetic load, which increases over generations
• Deleterious alleles can eventually become fixed within the asexual population

Red Queen Effect

• The Red Queen Effect and its connection to the benefits of sexual reproduction in context of post-parasite coevolution
• Hosts must constantly evolve to defend against parasites that also evolve counter-mutations, this creates a co-evolutionary arms race.
• Parasite adaptations to infect hosts leads to hosts with genetic variations that provide resistance being favoured by natural selection.
• Drives reciprocal evolutionary change in both host and parasite populations.
• Generates genetic variation in offspring through recombination, increasing the likelihood of resistance to prevalent parasites.
• Asexual populations are at a disadvantage due to producing genetically identical offspring
• Snails in New Zealand show that populations with high parasitism have more sexual reproduction
• Sex is favoured in environments with strong parasitic pressure.

Darwin's Sexual Selection Theory

• Darwin's Dilemma includes different parental investment and puzzling patterns like sexual dimorphism
• Females should be choosy due to anisogamy and offspring care: males should not be choosy
• Females have a slower reproduction rate, leading to a male-biased operational sex ratio (OSR)
• The sex that invests more in reproduction, often females, is typically the choosier sex.
• Males compete for access to females with lower initial investment, alleles should increase male attractiveness.
• Sexual selection is commonly stronger on males due to the male-biased OSR.
• Sexual dimorphisms, like ornaments and armaments, result from sexual selection.

Sexual Selection

• Includes intrasexual selection (competition) and intersexual selection (mate choice).
• Sexual dimorphisms are physical differences between males and females, often from sexual selection.
• Ornaments (attractive traits) and armaments (weaponry to outcompete)
• Sexual selection is commonly stronger on males due to a male-biased operational sex ratio (OSR)

Reproductive Limitations

• Females are often linked by fecundity (their capacity to produce offspring)
• Males are often limited by the number of mates they can acquire.
• The operational sex ratio (OSR) is the ratio of males to females capable of reproducing at a given time
• A slower rate of reproduction by females typically leads to a male-biased OSR

Female Choice

• Provides direct benefits to the female (e.g., food, nest sites, protection)
• Provides indirect benefits (genetic quality of offspring)

Role Reversal in Sexual Selection

• This can occur when the typical factors are reversed.
• The sex that invests more into reproduction is often the choosy sex
• Male dance flies are choosy, and females are more 'showy', males likely invest more in reproduction.

Sexual Selection and Reproductive Success

• Sexual selection is differential reproductive success from competition for mates
• It is related to the variance in reproductive success within a sex
• A male-biased operational sex ratio (OSR) often leads to males competing for mates.
• Systems where some males acquire larger harems indicate variance in male reproductive success
• Sexual dimorphisms are often the result of sexual selection, typically acting on males due to factors like the OSR.

Fundamental Concepts

• Describing average patterns in nature doesn't mean you are describing all patterns
• What is (on average) across non-human taxa does not define what ought to be in humans (Hume's Law).
• Avoid the naturalistic fallacy, natural properties are not the same as moral properties
• Asexual reproduction requires mating with a half of the offspring will be female, leading to a two-fold cost of sex.
• Muller's ratchet suggests that asexual populations accumulate irreversible, deleterious mutations.
• Sexual populations can purge deleterious alleles.
• The Red Queen Effect proposes that sex is beneficial because genetic variation in offspring increases resistance to evolving parasites.