Sex and Sexual Selection

Variation & Section

• Origin of Sex and Diversity of Mechanisms: Understanding the evolution and various methods of sexual reproduction.

• Sex Determination: Mechanisms defining how sex is determined genetically.

• Cost of Sex: Understanding the reproductive costs associated with sexual reproduction, and why it is maintained.

• Evolutionary Consequences of Sexual Reproduction: Exploring how sexual reproduction affects evolution.

• Darwinian Sexual Selection: Analyzing parental investment and differences in gametes.

• Mechanisms of Sexual Selection: Differentiating how selection occurs based on reproductive traits.

Reproductive Strategies

• Asexual Reproduction:

  • Produces genetically identical cells from a single parent via mitosis.

  • Examples: Fission and budding.

• Parthenogenesis:

  • Females create offspring from unfertilized eggs; involves incomplete reductive division.

• Hermaphroditism:

  • Individuals possess both male and female reproductive tissues.

  • Can be sequential (e.g., rock shrimp, clownfish).

  • Protogyny: Female-to-male transition.

  • Protandry: Male-to-female transition.

• Gonochorism (Dioecious):

  • Distinct sexes, with some species capable of sex changes.

Sexual Reproduction

• Overview:

  • Sexual reproduction dates back at least 2000 million years, suggesting a significant selective advantage.

  • Misconceptions: Not universally adopted upon first appearance; sex evolved multiple times, with some groups reverting to asexual reproductive strategies.

Sex Determination

• Genetic Systems:

  • XX/XY: Typical in humans and mammals; XX for females, XY for males (SRY gene on Y triggers male development).

  • XX/XO: Seen in crickets, cockroaches, and some mammals.

  • ZW: Found in birds and some fish; females ZW, males ZZ.

  • Haplodiploid: Typically males are haploid; females are diploid or sterile males.

  • Environmental Temperature-sensitive: Common in certain fish and reptiles, can be a combination of genetic and environmental factors.

Competing Theories: Origin of Sexual Reproduction (Syngamy)

• Neomuran Revolution:

  • Proposes that eukaryotic recombination machinery evolved from archaea ancestors.

  • Meiosis adapted later, necessitated by accurate DNA replication.

• Viral Eukaryogenesis:

  • Suggests that proto-eukaryotic cells emerged from infections of viruses into bacteria, leading to meiosis as a process for genetic exchange.

Evolution of Sexual Reproduction

• Importance of Sex in Evolutionary Biology:

  • Graham Bell notes the confusion surrounding the selective advantages of reproduction.

  • Darwinism emphasizes gene transmission: asexual reproduction preserves full genetic contribution, whereas sexual reproduction halves it, posing a puzzling evolutionary question.

Advantages of Reproductive Modes

• John Maynard Smith's Analysis:

  • Mathematical model outlining the two-fold cost of producing males (1978).

  • Assumptions: Female's reproductive strategy does not impact offspring number or survival probabilities.

  • Asexual reproduction leads to more grandchildren; thus, sexual reproduction must confer significant advantages to persist.

• Research on Tribolium Beetles:

  • Found that sexual strains consistently outperformed asexual strains despite higher immediate costs.

Genetic Health and Evolutionary Hypotheses

• Mutation Elimination Hypothesis:

  • Muller’s Ratchet: Sexual reproduction helps to purge deleterious mutations from the gene pool, crucial for the fitness of populations in the face of evolving threats.

• Fisher-Muller Hypothesis:

  • Proposes sex allows for adaptive combinations of favorable mutations to cope with environmental changes.

  • Evidence suggests that larger populations tend to benefit more from sexual reproduction.

Sex and Evolution

• Sexual Selection:

  • A unique form of natural selection that only applies to traits related to sexual reproduction.

• Differential Parental Investment:

  • Females invest significantly in few large gametes (e.g., yolk-nourished eggs), whereas males invest less, producing many inexpensive gametes.

Natural vs. Sexual Selection

• Natural Selection:

  • Practical, addresses survival: utilitarian and economical.

• Sexual Selection:

  • Often colorful and extravagant in form, it may lead to change in traits that don’t have direct survival benefits.

Anisogamy and Investment Differences

• Sperm and Egg Production:

  • Males can produce 100-200 million sperm daily, summing to about 4.5 trillion over a lifetime.

  • Females are born with approximately 2 million eggs, of which only a few mature.

Mate Selection and Female Choosiness

• Male Fitness:

  • Depends directly on how many females he fertilizes.

• Female Choosiness:

  • Females exhibit more discretion due to their limited investment; low-quality mates can waste precious reproductive resources.

  • Example: Latrodectus hesperus demonstrated varying mate selection based on their environment.

Hierarchies and Mating Strategies

• Intra-sexual Selection:

  • Males often compete amongst themselves for fertilization opportunities via direct competition.

• Inter-sexual Selection:

  • Traits favored in one sex (often males) attract the opposite sex.

Sexual Dimorphism and Genetic Quality

• Mate Selection:

  • Often based on genetic markers that indicate the quality of potential mates.

• Sensory Bias:

  • Some evolutionary traits emerge from biased female selection prior to their expression (e.g., male displays).

The Handicap Principle

• Sexual Displays:

  • Reliable indicators of genetic fitness, showcasing resistance to disease.

• Example: Long-tailed swallows where longer tails correlated with mating success and offspring viability.