Sex and Sexual Selection

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.

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.

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.

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.

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.

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.

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.

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.

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 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.

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.

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.

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.

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).

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.