Sexual Reproduction and Selection

Sex

  • Definition of Sex

    • Sex = syngamy followed by reduction (diploid to haploid for gametes)

    • Sex (as a process): where gametes meet and reform diploid cells

    • Sexes are defined by anisogamy

    • Isogamous: organisms that do not have distinct sexes (no difference in gametes)

  • Forms of Reproduction

    • Dioecious vs. Hermaphroditic

    • Dioecious: distinct male and female individuals (most animals are dioecious)

    • Hermaphroditic: individuals possess both male and female reproductive organs, producing both types of gametes

    • Outcrossing vs. Selfing

    • Outcrossing: mating with individuals from different genetic backgrounds

    • Selfing: self-fertilization, typically in hermaphroditic species

    • Asexual Reproduction

    • Vegetative Propagation: a form of asexual reproduction, where a piece of the body becomes a new, genetically identical individual (budding)

    • Parthenogenesis: when unfertilized eggs develop into individuals

    • Examples:

    • Cnidarians can reproduce via parthenogenesis

    • Whiptail Lizards: all females, pseudo-copulate with each other as they only release ova during copulation, utilizing both sexual and asexual reproduction

The Cost of Sex

  • Challenges and Costs of Sexual Reproduction: - Breakdown of successful gene combinations

    • Production of only half the descendants (compared to asexual reproduction) as males don't make babies

    • Exposure to sexually transmitted diseases (STDs)

    • Additional complexities: courtship and other social hassles

Questioning the Value of Sex

  • Why Engage in Sexual Reproduction?

Benefits of Sexual Reproduction

  • Genetic Shuffling: - Sex allows for a reshuffling of genetic material which provides adaptability

  • Adapting to Variability: - Sexual reproduction enables lineages to cope with unpredictable environmental changes:

    • Abiotic factors: climate, geological changes

    • Biotic factors: competition, presence of predators, food availability, and pathogens (coevolution between parasites and hosts)

      • Red-Queen Hypothesis: describes how species must continuously evolve to maintain their fitness relative to other co-evolving species, such as parasites. Parasites evolve fast due to their short lifespans and generations, driving evolution in their host species.

Sex Ratio Dynamics

  • Typical Birth Sex Ratio: - Generally 1:1 at birth, as evolution favors the rarer sex, making 1:1 the most evolutionarily stable strategy

  • Rare Situations Favoring One Sex: - Situations of mate competition focusing in localized areas, or local competition and sexual selection can alter the ratio

    • Example: In fig wasps,

      • The first female lays a brood heavily skewed towards females (mostly daughters).

      • Subsequent females respond by laying more males compared to the first, indicating mating skew is dependent on male quality.

      • Eggs hatch and offspring mate strictly with other wasps in the same fig, after which females set out to find more figs to lay eggs.

    • Example: Female Collared flycatchers with “sexier” mates tend to have more sons.

Discussion and Exploration

  • Activity: - Take turns explaining to each other why most species maintain a 1:1 sex ratio

Hermaphroditism Characteristics

  • Tetrapods: - No true hermaphroditism (amphibians, reptiles, birds, mammals). However, intersex variations are observed and rising in fish, amphibians, and humans due to exposure to endocrine-disrupting chemicals.

  • Types of Hermaphroditism: - Simultaneous: Found in earthworms, gastropods (snails and slugs), and hamlet fish where self-fertilization is common

    • Sequential: Seen in teleosts (a group of fish species), typically irreversible. Sequential hermaphrodites often receive different reproductive benefits/requirements according to their body size:

    • Male to Female Transition: Common in circumstances such as loss of a harem male

    • Female to Male Transition: In species such as sea bass where larger females produce more viable offspring

Anisogamy and Consequences

  • Definition: - Anisogamy refers to the differentiation between female and male gametes.

  • Evolution of Anisogamy: - Separate evolutionary paths for females and males. It's more efficient and economical to have only one mobile gamete for gametes to meet, resulting in one gamete becoming stationary and bigger (egg, providing nutrition) and the other becoming smaller and mobile (sperm).

  • Consequences of Anisogamy: - Influences parental investment decisions

    • Creates variance in reproductive success and fitness. Females have a higher investment to produce gametes.

    • Impacts embryo location and potential abandonment scenarios

Sexual Selection

  • Definition: - Sexual selection refers to the selection for access to the gametes of the opposite sex, significantly impacting organism traits. This concept was proposed by Darwin but wasn't popular for a long time.

  • Secondary Sexual Characteristics: - Many traits evolved as a result of sexual selection

    • Primary sexual characteristics are testes and ovaries.

    • Secondary sexual characteristics are important in sexual selection and therefore reproductive success but are not directly involved in sex.

  • Mating Success Correlation: - Male mating success generally correlates with reproductive success, demonstrated by Bateman's gradient (graph). Male reproductive success is limited by the number of mates it has, while females are limited by the number of eggs produced. As a result, males invest more in competition.

    • Notably, there is often a male-biased operational sex ratio

    • Females tend to be choosy, selecting males based on certain attributes often correlated with care and aggression.

    • Females have a longer reproductive cycle than males (pregnancy, nursing, etc. before being fertile again).

    • Female care strengthens the intersexual difference while male care weakens it.

Reproductive Variance and Concepts of Competition

  • Males vs. Females: - Males exhibit greater variance in reproductive success

  • Polls and Quizzes: - Statistical representations of male mating variance across various species to analyze reproductive successes. Variance is the raw material for sexual selection and corresponds with the strength of selection.

    • Examples of Quiz Questions:

    • Consider contexts like lek polygynous birds versus socially monogamous with extra-pair matings

    • Example: Bowerbirds show much larger variance in the number of mates between males and females, indicating strong selection as males provide no care.

    • The average distance (always positive) to the mean in a distribution is a measure of variability.

Types of Sexual Selection

  • Intrasexual Selection: (Intra-: within)

    • Types: exploitation competition (scramble competition) and interference competition

    • Exploitation (Scramble) Competition: competing for who gets the resource first (e.g., Horse-shoe crabs), reliant on locomotion and mate-seeking

    • Interference Competition: physical competition, such as armament development, or guarding the resource (mate, food, territory, etc.) and preventing others from accessing it

    • Size and Guarding: males develop strategies for mate guarding and alternative tactics such as sneaking or impersonation

  • Sperm Competition: - Mechanisms by which male reproductive success is influenced through competition within female reproductive tracts

  • Intersexual Selection: - Predominantly defined by mate choice which can include preferences for ornamentation or displays

Intrasexual Selection Dynamics

  • Male Strategies: - Males typically compete due to females being more passive in mate choices

    • Particularly effective when males are capable of monopolizing multiple females, typically seen in female-aggregated communities

Male-Male Competition and Evolution

  • Evolutionary Arms Race: - Directional selection leads to an increase in male size and armaments

    • Examples: deer, sheep, and elephant seals (which aggregate on certain beaches to reproduce, where the strongest males monopolize access to many mates and guard them)

    • Trades off between male viability and reproductive success

Mate Guarding Examples

  • Behavioral Strategies: - In species such as the Seychelles warbler (an endangered species living on an island where researchers banded all members), males may latch onto females in copulation guarding. Less intrusions and EPC attempts occur during the female's fertile window due to male efforts to prevent other males from accessing her.

    • Observed phenomena include copulatory plugs made by anurans, dragonflies (male dragonflies won't let females go until they have laid eggs and have a special structure on their penis for sperm scrubbing), and rodents.

    • In amphibians, fertilization happens outside the body, and some males have a specialized finger to grab onto females.

Alternative Mating Tactics

  • Adaptive Strategies: - Non-territorial males, such as gray seals, father offspring while at sea

    • Strategies where smaller males utilize "pre-ejaculation" to enhance mating success

    • Example: In a species living inside a sponge, there are 3 morphs of males:

      • Alphas defend sponge territory and mates.

      • Betas pretend to be females to avoid attack from alphas and enter the sponge to mate with females (female impersonation exists in many taxa).

      • Gammas sneak into territories and mate with females.

      • The rarer type has a higher fitness, therefore all morphs are maintained in the population.

Sperm Competition Strategies

  • Female Genital Tract Dynamics: - Competition among sperm within a female's tract is prevalent, leading to frequent copulation sessions

    • Adaptive traits include sperm scrubbing behaviors observed in species like Dunnocks and dragonflies (male dragonflies have a special structure on their penis to do this).

    • Cooperative sperm: sperms from the same male recognize each other and join together.

    • In polyandrous species, males often have relatively larger testes.

Mate Choice Dynamics

  • Factors Influencing Choice: - Females often show preferences for specific traits among males, leading to trait dimorphism.

    • Example: Females prefer males with large, bright red patches in House Finches.

  • Understanding Benefits of Being Choosy: - Rationales for female choosiness include costs associated with mate selection; leading to potential benefits to offspring viability or compatibility.

    • First egg date is a proxy for reproductive success, as females who lay eggs earlier in the breeding season are more likely to succeed.

Sensory Bias in Mate Choice

  • Origin of Choosing: - Often a practical origin with traits or preferences developing over generations

  • Examples:

    • In guppies, the color orange helps them find food; as a result, females prefer males with orange spots, even if this trait does not indicate male quality (utilitarian coloration).

    • "Sensory trap" observed in frog species where females exhibit a preference for a "chuck" sound that does not yet exist in their species, indicating the preference existed before the vocalization evolved. Females are sensitive to the frequency range where the chuck sound lies; this is an ancestral trait but the vocalization evolved later.

Runaway Selection Model

  • Model Dynamics: - Changes in trait incidence due to genetic linkage

    • Examines how female preferences and associated traits become interdependent over generations (e.g., females with a preference for a trait will benefit from having "sexier" sons).

    • As females with preference increase in population, males expressing the trait become increasingly advantaged.