Why Did Sex Evolve?

Why Did Sex Evolve?

Objectives

  • Define what a "reproductive handicap" is.

  • Examine the pros and cons of sexual and asexual reproduction.

  • Analyze how sexual reproduction increases genetic variation.

  • Investigate how stable/changing environments affect selection for sex.

  • Define hermaphrodites and discuss two advantages of this strategy in low population densities.

  • Explore the C-fern life cycle and consider a significant question for lab investigation.

Definition of Sexual Reproduction

  • Sexual Reproduction: A biological process wherein offspring are produced through the combination of genetic material from two individuals.

  • Processes involved in sexual reproduction absent in asexual reproduction include:

    • Meiosis: Reduction division leading to the formation of gametes.

    • Fertilization: Fusion of male and female gametes.

Reproductive Handicap

  • Twofold Cost of Sex:

    • Requires two parents for reproduction.

    • Only half of the population (females) can produce offspring, while asexual reproduction allows for full population contribution.

  • Implication: In populations capable of both sexual and asexual reproduction, the number of asexual offspring will greatly outnumber sexual ones.

  • Thus, from a natural selection standpoint, sexual reproduction appears to be less fit, creating the concept of a reproductive handicap.

Pros and Cons of Reproductive Strategies

  • Asexual Reproduction

    • Pros:

    • No mate is required for reproduction (DIY).

    • Offspring inherit all genes from a single parent, resulting in a stable genetic lineage.

    • Cons:

    • Limits genetic variation, potentially making populations less adaptable to environmental changes.

  • Sexual Reproduction

    • Pros:

    • Increases genetic variation in offspring, enhancing adaptability.

    • Helps eliminate deleterious mutations over generations through recombination.

    • Cons:

    • Offspring inherit only a fraction of each parent's genes, thus potentially reducing overall fitness.

    • Higher vulnerability to predators and diseases.

    • Greater resource expenditure in courting mates and attracting partners.

    • Increased risk of sexually transmitted infections (STIs) that can spread during mating.

Mechanisms of Genetic Variation Through Sexual Reproduction

  1. Independent Assortment: The random distribution of maternal and paternal chromosomes during gamete formation.

    • Possible combinations described by 2^n, where n is the number of chromosome pairs.

    • Example: For humans (n=23), results in about 8,388,608 genetically unique gametes.

    • Random fertilization can further increase genetic diversity, leading to over 70 trillion unique zygotes.

  2. Crossing Over (Recombination): Occurs during meiosis when homologous chromosomes exchange genetic material, producing recombinant chromosomes.

    • Enhances genetic variation by mixing genes from both parents.

  3. Random Fertilization: The combination of any sperm with any egg leads to numerous possible genetic outcomes.

Influence of Environment on Selection for Sexual Reproduction

  • Research Question: When is genetic variation adaptive?

  • Investigate the type of environments that favor sexual reproduction over asexual reproduction, contrasting stable environments with fluctuating ones.

Case Study: Snails and Trematodes

  • Species: Potamopyrgus antipodarum

    • Acts as a host to over 12 species of parasitic worms (trematodes).

    • Males reproduce sexually (diploid) while females can be either sexual (diploid) or asexual parthenogenic (triploid).

  • Selective Pressure from Trematodes: Infection from trematodes results in the destruction of gonads, hindering reproductive capability.

    • This pressure drives the evolution of trematode resistance, creating a selective environment that favors sexual reproduction due to increased genetic diversity and adaptability.

Investigative Approach: New Zealand Lakes Study

  • Hypothesis: A higher frequency of males correlates with increased numbers of sexually reproducing females.

  • Data Collection: Observe patterns in male frequency versus total infection rates in snail populations.

Red Queen Hypothesis

  • Genetic variation from sexual reproduction empowers host organisms to adapt to changing environments, particularly in response to internal parasites and pathogens.

  • The coevolution of hosts and parasites creates a significant selective pressure favoring sexual reproduction.

Key Takeaways

  1. Both sexual and asexual reproduction possess advantages and disadvantages, influencing evolutionary strategies.

  2. Sexual reproduction's handicap in terms of offspring quantity contrasts with its benefits in terms of genetic diversity.

  3. Variable environments will likely select for sexual reproduction as organisms adapt to survive against pathogens and allergens.

Hermaphrodites

  • Hermaphroditic individuals are capable of producing both eggs and sperm.

  • Inquiry: Is self-fertilization considered sexual or asexual reproduction?

  • Discuss whether hermaphrodites evolve in high or low population densities.

  • Advantages of Hermaphroditism in Low Density:

    • Ability to self-fertilize when no mate is found.

    • Capability to mate with any encountered individual of the species, enhancing reproductive opportunities in sparse populations.

C-Fern Life Cycle: Ceratopteris richardii

  • Gametophytes play a crucial role in reproduction, facilitating the fertilization process between sperm and egg.

  • Investigate the correlation between spore density and the sex ratio of gametophytes:

    • Formulate hypotheses and predictions regarding the percentage of hermaphrodites or males relative to spore density.

Broader Inquiry

  • Ultimately, what evolutionary advantage do males contribute to the reproductive landscape?