Overview
This section explains the evolutionary advantages of sexual reproduction, discusses the role of genetic variation, and describes three different life-cycle strategies found in sexually reproducing multicellular organisms.
Evolutionary Significance of Sexual Reproduction
- Sexual reproduction is a highly successful and early evolutionary trait in eukaryotes.
- Asexual reproduction offers benefits like rapid population growth and not requiring a partner.
- Despite these advantages, asexual-only multicellular organisms are rare.
- Sexual reproduction may be favored due to the genetic variation it produces among offspring.
Advantages of Genetic Variation
- Variation in sexually produced offspring increases population adaptability and survival.
- Asexual populations rely solely on mutation for variation, while sexual populations reshuffle genes each generation.
- Crossing over and independent assortment during meiosis further increase genetic diversity.
The Red Queen Hypothesis
- The Red Queen hypothesis states that ongoing genetic variation is necessary because species coevolve with competitors, predators, and parasites.
- Constant genetic improvement is needed to maintain reproductive success in changing environments.
- Coevolving species must continually adapt or risk extinction.
Sexual Life-Cycle Strategies
- Sexual life cycles alternate between meiosis (reduction division) and fertilization (restoring diploid condition).
- Animals use a diploid-dominant cycle: multicellular diploid adults produce haploid gametes, which fuse to make a zygote.
- Fungi and some algae have a haploid-dominant cycle: multicellular bodies are haploid; diploid zygote undergoes meiosis immediately.
- Plants and some algae show alternation of generations, with both multicellular haploid (gametophyte) and diploid (sporophyte) stages.
Examples of Life Cycle Types
- In most animals, only gametes are haploid; no multicellular haploid stage exists.
- In fungi, the main body is haploid; zygote is the only diploid stage.
- In plants, sporophytes (diploid) produce haploid spores, and gametophytes (haploid) produce gametes.
Key Terms & Definitions
- Meiosis — cell division that halves chromosome number, producing gametes or spores.
- Genetic variation — differences in DNA among individuals in a population.
- Red Queen hypothesis — evolutionary theory explaining continual adaptation for species survival.
- Germ cells — specialized diploid cells that divide to form gametes.
- Gametes — haploid sex cells (sperm or egg).
- Zygote — diploid cell formed from the fusion of two gametes.
- Sporophyte — diploid multicellular stage in plants that produces spores.
- Gametophyte — haploid multicellular stage in plants that produces gametes.
Action Items / Next Steps
- Review the main types of sexual life cycles: diploid-dominant, haploid-dominant, and alternation of generations.
- Be prepared to compare advantages of sexual vs. asexual reproduction for upcoming discussions or exams.