11.2 Sexual Reproduction - Biology 2e | OpenStax

Learning Objectives

• Explain meiosis and sexual reproduction as highly evolved traits.
• Identify variation among offspring as an evolutionary advantage of sexual reproduction.
• Describe the three life-cycle types among sexually reproducing multicellular organisms.

Introduction to Sexual Reproduction

• Sexual reproduction likely evolved after eukaryotic cells.
• Most eukaryotes can reproduce sexually; some rely solely on this mode.
• Asexual reproduction: offspring are genetic clones; advantageous in stable environments.
• Sexual reproduction: provides genetic diversity important for population survival.
• Disadvantages: slower population growth compared to asexual reproduction.

Advantages of Sexual Reproduction

• Variation from sexual reproduction is critical for survival and reproduction.
• Mutations and genetic shuffling increase diversity.
• In sexually reproducing populations, variation is not limited to mutations, unlike asexual populations.

Evolution Connection: The Red Queen Hypothesis

• Proposed by Leigh Van Valen in 1973.
• Species coevolve with others (e.g., predators, prey, parasites).
• Constant adaptation is necessary for survival; species must improve fitness continuously to compete.
• Genetic variation from sexual reproduction helps species keep up with evolving challenges.

Life Cycles of Sexually Reproducing Organisms

Diploid-Dominant Life Cycle (e.g., Animals)

• Only haploid cells are gametes.
• Diploid germ cells undergo meiosis to produce gametes.
• No multicellular haploid stage; fertilization restores diploid state.

Haploid-Dominant Life Cycle (e.g., Fungi, Algae)

• The multicellular stage is haploid.
• Haploid cells form diploid zygotes, which undergo meiosis to create spores.
• Spores form new haploid organisms.

Alternation of Generations (e.g., Plants, Some Algae)

• Both haploid and diploid multicellular organisms exist.
• Haploid gametophytes produce gametes; diploid sporophytes arise from fertilization.
• Sporophyte produces spores through meiosis, developing into gametophytes.

Visual Connections

• Images illustrate life cycles of animals, fungi, and plants.
• Importance of different genetic combinations through life cycles.

Conclusion

• Sexual reproduction is predominant due to its advantage in creating genetic diversity.
• Nearly all multicellular organisms employ sexual reproduction, supporting its evolutionary benefits.

References

• Leigh Van Valen's Red Queen Hypothesis.
• Illustrations of animal, fungi, and plant life cycles.