Rucete ✏ Campbell Biology In a Nutshell
Unit 7 ANIMAL FORM AND FUNCTION — Concept 46.1 Both Asexual and Sexual Reproduction Occur in the Animal Kingdom
Animal reproduction involves both sexual and asexual strategies, each offering advantages in specific environments. These strategies reflect evolutionary adaptations that maximize reproductive success under different ecological and physiological constraints.
1. Sexual and Asexual Reproduction
- Sexual reproduction involves fusion of haploid gametes (egg and sperm) to form a diploid zygote
- Asexual reproduction produces offspring without gamete fusion, usually via mitotic division
- Common asexual mechanisms include:
- Budding (e.g. corals)
- Fission (e.g. flatworms)
- Fragmentation and regeneration (e.g. sponges, annelids)
- Parthenogenesis: development from an unfertilized egg (e.g. honeybee drones, some lizards)
2. Variation in Sexual Strategies
- Hermaphroditism: each individual has both reproductive systems; common in sessile or parasitic species
- Hermaphrodites can self-fertilize or exchange gametes with others (e.g. sea slugs)
- Sex reversal occurs in species like:
- Bluehead wrasse: large female changes to male after dominant male dies
- Oysters: start as male, become female later to maximize egg output
3. Reproductive Cycles and Environmental Cues
- Most animals have reproductive cycles linked to seasons and regulated by hormones
- Reproduction is timed for optimal survival of offspring (e.g. spring lamb births)
- Climate change can disrupt timing—e.g. caribou birth no longer matches plant sprouting, lowering survival
- Some species, like Daphnia, switch between asexual and sexual reproduction based on conditions
4. Behavior and Hormonal Triggers in Asexual Species
- Aspidoscelis lizards (all-female) show mating behaviors despite reproducing by parthenogenesis
- Hormone cycles (estradiol and progesterone) drive male- and female-like behaviors, stimulating ovulation
- Suggests evolution from sexual ancestors; behavior enhances reproductive success
5. Evolutionary Enigma of Sexual Reproduction
- Sexual reproduction has a twofold cost: only half of offspring are female, slowing population growth
- However, sexual reproduction is widespread due to benefits like:
- Genetic variation through recombination and fertilization
- Increased adaptability in changing or pathogen-rich environments
- Faster elimination of harmful mutations
6. Fertilization Strategies
- External fertilization: eggs and sperm released into environment (e.g. frogs, corals); requires moist habitat and precise timing
- Spawning: synchronized release of gametes, often triggered by cues like lunar phase or temperature
- Internal fertilization: sperm deposited in/near female tract; increases survival chances but often requires more complex behavior
In a Nutshell
Animals reproduce through a wide variety of sexual and asexual strategies. Sexual reproduction introduces genetic diversity but carries a reproductive cost. Asexual modes like budding and parthenogenesis are efficient in stable environments. Evolution has also shaped unique adaptations such as hermaphroditism and sex reversal. Fertilization, whether external or internal, must align with environmental conditions to ensure reproductive success.