Overview
This lecture covers the processes and adaptations involved in fertilization, comparing internal and external fertilization, and discusses the evolution of reproductive systems in animals.
Methods of Fertilization
- Fertilization is the fusion of sperm and egg, occurring either internally or externally.
- Humans are an example of internal fertilization, while many aquatic animals use external fertilization.
External Fertilization
- Occurs mainly in aquatic environments where eggs and sperm are released into water (spawning).
- Spawning is often synchronized by environmental triggers like temperature or daylight.
- Broadcast spawning leads to genetic diversity but low offspring survival, requiring millions of eggs.
- Some fish use courtship to target fertilization and enhance chances for offspring survival.
- Eggs are protected from drying out in water but face high rates of predation.
Internal Fertilization
- Most common in land animals, but some aquatic animals also use internal fertilization.
- Oviparity: Eggs are laid outside, nourished by yolk (seen in most birds, fish, reptiles, and some mammals).
- Ovoviviparity: Eggs develop inside parent, embryos nourished by yolk, hatch fully formed (some fish, sharks, lizards, snakes, and insects).
- Viviparity: Young develop inside female, nourished by placenta, and are born alive (most mammals, some fish, a few reptiles).
- Internal fertilization increases survival by protecting eggs and limiting predation, but typically produces fewer offspring.
Evolution of Reproductive Systems
- Early multicellular animals developed organs for reproduction, starting with Annelids.
- Reproductive organs (gonads) evolved to produce eggs and sperm via meiosis.
- Insects have complete reproductive systems, sperm storage (spermatheca), and time fertilization for optimal survival.
- Birds and reptiles have a cloaca for reproduction, excretion, and digestion; mammals have separate openings and a uterus for offspring development.
- Sperm transfer methods include external release, cloacal contact in birds, or direct delivery via penis in mammals.
Key Terms & Definitions
- Fertilization — The union of sperm and egg to form a zygote.
- External fertilization — Fertilization outside the body, usually in water.
- Spawning — The simultaneous release of eggs and sperm into the water.
- Internal fertilization — Fertilization inside the female’s body.
- Oviparity — Laying eggs that develop outside the body.
- Ovoviviparity — Retaining eggs inside the body until they hatch.
- Viviparity — Giving birth to live young developed inside the parent.
- Gonads — Organs that produce gametes (sperm and eggs).
- Cloaca — A common opening for digestive, excretory, and reproductive tracts in some animals.
- Spermatheca — Storage sac for sperm in female insects.
Action Items / Next Steps
- Review the differences between oviparity, ovoviviparity, and viviparity.
- Study the anatomical adaptations supporting internal and external fertilization.
- Prepare for questions comparing advantages and disadvantages of each fertilization method.