Sexual versus Asexual Reproduction

1. Sexual versus Asexual Reproduction

2. Definitions: - Diversity in offspring > def. – Diversity - the relative uniqueness of each individual in the population - offspring less likely to have mutations show up > def. – Mutation – changes in DNA

3. Identical? • Look at the following - it shows two sequences how identical and non-identical twins are formed • Can you tell which is which? Can you explain how you know?

4. Identical? • Identical twins can form because once a sperm has fertilized the egg, the ball of cells (embryo) can split into two. If this happens you get two identical twins • Non-identical twins can form, because sometimes a female can ovulate and produce two eggs in a month. These are then fertilized by a different sperm each, so you produce non-identical twins • Unique individuals are due to individual sperm and eggs fertilizing one another

5. Internal & External Fertilization D. Crowley, 2007

6. Internal & External fertilization • To know the difference between internal and external fertilization

7. fertilization • Different organisms fertilize in different ways • Can you think of any organisms which fertilize differently to humans (e.g. think about frogs and fish) • For fertilization to occur, the male sperm cells must get to the female egg cells. • There are two main ways for this to occur - internal and external fertilization

8. External fertilization • External fertilization occurs outside the female - the male releases sperm cells onto the egg cells as the female lays them • External fertilization occurs in fish and most water dwelling animals (e.g. amphibians). These organisms usually produce a large number of sex cells which are released into the water. • There are usually courtship rituals which make sure the male and female sex cells are released near each other. • However, the chances of fertilization are very low, so huge numbers of eggs and sperm are needed to ensure enough offspring are produced

9. Internal fertilization • Internal fertilization occurs inside the female • Internal fertilization occurs in most land dwelling organisms (such as mammals) - this is necessary as sperm need fluid in order to be able to swim to the egg (the fluid comes from semen, ejaculated along with the sperm from the male). • These organisms produce far fewer sex cells, as the chances of successful fertilization are much higher (the sex cells are much closer together when they are released)

10. How Many? • Why is it that organisms which produce by external fertilization produce so many more sex cells? • The chances of fertilization are much lower in externally fertilized organisms, as the sex cells are further apart from each other: - • This means the sex cells are more likely to be predated • They can also be separated from one another (due to currents) • In external fertilization there is also less parental care, so the offspring are more likely to be killed (hence producing lots of them)

11. How Many? • Why then are there so few sex cells produced in internal fertilization? • As the likelihood of successful fertilization is higher in internally fertilized organisms, because the sex cells are so much closer together, producing a great many sex cells would cause problems because: - • There is only a limited amount of space within the female’s uterus to accommodate a growing foetus • In internal fertilization there is a greater deal of parental care, so the offspring are more likely to survive (hence there is no need to produce lots of them)

12. Understanding • A bullfrog produces 20’000 eggs - because it reproduces externally, so needs many eggs as a great deal will be predated / may become separated • The Lake Malawi mouthbrooder probably got its name because it cares for its young (after they have been fertilized) within its mouth, where they grow. Brood can mean to be anxious about something, as well as meaning the young that are all born and reared at a similar time. So the mouthbrooder is anxious for her young which have all been born at a similar time, and looks after them in her mouth!

13. Internal & External • Internal fertilization • Eggs produced in small numbers • Method usually by land animals (water is not available for the sperm to swim) • Method has a better chance of the offspring surviving due to improved protection • Human, butterfly, horse • External fertilization • Eggs produced in very large numbers • Used by fish because water is available for sperm to swim in • Sperm can get eaten by aquatic predators • Fish, frog