1. Sexual versus Asexual Reproduction
Type of Reproduction
Methods
Advantages
Disadvantages
Sexual
Internal fertilization
Diversity in offspring
Offspring less likely to have mutations show up
Requires a mate to reproduce
Population increases are limited
External fertilization
Conjugation
Asexual
Budding
Can increase populations rapidly
Does not require a mate for reproduction to take place
Lack of diversity in offspring
Because they reproduce offspring genetically identical to parents, the offspring inherit any mutations of the parent.
Spores
Fission

2. - Diversity in offspring
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
Sunday, April 02, 2017
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!
Animal
Number of egg cells
fertilization type
Bat 1
Internal
Bullfrog
20’000
External
Cat 4
Cod
5’000’000
Tiger 2

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