Sexual versus Asexual Reproduction
Type of Reproduction
Sexual
Methods
Disadvantages
Internal fertilization
Diversity in offspring
External fertilization
Offspring less likely
to have mutations
show up
Population increases
are limited
Can increase
populations rapidly
Lack of diversity in
offspring
Does not require a
mate for
reproduction to
take place
Because they
reproduce offspring
genetically identical
to parents, the
offspring inherit any
mutations of the
parent.
Conjugation
Budding
Asexual
Advantages
Requires a mate to
reproduce
Spores
Fission
• 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
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?
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
Internal & External Fertilization
D. Crowley, 2007
Wednesday, March 23, 2016
Internal & External fertilization
• To know the difference between internal and external
fertilization
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
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
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)
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)
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)
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
Animal
Number of egg cells
fertilization type
Bat
1
Internal
Bullfrog
20’000
External
Cat
4
Internal
Cod
5’000’000
External
Tiger
2
Internal
– 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!
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