Download Lesson 6: Reproduction and Variation

Lesson 1.20: Reproduction and
Variation
Key Points:
1. Distinguish between sexual and asexual reproduction.
2. Identify examples of each type of reproduction.
3. Understand how variation is produced in each type.
4. Evaluate the benefits and drawbacks of each type in
terms of adaptation to the environment and production of
variation.
Asexual Reproduction
Strictly speaking “like begets
like” only applies to organsisms
that reproduce asexually. In
asexual reproduction, a
single individual is the sole
parent and passes copies of all
its genes on to its offspring.
For example, single celled
organisms can reproduce
asexually by mitosis (cell
division), in which DNA is
copied and allocated equally to
two daughter cells. The
offspring are exact copies of
the parent.
Asexual Reproduction
Some multicellular organisms
are also capable of
reproducing asexually. Hydra
a relative of the jellyfish, can
reproduce by budding (pic).
Since the cells of the bud were
derived by mitotic cell division
from the parent, the “chip off
the old block” is usually (99%
of the time) genetically
identical to the parent.
Very rarely genetic differences
caused by changes in the
DNA of the parent (called
mutations) cause small
differences.
Asexual Reproduction
Some plants and other animals can reproduce asexually
as well. For example potatoes produce “little eyes” that
are actually buds – groups of cells that undergo rapid
cell division and develop into new plants. When
potatoes are not used the buds begin to sprout and
grow. In plants asexual reproduction is called Vegetative
Propagation.
Asexual Reproduction
Another example is the starfish. Starfish prey on oysters so
divers would collect the starfish in an attempt to save the
oyster beds. After the divers collect the starfish they would
tear them apart and throw the pieces back in the water. A
sea star arm however, can regenerate an entire sea star if
part of the central body is attached to it. So in an effort to rid
the oyster beds of predators the divers actually made their
problem worse!
Any individual that reproduces asexually gives rise to a
clone, a group of genetically identical individuals.
Other Examples of Asexual Reproduction
Ameoba
Mold
Bamboo
Sexual Reproduction
(Lets keep our minds out of the gutter please!)
Compared to asexual reproduction, sexual reproduction usually results
in greater variation; two parents give rise to offspring that have unique
combinations of genes inherited from both parents, for example half of
your genes came from your mother and half from your father. In contrast
to a clone, offspring of sexual reproduction vary genetically from their
siblings and both parents. What are some advantages of both types of
reproduction?
Sexual Reproduction
One way sexual reproduction generates genetic variation is
shown in the figure below which color codes the
chromosomes so that we can track them as they are
packaged in gametes (eggs and sperm). The two colors are
used to distinguish which chromosomes are inherited from
the mother and from the father.
So there is a
fifty/fifty chance that
a gamete produced
by an offspring will
have a maternal
chromosome or a
paternal one.
Sexual Reproduction
In the case of humans the
number of possible
combinations of maternal
and paternal
chromosomes in the
resulting gametes is 223,
or about 8million. Thus
each gamete that a
human produces contains
one of 8 million possible
assortments of
chromosomes inherited
from that individuals
mother and father.
Crossing over produces
even more variation
Some organisms use both methods
Some organisms (like the paramecium) and most plants
can reproduce using either method. This can be highly
advantageous, Why?