Download Natural selection on single gene traits

16.2 Evolution as Genetic change
Biology
Mr. Hines
A large part of genetics is understanding that
reproduction and passing on “fit” traits is
vital.
If an organism lives to reproduce, it was
obviously successful at surviving.
The traits that were necessary to survive will
be passed on to another generation.
Natural selection works on survival and
reproduction.
If an organism does not possess the traits for
survival, it will die, and therefore not pass
on its genes.
This process enables perfection over time.
What is evolution?
Evolution is any change over time in the
relative frequencies of alleles in a
population.
Individual animals do not evolve, entire
populations do.
Natural selection on single gene
traits
Natural selection on single-gene traits can
lead to changes in allele frequencies and
thus to evolution.
Take for example a hypothetical population of
lizards.
80% of the lizards are brown
10% are red
10% are black
If red lizards are more visible to predators,
they would be less likely to survive.
Black lizards might absorb more sunlight and
warm up faster which could allow them to
escape a predator more effectively.
The red lizard can not reproduce if it is dead
and will not affect the gene frequency.
The black lizard will live to pass on its trait.
This could change the gene frequency.
When a gene frequency of an allele changes,
we have evolution
Change of gene frequency =
Evolution
•
Natural Selection on Polygenic
traits
When traits are controlled by more than one
gene, the affects of natural selection are
more complex.
Human height is an example of a polygenic
trait.
Natural selection can affect the distributions
of phenotypes in any of 3 ways.
1. Directional selection
2. Stabilizing selection
3. Disruptive selection
Directional selection
We will use beak size to demonstrate this
idea.
The dotted bell curve represents a wide range
of beak sizes.
The bell curve peaks where beak size is most
common.
If the food supply of small seeds diminishes, birds
with larger beaks would have an advantage.
This would cause beak size in a population to shift
towards large beaks.
The bell curve would move to the right in this case.
This is called directional selection
Stabilizing Selection
We will use human birth weight to
demonstrate this idea.
Human babies can be born in a range of birth
weights.
If a baby is born too small, its survival chances are
lower.
If the baby is born too large, complications in birth
arise, and its survival chances are lower.
A range of birth weights will narrow based on
survival rates.
The bell curve will become more narrow and steeper
This is called stabilizing selection
Disruptive selection
We will use beak size to demonstrate this
idea.
If a population of birds has a wide range of
beak sizes, it will have a normal bell curve
This would mean that the food supply is very
diverse.
If the food supply changed so that medium
sized seeds disappeared, and small and
large seeds became more common, small
beaks and large beaks would be the
strongest traits
This would cause the bell curve to be divided.
The population would split into 2 subgroups.
This can also lead to new species of birds –
birds with large beaks are one species, and
birds with small beaks are a different
species.
This is called disruptive selection
Genetic drift
Natural selection is not the only force in
evolution.
Genetics are controlled by the laws of
probability – or chance.
Alleles can grow in number in a gene pool
simply by chance.
This is called genetic drift.
Genetic drift happens more quickly in a small
population than a large population.
We will use beetles to demonstrate this idea.
Lets say that this sample of beetles lives on
the continent of South America.
•
A Typhoon blows through the
area and carries a different
sample of beetles to the
Galapagos.
Notice that sample of beetles is different on
each island
If these beetles reproduce, it would produce
the following outcome.
Therefore, the population of beetles on each
island is different based and random
chance.
This is genetic drift.
Evolution vs Genetic equilibrium
Another look at evolution is to determine
what happens when no change takes place.
Are there conditions when evolution will not
occur?
The Hardy-Weinberg principle states that if
evolution does not occur, the allele
frequency will remain constant.
That can be stated in reverse. If the allele
frequency remains constant, evolution is
not occurring.
This condition is called genetic equilibrium.
What conditions will cause genetic
equilibrium?
1. Random mating
2. The population must be very large
3. There can be no movement in and out of
the population
4. No mutations
5. No natural selection
Random mating
All individuals have an equal chance of
producing offspring.
This is uncommon in a population – usually
the female will decide who she will mate
with.
This is called sexual selection**sexual
selection is a major force in evolution – it is
barely mentioned in the book.
Large populations
Some populations are so large that genetic
drift never occurs.
No movement in or out of a
population
Any organism moving in or out of a
population will certainly change the gene
frequency
No mutations
No mutations can occur if an allele frequency
is to remain unchanged.
No natural selection
All genotypes must have an equal chance of
survival and reproduction
If all 5 of these conditions are not met, a
population will evolve.
There is no known population that meets all 5
conditions, therefore all populations are
evolving.
This can be restated that non-evolution is
impossible!