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Regents Living Environment
16-2: Evolution as Genetic Change
Oswego High School
Looking at evolution from a genetic standpoint offers a new way to look at key
evolutionary concepts. Every time organisms reproduce, they pass on their
genes to their offspring. We can look at evolutionary fitness as how successful
an organism is in passing on its genes. We can also view adaptations as any
genetically controlled trait that increases an organism’s chances of survival.
Natural selection rarely acts on genes alone because it is an entire organism that
survives to reproduce or dies without reproducing. Therefore, natural selection can only
affect which individuals survive and reproduce and those who don’t. If an individual
dies without reproducing, they do not contribute their genes to the population. If an
individual produces many offspring, its alleles will stay in the gene pool and may
increase in frequency.
Recall that evolution is the change in relative frequency of alleles in a
population. This means that evolution can only occur in populations and not
individual organisms
Natural Selection on Single-Gene Traits
Natural selection on single gene traits can lead to changes in allele frequencies and
thus can lead to evolution.
For Example:
There is a population of brown lizards. A mutation occurs that turns some of the
lizards black and red. The red lizards become visible to predators and their alleles will
be less likely to be passed on. The black color may allow the lizards to absorb
sunlight and cause them to warm up faster on cold days. If high body temperature
means that the lizard will escape predators, the black lizards may produce more
offspring than the brown lizards. The black allele may increase in frequency over
time.
If a change in color does not affect fitness, the allele that produces the color change will
not be under pressure from natural selection.
When traits are controlled by more than one gene, the effects of natural selection are
more complex. As we said before, the action of multiple alleles on traits can produce
a wide range of phenotypes that fit a bell curve.
The fitness of those close to each other on the curve will not be much different.
However, fitness can vary from one end of the curve to another. Natural selection acts
where fitness varies.
Natural selection affects allele distributions in one of three different ways.
Stabilizing Selection
Stabilizing selection occurs when individuals in the Middle of the curve have a higher
fitness than those at the ends. Stabilizing selection selects for the intermediate and
against the extremes.
Example: The mass of human babies is under the influence of stabilizing selection.
Babies of below average mass are more susceptible to illness and are less likely to
survive. Babies born of above average mass will be more difficult to pass through
the birth canal. These babies will be less fit than those who are born of average
mass.
Directional Selection
Directional selection occurs when individuals on one end of the curve have a higher
fitness than those in the middle and the other end of the curve. Directional selection
selects for one extreme and against the middle and the other extreme.
Example: Finches with larger beak sizes will be able to feed on harder and thicker
seeds. A food shortage causes the number of small and medium sized seeds to
decrease. Finches with larger beaks will have better access to food than those with
small and medium sized beaks. This results in the large beaked finches having a higher
fitness than those with small and medium sized beaks and the average beak size of
the population would increase.
Disruptive Selection
Disruptive selection occurs when individuals at both ends of the curve have a higher
fitness than those in the middle. Disruptive selection selects for both extremes and
against the middle
If natural selection is strong enough and lasts long enough, two distinct phenotypes
can be created.
Example: Finches live in an area where medium sized seeds are less common, while
small and large seeds become more common. This means that birds with
______________ and ______________ beaks will have a higher fitness than those with
________________ sized beaks.
Genetic Drift
Natural selection is not the only source of evolution. Sometimes, in small populations,
evolution can occur by alleles becoming more or less common by chance. This
random change in the allele frequency is called genetic drift
In small populations individuals that have a certain allele may produce more offspring
that others strictly by chance.
Founder Effect
One way genetic drift can occur is when a small group of individuals from a population
colonize a new area. It is possible that these individuals have a different allele
frequency than the larger population. As a result of this, the new population will be
genetically different than the original population.
The situation where a small number of individuals from a larger population colonize a
new habitat, creating a new gene pool, is called the Founder Effect
Bottleneck Effect
Genetic drift can also occur as a result of a random event. These events may
include floods, earthquakes, hurricanes or forest fires
As a result of these chance events, the majority of the population is wiped out,
leaving few survivors. These survivors have a much smaller gene pool than the
original population. These individuals will mate with each other, causing the offspring to
be more related to each other than they would have in the original population.
Changes in allele frequency as a result of chance events is called Bottleneck
Effect
Evolution vs. Genetic Equilibrium
Scientists often find it helpful to determine what happens when no change takes
place in order to clarify how evolutionary change takes place.
The Hardy-Weinberg Principle states that allele frequencies in a population will
remain constant unless one or more factors cause them to change.
The situation in which allele frequencies remain constant is called Genetic
Equilibrium
There are five factors that are required to maintain genetic equilibrium.
1. Random Mating
Ensures that all individuals in a population have an __________________ chance of
reproducing and passing their alleles on to the next generation.
In natural populations, mating is almost __________________ random. Many species
select mates based on certain __________________ traits like size and strength.
This is called __________________ _____________________.
If there is non-random mating, genes for certain traits are not in equilibrium and are
under ______________ __________________.
2. Large Population
Large populations help maintain genetic equilibrium because genetic drift has
_____________ effect on larger populations than on small populations.
3. No movement into or out of the population
If new individuals enter a population, they bring ___________ alleles into the
population.
In order to stay in equilibrium, a population’s gene pool must be kept _______________
and __________________ from the gene pools of other populations.
4. No Mutations
Mutations cause __________ alleles to be introduced into a population, causing a
change in ________________ ____________________.
5. No Natural Selection
All genotypes need to have an ______________ chance of survival in the population.
No phenotype can have a selective _____________________ over others.
All of these conditions must be met in order for genetic equilibrium to take place.
If any or all of these are disrupted, evolution will occur.