CHANGES IN GENE POOLSGene pool: all of the alleles in a population.

Changes in a gene pool are the basic requirement for evolution to be able to occur.

What can cause a change in a gene pool?

Mutation

Natural Selection

Mate Selection

Migration

Genetic drift

Keywords

• Selection pressure: An environmental factor that drives evolution in a particular direction.

• Directional selection: Occurs when natural selection favors one of the extreme variations of a trait

• Disruptive selection: Natural selection that favors individuals with either extreme of a trait

• Stabilising selection: A type of natural selection in which genetic diversity decreases as the population stabilises on a particular trait value

Starter quiz

1. What two observation did Darwin make about populations that lead to natural selection?

2. What is a deme?

3. When a red flower and white flower combine to make a pink flower, what type of allele interaction is this?

4. What type of genes can be separated by recombination?

NATURAL SELECTIONIn a population there is variation in alleles, some combinations will result in a more beneficial phenotype.

The individuals with the more beneficial alleles will most likely be the ones to survive and pass on their alleles to their offspring (“selected for”). Other individuals may die (or not breed) because of their less beneficial phenotype (“selected against”).

This causes a change in the gene pool.

Natural Selection may be:Directional StabilisingDisruptive

Normal

Mutant

DIRECTIONAL SELECTIONExample: Peppered moths

Two forms of peppered moth exist in England.

The normal form, which is well adapted to be camouflaged amongst lichen, and a naturally occurring black mutant.

After the beginning of the industrial revolution, the black form began to become more prevalent near industrial areas until it made up 90% of the population.

Predated on (selection pressure)

STABILISING SELECTIONExample: Human birth weight

There is an optimum weight for human babies.

Too much lower and the baby may be too weak to survive.

Too high increases the chance of complications during birth.

See Biozone pg 336

Birth weight Chance of mortality (selection pressure)

DISRUPTIVE SELECTIONExample: Seed eating finches

In a population of finches there is variation in beak size. This allows the finches to feed on a variety of seeds.

If a species of seed tree were to die out, so that the middle sized finch had a large reduction in food source, the largest and smallest finches would be selected for.

Biozone pg 334Food shortage

(selection pressure)

MATE SELECTIONMany populations of animals do not choose their mates randomly. Mates may be chosen based on size, colouration, displays and courting rituals etc.

Mate selection causes change in gene pools as some individuals are selected for and others against.

Example: Gorilla

The dominant male (silverback) gorilla has all of the breeding rights in the group. Only the alleles leading to the phenotype of strength and size will be passed on.

This situation would also result in sexual dimorphism (males and females look different. Biozone pg 335

MIGRATIONNew individuals can bring new alleles into a population (immigration), or rare alleles can be removed from the population when individuals leave (emigration).

Both of these will change the allele proportions in the gene pool.

The movement of alleles between populations is called gene flow. Usually gene flow between two populations causes them to remain genetically similar.

Example: Tussocks

An alpine tussock and lowland tussock will diverge due to differing selection pressures.

However, if the prevailing winds blow pollen from one population to the other, gene flow will occur and the divergence may be lessened.

GENETIC DRIFTWhen the number of individuals in a population is small, random events or chance may cause a significant change in the gene pool. Large populations are much less subject to genetic drift.

There are 2 main situations that can lead to small populations and, therefore, genetic drift:

The Founder Effect

Population Bottlenecks

Small population (10):

Allele frequencies fluctuate wildly and most are lost by chance.

Larger Population (100):

Allele frequencies are much more stable and few are lost. Variation is retained.

Biozone pg 339

THE FOUNDER EFFECTA small number of individuals colonise a new area.

The alleles they carry may be not a fair representation of the original population.

Rare alleles may become common, some others may be eliminated entirely in the new population.

Example: The Amish

Amongst the (200) founders of the Amish population were one couple who carried a recessive gene mutation for Ellis-van Creveld syndrome. Symptoms include short stature, polydactyly (extra digits), and many have a hole between the top 2 chambers of the heart.

Because of their closed population, members of the population intermarry and the recessive traits show up much more commonly (gene pool changed: genetic drift).

The EVC gene is located on the short (p) arm of chromosome 4 at position 16.More precisely, the EVC gene is located from base pair 5,763,824 to base pair 5,881,684 on chromosome 4.

THE FOUNDER EFFECT

Polydactyly

Biozone pg 337

POPULATION BOTTLENECKSome populations have been subjected to events that have almost wiped them out e.g. disease, drought, flood, fire, climatic changes.

As in the founder effect, the small number of individuals remaining in the population may be not a representative sample of the original alleles. This can cause genetic drift.

Example: Cheetahs

It is believed that around the last ice-age the cheetah population was decimated (along with the mammoth, sabre toothed tigers, etc).

Only a handful survived, and went on to repopulate to the 12,000 cheetah now surviving.

These individuals show remarkable genetic similarities, so much so that they often can accept each others’ skin grafts.

An NZ example is the Chatham Is. Black robin (pop. Decreased to 7, inc. one female).

Biozone pg 338