Gene Pools and Evolution

Evolution (What is it?)

• the change in characteristics over time

• is a slow process

Species (What is it?)

• is a group of organisms which can interbreed and produce fertile offspring

Population (What is it?)

• A group of the same species

• a population can be further broken down into gene pools

Gene Pools (What is it?)

• The term ‘gene pool’ describes all the genes within a population

• Across the planet, many human gene pools exist of varying size and composition

• affected by evolutionary mechanisms including natural selection and chance occurences

• contain a variety of alleles

• constantly changing due to changes in allele frequency

Allele frequency (What is it?)

When scientists study a population, they look at how often an allele of a gene is common in a gene pool.

1. Factors that influence allele frequencies (Mutations)

• Mutations are a sudden and permanent change that can occur at the gene or chromosome level

• Can occur in somatic or germline cells

Factors that influence allele frequencies (Mutations) (Mutations lead to changes in allele frequencies)

1. Mutations introduce new alleles into the population/muations cause variation.

2. Alleles may produce traits favourable to survival.

3. Favourable alleles are passed on to offspring

4. Favourable traits will increase in number within the gene pool/natural selection will produce changes to gene pool.

2. Factors that influence allele frequencies (Natural Selection)

• Natural Selection=survival of the fittest

• Individuals who have more heritable traits (likely to survive and reproduce) will pass those favourable characteristics onto the next generation.

• less favourable won’t be passed on

• This not by chance

Factors that influence allele frequencies (Natural Selection) (Variation)

• Individuals are not all the same because of differences in their genes

• Some of these differences may give some individuals a selective advantage over others

Factors that influence allele frequencies (Natural Selection) (Struggle for existence)

• Due to excessive birth rate and limited resources, there will be a struggle between individuals.

Factors that influence allele frequencies (Natural Selection) (Overpopulation/Struggle to survive)

• Populations don’t increase quickly in size because individuals have to compete with each other for resources that are limited in supply.

• If the climate changes faster than the population of a species can change, then the species will die out.

• Predation or disease could be a factor

Factors that influence allele frequencies (Natural Selection) (Survival of the fittest)

• Only the best adapted individuals survive to reach maturity and reproduce.

• Usually these individuals will have alleles that give them a survival advantage which will be passed on.

Factors that influence allele frequencies (Natural Selection) (Like produces like)

• The favourable/advantageous characteristics are passed on to the next generation.

• Where individuals are less adapted to the environment and less able to compete are less likely to survive and reproduce.

Factors that influence allele frequencies (Natural Selection) (Over Time)

• The proportion of the favourable alleles increases and the less favourable alleles decreases.

Factors that influence allele frequencies (Random Genetic Drift)

• Random genetic drift= is different to natural selection.

• RGD is the change in the alleles frequency in a population due to a result of chance events.

• Mainly occurs in small populations but can happen in large populations, it’s just not noticeable or has much of an influence on that large population

• Also occurs in isolated populations

Factors that influence allele frequencies (Random Genetic Drift) (Founder Effect)- What is it?

• Founder effect is a type of genetic drift where a small isolated population moves/splits itself away from the main population.

• By chance- breeding is not equal among the population

  • One individual breeds more than others.

  • Some individuals die before breeding.

Factors that influence allele frequencies (Random Genetic Drift) (Founder Effect)- How it works?

• They will form a new community

• This new colony is started by members of the ‘original’ population.

• The small population size means that the new colony may have

  • Reduced genetic variation from the original population

  • A non-random sample of the gene in the original population

Factors that influence allele frequencies (Random Genetic Drift) (Founder Effect) Example: The Dunkers

• Religious population

• originated in Germany

• migrated to America (Pennsylvania)

• nearly 60% of the Dunkers had blood type A

• USA has a population of 40% blood type A

• They do not permit to marry outside their group.

• Isolated population within USA

Factors that influence allele frequencies (Random Genetic Drift) (Founder Effect) Example: The Ashkenazi Jews

• Ashkenazi Jews originated from a small community in Central/Eastern Europe

• Group geographically isolated

• married within their group

• Tay-Sachs will be prevelant

Factors that influence allele frequencies (Random Genetic Drift) (Bottleneck Effect)- What is it?

• Is when a portion of the population is randomly eliminated by a catastrophic event such as flooding, earthquake, volcanic eruption or destruction of habitat.

Factors that influence allele frequencies (Random Genetic Drift) (Bottleneck Effect)- How it works?

• Natural disaster e.g. Tsunami

• A Tsunami occurs by chance/randomness. We have no idea it is going to happen.

• Those who survive may have a different frequency of alleles in the gene pool from the original population.

• This could result in a change of the phenotype which will reflect the genetics of the surviving population.

Factors that influence allele frequencies (Random Genetic Drift) (Bottleneck Effect) Example: Micronesian Island of Pingelap

• A typhoon killed 90%

• left 20 survivors, one had a rare eye disorder called Achromatopsia

• Colour blindness, extreme sensitivity of light.

Factors that influence allele frequencies (Migration)- What is it?

• gene flow is the movement of genetic material from one population to another.

• Individuals that are new in a population may introduce new alleles or increase the frequency of other alleles within a gene pool.

Factors that influence allele frequencies (Migration)- How it works?

• People from other countries bring certain diseases to Australia that haven’t been around for sometime.

• HIV/AIDS is on the increase as people from Africa are leaving their country and brining certain/different diseases.

Factors that influence allele frequencies (Migration) Population Example: The increase of the negative blood.

• The Chinese all had positive blood (Rh⁺)

• When the Chinese started trading with the European countries in the 16th century, the European immigrants introduced the (Rh⁻) blood type into the Chinese population.

• The frequency of this blood type is still considered low today.

Factors that influence allele frequencies (Migration) Population Example: The increase of the antigen I^B across Europe and Asia.

• Mongols have a higher proportion of this allele, but because the Mongols invaded Europe in the 12th and 13th centuries, there was an increase in the occurence of this allele in the rest of Asia and Europe.

Factors that influence allele frequencies (Barriers to Gene flow)

• Barriers can prevent two populations from interbreeding and exchanging alleles

• No two environments are the same

• Characteristics can change in one group and benefit them which over time the allele frequency of that gene pool will change to become beneficial and favourable for survival.

• Over time the two populations will become less alike

Factors that influence allele frequencies (Barriers to Gene flow) Example: Geographical barrier (physical)

• Oceans, mountain ranges, large lake systems, deserts.

• Australia is isolated from other countries

Factors that influence allele frequencies (Barriers to Gene flow) Example: Sociocultural Barrier

• Australians tend to marry people of the same education status background and same religious background.

• Don’t “tend” to find people who are wealthy marrying someone who has low socio economic status.

Factors that influence allele frequencies (Genetic Diseases)

• Alleles that cause fatal disease are often gradually eliminated from the gene pool, as the people who inherit them, die out before they get the chance to reproduce and pass it on to the next generation.

• Sometimes disease-causing alleles persist because they offer a survival advantage under particular selection pressure.

Factors that influence allele frequencies (Genetic Diseases) Example: Tay-Sachs Disease [Population effect]

• Ashkenazi Jewish, French Canadian

Factors that influence allele frequencies (Genetic Diseases) Example: Tay-Sachs Disease [Cause]

• Mutation of the HexA gene, codes for enzyme beta-hexosaminidase.

• Enzyme breaks down toxic and fatty substances in brain/spinal cord.

• brain/spinal cord cloggs destroying nerves

Factors that influence allele frequencies (Genetic Diseases) Example: Tay-Sachs Disease [Symptoms of Disease]

• Interlectual and physical disabilities

• Death occurs in first few months (lethal)

Factors that influence allele frequencies (Genetic Diseases) Example: Tay-Sachs Disease [Inheritance Pattern]

• Autosomal recessive

• tt

Factors that influence allele frequencies (Genetic Diseases) Example: Tay-Sachs Disease [Effect on the gene pool]

• Heterozygous parents that carry the recessive allele have a 1 in 4 chance of a child with Tay-Sachs.

• Heterozygous (Tt) have selective advantage of not contracting TB.

• Therefore can pass suitable gene to offspring, tt will die off.

• Population isolated due to geographical barrier/barrier of some sort.

Factors that influence allele frequencies (Genetic Diseases) Example: Sickle-Cell anaemia [Population effect]

• Certain ethnic groups: African decent includes African-Americans.

Factors that influence allele frequencies (Genetic Diseases) Example: Sickle-Cell anaemia [Cause]

• Affects red blood cells (Carry oxygen from lungs to rest of body).

• DNA molecule that codes for beta amino acids in haemoglobin has a mutation.

• Mutation: Adenine replaced with Thymine; causes red blood cell to be sickle shaped.

• RBC cannot fit through narrow blood vessels as they are now long and stiff versus round and flexible.

Factors that influence allele frequencies (Genetic Diseases) Example: Sickle-Cell anaemia [Treatment]

• No cure

• Bone marrow transplant

• Gene Therapy

Factors that influence allele frequencies (Genetic Diseases) Example: Sickle-Cell anaemia [Inheritance Pattern]

• Autosomal recessive

• 1 in 4 chance

• Effect on the gene pool

• Homozygous recessive (aa) die out before reproducing (lethal).

• Homozygous dominant (AA) survive sickle-cell but have no resistance to Malaria (will die if they have Malaria).

• Heterozygous (Aa) “Sickle-cell trait” - occurs when oxygen is in short supply

• “Sickle-cell trait” reduces the risks of contracting Malaria (selective advantage).

• sickle cell allele maintained in places where:

• Malaria is a problem

Population: Jewish

Disease: Tay-Sachs

Population: Pingelap

Disease: Achromatospia (Colour blindness)

Population: Greek

Disease: Thalassaemia

Population: Amish

Diseases: Polydactyly, Dwarfism

Population: African

Disease: Sickle-cell anaemia

Survival Advantage DEF:

A trait that allows an organism to survive and reproduce (matured) while passing on advantageous trait to offspring.