0.7 Gene Pools and Mechanisms for Evolution

Population

• a group of organisms of the same species that are found in the same area and can interbreed

Gene pool

• total sum of all alleles of all genes of all individuals in a population at any one time

• Can be used to compare populations at different times or locations

Allele

• a version of a gene, a heritable unit that controls a particular feature of an orgamism

Allele frequency

• refers to how common an allele is in a population

• Determined by counting how many times the allele appears in the population then dividing by the total number of copies of the gene

Genetic variation

• variation in alleles of genes that occur both within and among a population

Genetic variation causes

1. Mutation

2. Random (independent) assortment

3. Crossing over of homologous chromosomes during meiosis

4. Random fertilisation

5. Random mating of individuals with varying traits

1. Mutations

• permanent structural alteration in an organisms dna

• Considered the ultimate source of genetic variation because they are the only process that creates new alleles. Other mechanisms such as crossing over an independent assortment only reshuffle existing alleles.

• They are random - whether a particular mutation happens or not is unrelated to how useful that mutation would be

• Can be beneficial neutral or harmful for the organism but mutations do not “try” to supply what the organism “needs”

• Not all matter for evolution

• Somatic mutations occur in non-reproductive cells and won’t be passed on offspring

2. Random (independent) assortment

• as the chromosomes move apart the opposite side of the cell during the first division of meiosis each pair moves independently and does not affect or influence the movement of any other pair

• In humans (23 chromosome pairs) this creates over 8.4 million possible chromosome combinations in the ovum and the sperm cells

3. Crossing over of homologous chromosomes during meiosis

• Results in a recombination of the order of the alleles on chromatids

4. Random fertilisation

• One of millions of sperm cells may fertilise the ovum

5. Random mating of individuals with varying traits

• A random meeting between individuals with different genetic characteristics can increase variation within a population

5. Random variation example

• Despite snail shells being different colours they’re all belong to a single species

• The range of colours and banding pattern is only part of the variation that makes each of them unique

• That variation is the raw material upon which natural selection acts and is the basis for evolutionary change

What is evolution?

• The gradual process of changed in the inherited traits of a population of organisms from one generation to the next

• OR

• A permanent change in a population gene pool from one generation to the next

Mechanisms for evolution

Evolution only occurs when there is a change in gene frequency within a population overtime

Processes that provide the basis for evolutionary change

1. Natural selection

2. Genetic drift

3. Migration (gene flow)

Natural selection

• A scientific theory that organisms best adapt to their environment tend to survive and out multiply (reproduce) those that are less well adapted

• Is the process by which random evolutionary changes are selected four by nature and a consistent orderly non-random way

Selection

• when some individuals with particular features have a greater chance than others of leaving fertile offspring

Survival of the fittest

The “fittest” are those phenotypes best city to the environment relative to other phenotypes present

Biological fitness

• Measured by the relative proportion of fertile offspring left by an individual in the next generation

Evolutionary success of an individual

• Depends on how fit he/she is

• Fit individuals are well adapted

Allele frequency and natural selection

• natural selection affects a frequency of alleles in a population gene pool

• The frequency of alleles determining favourable traits increases and the frequency of the alleles determining unfavourable traits decreases

The process of natural selection

1. Overproduction

2. Variation

3. Struggle for survival

4. Adaptation

5. Survival of the fittest

6. Reproductive fitness

7. Allele carryover

8. Changes to the gene pool (evolution)

1. Overproduction

• more younger born than survived to maturity

2. Variation

• Sexually Reproducing species show variation in all inherited features

• via random assortment, crossing over, mutations and random mating

3. Struggle for survival

• Individuals and a population and complete with one other for survival

• Example: Competition for resources, (food, water, shelter), mates or against predators and disease

4. Adaptation

• Any characteristic/feature that helps an organism survive in its environment

• It is the process by which an organism becomes better suited with environment

5. Survival of the fittest

Some individuals are better adapted to their environment and they are more likely to survive than others

Reproductive fitness

• those who survive longest are likely to successfully raise the most offspring

7. Allele carryover

• offspring that survival carry the alleles that determine adaptive features

8. Changes to the gene pool (evolution)

A little that determine favourable features will increase in the gene pool, whereas unfavourable alleles will decrease

What does natural selection determine?

• Which mutations remain in gene pool and which ones are eliminated

• It does not create the variation

• It acts in the variation that is already present in the population

Selective pressures

• Drives natural selection

• External agents which affect an organisms ability to survive in a given environment

• Can be negative, which decreases the occur occurrence of a trait or positive which increases the proportion of a trait

• Can be density density-dependent (affected by population size) or density-independent (unaffected by population)

1. Resource availability

Presence of sufficient food, habitat (shelter/territory) and mates

1. Resource availability

Presence of sufficient food, habitat (shelter/territory) and mates

2. Environmental conditions

Temperature, weather conditions or geographical access

3. Biological factors

Predators and pathogens diseases

Examples of density dependent selection pressure factors

• P redators

• A vailability of recourses (eg shelter, water)

• N utrient supply (eg food source)

• D isease/ pathogenic spread

• A ccumilation of wastes

Examples of density independence selection pressure factors

• P henomena (eg natural disasters)

• A biotic factors (eg temoerature, CO2 levels)

• W rather conditions (eg floods, storms)

Genetic drift

• refers to the random change in the allele frequencies in a population over generations

• Due to chance events

• Affects small populations more than large populations

Example of genetic drift

• the frequency of our particular trait could for no obvious reason drift from 2% in generation one to 11% in generation 2 to 5% in generation three

The effects of genetic drift

• traits being lost from small populations

• Unusual traits or mutations not commonly found in the current population and that are often non-adaptive becoming established

Effects of genetic drift on small breeding population example

• When Woman and their mates are both heterozygous (Aa) for a trait, we would expect that 25% of that children will be homozygous recessive (aa).

• By chance, however, a particular couple might not have any children with this genotype

• Unless other families have an unpredictably large number of homozygous recessive children for this chat to counter the random deviation, the populations gene pool frequencies will change in the direction of having fewer recessive alleles - genetic drift will occur

Two forms of genetic drift

1. Bottleneck effect

2. Founder effect

• both effects show how chance events can lead to significant changes in all frequencies and genetic diversity, especially in small populations.

Large population size

• Individuals make only a small contribution to the gene pool

• Evolution occurs slowly

• Most changes are adaptive (have a survival advantage)

• Natural selection is the main driving force

Small population size

• Individuals make a relatively large contribution to the gene pool

• Evolution can occur rapidly

• Many changes are non-adaptive (provide no survival advantage)

• Many changes are due to chance events (genetic drift, founder effect)

1. Bottleneck effect

• Occurs when the abundance and distribution of an existing population dramatically reduces

• This can be caused by an event that results in a drastic decrease in a population e.g. natural disaster, deforestation, and pollution

• Increases the rate of genetic drift and decreases genetic diversity

1. Bottleneck effect example

• In the original population there are green, orange, and red individuals living in a habitat

• Orange makes out the majority of the population. The colours represent alleles for a colour gene.

• When disaster strikes, only a few random individuals make it out alive

• By pure luck, most of the survivors are red with a few green and one orange

• Overtime the population grows again but now the major majority of members are red and there are just a few orange. This is very different from the original population before the disaster happened.

• This illustrates that when a bottleneck event happens the variety in a population can decrease and the proportions of different alleles can change completely

2. The founder effect

• Occurs when a small number of people migrate away from the ‘parent’ population and settle in a new area

• The founding population carry only a small fracture of the original ‘parent’ populations, genetic variation (e.g. they are not representative of the entire gene pool)

• They increase in number and retain their frequencies, maintaining the differences to the original group

• As a result they may differ both genetically and in appearance compared to the parent population

• The smaller the number of individuals found in the new colony the smaller the genetic diversity

• This can lead to speciation if the colony is separated by some barrier/isolated

2. Founder effect example

• an island in the Pacific was colonised in 1790

• Today the population of the island is 44 people

• The population exhibits reduced genetic diversity because it descended from a small number of founders

• This small initial population means that certain alleles including those genetic conditions were present higher frequencies by chance alone

• Therefore, they have higher incidences of certain genetic disorders that are rare in the general population

Migration – gene flow

• Gene flow is when there are changes in a little frequencies in a gene pool due to Migration

• Gene flow may influence the genetic composition of a population if there are differences in allelic frequencies between native and immigrant populations

• The greater the immigration rate and differences in allelic frequencies between populations, the greater the impact of gene flow

Barriers to gene flow

• populations are often kept apart by geographical barriers, (oceans, mountain ranges, deserts etc), mechanical, behavioural, (religious, cultural Barriers, language, and race in humans) that inhibit the amount of interbreeding between them

• Due to the difference as an environment, the environmental pressure on one population will differ from the pressure on another

• This results in slightly different characteristics being favoured in one population compared to another

• Overtime the allele frequencies of each gene pool will change depending on which character characteristics are favoured for survival

• Isolation results in the development of separate gene pools

Coin flipping experiment

• The expectations that heads will turn up 50% of the time because there are only two sides to a coin, heads and tails

• If you flip a coin 10 times, it may or may not result in five heads

• The more times that you flip your coin, the more likely it will approach the expected 50% heads

• If you do an infinite number of times it will be 50%. In other words when a sample is very small the probable outcome may not occur.

• As a sample increase increases in size, it will get progressively closer to it

In-breeding

• Reduce mating opportunities often lead to an increase in in–breeding

• Results in reduced genetic diversity in the populations gene pool

Consequences of in–breeding

• Can result in non-adaptive changes occurring

• Can amplify undesirable traits

• Can (more frequently) greatly increase the risk of unusual, often harmful traits being expressed