Biology 4.4 - Variation and Evolution

Variation = the differences in phenotype between organisms of the same species. 

Variation can occur because:

• Organisms have different genotypes.
• Organisms have the same genotype but different epigenetic modifications.
• Organisms have different environments.

There are two types of variation: heritable and non-heritable. 

Heritable variation = differences that can be inherited.

E.G. differences in phenotype that occur due to different DNA nucleotide sequences or different epigenetic modifications. 

In asexually reproducing organisms, heritable variation can only be increased by mutation. However, in sexual reproduction there are several mechanisms that generate heritable variation:

• Crossing over at prophase I.
• Independent assortment at metaphase I and II.
• The combination of gametes at fertilisation.

Non-heritable variation = differences that cannot be inherited. 

For example, those caused by the environment. 

Discontinuous variation is where characteristics fall into a number of distinct categories. There are no intermediates. 

For example, plant height and the number of fingers humans have.

• The characteristics are controlled by single genes = MONOGENIC.
• Environment does not alter gene expression. 
• Discontinuous can show heritable variation.
• Best represented as a bar chart.

Continuous variation is where characteristics can hold a range of values between two extremes. 

For example, the number of plant leaves and the height of humans.

• Controlled by several genes = POLYGENIC. 
• The environment can influence gene expression.
• Can show both heritable and non-heritable variation. 
• Best represented by a smooth bell curve.

Inter specific competition occurs between individuals of DIFFERENT SPECIES competing for the same resources.

Intra specific competition occurs between individuals of the SAME species competing for the same resources. 

Plants will compete for light, water, mineral ions and soil.

Animals will compete for food, shelter, mates etc...

How does this affect the breeding success/survival of a population?

Population growth is limited: fewer offspring will be produced or fewer offspring will survive to maturity, and so not reproduce.

A selection pressure is an environmental factor that alters the frequency of alleles in a population, when the factor is limited. 

EXAMPLE: if food was in short supply, rabbits that were better at finding food would have an advantage. They are more likely to survive and reproduce. The advantageous allele is therefore passed on to future generations. 

This is an example of natural selection.

Natural selection = the increased chance of survival and reproduction of organisms with phenotypes suited to their environment, enhancing the transfer of favourable alleles from one generation to the next.

As a result, selective pressures/agencies can enhance the chance of survival and reproduction if the factor is not optimal. 

Selective pressures increases the chances of some phenotypes and alleles being passed on to future generations.

Selection pressures can be environmental factos, such as:

• Availability of nesting sites.
• The day lengths can effect reproduction in rabbits - females usually become pregnant in May as a result of the males testes enlarging during November in response to the shorter days.
• Overcrowding allows diseases to spread more easily.
• Predation - some individuals in the prey population may be more likely to survive (e.g. they are better camouflaged or they are mimic species).
• The temperature can affect survival.
• Human impact - habitat loss can destroy breeding grounds. 

• Some phenotypes are better suited to an environment. Their phenotype is controlled by their genotype.
• If the phenotype provides an advantage, the individuals have a better chance of survival and so will successfully breed and transmit the advantageous alleles to the next generation.
• For example, most hares have alleles that will produce a brown coat. A small percentage of the population will be homozygous recessive for this allele and so have a white coat.
• They will stand out and so are more likely to be caught by predators.
• They remain rare within the population.
• HOWEVER, if the environment changed and the hares lived in the Artic, then the white coat would be advantageous. 

A gene pool is all the alleles in a population at a given time. 

The gene pool is constantly changing as some alleles become more frequent than others, as a result of a changing environment. Some alleles will provide a favourable phenotype, and so will be selected for and transmitted into the next generation. Others will be selected against. 

The frequency of an allele refers to the percentage of all the alleles of a gene in a specific gene pool.

Genetic drift = random change in the allele frequency of a gene pool. 

They occur as a result of the random nature of reproduction and are most significant in smaller populations.

Why?

In large populations, if the individuals with a particular allele mate randomly, they are 'buffered' against the effects. 

In a small populatioj, if some individuals with the particular allele fail to breed, then the allele frequency in the populatiuon will decline significantly. 

As a result genetic drift can bring about a loss of genetic variation in smaller populations.

This states that the frequency of dominant and recessive alleles and genotypes will remain the same from one generation to the next if the following conditions are met:

• Organisms are diploid (2n).
• The allele frequency is the same for both sexes.
• They reproduce sexually.
• Mating is random.
• The population is large (n>100).
• Generations do not overlap.
• There is no selection for or against different phenotypes within the population.
• There is no migration.
• There is no mutation. 

• P = the frequency of the dominant allele (A).
• P² = the frequency of AA.
• q = the frequecny of the recessive allele (a).
• q² = the frequency of aa.
• 2pq = the frequency of Aa.

The founder effect: when a new population is formed by a small number of individuals, and it is likely that by chance alone the allele frequencies in the new population will differ from the source population. 

How does this work?

1. A founder population is formed by a small number of individuals.

2. Chance variation/genetic drift in allele frequencies from one generation to the next can lead to drastic changes of phenotypes for a large proportion of the population.

3. This causes divergence away from the original population. 

NOTE: the smaller the population, the more dramatic/rapid the effect.

Evolution = a change in the average phenotype of a population.

If the change is profound enough then the individuals with the altered phenotype will no longer be able to breed with the initial population = speciation. 

Speciation = the formation of a new species.

Species = a group of organisms that can interbreed to produce fertile offspring.

Speciation can occur gradually by the isolation of individuals. The only way it can occur rapidly is by polyploidy: when the chromsome number doubles my endomitosis. This is more common in plants.

Within a population, sub-groups will breed more often with each other than with the rest of the population. These are called DEMES. 

If a deme becomes isolated it can no longer breed with other members of other demes: gene flow is prevent and the deme is reproductively isolated.

Reproductive isolation = the prevention of gene flow and reproduction between demes within a species.

If this deme is isolated for many generations the organisms will develop differences in their allele frequences and will undergo several different mutations. Eventually, they can no longer breed with the initial population and so speciation has occurred.

There is two types of reproductive isolation:

1. Pre-zygotic = gametes are prevented from fusing and so a zygote does not form.

2. Post-zygotic = gamates fuse and a zygote forms. 

Pre-zygotic isolation:

• Geographical
• Behavioural
• Morphological (mechanical)
• Gametic
• Seasonal (temporal)

Post-zygotic:

• Hybrid inviability
• Hybrid sterility 
• Hybrid breakdown

Geographical isolation occurs when individuals of a population become split by a physical barrier into seperate demes. 

For example, sticky cinquefoil grow in California. Some grow on the lowlands, and are much bushier and taller than those at high altitude.

This is an example of ALLOPATRIC SPECIATION - the evolution of a new species by demes isolated in different geographical locations.

• E.g. Grasshoppers and their mating rituals. 
• This is an example of SYMPATRIC SPECIATION - the evolution of a new species from demes sharing a geographical location.

• E.g. the exoskeleton of insects is rigid, and so male and female genitalia must be complementary for reproduction to occur.
• This is an example of sympatric speciation.

• E.g. Gametes in open environments often meet other gametes. In coral reefs, many coral species release their gametes into the water. However, two thirds are incompatible. 
• This  is an example of symaptric speciation.

• Also called temporal isolation.
• Reproductive organs can mature at different times of the year.
• E.g. The creeping buttercup flowers in June, whilst the lesser celanoine flowers in April. They can not interbreed. If they did, their distinctive characteristics would be lost. 
• This is an example of sympatric speciation. 

Hybrid inviability 

Hybrid = the offspring of a cross between members of two different species.

In this case, fertilisation occurs, but incompatibility between the genes means that an embryo cannot develop. E.g. The Northern leapord frog and the wood frog. 

Hybrid sterility 

If the chromosomes are not sufficiently similar, chromosomes are unable to pair at prophase I of meiosis I and so gametes cannot form: the offspring is sterile. E.g. A mule is a cross between a female donkey and a male horse, and has 63 chromosomes: a horse has 64 chromosomes, whilst a donkey has 62. 

Hybrid breakdown

Some F1 hybrids are fertile, but their F2 are infertile. E.g. cotton, legumes.,

EXTRA: Hybrid fertility in wheat - continuous hybridisation and chromosome doubling has resulted in hexaploid wheat.

• There is variation within a population.
• This is due to mutations.
• Some individuals will have alleles that give them a phenotype that is better adapted to the environment.
• The individuals therefore have an increased chance of survival.
• They will breed and pass on this advantageous gene to the offspring.
• Over several generations, the frequency of this advantageous allele within the population will increase.

NOTE: if the change in phenotype is significant enough, the individuals possessing the alleles may no longer be able to breed with the original population. Speciation has occurred.

Stabilising selection is a type of natural selection where the population mean stabilises at a non-extreme value. 

• Continuous variation has a range of values.
• In a particular environment, the average phenotype may provide an advantage over the extremes.
• The extreme values are selected against.
• In future generations, the average phenotype will remain the same, but a greater proportion of the population will possess this phenotype.
• If the environment remains unchanged this will continue until most members of the population share the same value.

E.g. Birth weight in humans.

This is a type of natural selection where an extreme phenotype is favoured over other phenotypes.

• In a changing environment an extreme phenotype may provide an advantage.
• Other values are selected against.
• Over time this changes the average phenotype within the population.

This is a type of natural selection where the average phenotype does not provide an advantage and instead higher and lower values are favoured.

• In a certain environment, the average phenotype may not provide an advantage. It is selected against.
• Over several generations, a higher and lower value are favoured, producing a bimodal curve (it has two peaks).