Variation and Evolution

Variation is the result of a combination of genetic and environmental factors.

Genetic variation is also known as heritable variation. Sexual reproduction uses three different ways to create variation.

• The mixing of two genotypes during cross variation

• The random distribution of chromosomes during metaphase I of meiosis.

• the crossing over between homologous pairs during prophase I of meiosis.

These can create variation within a generation, but mutation creates novel variation that is long lasting.

Continuous variation

• most characteristics are controlled by a number of genes and aren't clean cut.
• A character within a population showing a gradation from two extremes is continuous.
• If an individual inherits the characteristics, he may not reach his full potential height due to lack of nutrition etc.
• Environmental influences are largely responsible for continuous variation in a population.

Discontinuous variation

characteristics that are clear cut are controlled by one gene. E.g. eye colour.

Environmental influences plays a role in phenotypic variation.

It affects the expression of genes, e.g. diet and exercise, temperature and light.

Seedlings which germinate in the dark are white as they needed light to make green chlorophyll. They are called etiolated.

 Competiton for breeding success and survival

• Although all organisms have the reproduction potential to increase their population from generation to generation, this does not happen.
• Environmental factors limit numbers, as competition cuts out those who aren't efficient.

Intra-specific competition- between individuals of the same species (for mates, dominance)

Inter-specific compeition- between different species e.g. for resources (food, space)

Population increases

• If a population increases , various Environmental pressures will bring it down again.
• Food supply will run short, overcrowding will encourage disease, predator numbers will increase.

Population decreases?

• Enviromental resistance is relived, and population will increase again.
• More food, space available, predators die out due to lack of food.

This process causes populations to oscillate around a mean level.

• Individuals which are best adapted to the environment are more likely to survive
• a.k.a their alleles are passed on to the next generation.
• Foxes are a selection pressure, as they will kill the weakest rabbits, and so the next generation is fitter.

Allele frequency 

• In different environments, different alleles will be more or less rare in the gene pool.
• A white coat will be rare in a green forest as it would stand out to predators, but very common in the arctic.

Population: a group of interbreeding individuals of the same species,occupying a habitat at the same time.

gene pool- all the alleles in a population at one time.

allele frequency: the number of times a allele occurs in the gene pool.

Population genetics is concerned with determining the relative proportions of various genotypes in a population.

if the environment isn't stable the gene pool will change, as certain alleles will be more advantageous.

A mathematical equation that can be used to calculate the frequencies of alleles.

It states that in large, randomly mating population, on absences of migration, mutation and selection, the gene pool remains constant. The proportion of dominant and recessive alleles of a particular gene remains ththe same.

it can use the frequency of one allele to predict expected proportions of the genotype in the population. It demonstrates the large amount of recessive alleles in heterozygotes, who are the reservoir of genetic variability.

P^(2) + 2pq+ q^(2)= 1

P is frequency of dominant allele, q is frequency of recessive allele. P + q = q, but the next generation:

PP = p^(2), Pq= 2pq, pp= q^(2)

Random genetic drift- change in frequency of allele in gene pool due to random sampling.

• Especially influential in small populations.
• If only 1% of a population contains a certain allele, and the population only has 1000 individuals, those 10 may randomly fail to mate--- the allele is lost.

The founder effect-

• If a small group is isolated from the original population, and contains different allele frequency.
• Genetic drift would then cause new isolated group to become very distinct from the initial population.
• E.g. Galapagos finches.

Natural selection: the process that encourages the transmission of favourable alleles and hinders the transmission of unfavourable ones, contributing to evolution. 

Natural Selection also maintains variation as well as causing change in population. It is a continuous process as competition doesn't seize.

Darwin's theory:

• Any population has variation
• Despite ability to increase population size, the number of adults remain stable.

This lead to two deductions:

• There is a struggle for survival with only the fittest offspring surviving.
• Those that survive pass on their advantageous characteristics to their offspring.

• Stabilising Selection- extreme variation/alleles are eliminated as they give no advantage.
• Directional selection- one extreme within variation is favoured
• Disruptive.- the two extremes are favoured, and average is eliminated. LEADS TO SPECIATION.

ISOLATION AND SPECIATION.

Breeding sub units (demes) can appear within a population due to isolation. If the subunits are later reunited and cannot mate, they have become a new species.

SPECIATION

Individuals within a deme are more likely to mate with eachother than with another deme. New species arise when a barrier stops demes from mating at all. They will evolve separately and won't be able to breed anymore.

Geographical isolation/ allopatric speciation

Population is physically split. Could be a mountain or river.

• E.g weather and climate separate bird population to two peaks of a mountain. Each peak may have its own selection pressures, and cause so much genetic difference the two demes cannot breed.
• E.g. different food sources = different beak shape= different mating calls=no breeding.

Reproductive isolation/ sympatric speciation.

Demes form in the same geographic area. Important in preventing dilution of gene pool.

• Behavioural: subspecies have different mating calls. 
• mechanical: genitalia are incompatible. 
• gametric: pollen grains fail to germinate on a stigma, or sperm die in oviduct. 
• Hybrid inviability: chromosomes no longer match so embryo does not develop.
• Hybrid sterility: hybrid produced cannot go through meiosis as chromosomes don't pair up. Hybrid cannot reproduce