Topic 7 Biology aqa

What are the two forces that affect genetic variation

Genetic drift and natural selection.

Genetic drift

Can cause changes in allele frequency in small populations.

Natural selection

When alleles that enhance the fitness of the individuals that carry them rise in frequency.

Evolution

Change in the allele frequency

Genotype

Genetic constitution of an organism

The phenotypes

The expression of this genetic constitution and its interaction with the environment

Population

A group of organisms of the same species occupying a particular space at a particular time that can potentially interbreed.

Primary sources of genetic variation

Mutation

Another cause of genetic variation

• meiosis

• Random fusion of gametes during sexual reproduction

Natural selection

Predation, disease and competition for the means of survival result in differential survival and reproduction,

Those organisms with phenotypes providing selective advantages are likely to produce more offspring and pass on their favourable alleles to the next generation. The effect of this differential reproductive success on the allele frequencies within a gene pool.

Consequence or reproductive separation of two populations

Can result in the accumulation of difference in their gene pools. New species arise when there genetic differences lead to an inability of members of the population to interbreed and produce fertile offspring.

Allopatric speciation

Reproductively isolated population due to geographical isolation

Sympathetic speciation

Reproductively isolated population not caused due to geographical barrier

• explain why individuals within a population of a species may show a wide range of variation in phenotype

• Genetic factors: mutations create new alleles.

• Meiosis: crossing over and independent segregation produce different combinations of alleles.

• Random fertilisation produces genetic variation.

• Environmental factors also affect phenotype.

• explain why genetic drift is important only in small populations

• Random changes in allele frequency occur by chance.

• Each individual represents a larger proportion of a small population.

• Therefore, chance events have a greater effect on allele frequencies.

• explain how evolutionary change over a long period of time has resulted in a great diversity of species.

1. Mutations produce genetic variation.

2. Individuals show different phenotypes.

3. Selection pressures favour individuals with advantageous phenotypes.

4. These individuals survive and reproduce more successfully.

5. They pass on their advantageous alleles to offspring.

6. The frequency of advantageous alleles increases over generations.

7. The frequency of associated phenotypes increases.

• explain how evolutionary change over a long period of time has resulted in a great diversity of species.

1. A population becomes isolated, preventing gene flow.

2. Mutations produce genetic variation in each population.

3. Different environments create different selection pressures.

4. Natural selection changes allele frequencies differently in each population.

5. Genetic differences accumulate over generations.

6. Eventually, the populations cannot interbreed to produce fertile offspring.

7. A new species has formed.

What is a community

Populations of different species living together in the same area

What is an ecosystem?

A community and the non-living components of its environment

What is a niche

The role of a species in its habitat, determines by adaptations to abiotic and biotic conditions

.

What is carrying capacity

The population size an ecosystem can support under its current conditions.

What affects carrying capacity

Abiotic factors, intraspeficic and inter specific competition and predation.

Interspecific competition

Competition between different species

Intraspecific competition

Competition between same species

How are slow-moving organisms samples

Use randomly placed quadrants, use a belt transect to investigate distribution across a gradient

How are motile organisms sampled

Use mark-release-recapture to estimate the population size of motile organisms.

MRR assumptions

Marking must not affect survival or recapture; it must not be toxic or increase predation.

MRR assumptions of population

Population remains approximately constant; no significant births, deaths, immigration or emigration.

Primary succession

1. Pioneer species colonise the area first.

2. Pioneer species change the abiotic conditions, e.g. by forming soil and adding organic matter.

3. The environment becomes less hostile / more suitable for other species.

4. New species colonise and outcompete previous species, which may become less abundant.

5. This process repeats, causing changes in species diversity and community composition.

6. Succession eventually leads to a climax community, where the community remains relatively stable.

Describe how to estimate the number of plants in a field using quadrats. (5 marks)

1. Divide the field into a grid. 2. Generate random coordinates. 3. Place quadrats and count plants. 4. Calculate mean per quadrat. 5. Multiply mean by total area ÷ quadrat area.