A Level Biology, 3.7 - Genetics, Populations, Evolution & Ecosystems

The theory of evolution underpins modern Biology. All new species arise from an existing species. This results in different species sharing a common ancestry, as represented in phylogenetic classification. Common ancestry can explain the similarities between all living organisms, such as common chemistry (eg all proteins made from the same 20 or so amino acids), physiological pathways (eg anaerobic respiration), cell structure, DNA as the genetic material and a ‘universal’ genetic code. The individuals of a species share the same genes but (usually) different combinations of alleles of these genes. An individual inherits alleles from their parent or parents. A species exists as one or more populations. There is variation in the phenotypes of organisms in a population, due to genetic and environmental factors. Two forces affect genetic variation in populations: genetic drift and natural selection. Genetic drift can cause changes in allele frequency in small populations. Natural selection occurs when alleles that enhance the fitness of the individuals that carry them rise in frequency. A change in the allele frequency of a population is evolution. If a population becomes isolated from other populations of the same species, there will be no gene flow between the isolated population and the others. This may lead to the accumulation of genetic differences in the isolated population, compared with the other populations. These differences may ultimately lead to organisms in the isolated population becoming unable to breed and produce fertile offspring with organisms from the other populations. This reproductive isolation means that a new species has evolved. Populations of different species live in communities. Competition occurs within and between these populations for the means of survival. Within a single community, one population is affected by other populations, the biotic factors, in its environment. Populations within communities are also affected by, and in turn affect, the abiotic (physicochemical) factors in an ecosystem.

What is a codominant allele interaction?

both alleles for same characteristic simultaneously expressed

What is the definition of a phenotype?

genetic expression & its interaction w environment

Genetic Diagrams - _____________________

• Inheritance of a characteristic controlled by _______________.

• Show the likelihood of the different alleles of that gene (and so different versions of the characteristic) being inherited by offspring of certain parents.

monohybrid Inheritance / single gene

Mendel's Law

• What does Mendel’s law of independent assortment state?

• What happens to some allele combinations?

• What are genes that are located close to each other on the same chromosome being more likely to be inherited as a pair called?

genes don’t influence allele sorting into gametes / not inherited independently / linkage

What is expected of genes that are close together on the same autosome?

more likely to be transmitted to gamete together

What happens during epistasis?

gene at locus inhibits expression at another locus

When is the Chi-squared test used?

discrete variation / absolute numbers

What are the assumptions of the Hardy-Weinberg principle?

no mutations, migration or emigration / no selective pressure towards specific genotype / infinite population size

Hardy-Weinberg Equation - p² + 2pq + q² = 1

• The components of the equation allows us to calculate the number of individuals in a population that have each genotype.

  • p² = pp = ____________________

  • q² = qq = _____________________

  • 2pq = _________________

homozygous dominant / homozygous recessive / heterozygous

Evolution

• What is its definition?

• How can it occur by?

allele frequency change over time / genetic drift / natural selection

Natural Selection Types

• ____________________ - individuals with alleles for characteristics towards the middle of the range are more likely to survive and reproduce; occurs when environment isn’t changing and reduces the range of possible phenotypes.

• ____________________ - individuals with alleles for a single extreme phenotype are more likely to survive and reproduce; could be in response to an environmental change.

• ____________________ - individuals with alleles for extreme phenotypes are more likely to survive and reproduce; opposite of stabilising selection because characteristics towards the middle of the range are lost; occurs when the environment favours more than one phenotype.

stabilising / directional / disruptive

______________________

1. Capture a sample of a species using an appropriate technique and count them.

2. Mark them in a harmless way eg by putting a spot of paint on them or by using an _____________________.

3. Release them back into their habitat.

4. Wait a week, then take a second sample from the same population.

5. Count how many of the second sample are marked.

6. Calculate estimated total population size. Equation: _______________________

Mark-release-recapture / identification tag / (n from 1st sample X n from 2nd sample) / n marked from 2nd sample

Transects

• _______ - quadrats are placed next to each other along the transect to work out species frequency and _____________________ along the transect.

• __________________ - instead of investigating the whole transect you can take measurements using a quadrat placed at __________________, eg every 2 metres; this can make it easier to cover a large distance.

belt / percentage cover / interrupted belt / regular intervals

Primary Succession

1. __________________, specialised to harsh conditions, are the first to colonise the newly exposed land.

2. These species help to break down substances on the exposed land to form soil. When they die, they are also decomposed which contributes to soil formation.

3. The formation of the soil makes the environment ______________. New species colonise this. When the new species die, the organic matter within the species is released into the soil by ________________.

4. When species die, the composition of the soil is changed. When new species colonise the area, they are also altering the abiotic conditions. New species may also alter the environment so that the pioneer species can no longer survive.

pioneer species / less hostile / decomposition

__________________

• Happens on land that’s been cleared of all the plants, but where the ______________, eg after a forest fire or where a forest has been cut down by humans.

• The established community of species is usually destroyed, but without too much disturbance to the soil. It can occur during any stage (including the climax community) __________________.

• Process similar to primary succession, but because there’s already a soil layer, starts at a later stage and the pioneer species are _________________ eg shrubs.

secondary succession / soil remains / after pioneer stage / larger plants