Diversity and Selection

Genetic diversity is the number of different alleles of geens in a species population.

• The larger the population, the greater genetic diversity.

• Genetic diversity increases by...
  • Mutations in DNA, forms new alleles - can be good or bad.
  • Different alleles being introduced into another population when an individual from another population migrates and reproduced - gene flow.

The gene pool is a collectin of all the different alleles if a gene in a population.

• Increase in alleles: survive and reproduction, immigration.
• Decrease in alleles: deathe, disease, lack of food, competition, emigration, predation.

1) Genetic Bottlenecks

• An event that causes a big decrease in a population.
• e.g. large number of organisms within a population die before reproducing.
• Decrease in the number of different alles in a gene pool and genetic diversity.
• Survivors reproduce and a larger population is created.

2) The Founder Effect

• The reduction in genetic diversity that occurs when just a few organisms from a population start a new colony.
• The frequency of each allele in the new colony is different to the frequency in the original colony.
• e.g. rare allele in original may be more common in the new colony - disease.

1) Gene mutation

• Changes to genes and chromosomes - new alleles.

2) Meiosis

• Independent segregation of homologous chromosomes and crossing over - new combination of alleles.

3) Random fertilisation

• Gametes are genetically unique, fertilisation is a random event so offspring is different to parents.

4) Environmental factors

• Exert an influence on gene expression in all organisms.

Adaptations are features that help organisms to survive in their environment.

Behavioural

• The way an organisms acts to increase chance of survival.
• e.g. possum 'plays' dead, scorpions dance before mating - so attract same species.

Physiological

• Processes inside an organisms body to increase chance of survival.
• e.g. brown bears - hibernate, lowers rate of metabolism in winter to conserve energy and don't have to look for food as it is scarce.

Anatomical

• Structural features of an organisms body to increase chance of survival.
• e.g. otters are streamlined so easier to glide through water, whales have thick layer of fat to keep warm in the cold sea.

Natural selection is the process where an allele becomes common in a population becasue it codes for a characteristic that makes an organism more likely to survive and reproduce to pass their genes on to its offspring.

• Variation
  • A population has geentic variation due to mutation.
• Alleles
  • Some individuals will have favourable alleles.
• Change
  • In environment.
• Selection
  • Individuals with favourable alleles will survive.
• Reproduce
  • Individuals who survive will reproduce and pass on their favourable alleles to their offspring.
• Frequency
  • Allele frequency for the favourable allele increases over many generations.

Directional selection is where individuals with alleles for characteristics of an extreme type are more likely to survive, reproduce and pass on their alleles.

• Response to environmental change.
• Selection of alleles for extreme characteristics.

• e.g. bacteria evolving antibiotic resistance.
  • Some individuals in bacterial population have alleles which gives them resistance.
  • Population exposed to antibiotic killing bacteria without allele.
  • Resistant bacteria survive and reproduce without competition.

• Change in mean.
• Range on characteristics is still the same.

Stabalising direction is where individuals with alleles for characteristics towards the middle of the range are more likely to survive, reproduce and pass on alleles.

• No change in environment.
• Selection of middle range characteristics.

• e.g. weight of babies.
• Very small babies less likely to survive - high SA:V therefore find it hard to maintain body temperature (pressure on respiratory and cardiac systems).
• Very large babies less likely to survive - giving birth is hard as it's hard to fit through mother's pelvis.
• Conditions more favourable for medium sized babies.

• No change in mean.
• Narrowing range of characteristics.