factors affecting evolution. Hardy-Weinberg principle

types of natural selection

stabilising selection

directional selection

stabilising selection

→occurs when.. [mention effect on allele frequency]

→when does it occur [in terms of environment]

→effects to population

-individuals with alleles for an average phenotype are more likely to survive

→Increases allele frequency for the average phenotype

-Occurs when the environment does not change (stable)

-selects against extreme phenotypes

→Individuals with extreme phenotypes are less likely to survive or reproduce

→Reduces variation in the population

[e.g. Human baby weight

Very small babies → less likely to survive

Very big babies → difficult birth

Average weight babies are more likely to survive, reproduce and pass on their alleles to offspring]

directional selection

→occurs when.. [mention effect on allele frequency]

→when does it occur [in terms of environment]

-individuals with alleles for an extreme phenotype are more likely to survive

→Increases allele frequency for an extreme phenotype

-occurs when the environment changes

the data of stabilising and directional selection is shown in what graph. draw graph for each selection.

bell-shaped curve

[look at the axes]

effect of stabilising and directional selection on bell-shaped curve

stabilising selection

-mean remains the same

-curve becomes narrower

→standard deviation decreases

→variation decreases as individuals with extreme phenotypes decrease

[peak of curve is slightly higher than original peak, not too high tho→ when drawing keep this in mind]

directional selection

-Curve shifts toward the favoured extreme

-Mean of population shifts toward the favoured extreme

-curve remains roughly the same

allele frequency

how common an allele is in a population.

e.g. 70% of alleles are A [frequency of A = 0.7]

30% are a [frequency of a = 0.3]

[not the number of people, it’s the proportion of a specific allele]

gene pool

the complete range of alleles in a population

gene flow

the transfer of alleles between populations

genetic drift

random changes in allele frequencies in a population due to chance events

[does not occur as a result of natural selection]

Which populations will genetic drift have a larger effect in? why?

small population

-each allele has a proportionally larger effect in a small population.

-alleles are more likely to be lost from population

-population becomes more vulnerable due to environmental change

what does genetic drift cause

reduces the genetic diversity of a population because alleles are lost from the gene pool

types of genetic drift

genetic bottleneck

founder effect

genetic bottleneck [include effects]

it’s a large decrease in population size due to an event such as natural disasters and disease

→ leads to a reduced gene pool and decreased genetic diversity

→alleles are lost from original population

→new population is descended from few survivors

→population may struggle to adapt to future changes in their environment

founder effect [include effects]

A small group of individuals leaves a population and starts a new population elsewhere

→ leads to a reduced gene pool and decreased genetic diversity

→rare alleles from the original population may become more common in the new population (may be beneficial, harmful or neutral)

→allele frequencies in new population are different to the original population

The size of a population influences its genetic variation. Factors that can limit population size?

Density-dependent factors [depends on the population density – the more crowded the population, the stronger the effect]

-competition

-predation

-disease

Density-independent factors [affect a population regardless of its size]

-natural disasters

genetic diversity in:

small population

large population

small

→small gene pools

→genetic diversity is low

large

→large gene pools

→genetic diversity is high

population

a group of organisms of the same species that live in the same habitat at the same time and can interbreed.

Hardy-Weinberg principle

states that allele frequencies remain constant across generations in a population that is not evolving.

state assumptions

the population must have:

No mutation

Large population size

Random mating

No migration into or out of the population (no gene flow)

No selection (all individuals survive and reproduce equally)

p + q=1

whats p and q

variable for:

homozygous dominant

homozygous recessive

heterozygous

p= dominant allele [tip: p is before q in alphabet]

q= recessive allele

p² = frequency of homozygous dominant [pp]

2pq = frequency of heterozygous individuals [pq or pq]

q² = frequency of homozygous recessive [qq]

→p²+2pq+q² = 1

[read carefully if q asks to find probability/proportion or number of individuals

→ for number of individuals multiply probability by total number of species]

when would using Hardy-Weinberg principle be not appropriate? (2)

-population is small

-mating is not random

factors that cause allele frequencies to change in a population that meets the Hardy-Weinberg principle (4)

natural selection

genetic drift

mutation

migration