individuals with in a population vary because they have differen alleles, predation, diease and competition(SELECTION PRESSURES) create a struggle for survival.
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how does natural selection work? stage 2
because some individuals vary, some are better adapted to the selection pressures than others, these individuals are more likley to survive, reproduce and pass on this advantagous allele.
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how does natural selection work? stage 3
this means that a greater proportion of the next generation inherit the advantagous allele, the frequency of this allele increases in generation to generation.
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what is stabalising selection?
when the environment isnt changing much, the individulas towards the middle of the range are more likeley to survive and reproduce. this also decreases the range of possible phenotypes.
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what is directional selection?
when they is a change in environment, individuals with an extreme type of allele.
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what is genetic drift?
when certain alleles are passed on by chance and become the majority of the population.
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what is a genetic bottleneck?
is an event that cuases a big reduction in a populations size, leading to a reduction in the gene pool (e.g.natural disaster).
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what does this do for genetic drift?
the genetic drift has a greater effect on smaller populations.
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what is the founder effect?
dscribes what happens when just a few organisms from a opulation start a new population and there are only a small number of alleles in the initial gene pool.
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what does the Hardy-Weinberg principle suggest?
that the frequencies of alleles in a population wont change from one generation to the next.
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what kind of population does this only apply to?
large populations, with random mating.
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what is the equation for allele frequency?
p+q=1
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what is p?
frequency of the dominant allele
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what is q?
frequency of the recessive allele
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what is the equasion for genotype frequency?
p squared + 2pq+q squared=1
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what is p squared?
the frequency of homozygous dominant genotype
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2pq?
frequency of the heterozygous genotype
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q squared?
frequency of the homozygous ressecive genotype.
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what is artificial selection?
when humans select individuals in a population to breed together to get desirable traits
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what are some examples?
modern dairy cow and bread wheat.
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how can this cause problems for animals involved?
it can exaggerate certain taits, leading to health problems
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what is speciation?
it is the development of a new species.
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when does it occur?
when populations of the same species become reproductivley isolated- changes in allele frequencies cause changes in pheneotype that mean they can no longer breed togetherto produce fertile offspring.
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what is allopatric speciation?
when geographical isolation and natural slection lead to 2 new species.
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what is reproductive isolation?
occurs becuae the changes in the alleles and pheneotypes of 2 populations prevent them from successfully breeding together.
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what are the 3 ways this occurs?
seasonal changes- become sexually active at different times of the year. mechanical changes- changes in genitalia prevent successful mating. behavioural changes- develop courtship behaviour that isnt atttractive to the main population.
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what is sympatric speciation?
speciation without geographical isolation e.g.random mutations
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