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Biology IB HL - 10.3 Speciation Part 2

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This deck covers key concepts of speciation, allele frequencies, and types of natural selection, including examples and effects on populations.

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What is an example of the founder effect?

Example: Certain Amish communities have a higher incidence of polydactyly because of inter-marriage within the community
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Key Terms

Term
Definition
What is an example of the founder effect?
Example: Certain Amish communities have a higher incidence of polydactyly because of inter-marriage within the community
What do allele frequencies represent?
Allele frequencies represent the prevalence of a particular allele in a population, as a proportion of all the alleles for that gene
How can allele frequencies be represented?
Consequently, allele frequencies are either represented as a percentage or as a value from 0 to 1.0
What can allele frequencies reflect (2)?
Changes in allele frequency can reflect either random processes (genetic drift) or differential processes (natural selection)
What will the founder effect and population bottlenecks do in terms of genetic differences?
Population bottlenecks and the founder effect will exacerbate genetic differences between geographically isolated populations
What is natural selection?
Natural selection is the change in the composition of a gene pool in response to a differentially selective environmental pressure
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Biology IB HL - 10.3 Speciation Part 2
TermDefinition
What is an example of the founder effect?
Example: Certain Amish communities have a higher incidence of polydactyly because of inter-marriage within the community
What do allele frequencies represent?
Allele frequencies represent the prevalence of a particular allele in a population, as a proportion of all the alleles for that gene
How can allele frequencies be represented?
Consequently, allele frequencies are either represented as a percentage or as a value from 0 to 1.0
What can allele frequencies reflect (2)?
Changes in allele frequency can reflect either random processes (genetic drift) or differential processes (natural selection)
What will the founder effect and population bottlenecks do in terms of genetic differences?
Population bottlenecks and the founder effect will exacerbate genetic differences between geographically isolated populations
What is natural selection?
Natural selection is the change in the composition of a gene pool in response to a differentially selective environmental pressure
How does natural selection affect the allele frequency?
The frequency of one particular phenotype in relation to another will be a product of the type of selection that is occurring
What 3 types of selection are there?
stabilising selection; directional selection; disruptive selection
What phenotype is favoured in stabilising selection?
Where an intermediate phenotype is favoured at the expense of both phenotypic extremes
What phenotype is removed in stabilising selection?
This results in the removal of extreme phenotypes (phenotypic distribution becomes centrally clustered to reflect homogeneity)
When does stabilising selection operate?
Operates when environmental conditions are stable and competition is low
What is an example of stabilising selection?
An example of stabilising selection is human birth weights (too large = birthing complications ; too small = risk of infant mortality)
What phenotype is favoured in directional selection?
Where one phenotypic extreme is selected at the cost of the other phenotypic extreme
How does the phenotypic distribution shift in directional selection?
This causes the phenotypic distribution to clearly shift in one direction (towards the beneficial extreme)
What does directional selection operate with regards to?
Operates in response to gradual or sustained changes in environmental conditions
What will directional selection typically be followed by?
Directional selection will typically be followed by stabilising selection once an optimal phenotype has been normalised
What is an example of directional selection?
An example of directional selection is the development of antibiotic resistance in bacterial populations
What phenotypes are favoured in disruptive selection?
Where both phenotypic extremes are favoured at the expense of the intermediate phenotypic ranges
How does the phenotypic distribution shift for disruptive selection?
This causes the phenotypic distribution to deviate from the centre and results in a bimodal spread
When may disruptive selection occur?
This occurs when fluctuating environmental conditions (e.g. seasons) favour the presence of two different phenotypes
What may continued separation lead to in disruptive selection?
Continued separation of phenotypic variants may eventually split the population into two distinct sub-populations (speciation)
What is an example of disruptive selection?
An example of disruptive selection is the proliferation of black or white moths in regions of sharply contrasting colour extremes
When does reproductive isolation occur?
Reproductive isolation occurs when barriers prevent two populations from interbreeding – keeping their gene pools separate
What are the two main categories of reproductive isolation barriers?
prezygotic and postzygotic
What is prezygotic isolation?
Prezygotic isolation – occurs before fertilisation can occur (no offspring are produced)