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Gene pools and speciation

Topic 10
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Several generations later, the
proportions of alleles may not be the same
➢ In case of immigration and emigration, the allele frequency usually changes
➢ No change in allele frequency means no evolution
➢ Variations in a population can lead to speciation
○

Speciation is the process of an evolving population changing significantly
enough so that production of offspring with the original population
becomes impossible
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Formation of new species

Hardy-Weinberg equation
➢ Used to calculate the frequency of alleles, genotypes or phenotypes within a
population
➢ Useful for determining how fast a population is changing or predicting the
outcomes of matings or crosses
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Gene pools and speciation

Topic 10
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3

Biology HL

Selection

➢ Directional selection is when a phenotype is favoured over another by natural
selection
➢ The frequency of one phenotype is increasing over time, while the other phenotype
is decreasing
○

Due to environmental changes

○

Examples: peppered moth, Biston betularia

➢ Stabilizing selection is when one phenotype is favoured over two extreme
phenotypes
○

A selection away from two extremes, a selection towards the mean

○

Example: a flower producing a balanced quantity of nectar; not too much not
too little

➢ Disruptive selection is when two extreme phenotypes are favoured over one
intermediate phenotype
○

A selection against the mean

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Example: birds with small beaks and birds with large beaks

○

Having two separate morphologies gives these species a better chance of
survival in places where water supply and food sources are variable

○

The idea is to maintain two different phenotypes within a population
■

If the differences are extreme and two populations occupy different
environments, it is possible for speciation to occur

Polyploidy

➢ Monoploidy is when the cells contain one set of chromosomes
○

Haploid cells (n)

➢ Polyploidy refers to the situation in which a cell contains three or more sets of
chromosomes
○

3n = triploid, 4n = tetraploid, 5n = pentaploid

➢ Arise when cell division does not completely separate the copies of chromosomes
into distinct nuclei
➢ Most common in plants than animals
○

More vigorous plants that produce bigger fruits or food storage organs and
are more resistant to diseases

➢ A triploid individual cannot mate with a diploid individual
○

A new species is formed

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Gene pools and speciation

Topic 10

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