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Biology F215
Variation and Selection
Key Words

Key Term Definition

Speciation due to organisms of a species being separated by geographical barriers so
Allopatric speciation that over time members of the two populations become so different that they cannot
interbreed and are considered to be two different species.

Organisms with…

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Genotype Alleles present within cells of an individual, for a particular trait/characteristic.

The concept that both genotype frequencies and gene frequencies will stay constant
from generation to generation, within a large interbreeding population where mating is
random, there is no mutation and no selection or migration.

Isolating mechanism…

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What is variation?

Variation is the differences in phenotype between individuals of the same or different species
o Phenotype is the observable characteristics of an organism

What is discontinuous variation?

Qualitative differences between phenotypes
o Discrete categories with no intermediate phenotypes
Individuals may be evenly or unevenly distributed between the…

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What is the genetic basis for continuous variation?

Characteristic controlled by two or more genes (polygenic characteristic)
o Each gene contributes to the phenotype
Different alleles have a small effect on the phenotype
An example is human eye colour

How do the variations in the environment and genes contribute to…

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What is a population?

A population is a group of individuals of the same species that can interbreed
o Populations change in size due to birth, death and migration
All the alleles in a population represents the gene pool
Individuals have genomes and Populations have gene pools

What are allele…

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Put into practise
o The frequency of cystic fibrosis (genotype ff) in the UK is currently approximately 1 birth in 2000.
Estimate the proportion of people in the UK that are carriers of the allele for cystic fibrosis.
1 in 2000 is recessive = q2 = 1/2000 = 0.0005 so:…

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In natural selection, the environment selects the individuals that survive and reproduce
o These selection pressures act on individual organisms with different alleles
This changes the frequency of alleles in a population

What types of natural selection are there?

Graph section 1 is stabilising selection which reduces variation but does…

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What is genetic drift?

Genetic drift in a small population is
when a chance event may cause a
large change in allele frequencies
o Even large populations may
pass through bottlenecks,
when population levels fall
What may produce
such a small
A natural disaster e.g. volcanic eruption, disease…

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What are isolating mechanisms?

Isolating mechanisms are when a population becomes split into sub groups by an isolating mechanism that
prevents individuals interbreeding
o The allele frequencies in each sub-group will change independently
This is due to mutation and natural selection
And may lead to speciation
Examples of this are:…

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Mechanisms exist that can prevent two populations from interbreeding, even though they exist in the same
o Biological isolation
Biological isolation prevents two populations from interbreeding, even though they exist in the
same environment
Mutations occur independently
Selection pressures differ
Allele frequencies change and gene pools diverge
o Speciation…


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