Transcript Evolution and Natural Selection

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Evolution occurs via natural selection:
How to describe natural selection
- Individuals within a population
show variation due to changes in
their DNA (mutations) (state the
differences).
- Those with features best
adapted to an
environment survive and reproduce
(name the best feature and say
why).
- These genes/features get
passed on to the
offspring.
- These features become more
common in the
population
The origin of genetic variation
Genetic variation is heritable. It is this variation that natural
selection acts upon. The causes of genetic variation are:
Mutation
Sexual recombination

deletion, addition or
substitution of a nucleotide

independent assortment of
chromosomes in meiosis

deletion or translocation
of part of a chromosome

crossing-over during meiosis

aneuploidy – loss or gain
of a single chromosome

random fertilization.

polyploidy – the addition
of whole chromosome sets.
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Environmental causes of variation
Organisms can be affected by their environment. Variation
caused by the environment is not heritable, so it is not
subject to natural selection.
However, the ability of organisms to develop differently
in different environments can be genetic. This means
organisms can evolve to be flexible.
Plants are a good example
of this. The number of
leaves, growth pattern and
size of any individual plant
is dependent on the
environment, e.g. availability
of light and nutrients.
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Variation
All species exhibit variation between individuals.
discontinuous
continuous
limited number of traits,
no intermediate forms
range or gradation
of characteristics
controlled by a few genes
usually with multiple alleles
controlled by many
genes (polygenic)
environment has little effect
on these characteristics
environment has a
significant effect
e.g. human blood groups,
antibiotic resistance
e.g. stem height in plants,
milk yield in cows
Genotype and environment can both determine phenotype.
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What is a gene pool?
The gene pool can be defined as:
The total information from all the genes and alleles of the
breeding individuals in a population at a particular time.
The gene pool’s composition changes
from one generation to the next as the
relative proportions of alleles vary.
If there is a consistent change in
allele frequency (the proportion
of organisms in the population
carrying a particular allele) then
a population is evolving.
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The Hardy-Weinberg principle
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Introducing the Hardy-Weinberg principle
The Hardy-Weinberg principle is a mathematical model
used to calculate the allele frequencies of traits with
dominant and recessive alleles.
The model assumes that the population:

is large

has random mating

is experiencing no selection

has no mutation, emigration or immigration.
If these assumptions are met then the allele frequencies
of the population will remain stable over time.
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Calculating allele frequencies
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