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Natural selection and Gene
• Evolution is ‘ a change in the gene pool of
a population from generation to generation
over time’.
• Although individuals are selected for, it is
populations that evolve.
• For natural selection to work there has to
be variation in the population for selection
to act on.
Sources of heritable variation
• Sexual recombination: meiosis mixes genes into
new combinations, there is random joining of
gametes in fertilisation.
• Crossing over: as well as mixing in meiosis the
homologous pairs can swap pieces.
• Mutations: new gene variations for evolution to
work on. Only source of new allelles, often
harmful and carried as reccessive allelles.
Agents that change gene
• Natural Selection
This is responsible for most evolutionary
change by reducing and changing genetic
• the range of phenotypes associated with a
characteristic in a population tend to be
normally distributed. Selection pressures
from the environment make some
phenotypes more favourable which
changes the alleles in the population
(decreases genetic variability).
• 3 types;
Stabilising natural selection
Disruptive natural selection
Directional natural selection
• Genetic drift: change in allele frequencies
due to chance. Happens in small
populations where chance will influence
more. Often happens when arm of sea
gets blocked off.
• Founder effect: limited number of
ancestors colonise an area = reduced
number of alleles in the gene pool.
• The bottleneck effect: disasters reduce
population to low level but the survivors
are random and not a representative
• Mutation: ultimate source of variation
within a population (increases genetic
• Gene flow/migration: this is movement of
individuals between populations.
Immigration (increase genetic variability),
migration (decreases).