Download Natural Selection and Specation

Natural Selection and Specation
The Peppered Moth – example of evolution
Sources of Genetic Variation
• The 2 main sources of genetic
variation are mutations and the
genetic shuffling that results from
sexual reproduction
Mutations
• Any change in a sequence of DNA
• Mutations can occur because of
– Mistakes in DNA replication
– Radiation or chemicals in the environment
• Some mutations dont affect the phenotype but some
do
Variation in Populations
• Variation via different phenotypes naturally
exist within a population
• Some genotypes may offer a selective
advantage, while others a disadvantage
• Variations within a population can be small,
but are the basis of evolution
Gene Pools
• The sum of all alleles within a population is referred to as the gene
pool
• Biologically, a gene pool is a group of individuals of the same
species that live in the same geographical area and interbreed to
produce fertile offspring
• Range of gene pool restricted by alleles available – not all organisms
have the genes to produce tails, wings etc
• Some genes do not contribute to variation – they are fixed within
the population
• Only 15-20% of genes are used in evolutionary change
Allele Frequencies
• Genetic polymorphs – different phenotypes that exist
due to different alleles
• Polymorph frequency affected by:
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Mutations in alleles
Immigration
Emigration
Reproduction rate
Genetic drift
The bottleneck effect
The founder effect
Migration and Gene Flow
• Populations in a genetic sense are
defined by reproductive and
genetic isolation
• Although some populations exist
in isolation, migration usually
takes place into and out of a
population
• If interbreeding occurs, gene flow
takes place
Migration and Gene Flow
• Indigenous Australians have some alleles for blood group
gene that exist in frequencies different from other
populations
• Do not possess allele IB therefore cannot be B or AB blood
groups
• Isolation for over 50,000 years means limited gene flow
• Increased genetic flow has lead this to change
Genetic Drift
• Natural Selection is not the only source of
evolutionary change
• In small populations, an allele can become more or
less common by chance
Genetic Drift
• These individuals may carry alleles in different
relative frequencies than did the larger population
from which they came
• If so, the population that they found will be
genetically different from the parent population
• This cause is not natural selection, but chance
Bottleneck Effect
• Catastrophic events can
severely reduce the size
of a gene pool
• Surviving population
may persist, however
this means that genes
are lost and surviving
population not
representative of
founding gene pool
Founder effect
• A situation in which allele frequency changes as a
result of the migration of a small subgroup of a
population
• Results in less genetic diversity in new population
Questions
• In groups, or with a partner, discuss questions
1 – 8 to test your knowledge of what we have
learned so far. Use the terminology in this
section to articulate your answers.
• Discussion helps retention of new concepts, so
get involved!!