Transcript Evolution as genetic change

Evolution as genetic change
Evolution: Genetic definition
• Evolution is any change in the relative
frequencies of alleles in the gene pool of a
species
• Sexual reproduction alone cannot cause
such a change
Natural selection on single gene
traits
• Harmful mutations
– Fewer copies of the allele are passed on
because the individual does not survive or does
not reproduce
– Allele becomes less common in the population
Natural selection on single gene
traits
• Helpful mutations
– Carriers of the allele produce more offspring
that survive than non-carriers
– Relative frequency of this allele increases in a
few generations
– An allele with no negative effects will be found
after several generations in nearly all members
of the species
Natural selection on polygenic
traits
• Variation of polygenic traits is a normal
distribution
• Fitness of individuals near each other in the
distribution is similar
• BUT can vary considerable from one side of
the curve to the other
Natural selection on polygenic
traits
• Natural selection can affect this distribution
in three ways:
• 1. Stabilizing selection
• 2. Directional selection
• 3. Disruptive selection
Stabilizing selection
• Organisms near the center of the curve are
more fit than those at the ends
• Look at figure on page 489
• Example: human baby weight at birth
• Evolution is minor or absent
Directional selection
• Organisms at one end of the curve have
higher fitness than those in the middle or
the other end
• Entire curve moves as the trait changes
• Evolution occurs, change in allele
frequency
• Example: finches
Disruptive selection
• Organisms at the ends of the curve have
higher fitness than those in the middle
• Single curve can split into two curves if the
selective pressure is high
• Example: insects that resemble poisonous
insects
Other mechanisms of evolution
• Role of chance
– In small population allele frequency can change
due to chance
• Random change in allele frequency is called
genetic drift
– Bottleneck and founder effects
• Look at figure on page 490 for example
Speciation (1)
• A new species evolves when:
• A population changes enough (genetic
changes) so that breeding cannot occur
between members of the original population
and the changing population
Speciation (2)
• One major way is when two populations are
isolated from each other geographically
• Other ways:
– Behavioral isolation
– Temporal isolation
Pace of evolution
• Gradualism
– Evolutionary change takes long periods of time
– Darwin proposed this
• Punctuated equilibrium
– Evolution proceeds rapidly followed by periods
of stability, then more evolution, then stability
– Modern view, along with gradualism
How do you test if evolution is
happening?
• Scientific method again…..
– Hypothesis is evolution is not happening (=
allele frequencies are not changing)
– Measure allele frequencies over time and see if
they change or not
• Five conditions can result in allele
frequency change
Hardy Weinberg principles
• The five condition are:
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Non random mating
Immigration or emigration
Small population sizes
Mutations
Natural selection