Download Mechanisms of Evolution

Mechanisms of Evolution
• 1) Micro-evolution
– change within a species
• 2) Macro-evolution
– change between species
Micro-evolution
• Hardy-Weinberg (2 mathmaticians)
– “Frequency of alleles in a population
stays the same generation after
generation unless acted upon by
some outside force.”
• So, what kind of things can
change the frequency of alleles in
a population?
Hardy-Weinberg (Cont)
• All alleles contribute by males (p+q)
• All alleles contributed by females
(p+q)
• Together males and females mate
• (p+q)(p+q) = 1
– all combinations =100%
• p2 + 2pq + q2 = 1 (Also show Punnett)
Hardy-Weinberg (Cont)
• The p = Allele F = Free Earlobes
• The q = Allele a = Attached earlobes
• Since q (or a) is the only phenotype
with a known genotype (aa), then we
can calculate the frequencies of
alleles in a population
• aa -> a2 -> a = (a2)1/2
• F = 1-a (since both a and F = 100%)
Hardy-Weinberg
• For a Non-Evolving population,
these conditions must be met:
• 1)
• 2)
• 3)
• 4)
• 5)
No mutations
No migration
No Genetic Drift-Population Large
Random Mating
No Natural Selection
• If a population changes (gene
frequency) or evolves, it must be
due to some factor that violates
one of the conditions stated
previously.
Gene Flow: Alleles
introduced to a gene pool
– 1) Mutation
– 2) Migration (immigration/emigration)
• Animals leave area for another
– contribute genes to neighbor’s gene pool
• Plants have seeds dispersed by high winds
• Immigration/Emigration
– Very Common
– 2nd only to Natural Selection as cause of
evolution
Conflicting Forces
• Gene flow - increase similarity
between populations
• Natural Selection - decrease
similarity between populations
• Possible Outcome:
– gradient of variation from one population
to another
– Increase distance = increase variation
Genetic Drift: Evolution by
chance
• 1) Bottleneck Effect: Most of the
population is destroyed by a natural
event (volcano, earthquake, etc.) and
a few individuals survive to reproduce
• 2) Founder Effect: Few people leave
an area to colonize new area
– Good chance not carry
representative sample of all genes
present in original population
– Environment is different
• different selection pressures
• Therefore, evolve in new
direction
– In practice, difficult to tell
how much of genetic
differences between old and
new populations is due to:
• 1) Founders effect
• 2) Different selection pressures
in two environments
– Founder Effect is great
• 1) Population of plants that populate
an island from a single seed
• 2) Animals which have descended from
one original wild female
– domestic hamsters
Mating Preferences
• Females consistently choose one trait over
another
– Exert selection pressure for one gene
• tails of peacocks
– Female preferences is a form
of Natural Selection
• One genotype has reproductive
advantage over another