Download Mechanisms of Evolution

Measuring Evolution
Populations evolve; individual
organisms do not
Natural selection acts upon an individual’s phenotype.
Within the lifetime of that individual, this phenotype
cannot change.
• Therefore natural selection operates only on
populations over many generations
Vocab:
• Gene pool: the entire collection of alleles among a
population.
• Allelic frequency: The percentage of a particular
allele in the gene pool of a population.
– Use % b/c population size changes from gen to
gen
Example snapdragon population
Generation
Phenotype
Amount
Red
8
Pink
8
White
0
Red
15
Pink
6
White
3
1
5
Frequency
R
R’
Allele frequency
The tool biologists use to see if populations have
evolved.
If allele frequency has not changed over
generations, then population is in genetic
equilibrium (not evolving)
- Relate to pop growth?
- Relate to succession?
If allele frequency has changed over
generations, then population is evolving
Ways Genetic Eqilibrium can
Change
(Mechanisms of Evolution)
1) Genetic Drift
• Change in gene pool of a small population due
to chance.
• Ex- Lizards: WW & Ww= non-webbed feet
• ww= webbed
• Pop= 10 individuals and 3 WW are killed by
earthquake, what happens to allele
frequency?
• Allelic frequency of W will lower in this pop
and w will increase.
Genetic Drift
2) Gene Flow
• The gain or loss of
alleles from a pop
by migration
• Tends to reduce
genetic differences
b/w populations
• Ex: Humans
3) Mutations
• Change in an organisms DNA that creates a
new allele which leads to new phenotypes.
• The source of genetic variability.
• Need to be recombined
• Rare; take long time to develop, can reduce
fitness
• (e.g.) Sickle cell anemia
4) Non-random mating
• Selection of mates other than by chance.
Selective breeding by populations (most do
this).
• Ex. Herd of elk, elephant seals, peafowl, red
cross-bills.
5) Natural Selection
• The factor that is likely to result in adaptive
changes in the gene pool.
• Acts upon the ranges of variation
Pattern of Inheritance?
• 3 types
1) Stabilizing Selection:
• Favors the average individual in a population
for a particular environment.
• Spider size; small= Can’t catch enough prey
• Large= easy to see and eaten
• Medium= Favored
• Variation is reduced
• Human babies born at an average mass are
more likely to survive than babies born either
much smaller or much larger than average.
Copyright Pearson Prentice Hall
2) Directional Selection
• When one of the extreme forms of a trait is
favored.
• Ex. Wood pecker beak size: Short, Med, Long
• In this case, birds with larger beaks have
higher fitness. Therefore, the average beak
size increases.
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3) Disruptive Selection
• When both extreme forms of a
trait are at an advantage
• Ex: Limpets shell color white &
Dark Brown
• On light colored rocks white
shell adv.
• On darker rocks dark brown adv.
• But tan shells easy to see
regardless of rock color, most
preyed upon.
• ** can lead to the evolution of 2
new species
• If average-sized seeds become scarce, a bird
population will split into two groups: one that
eats small seeds and one that eats large seeds.
Copyright Pearson Prentice Hall