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MECHANISMS FOR EVOLUTION
CHAPTER 20
• Objectives
– State the Hardy-Weinburg theorem
– Write the Hardy-Weinburg equation and be able
to use it to calculate allele and genotype
frequencies
– List the conditions that must be met to maintain
Hardy Weinburg equilibrium
VOCABULARY
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POPULATION
SPECIES
GENE POOL
GENE FLOW
BOTTLENECK EFFECT
FOUNDER EFFECT
HETEROZYGOTE
ADVANTAGE
• HYBRID VIGOR
• STABILIZING SELECTION
• DIRECTIONAL
SELECTION
• DIVERSIFYING
SELECTION
• SEXUAL DIMORPHISM
• POPULATION
– Localized group belonging to the same species
• SPECIES
– Naturally breeding group of organisms that
produce fertile offspring
• GENE POOL
– Total aggregate of genes in a population at any
one time
Most species are not evenly distributed over a
geographic range. Individuals are more likely
to breed with others from their population
center
HARDY – WEINBURG THEOREM
• A theory that describes a NON-EVOLVING
population. This could be used to compare a
population to at a later time to look for
changes.
– In the absence of other factors the segregation
and recombination of alleles during meiosis and
fertilization will not alter the overall genetic makeup of a population. (i.e. A population will stay the
same over time)
Conditions for Hardy-Weinburg
• In order for a non-evolving population to exist.
All of the following conditions must be met.
– Large population
– No net mutation
– Isolated population
– Random mating
– No natural selection
• In order to detect changes we must know
the current genetic make-up.
• Let’s test
• Imagine an isolated wildflower population with
the following characteristics:
– Diploid flowers with a mix of pink and white flowers.
– Pink is dominant A and white is recessive a
– There are 480 pink flowers and 20 white.
• What are the possible alleles?
– A (pink allele) or a (white allele)
• What do we know about their genotypes?
Hardy-Weinburg Equation
• Convert all numbers to ratios and let:
p = dominant allele & q = recessive allele
 this is the Allele ratio
For all the potential genotypes:
or 
• p2 = frequency of AA
• 2pq = frequency of Aa
• q2 = frequency of aa
• Go with what we know first:
– Calculate q2 : We can see the recessive trait.
– q2= 20/500 = 0.04 - or 4% of the population
• Use the √ q2 to find q
• √ 0.04 = 0.2
So: q = 0.2 or (20%)
• Then use q to find p
• The allele frequencies are:
• P=1–q
• P = 1 – 0.2
• P = 0.8 or 80%
• Now to find out the genotype ratio’s in this
population:
• p2 = frequency of AA
• 2pq = frequency of Aa
• q2 = frequency of aa
MICROEVOLUTION
LEADS TO
MACROEVOLUTION
MECHANISMS FOR EVOLUTION
• DO POPULATIONS OR INDIVIDUALS EVOLVE?
• WHAT IS A GENE POOL
• HOW CAN THE GENE POOL CHANGE?
– MUTATION
– GENETIC DRIFT
– GENE FLOW
– NON-RANDOM MATING
– NATURAL SELECTION
• WOULD THESE THINGS EFFECT A LARGE POPULATION OR A SMALL
POPULATION MORE?
Life Sciences-HHMI Outreach. Copyright 2006 President and Fellows of Harvard College.
CAUSES OF MICROEVOLUTION
• MUTATIONS – provide new alleles in a
population and provide the variation for
evolution to occur, should the mutation lead
to some adaptive advantage.
– Mutation alone does not cause evolution, but
provide a selective advantage that due to natural
selection can lead to a shift in allele frequency.
CAUSES OF MICROEVOLUTION
• GENETIC DRIFT – in small populations the
frequencies of alleles can be drastically affected
by chance events
– BOTTLENECK EFFECT – if populations are driven to the
point of extinction the remaining individuals do not
carry a true representation of the original gene pool.
– FOUNDER EFFECT – when a small number of
individuals colonize a new area they only carry with
them a small representation of the total number of
the alleles from the gene pool.
• GENE FLOW – genetic exchange between
populations due to migration
• NONRANDON MATING
– Breed with other members of the
“neighborhood” promotes inbreeding
– Assortative mating – mate with others like
themselves . This is the premise behind
artificial selection.
CAUSES OF MICROEVOLUTION
• NATURAL SELECTION – any environmental
factor that leads to a particular allele having
some adaptive advantage.
• There are three ways that natural selection
can affect the frequency of traits:
– Stabilizing selection
– Directional selection
– Disruptive selection
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Variation within Populations
• Most heritable variation is measured by
– Quantitative characters (vary along a continuum
ie. Height) are polygenetic
– Discrete characters (pink or white) are located on
a single gene
• Polymorphism – two or more forms of a
discrete character are represented in a
population
GEOGRAPHICAL VARIATION
A cline is a graded change in some trait along a
geographical axis.
MODES OF SELECTION