Transcript natural selection

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
• Describes a NON-EVOLVING population
• In the absence of other factors the
segregation and recombination of alleles
during meiosis and fertilization will not alter
the overall genetic make-up of a population
• Imagine an isolated
wildflower population
with the following
characteristics
– Diploid with both pink
and white flowers
– Pink is dominant A and
white is recessive a
– There are 480 pink
flowers and 20 white
• 320 are AA
• 160 are Aa
(p + q)2 = 1
p+q=1
p2 + 2pq +q2 = 1
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p2 = frequency of AA
2pq = frequency of Aa
q2 = frequency of aa
Calculate q2 first
There are 1000 alleles
– AA -- 320 x 2/plant = 640
– Aa --160 x 1/plant = 160
800
– aa -- 20 x 2/plant = 40
– Aa – 160 x 1/plant =160
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200
– Frequency of A = 80% and a = 20%
Condition for Hardy-Weinburg
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Large population
No net mutation
Isolated population
Random mating
No natural selection
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