Download Modern theory of evolution o Bottleneck Mutation

Modern theory of evolution
r Changes in gene pool as a result of natural selection. Natural selection leads to changes over
time, changes in allelic frequency.
u Gene - paft of the chromosome containing a trait
o Alleles - alternate forms of a trait
u Gene pool - all of the genes in a population
u Allelic frequency - propoftion of specific genes in a gene pool
Population size
r If population is large, then free gene flow
a Migration - immigration or emigration out of an area
r If population is localized or fragmented then genetic drift occurs
r Genetic drift- any change in genetic make up due to chance. It is a non random change in
gene frequency, In small populations ceftain alleles, present in the larger population, may not
be present.
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o
Founder effect
Bottleneck
Founder effect
r when a small pop gets isolated from their place of origin, rare alleles may be enhanced. Some
alleles may be eliminated.
Bottleneck
n If a pop is reduced in size due to flood etc. the survivors may have only a paft of the total
alleles causing a reduction in genetic variability of the population (A sharp reduction in the
gene pool)
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Mutation
r
Random; onlysource of new alleles
u Can be lethal, neutralor beneficial
u Caused by
Mistakes made when DNA replicates
Environmental factors such as gamma rays, chemicals, temperature
r
r
What are the results of Natural Selection?
r Natural selection causes deviations from Hardy Weinberg by changing allele frequenry.
o Adaptations to living and physical conditions enables organisms to survive under a given
set of conditions and live to reproduce
Natural selection is a main cause of evolution
r The environment imposes the conditions that result in selection and thus the direction of
r
r
evolution.
Natural selection acts on the phenotype of the organisms; this in turn affects the genotype.
Natural selection on single genes can affect allelic frequency of that gene. Fitness depends
on the effect of that one gene.
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When polygenic traits are involved there is a range of phenotypes that may be fit.
Natural selection acts upon variation in the population
Three main types of selection
o Stabilizing selection
a Disruptive selection
Directional selection
o
Stabilizing selection
r
Selects against extremes. Acts to eliminate both extremes from the phenotypes; individuals at
the center of the curve show highest fitness.
q Ex: babies born to humans tend to be around the same size and birth weight. Larger and
smaller infants are not as fit.
In ducks and chickens eggs those with intermediate weights have highest fitness..,
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Disruptive selection
r
Selects for extremes, acts to eliminate the intermediate type.
a In a bird population where large and small seeds are more common than medium seeds,
birds at the extremes have highest fitness,
Directional selection
r
Shift in population; acts to eliminate one extreme from an array of phenotypes.
o Ex: beak size when only large seeds are available, those birds with larger beaks are more
fit.
Speciation
r Speciation - formation of new species
r Gradualism (Phyletic evolution) - gradual changes take place through time.
I Allopatric speciation - Divergent evolution - ancestral species gives rise to others.
Punctuated equi Iibrium.
o Geographic isolation leads to reproductive isolation. Genetic drift
o Adaptive radiation - organisms evolve to fill unoccupied niches especially after major
extinction. Key brea kthroug hs.
a Lineages may remain unchanged for long periods of time
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Name
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Variation in P€)pulations
with Chapter
Use
15, Section 15.2
Stabilizing Selection
Directiona! Selection
Selection
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Disruptive Selection
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UNIT
5
CHAPTER
1
5
The Theory of Evolution
Name
Variation in Populations
Use
with Chapter lS, Section lS.2
1. Study the_graph for stabilizing selection. Explain the effect of this qrpe of
natural selection.
2. use the transparency to explain the effect of directional selection.
3. use the tra,sparency to explain the effect of disruptive selection.
a. why might average-sized spiders be favored in a given environment?
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5' Why mirhl woodpeckers with longer
shorter beaks?
beaks have a selective advantage over those
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6. Limpets are marine organisms that attach themselves to rocks. Under what
environmental conditions_might intermediate forms of limpets-those that are tan,
rather than white or dark brown-be at a disadvantage?
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7. what do the three gpes of natural serection have in common?
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CHAPTER
15
The Theorv of Evolution
UNIT 5