Transcript AP Biology – Ch. 23 – The Evolution of Populations

AP Biology – Ch. 23 – The Evolution of
Populations
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The smallest unit of evolution?
Microevolution
3 main mechanisms responsible for change in
allele frequency
Which one of these consistently improves the
match between organisms and their environment?
Microevolution
Microevolution
Microevolution
Microevolution
Five Fingers of Evolution
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https://www.youtube.com/watch?fe
ature=player_embedded&v=5NdMnl
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23.1
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How do the variations that are the
raw material for evolutionary
change arise?
Mutation and sexual reproduction
Genetic Variation
A) Variation Within a Population
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Discrete or quantitative
Either-or-basis vs on a continuum
Single gene vs two or more genes
Measuring Genetic Variation
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Gene variability vs nucleotide
variability
Average heterozygosity – gel
electrophoresis, restriction fragment
analysis
Nucleotide variability – compare
DNA sequences
Why is this information important?
Variation between populations
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Geographic variation-differences in
the genetic composition of separate
populations
Ex) mice populations separated by a
mountain range, mummichog fish
and a cold-adaptive allele(cline)
Mutation
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The ultimate source of new alleles
Change in the nucleotide sequence
of an organism’s DNA
Point mutation
Alter gene number or sequence
Mutation rates – low in plants and
animals
- even lower in prokaryotes and
viruses(but shorter generation time)
- RNA viruses – mutations accumulate
faster – how does this impact treatment?
Sexual Reproduction
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variation results from the unique
combination of alleles that each
individual receives due to:
- crossing over
- independent assortment
- fertilization
23.2 – Hardy-Weinberg
-population
-gene pool/fixed allele
-conditions for HW equilibrium?
1. no mutations
2. random mating
3. no natural selection
4. extremely large population
5. no gene flow
23.3
Natural selection, genetic drift, and gene flow – alter
allele frequencies directly
1. natural selection – favoring some alleles over
others can result in adaptive evolution
2. genetic drift – chance events causing allele
frequencies to fluctuate
a. founder effect – Ex) Tristan da Cunha
b. bottleneck effect – a severe drop in
population size due to a change in the
environment
- case study – prairie chickens
Genetic drift: A Summary
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significant in small populations
can cause allele frequencies to
change at random
can lead to loss of genetic
variation within populations
can cause harmful alleles to
become fixed
Gene Flow
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the transfer of alleles into or out of
a population due to movement of
fertile individuals or gametes
serves to reduce genetic differences
between populations
Fig. 23.12 – Agrotis tenuis – What
if?
Modes of selection
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http://wps.pearsoncustom.com/wps
/media/objects/3014/3087289/Web
_Tutorials/17_A02.swf
Sexual Selection
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A form of natural selection in which
individuals with certain inherited
characteristics are more likely than
other individuals to obtain mates
http://evolution.berkeley.edu/evosit
e/evo101/IIIE3Sexualselection.shtm
l
http://www.pbs.org/wgbh/evolution
/library/01/6/l_016_09.html
The Preservation of Genetic
Variation
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1. Diploidy
2. Balancing Selection
a. Heterozygote Advantage
b. Frequency-dependent
selection
3. Neutral Variation
Frequency-Dependent Selection
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Situation 1 – Butterfly mimics poisonous butterfly
Situation 2 - Poisonous butterfly has several
morphs
Situation 1 – The fitness of the mimics is
positively/negatively frequency-dependent – The
fitness of a genotype increases as it becomes
more/less frequent.
Situation 2 – The fitness of each morph is
positively/negatively frequency-dependent – Each
morph gets fitter as it becomes more/less common.
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