Transcript Chapter 1 Notes

The Evolution of
Populations
Darwin and Mendel were contemporaries of the
19th century
- at the time both were unappreciated for
their work
The turning point for evolutionary theory was
the development of population genetics
- emphasizes genetic variation and
recognizes the importance of quantitative
characters
A population’s gene pool is defined by its allele
frequencies
Population: a localized group of individuals
belonging to the same species
Species: individuals that have the potential to
interbreed and produce fertile offspring in
nature
The total aggregate of genes in a population at
any one time is called the population’s gene
pool
- all the alleles of a gene of all the individuals
in a population
Example of allele frequency
- population is 500 plants
- 20 are white (rr)
- 320 are red (RR), 160 are red (Rr)
Allele frequency is .8 or 80%
- 320 X 2 (for RR) = 640 + 160 (for Rr) ;
800/1000 = .8
The Hardy-Weinberg theorem describes a
nonevolving population
- the frequencies of alleles and genotypes in
a population’s gene pool remain constant
unless acted upon by outside factors
- the shuffling of alleles has no effect on a
population’s gene pool
This idea was independently discovered by
both Hardy and Weinberg in 1908
Uses 2 equations simultaneously
-P+Q=1
- p2 + 2pq + q2 = 1
For the HW equation to work, 5 conditions
must be met
- large population size
- no migration
- no mutations
- random mating
- no natural selection
Mutations and sexual recombination generate
genetic variation
Only mutations that occur in gametes can be
passed along to offspring
A mutation that alters a protein is more likely
to be harmful
Mutation: a change in a organism’s DNA
- if mutation is in gametes, immediate
change can be seen in the gene pool
- if the new allele produced by a mutation
increases in frequency, it is because the
mutant alleles are producing a
disproportionate number of offspring by NS
or genetic drift
Unique recombinations of existing alleles in a
gene pool are produced through meiosis
- the effect of crossing over
Microevolution: the generation-to-generation
change in a population’s frequencies of
alleles
The two main causes of microevolution are
genetic drift and natural selection
Genetic drift: a change in a population’s allele
frequencies due to chance
- the smaller the sample size, the greater the
chance of deviation for idealized results
- ex. coin toss
Bottleneck effect: genetic drift resulting from
the reduction of a population such that the
surviving population is not representative of
the original population
- generally caused by natural disaster
Founder effect: genetic drift in a new colony
- a few individuals from a larger population
colonize an isolated new habitat
- ex. from mainland to island
Natural Selection: the differential success in
reproduction
- the alleles passed on to the next generation
are disproportionate to the frequencies in the
present generation
- ex. Wildflower population
Gene flow: genetic exchange due to the
migration of fertile individuals or gametes
between populations
- ex. Wildflower population in a windstorm
Genetic variation occurs within and between
populations
Both quantitative and discrete characters
contribute to variation within a population
- quantitative variation indicates polygenic
inheritance
- discrete characters can be classified on an
either-or basis
Polymorphism: when two or more morphs
(variations) are represented in high enough
frequencies to be noticeable
Genetic variation can be measured at the level
of whole genes (gene diversity) and at the
molecular level of DNA (nucleotide diversity)
Gene diversity: the average percent of loci that
are heterozygous
Nucleotide diversity: comparing the nucleotide
sequence of DNA samples
Geographic variation: differences in gene pools
between populations or subgroups.
- NS can contribute to geographic variation
Diploidy and balanced polymorphism preserve
variation
Genetic variation can be hidden from being
selected against by the use of heterozygotes
Balanced polymorphism: the ability of natural
selection to maintain stable frequencies of
phenotypic forms
- ex. heterozygote advantage as seen in
sickle-cell disease
- ex. frequency-dependent selection: survival
and production of any one morph declines if
that phenotype becomes too common in a
population
Populations can adapt to the environment in
various ways
Directional selection: shifts the frequency curve
for variations in one direction by favoring
individuals that deviate from the average
character
ex. size of black bears
Diversifying (disruptive) selection:
environmental conditions favor individuals on
both extremes of a phenotypic range
Stabilizing selection: acts against the extremes;
favors the more common intermediate
variants