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Chapter 23 Notes
The Evolution of
Populations
Concept 23.1
Darwin and Mendel were contemporaries
of the 19th century
- at the time both were unappreciated
for their work
Concept 23.1
The turning point for evolutionary theory
was the development of population
genetics
- emphasizes genetic variation and
recognizes the importance of
quantitative characters
Concept 23.1
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
Concept 23.1
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
Concept 23.1
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
Concept 23.1
Concept 23.1
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
Concept 23.1
This idea was independently discovered
by both Hardy and Weinberg in 1908
Uses 2 equations simultaneously
-P+Q=1
- p2 + 2pq + q2 = 1
Concept 23.1
Concept 23.1
For the HW equation to work, 5
conditions must be met
- large population size
- no migration
- no mutations
- random mating
- no natural selection
Concept 23.2
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
Concept 23.2
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
Concept 23.2
Unique recombinations of existing alleles
in a gene pool are produced through
meiosis
- the effect of crossing over
Concept 23.3
Microevolution: the generation-togeneration change in a population’s
frequencies of alleles
The two main causes of microevolution
are genetic drift and natural selection
Concept 23.3
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
Concept 23.3
Concept 23.3
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
Concept 23.3
Concept 23.3
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
Concept 23.4
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
Concept 23.4
Gene flow: genetic exchange due to the
migration of fertile individuals or
gametes between populations
- ex. Wildflower population in a
windstorm
Concept 23.4
Genetic variation occurs within and
between populations
Both quantitative and discrete characters
contribute to variation within a
population
- quantitative variation indicates
polygenic inheritance and
Concept 23.4
- 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
Concept 23.4
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
Concept 23.4
Geographic variation: differences in
gene pools between populations or
subgroups.
- NS can contribute to geographic
variation
Concept 23.4
Concept 23.4
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
Concept 23.4
- 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
Concept 23.4
Concept 23.4
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
Concept 23.4
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
Concept 23.4