Transcript CH24

Chapter 24
Evolutionary Genetics
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Chapter Outline
Genetic Variation in Natural Populations
Molecular and Evolution
Speciation
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Genetic Variation in Natural
Populations
--Variation in alleles of genes
--There are three primary sources of genetic variation
Mutations are changes in the DNA structure
Gene flow in genes’ movement
[Sex determination (non somatic cells)]
--It provides the raw material for natural selection
Genetic Variation~~phenotype
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Phenotypic Variation
Phenotypic differences or variations= polymorphisms
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Phenotypic Variation
Polymorphisms: two or more clearly
different phenotypes (forms) co-exist in
the same population
In Drosophila, many mutant alleles have been characterized
for phenotypes such as eye color.
In Human, the Duffy Blood Group system is characterized
by a different allele in Chromosome 1 (Duffy allele, Fya and Fyb).
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A Human Polymorphism
Polymorphism and ethnic groups
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Variation in Chromosome
Structure (genetic variability).
Drosophila polytene chromosomes afford
researchers an opportunity to look for variation in
chromosome structure in natural populations.
There have been identified many rearrangements
of the banding patterns in the polytene
chromosomes
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Puffs
Variation in Protein Structure
Amino acid differences in proteins can
be detected using gel electrophoresis.
Proteins differing in size and charge can
be separated by moving them through a
starch or polyacrylamide gel (matrix).
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Isocitrate Dehydrogenase
Variability in Trillium pusillium
Isocitrate dehydrogenase exists as a dimer.
Electrophoresis can distinguish dimers
containing two fast-moving subunits, two
slow-moving subunits, and a “hybrid” enzyme
with one fast and one slow enzyme subunit.

The fast- and slow-moving subunits are
allozymes (variant forms) encoded by
different alleles of the same locus.
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Protein Polymorphisms
Proteins that exhibit electrophoretic
variation are polymorphic if at least two of
the variants have frequencies greater than
1% in the population.
Proteins that do not exhibit electrophoretic
variation are monomorphic.
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Variation in Nucleotide Sequences
DNA sequencing can be used to study
genetic variation.
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Polymorphisms in the Alcohol
Dehydrogenase (Ddh) Gene of
Drosophila
11 Ddh genes
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Most of the polymorphisms in the
Alcohol dehydrogenase (Adh) gene are
in noncoding regions (introns and 3'
and 5' untranslated regions).
Most of the polymorphisms in the
coding region are silent- (it does not
alter a protein function and mobility).
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Techniques for Detecting
Nucleotide Polymorphisms
PCR followed by DNA sequencing
Gene chip technology for identification of
SNPs ( single-nucleotide polymorphisms) ~1
to 2kd.
Anti-sense RNA
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
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Molecular and Evolution
Analysis of DNA (and protein)
sequences provide information on the
phylogenetic relationships among
different organisms, and on their
evolutionary history.
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Molecular Evolution
Heredity depends on the sequence of
nucleotides in DNA and the transmission of
DNA molecules from parents to offspring.
When mutations occur, modified DNA
molecules are transmitted to the offspring.
Over time, mutations accumulate and the
DNA sequence is changed; chromosomal
rearrangements may also occur.
genotype
DNA RNA Protein
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phenotype
DNA and Protein Sequence Analysis vs.
Traditional Methods to Study Evolution
DNA and protein sequences follow simple
rules of heredity.
Molecular sequence data are easy to obtain
and are amenable to quantitative analyses
framed in evolutionary genetics theory.
Molecular sequence data allow analysis of
evolutionary relationships among organisms
that are phenotypically very dissimilar.
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DNA and Protein Sequence Analysis vs.
Traditional Methods to Study Evolution
It is difficult to obtain DNA or protein
sequence data from extinct organisms.
It is not always clear how molecular
sequence data relate to questions about
evolution at the phenotypic level.
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Molecular Phylogenies
Evolutionary relationships among organisms (at
the DNA level) are summarized in phylogenetic
trees, or phylogenies (branching diagram).
All organisms on earth have descended from a
common ancestor.
A phylogeny that shows only the relationships
among organisms is an unrooted tree.
A phylogeny that superimposes the
relationships on a time scale to show how
organisms evolved (variation) is a rooted tree.
Phylogenetic Trees
 Lineages bifurcate (nodes) to produce branches.
 The terminal branches lead to the organism being
studied.
 Each bifurcation represents a common ancestor.
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Homologous Sequences
The descendants of an (common) ancestral
DNA or protein sequence are homologous.
Sequences that resemble each other but are
derived from different ancestral sequences
are analogous.
Construction of phylogenetic trees should be
based on homologous sequences.
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Steps Involved in Constructing
Phylogenetic Trees
Aligning the sequences to allow comparisons
among them
Ascertaining the amount of similarity or
difference between any two sequences
Grouping the sequences on the basis of
similarity
Placing the sequences at the tips of trees
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Parsimony
The principle of parsimony or
SIMPLICITY is used to judge the merit
of a tree.
The best tree is the one that requires
the fewest mutational changes.
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Phylogenetic
Trees of Hominids
Based on
Mitochondrial (mt)
DNA
mt DNA:
circular with ~16.6 Kb
Sequence ~896 bp
Analyzed ~ 283 bp
---number of mutations
145, 147, 148
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Speciation
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What Is a Species?
A species is a group of organisms that share
characteristics.
species is a group of interbreeding, or
potentially interbreeding, producing fertile
organisms
species is a group reproductively isolated from
other species.
Species have been defined in different ways.
– Traditionally, species have been defined based on
phenotypic characteristics.
– Evolutionary genetics, species have a “defined” a
“shared genetic pool”
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Speciation
Species arise when a population
of organisms splits into genetically
distinct groups that can no longer
interbreed with each other.
Consequence?
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Reproductive Isolation
Is a feature or mechanism to prevent
breeding between species
Prezygotic isolating mechanisms
prevent members of different groups
from producing hybrid offspring.
Postzygotic isolating mechanisms
prevent hybrid offspring from passing on
their genes to subsequent generations.
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Prezygotic Isolating Mechanisms
Prezygotic isolating mechanisms prevent
mating between individuals form different
populations of organisms or by preventing the
gametes of these individuals to form zygotes.
These mechanisms include
– Ecological or Geographical isolation based on
habitat preference (different habitats in the same
geographical area)
– Temporal or behavioral factors (e.g., different
times sexual maturity or different courtship rituals)
– Mechanical Anatomical or chemical
incompatibilities in reproductive organs or
gametes (e.g., failure to mate successfully or to
form zygotes) © John Wiley & Sons, Inc.
Postzygotic Isolating Mechanisms
Postzygotic isolating mechanisms
operate after hybrid zygotes have been
formed.
Mechanisms include
– Reduction of hybrid viability (e.g., failure to
survive or to reach sexual maturity)
– Impaired hybrid fertility (failure to produce
functional gametes)
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Modes of speciation
Allopatric
Sympatic
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122) Which of the following accurately describes the Hardy-Weinberg genotype frequency expression?
a) P2 + 2p +q2
b) P2 + 2pq + q3
c) P2 +2pq+q2
d) P2 + 3pq+q3
e) (p+q)3
Answer: c
243) What effect does consanguineous mating and assortative mating have on genotypic frequencies in populations?
1. Reduce the frequency of homozyotes
2. Increase the frequency of homozygotes
3. Reduce the frequency of heterozygotes
a) 1
b) 2
c) 3
d) 1 and 3
e) 2 and 3
Answer: e
38) What is the ultimate source of all genetic variability?
a) Natural selection
b) Artificial selection
c) Mutation
d) Natural selection and artificial selection
e) None of these
Answer: c