Sources of Variation:
Mutation
Recombination
VII. Mutations I: Changes in Chromosome Number and Structure
- Overview:
VII. Mutations I: Changes in Chromosome Number and Structure
- Overview:
1) A mutation is ….
VII. Mutations I: Changes in Chromosome Number and Structure
- Overview:
1) A mutation is a change in the genome of a cell.
VII. Mutations I: Changes in Chromosome Number and Structure
- Overview:
1) A mutation is a change in the genome of a cell.
2) Evolutionarily important mutations are heritable (not somatic). However,
the tendency for a gene to mutate in somatic tissue (cancer) as a result of
sensitivity to env conditions may be heritable.
VII. Mutations I: Changes in Chromosome Number and Structure
- Overview:
3) Changes occur at 4 scales (large to small)
- Change in the number of SETS of chromosomes (change in PLOIDY)
- Change in the number of chromosomes in a set (ANEUPLOIDY: trisomy, monosomy)
- Change in the number/arrangement of genes on a chromosome
- Change in the nitrogenous base sequence within a gene
In general, the LARGER the change, the more dramatic (and usually deleterious) the
effects. If you have a functioning genome, a big change is going to be MORE
LIKELY to disable it than a small change…
VII. Mutations I: Changes in Chromosome Number and Structure
A. Polyploidy
VII. Mutations I: Changes in Chromosome Number and Structure
A. Polyploidy
1. Mechanisms:
VII. Mutations I: Changes in Chromosome Number and Structure
A. Polyploidy
1. Mechanisms:
a. Autopolyploidy: production of a diploid gamete used in
reproduction within a species.
Failure of meiosis I or II
2n gamete
3n zygote
Correct meiosis in
other parent
1n gamete
VII. Mutations I: Changes in Chromosome Number and Structure
A. Polyploidy
1. Mechanisms:
a. Autopolyploidy: production of a diploid gamete used in
reproduction within a species.
Errors in mitosis can also contribute, in hermaphroditic species
2n
1) Consider a bud cell in
the flower bud of a plant.
2n
1) Consider a bud cell in
the flower bud of a plant.
4n
2) It replicates it’s DNA
but fails to divide... Now
it is a tetraploid bud cell.
2n
1) Consider a bud cell in
the flower bud of a plant.
3) A tetraploid flower develops
from this tetraploid cell; eventually
producing 2n SPERM and 2n EGG
4n
2) It replicates it’s DNA
but fails to divide... Now
it is a tetraploid bud cell.
2n
1) Consider a bud cell in
the flower bud of a plant.
4n
2) It replicates it’s DNA
but fails to divide... Now
it is a tetraploid bud cell.
3) A tetraploid flower develops
from this tetraploid cell; eventually
producing 2n SPERM and 2n EGG
4) If it is self-compatible, it can mate
with itself, producing 4n zygotes
that develop into a new 4n species.
Why is it a new species?
How do we define ‘species’?
“A group of organisms that reproduce with one another and are
reproductively isolated from other such groups”
(E. Mayr – ‘biological species concept’)
How do we define ‘species’?
Here, the tetraploid population is even reproductively isolated from its
own parent species…So speciation can be an instantaneous genetic event…
2n
4n
4n
1n
2n
2n
3n
Zygote
1n
2n
Gametes
Triploid is a dead-end…
so species are separate
Zygote
Gametes
VII. Mutations I: Changes in Chromosome Number and Structure
A. Polyploidy
1. Mechanisms:
a. Autopolyploidy: production of a diploid gamete used in
reproduction within a species.
b. Allopolyploidy: fusion of gametes from different species
(hybridization). These are usually sterile because the chromosomes are not
homologous and can’t pair during gamete formation. BUT… if the chromosomes
replicate and separate without cytokinesis, they create their own homologs and sexual
reproduction is then possible.
Spartina alterniflora from
NA colonized Europe
X
Spartina maritima native
to Europe
Sterile hybrid – Spartina x townsendii
Allopolyploidy – 1890’s
Spartina anglica – an
allopolyploid and a
worldwide invasive
outcompeting native
species
VII. Mutations I: Changes in Chromosome Number and Structure
A. Polyploidy
1. Mechanisms:
2. Frequency:
Polyploidy is common in plants; 50% of angiosperm species may be
the product of polyploid speciation events.
VII. Mutations I: Changes in Chromosome Number and Structure
A. Polyploidy
1. Mechanisms:
2. Frequency:
Polyploidy is common in plants; 50% of angiosperm species may be
the product of polyploid speciation events.
In vertebrates, polyploidy decreases in frequency from fish to
amphibians to reptiles, and is undocumented in birds. There is one tetraploid
mammal. (Red viscacha rat).
VII. Mutations I: Changes in Chromosome Number and Structure
A. Polyploidy
1. Mechanisms:
2. Frequency:
3. The effect of hermaphrodism:
VII. Mutations I: Changes in Chromosome Number and Structure
A. Polyploidy
1. Mechanisms:
2. Frequency:
3. The effect of hermaphrodism:
- when the sexes are separate, the rare, random mutation of producing a
diploid gamete is UNLIKELY to occur in two parents simultaneously. So, the rare
diploid gamete made by one parent (karyokinesis without cytokinesis doubling
chromosome number in a cell) will probably fertilize a normal haploid gamete. This
produces a TRIPLOID… which may live, but would be incapable of sexual reproduction.
2n
1n
3n
VII. Mutations I: Changes in Chromosome Number and Structure
A. Polyploidy
1. Mechanisms:
2. Frequency:
3. The effect of hermaphrodism:
- unless…. the new organism could ALSO produce eggs without
reduction..clonally… and these are the rare animals that we see – triploid ‘species’
that are composed of females that reproduce asexually. (Some may still mate with
their diploid ‘sibling’ species so that the sperm stimulated the egg to develop – but
without incorporation of sperm DNA.)
Like this Blue-spotted Salamander A. laterale,
which has a triploid sister species, A. tremblayi
C. Inornatus
C. neomexicanus
C. tigris
Parthenogenetic diploid
VII. Mutations I: Changes in Chromosome Number and Structure
A. Polyploidy
1. Mechanisms:
2. Frequency:
3. The effect of hermaphrodism:
- So, in species with separate sexes, polyploidy is probably rare because the
typical condition would be TRIPLOIDY, which is usually a sterile condition.
VII. Mutations I: Changes in Chromosome Number and Structure
A. Polyploidy
1. Mechanisms:
2. Frequency:
3. The effect of hermaphrodism:
- So, in species with separate sexes, polyploidy is probably rare because the
typical condition would be TRIPLOIDY, which is usually a sterile condition.
- But in hermaphroditic organisms (like many plants), a single mutation can
affect BOTH male and female gametes.
VII. Mutations I: Changes in Chromosome Number and Structure
A. Polyploidy
1. Mechanisms:
2. Frequency:
3. The effect of hermaphrodism:
SO! Polyploidy may be more frequent in plants because they are
hermaphroditic more often than animals; especially vertebrates. Most cases of
polyploidy in animals is usually where triploid females survive and reproduce
asexually.
Also, simpler development in plants means they may tolerate imbalances better.
VII. Mutations I: Changes in Chromosome Number and Structure
A. Polyploidy
1. Mechanisms:
2. Frequency:
3. The effect of hermaphrodism:
4. Evolutionary Importance:
- obviously can be an instant speciation event
- polyploidy is also a mechanism for “genome doubling” or “whole genome
duplication”
- this duplication allows for divergence of copied gene function and
evolutionary innovation. Eventually, the copies may be so different that they don’t
really represent duplicates any more… resulting in “diploidization”.
VII. Mutations I: Changes in Chromosome Number and Structure
A. Polyploidy
B. Aneuploidy
B. Aneuploidy
1. Mechanism: Non-disjunction during gamete formation
During either Meiosis I or II, segregation of (homologs or sister chromatids) does not
occur; both entities are pulled to the same pole.
B. Aneuploidy
1. Mechanism: Non-disjunction during gamete formation
During either Meiosis I or II, segregation of (homologs or sister chromatids) does not
occur; both entities are pulled to the same pole.
This produces gametes with one more (1n + 1) or one less (1n -1)
chromosome than they should have. Subsequent fertilization with a normal haploid
(1n) gamete produces a trisomy (2n+1) or monosomy (2n-1).
VII. Mutations I: Changes in Chromosome Number and Structure
A. Polyploidy
B. Aneuploidy
C. Changes in Gene Number and Arrangement
VII. Mutations I: Changes in Chromosome Number and Structure
A. Polyploidy
B. Aneuploidy
C. Changes in Gene Number and Arrangement
1. Deletions and Additions:
VII. Mutations I: Changes in Chromosome Number and Structure
A. Polyploidy
B. Aneuploidy
C. Changes in Gene Number and Arrangement
1. Deletions and Additions:
a. mechanisms:
i. unequal crossing over:
i. Unequal Crossing-Over
a. process:
If homologs line up askew:
A
B
a
b
i. Unequal Crossing-Over
a. process:
If homologs line up askew
And a cross-over occurs
A
B
a
b
i. Unequal Crossing-Over
a. process:
If homologs line up askew
And a cross-over occurs
Unequal pieces of DNA will be exchanged… the A locus has been duplicated on the
lower chromosome and deleted from the upper chromosome
B
A
a
b
i. Unequal Crossing-Over
a. process:
b. effects:
- can be bad:
deletions are usually bad – reveal deleterious recessives
additions can be bad – change protein concentration
i. Unequal Crossing-Over
a. process:
b. effects:
- can be bad:
deletions are usually bad – reveal deleterious recessives
additions can be bad – change protein concentration
- can be good:
more of a single protein could be advantageous
(r-RNA genes, melanin genes, etc.)
i. Unequal Crossing-Over
a. process:
b. effects:
- can be bad:
deletions are usually bad – reveal deleterious recessives
additions can be bad – change protein concentration
- can be good:
more of a single protein could be advantageous
(r-RNA genes, melanin genes, etc.)
source of evolutionary novelty (Ohno hypothesis - 1970)
where do new genes (new genetic information) come from?
Gene A
Duplicated A
generations
Mutation – may even render the protein
non-functional
But this organism is not selected against, relative to others in the
population that lack the duplication, because it still has the
original, functional, gene.
Gene A
Duplicated A
generations
Mutation – may even render the protein
non-functional
Mutation – other mutations may render the
protein functional in a new way
So, now we have a genome that can do all the ‘old stuff’
(with the original gene), but it can now do something NEW.
Selection may favor these organisms.
If so, then we’d expect many different neighboring genes to have
similar sequences. And non-functional pseudogenes (duplicates that
had been turned off by mutation).
These occur – Gene Families
And, if we can measure the rate of mutation in these genes, then we can
determine how much time must have elapsed since the duplication event…
Gene family trees…
VII. Mutations I: Changes in Chromosome Number and Structure
A. Polyploidy
B. Aneuploidy
C. Changes in Gene Number and Arrangement
1. Deletions and Additions:
a. mechanisms:
i. unequal crossing over: (both)
ii. Intercalary Deletion
B
A
C
1. Deletions and Additions:
a. mechanisms:
i. unequal crossing over: (both)
ii. Intercalary Deletion
B
A
C
1. Deletions and Additions:
a. mechanisms:
i. unequal crossing over: (both)
ii. Intercalary Deletion
A
C
1. Deletions and Additions:
a. mechanisms:
i. unequal crossing over: (both)
ii. Intercalary Deletion
-recognized by the formation of a ‘deletion loop’ in homolog during synapsis:
1. Deletions and Additions:
a. mechanisms:
i. unequal crossing over: (both)
ii. Intercalary Deletion
iii. Transposons (addition)
- transposons are copied (replicated) independent of the S
phase of interphase…the copy is inserted elsewhere in the genome. Create
homologus regions that lead to unequal crossing over and duplications
1. Deletions and Additions:
a. mechanisms:
i. unequal crossing over: (both)
ii. Intercalary Deletion
iii. Transposons (addition)
- OR, a transposon can be inserted within a gene, destroying it and
functionally ‘deleting’ it.
VI. Mutation
A. Overview
B. Changes in Ploidy
C. Changes in ‘Aneuploidy’ (changes in chromosome number)
D. Change in Gene Number/Arrangement
1. Deletions and Additions
2. Inversion (changes the order of genes on a chromosome)
VII. Mutation
A. Changes in Ploidy
B Changes in ‘Aneuploidy’ (changes in chromosome number)
C. Change in Gene Number/Arrangement
1. Deletions and Additions
2. Inversion (changes the order of genes on a chromosome)
VII. Mutation
A. Changes in Ploidy
B Changes in ‘Aneuploidy’ (changes in chromosome number)
C. Change in Gene Number/Arrangement
1. Deletions and Additions
2. Inversion (changes the order of genes on a chromosome)
Chromosomes are no longer homologous along entire length
B-C-D on top
d-c-b on bottom
VII. Mutation
A. Changes in Ploidy
B Changes in ‘Aneuploidy’ (changes in chromosome number)
C. Change in Gene Number/Arrangement
1. Deletions and Additions
2. Inversion (changes the order of genes on a chromosome)
Chromosomes are no longer homologous along entire length
ONE “loops” to get
genes across from
each other…
And if a crossover occurs….
VII. Mutation
A. Changes in Ploidy
B Changes in ‘Aneuploidy’ (changes in chromosome number)
C. Change in Gene Number/Arrangement
1. Deletions and Additions
2. Inversion (changes the order of genes on a chromosome)
The cross-over
products are nonfunctional, with
deletions AND
duplications
VII. Mutation
A. Changes in Ploidy
B Changes in ‘Aneuploidy’ (changes in chromosome number)
C. Change in Gene Number/Arrangement
1. Deletions and Additions
2. Inversion (changes the order of genes on a chromosome)
The only functional
gametes are those that
DID NOT cross over –
and preserve the
parental combination of
alleles
VII. Mutation
A. Changes in Ploidy
B Changes in ‘Aneuploidy’ (changes in chromosome number)
C. Change in Gene Number/Arrangement
1. Deletions and Additions
2. Inversion (changes the order of genes on a chromosome)
Net effect: stabilizes sets of
genes. This allows selection
to work on groups of alleles…
those that work well
TOGETHER are selected for
and can be inherited as a ‘coadapted gene complex’
VII. Mutation
A. Changes in Ploidy
B Changes in ‘Aneuploidy’ (changes in chromosome number)
C. Change in Gene Number/Arrangement
1. Deletions and Additions
2. Inversion (changes the order of genes on a chromosome)
3. Translocation
Translocation Downs.
Transfer of a 21
chromosome to a 14
chromosome
Can produce normal, carrier,
and Down’s child.
VII. Mutation
A. Changes in Ploidy
B Changes in ‘Aneuploidy’ (changes in chromosome number)
C. Change in Gene Number/Arrangement
D. Change in Gene Structure
1. Mechanism #1: Exon Shuffling
Crossing over WITHIN a gene, in introns, can recombine exons within a gene, producing
new alleles.
EXON 1a
EXON 2a
EXON 3a
Allele “a”
EXON 1A
EXON 2A
EXON 3A
Allele “A”
VII. Mutation
A. Changes in Ploidy
B Changes in ‘Aneuploidy’ (changes in chromosome number)
C. Change in Gene Number/Arrangement
D. Change in Gene Structure
1. Mechanism #1: Exon Shuffling
Crossing over WITHIN a gene, in introns, can recombine exons within a gene, producing
new alleles.
EXON 1a
EXON 2a
EXON 3a
Allele “a”
EXON 1A
EXON 2A
EXON 3A
Allele “A”
EXON 1A
EXON 2a
EXON 3a
Allele “α”
EXON 1a
EXON 2A
EXON 3A
Allele “ά”
VII. Mutation
A. Changes in Ploidy
B Changes in ‘Aneuploidy’ (changes in chromosome number)
C. Change in Gene Number/Arrangement
D. Change in Gene Structure
1. Mechanism #1: Exon Shuffling
2. Mechanism #2: Point Mutations
a. addition/deletion: “frameshift” mutations
Normal
Mutant: A inserted
…T C C G T A C G T ….
…A G G C A U G C A …
ARG
HIS
ALA
DNA
m-RNA
…T C C A G T A C G T ….
…A G G U C A U G C A …
ARG
SER
CYS
Throws off every 3-base codon from mutation point onward
VII. Mutation
A. Changes in Ploidy
B Changes in ‘Aneuploidy’ (changes in chromosome number)
C. Change in Gene Number/Arrangement
D. Change in Gene Structure
1. Mechanism #1: Exon Shuffling
2. Mechanism #2: Point Mutations
a. addition/deletion: “frameshift” mutations
b. substitution
Normal
Mutant: A for G
… T C C G T A C G T ….
…A G G C A U G C A …
ARG
HIS
ALA
DNA
m-RNA
…T C C A T A C G T ….
…A G G U A U G C A …
ARG
TYR
ALA
At most, only changes one AA (and may not change it…)
SOURCES OF VARIATION
MUTATION
RECOMBINATION
?
?
RECOMBINATION
Independent Assortment
Independent Assortment produces an amazing amount of
genetic variation.
Consider an organism, 2n = 4, with two pairs of homologs. They
can make 4 different gametes (long Blue, Short Red) (Long Blue,
Short Blue), (Long Red, Short Red), (Long Red, Short blue).
Gametes carry thousands of genes, so homologous
chromosomes will not be identical over their entire length, even
though they may be homozygous at particular loci.
Well, the number of gametes can be calculated as 2n
or
Independent Assortment produces an amazing amount of
genetic variation.
Consider an organism with 2n = 6 (AaBbCc) ….
There are 2n = 8 different gamete types.
ABC
Abc
aBC
AbC
abc
abC
Abc
aBc
Independent Assortment produces an amazing amount of
genetic variation.
Consider an organism with 2n = 6 (AaBbCc) ….
There are 2n = 8 different gamete types.
And humans, with 2n = 46?
ABC
Abc
aBC
AbC
abc
abC
Abc
aBc
Independent Assortment produces an amazing amount of
genetic variation.
Consider an organism with 2n = 6 (AaBbCc) ….
There are 2n = 8 different gamete types.
And humans, with 2n = 46?
223 = ~ 8 million different types of gametes.
And each can fertilize ONE of the ~ 8 million types of gametes of
the mate… for a total 246 = ~70 trillion different chromosomal
combinations possible in the offspring.
ABC
Abc
aBC
AbC
abc
abC
Abc
aBc
Independent Assortment produces an amazing amount of
genetic variation.
And each can fertilize ONE of the ~ 8 million types of gametes of
the mate… for a total 246 = 70 trillion different chromosomal
combinations possible in the offspring.
YOU are 1 of the 70 trillion combinations your own parents
could have made. IA creates a huge amount of genetic
variation, and that doesn’t include crossing over!!!!
VII. Mutation
A. Changes in Ploidy
B Changes in ‘Aneuploidy’ (changes in chromosome number)
C. Change in Gene Number/Arrangement
D. Change in Gene Structure
E. Summary
MUTATION:
-New Genes:
point mutation
exon shuffling
RECOMBINATION:
- New Genes:
crossing over
-New Genotypes:
crossing over
independent assortment
Causes of Evolutionary Change
V A R I A T I O N
Sources of Variation
Natural Selection
Mutation (polyploidy can make new
species)
SOURCES OF VARIATION
MUTATION
RECOMBINATION
Independent Assortment
Crossing Over – New combinations of genes on chromosomes
“Hey!! That solves my dilemma about how new
variation is produced each generation!!
Too bad I’m dead!