Download Evolution 1/e - SUNY Plattsburgh


Read chapter 6

Darwin unfortunately never understood
what genes were or how information
passed between generations.

Even though Darwin didn’t know it,
Gregor Mendel was figuring out the laws
of inheritance while Darwin was
developing his theory of evolution.

Mendel carried out classic research on
inheritance by crossing pure-breeding
strains of peas.

The results of his crosses revealed two
basic laws of inheritance.

Law of segregation – each individual has
two gene copies at each locus and
these are segregated (split up) during
the formation of gametes so only one
copy goes into each gamete.

Mendel also showed by carrying out crosses
in which he tracked multiple traits
simultaneously that different traits were
inherited independently.

Mendel’s Law of independent assortment –
alleles at different loci are inherited
independently i.e. what allele is passed
down at one locus has no influence on
what allele is passed down at another.

What Mendel’s work showed was that
copies of discrete particles (what we
today call genes) are passed from one
generation to the next.

Inheritance is thus particulate and is not
a process of blending (e.g. as occurs
when paints are mixed)

An early criticism of Darwin’s work was
that any new favorable mutation would
be so diluted by the effects of blending
inheritance that it could never become
common.

How then could evolution occur?

With particulate inheritance that
problem disappears. Genes do not
blend together and so a favorable
variant can increase over time as a result
of selection.

DNA made up of sequence of
nucleotides. Each nucleotide includes a
sugar, phosphate and one of four
possible nitrogenous bases (adenine and
guanine [both purines], and thymine and
cytosine [both pyrimidines]).
The opposite strands of the DNA molecule
are complementary because the strands
are held together by bonds between the
opposing bases and adenine bonds only
with thymine and cytosine only with
guanine.
 Thus, knowing the sequence on one strand
enables one to construct the sequence on
the other strand.

Sequence of bases in DNA codes for
protein structure as each three base
sequence codes for one amino acid in
the protein chain.
 [To refresh yourself on basic DNA
structure and protein synthesis see any
Introductory Biology textbook]


In the processes of transcription and
translation a section of DNA (a gene) is
first transcribed into to an RNA copy,
which then has all the all the non-coding
portions (the introns) removed, and is
then translated into a protein.

One gene codes for one protein.
Different versions of a gene are called
alleles.
 The combination of genes an individual
has at a locus (a genes physical location
on a chromosome) is referred to as its
genotype.
 [genotype may also refer to all the
alleles an individual has at all its loci]


An individual who has two different
alleles at its loci is a heterozygote.

An individual with two copies of the
same allele is referred to as a
homozygote.

If the heterozygote is phenotypically
identical to one of the homozygotes we
say the allele in that homozygote is
dominant and the other allele is
recessive.

If the heterozygote is phenotypically
intermediate between the two
homozygotes we say the alleles are
codominant.
A grid called a punnett square can be
used to figure out what genotypes will
be produced as a result of a cross
between individuals.
 Each side of the punnett square lists the
possible gametes an individual can
produce and the squares in the grid
show the genotypes different
combinations of gametes will produce.


A major source of genetic variability
comes from the production of new
combinations of genes as a result of
sexual reproduction.

The process of crossing-over in which
pieces of chromosomes are exchanged
between chromosomes produces new
and unique chromosomes.

Recombination remixes existing variation,
but where does variation ultimately
come from?

Mutations – changes in the DNA
sequence of an organism are the
ultimate source of all variation
When DNA is synthesized an enzyme
called DNA polymerase reads one
strand of DNA molecule and constructs
a complementary strand.
 If DNA polymerase makes a mistake and
it is not repaired, a mutation has
occurred.

A mistake that changes one base on a
DNA molecule is called a point mutation.
 Two forms:

› Transition: one pyrimidine (T or C) substituted for
the other pyrimidine or one purine substituted for
the other purine (A or G).
› Transversion: purine substituted for pyrimidine or
vice versa

Not all mutations cause a change in
amino acid coded for. These are called
silent mutations.

Mutations that do cause a change in
amino acid are called replacement
mutations.
Another type of mutation occurs when
bases are inserted or deleted from the
DNA molecule.
 This causes a change in how the whole
DNA strand is read (a frame shift
mutation) and produces a nonfunctional protein.


Duplication results from unequal crossing
over when chromosomes align
incorrectly during meiosis.

Result is a chromosome with an extra
section of DNA that contains duplicated
genes.

Extra sections of DNA are duplicates and
can accumulate mutations without being
selected against because the other copies
of the gene produce normal proteins.

Gene may completely change over time so
gene duplication creates new possibilities
for gene function.

Hemoglobin (the oxygen-carrying
molecule in red corpuscles) consists of
an iron-binding heme group and four
surrounding protein chains (two coded
for by genes in the Alpha cluster and two
in the Beta cluster).
Ancestral globin gene duplicated and
diverged into alpha and beta ancestral
genes about 450-500 mya.
 Later transposed to different
chromosomes and followed by further
subsequent duplications and mutations.

From Campbell and Reese Biology 7th ed.

Lengths and positions of exons and
introns in the globin genes are very
similar. Very unlikely such similarities
could be due to chance.
Exons (blue), introns (white), number in box is number of nucleotides.
4.9
Different genes in alpha and beta families
are expressed at different times in
development.
 For example, in very young human fetus,
zeta (from alpha cluster) and epsilon (from
beta cluster) chains are present initially then
replaced. Similarly G-gamma and Agamma chains present in older fetuses are
replaced by beta chains after birth.

4.8
Gestation (weeks)
Post-birth(weeks)
Fetal hemoglobin has a higher affinity for oxygen than adult hemoglobin.
Enhances oxygen transfer from mother to offspring.

It is important to realize that mutations
arise randomly and are not biased
towards making the organism better
adapted.

In fact, most mutations are deleterious.
The random nature of mutations was
demonstrated by an elegant
experiment by Luria and Delbruck in
1943.
 At the time it was unclear if mutations
occurred at random or if the
environment somehow induced
mutations when there was a need for
them.


Luria and Delbruck worked on the
bacterium Escherischia coli and a
bacteriophage virus that killed E. coli.

When E. coli on petri dishes was exposed
to the virus most E. coli were infected
and killed but a few resistant bacteria
survived and produced colonies.
L+D wanted to distinguish between two
competing hypotheses.
 1. Random mutation: before being
exposed to phage some bacteria would
have randomly developed mutations.
These would survive and produce
colonies.


2. Acquired hereditary resistance. At
time of exposure to phage all bacteria
would be vulnerable. However,
exposure to the phage would induce
resistance in some small percentage of
bacteria that would produce resistant
colonies.
L+D experiment
L+D grew phage sensitive bacteria to
high densities in a broth and spread
them on a plate covered with phage.
 They then counted the number of
colonies that grew after 24-48 hours.
 Using just this count data they could
distinguish between the two hypotheses.


How were they able to use the count
data to distinguish between the two
hypotheses?

Logic depends on the timing of when
mutations would have happened.
If the presence of phage induced
mutations in the bacteria then mutations
would not occur until after the bacteria
were exposed to the phage.
 Because there were lots of bacteria in
each culture and a nontrivial number
should develop resistance by the law of
large numbers we would expect a similar
number of colonies to develop on each
plate.


In contrast under the random mutation
model beneficial mutations could have
arisen at any time before phage
exposure.
Thus we would expect in some cases for
the mutation to have arisen well before
exposure to phage and for the bacteria
to have produced lots of descendents
with the mutation.
 In other cases the mutation may have
arisen more recently and so few
descendents with the mutation would be
present.


Therefore, under the random mutation
scenario we would expect there to be a
lot of variation from plate to plate in the
number of resistant colonies seen.
L+D repeated their experiment many
times and counted the resistant colonies
they found.
 When they analyzed their data they
found that the variance was much larger
than the mean which supported the
random mutation hypothesis and
allowed them to reject the induced
mutation hypothesis.


Very little data is available of the
proportions of mutations that are
beneficial versus deleterious.

However a recent study on a virus in
which specific point mutations were
artificially induced have shown that most
are deleterious, about 40% neutral and
only 2-3% beneficial.

Even though most mutations are neutral
or deletrious the few beneficial
mutations when combined with natural
selection are what lead to evolution
taking place.