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Neutral theory of evolution

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Evolucion GuevaraFiore

on 13 October 2015

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Transcript of Neutral theory of evolution

Neutral theory of molecular evolution
Is evolution only natural selection?
1968.
The vast majority of evolutionary changes at the molecular level are caused by
random drift
of selectively neutral mutants
(not affecting fitness).
Neutral theory
is compatible with Darwin's theory of evolution by
natural selection:
adaptive changes are acknowledged as present and important, but hypothesized to be a small
minority
of all the changes seen fixed in DNA sequences
.
Motoo Kimura
(1924-1994)
Genetic drift or allelic drift is the change in the frequency of a gene variant (allele) in a population due to random sampling.
Since then, this hypothesis has been tested using the McDonald-Kreitman test (looks for
ancient selection
over long periods, as opposed to the steady accumulation of mutations that confer no selective advantage), and has not been supported in all species
Protein sequence comparisons, studies of the Treffers mutator gene in E. coli, analysis of the genetic code to comparative immunology... they argue that most protein evolution is due to neutral mutations and genetic drift.
Non-Darwinian Evolution, 1969
Evolutionary Rate at the Molecular Level, 1968
When one compares the
genomes
of existing species, the vast
majority
of molecular differences are selectively "neutral", i.e. the molecular changes represented by these differences do not influence the
fitness
of the individual organism. As a result, the theory regards these genomic features as neither subject to, nor explicable by,
natural selection.
1960's-1970's
Macro
vs
Micro
evolution
DEVELOPMENTAL BIOLOGY
History & constraints
NS & drift
What happens to genes/traits that are not experiencing NS, because they make no difference to reproductive success?
1. Meiosis:
Probability of a gene into a gamete is 50%.

2. Fixation of neutral alleles:
you dont know which mutation will be fixed, but because there are so many of them you know how many events will happen in a unit of time. Law of large numbers for random events: if a lot of random effects = the average is predictable.

3. Regular fixation of random alleles:
if you look at the entire genome, over a given period of time, a certain, very predictable average of number of mutations will be fixed if they are neutral. If you can locate the neutral mutations in the genome, you can use them to estimate relationships and the time to the last common ancestors.
THINK ABOUT ENTIRE POPULATIONS THINK ABOUT DISTRIBUTIONS / FQS
NEUTRAL EVOLUTION:
1. Why neutrality arises
1. Many genotypes produce the same phenotype (many of the genes/DNA sequences no difference in the proteins they produce).

2. Many phenotypes have the same fitness.
Fitness
Phenotype
Genotype
F1
F2
P1
P2
P3
G1
G2
G4
G3
SPACE
G1, G2, G3 are neutral with respect to each other when measured in a certain environment, but they differ from G4
2. Why genetic variation does not always produce fitness variation? Why it is neutral?
1. Synonymous substitutions in DNA sequences:
two o more codes produce same aminoacid.

2. Pseudogenes and other non-transcribed DNA:

no longer
expressed: not transcribed & eroded by mutations. Regulatory function (RNA, signaling pathways).

3. Neutral aminoacid variation:
aminoacid substitutions that do not produce any significant change in the geometry or function of a protein
. Free to diverge at random
.

4. The canalization of development:
Not well understood. Developmental mechanisms that are limiting the range of phenotypic variation. Buffering - stable state (e.g. 5 fingers). Strong selection forces operating at the microlevel within cells on gene signalling pathways (phenotype buffered as a byproduct).
3. What causes random evolutionary change or genetic drift?
Mutation.
MOLECULAR EFFECT

The Mendellian lotery - meiosis is a fair coin.
CELLULAR EFFECT

Founder effects & genetic bottlenecks (passage through small population sizes).
POPULATION EFFECT


Variation in reproductive success in populations of all size.
DEMOGRAPHIC EFFECT
MUTATIONS
sometimes
non-random:
More frequently at some sites than others
Mutation rate in pathogenic bacteria increases in response to certain signals (neglect repairing)
Mutations do not produce random changes in phenotype space (perturbations in the inherited set of possibilities that a given evo. linage produces).
BUT...
MUTATIONS
are
random:
There is not systematic relationship bn their phenotypic effect and the needs of the organism in which they occur.
RANDOM WITH RESPECT TO FITNESS
Evolution cannot anticipate that a function was going to be useful.
MUTATIONS = VARIATION
Natural selection edits it
50%
Democracy is stable
Not all genes segregate
fairly.

Some genes can
bias
the odds in their favour.
Such genes are called
meiotic drivers.
They act against the
interests
of all the other genes in the genome.
They often function through a
poison
(long range) -
antidote
(short range) system.
Once the
diploid state
evolved (the possibility of meiotic drive), long history of invasions by
meiotic drivers.
Other genes evolved to
repress
such drive.
Result: ensured segregation - what we now call meiosis.
FOUNDER EFFECT:
A few individuals found a population, bringing with them only a small portion of the
genetic variation
of the original stock;
e.g. human genetic diseases.
BOTTLENECK:
A population crashes to very small size. Only a few alleles make it through because there are only a few individuals left alive to carry them
GENETIC DRIFT
is the random, aimless wandering of frequencies of
neutral genes.
It resembles the brownian motion of small particles pushed about by thermal motion of molecultes.

Neutral evolution occurs when genes
do not
experience natural selection because they have
no effect on reproductive success.


Neutrality
arises when mutations in an organism's genotype cause
no change in its phenotype,
or when changes in the genotype bring about changes in the phenotype that
do

not affect reproductive success.

Because neutral genes do not change in any particular direction over time and simply "drift," thanks in part to the randomness of meiosis, they can be used as a sort of
molecular clock
to determine common ancestors or places in the
phylogenetic tree of life.



Variation in family size:
Neutral gene can land in a individual with a small or big family

GENETIC DRIFT

Happens in populations of all sizes, not only small ones!

NEUTRAL THEORY OF EVOLUTION
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