Lecture 15: Darwin microevolution

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Lecture 15 Darwin microevolution
Assigned Readings Ch. 22 23.1 - 23.3
Main Ideas in the Origin descent with
modification and natural selection phylogenetic
trees, fossil, fossil record, heritable
variation overproduction of offspring
environmental selection, use and disuse of
characters, acquired characters lack of
inheritance of acquired character, artificial
selection, cultivation cultivated varieties,
adaptation  Aspects of biology explained by
Darwinian evolution . adaptation in real time
(observable in human lifetime or recorded human
history) homology (anatomical, embryological,
molecular) biogeography and convergent evolution
Microevolution genetic drift, mutation, natural
selection (next lecture) Genetic drift chance
allele frequency change  Mutations missense,
silent, insertions and deletions, frameshifts,
deleterious, neutral, beneficial allele tree
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Main Ideas in the Origin descent with
modification (phylogenetic trees, fossil, fossil
record) natural selection (overproduction,
heritable variation, environmental selection,
use and disuse of characters, artificial
selection, adaptation)
Fig. 22.7. One of Darwins two main contributions
in the Origin is the idea of Descent with
modification - as seen with the relationships of
elephants. This is a phylogeny (graph showing
relationships) or phylogenetic tree, based on
both living species and those known from fossils
(remnants of bones etc. from the past). The
whole story of the past is contained in the
fossil record.
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Main Ideas in the Origin descent with
modification (phylogenetic trees, fossil, fossil
record) natural selection (overproduction,
heritable variation, environmental selection,
use and disuse of characters, artificial
selection, adaptation)
Fig. 22.3. The fossil record provides much of
the documentation for Darwins idea of descent
with modification.
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Main Ideas in the Origin descent with
modification (phylogenetic trees, fossil, fossil
record) natural selection (overproduction,
heritable variation, environmental selection,
use and disuse of characters, artificial
selection, adaptation)
One of the observations that lead to the theory
of natural selection overproduction of
offspring. Malthus had argued that human
populations can increase exponentially, while
food production can only increase linearly, with
time.
Fig. 22.8
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Main Ideas in the Origin descent with
modification (phylogenetic trees, fossil, fossil
record) natural selection (overproduction,
heritable variation, environmental selection,
use and disuse of characters, artificial
selection, adaptation)
Fig. 22.9
A second observation that lead to the theory of
natural selection heritable variation exists for
all kinds of traits (features, characteristics)
of organisms. Putting observation one and two
together leads to the deduction of natural
selection genotypes that survive/reproduce best
replace other genotypes. The environment is
doing the selection.
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Main Ideas in the Origin descent with
modification (phylogenetic trees, fossil, fossil
record) natural selection (overproduction,
heritable variation, environmental selection,
use and disuse of characters, artificial
selection, adaptation)
Fig. 22.4. But one must be sure that the
variation is heritable. Evolution of acquired
characters (as proposed by Lamarck) has not been
shown to exist. (However, Darwin accepted this
because he had not read his copy of Mendels
paper- he referred to use and disuse of
characters as a part of his theory).
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Main Ideas in the Origin descent with
modification (phylogenetic trees, fossil, fossil
record) natural selection (overproduction,
heritable variation, environmental selection,
use and disuse of characters, artificial
selection, adaptation)
Fig. 22.10. Artificial selection provided
support for the idea of natural selection. These
are cultivated varieties of wild mustard.
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Main Ideas in the Origin descent with
modification (phylogenetic trees, fossil, fossil
record) natural selection (overproduction,
heritable variation, environmental selection,
use and disuse of characters, artificial
selection, adaptation)
Fig. 22.6. Adaptations to different diets in
Darwins finches
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Main Ideas in the Origin descent with
modification (phylogenetic trees, fossil, fossil
record) natural selection (overproduction,
heritable variation, environmental selection,
use and disuse of characters, artificial
selection, adaptation)
Fig. 22.11. Some spectacular adaptations.
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The study of microevolution - evolution on a
small scale - overlaps extensively with
population genetics - the two are almost the same
thing. There are some useful terms discrete
genetic variation - the phenotypes fall into
clear-cut types continuous variation - polygenic
variation (usually with some environmental
effects) polymorphism - another way of saying
there is discrete variation - there are different
morphs. Also polymorphic. heterozygosity - the
proportion of heterozygotes in a population - a
useful way to measure the amount of discrete
genetic variation.
Microevolution genetic drift mutation natural
selection (next lecture)
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Microevolution genetic drift mutation natural
selection (next lecture)
The study of microevolution - evolution on a
small scale - overlaps extensively with
population genetics - the two are almost the same
thing. There are several evolutionary forces
that act on allele frequencies and genetic
variation to produce change Genetic drift -
random change in allele frequencies Mutation -
adds new alleles Natural selection - non-random
change in allele frequencies
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Genetic drift chance allele frequency change
Allele frequencies do not remain exactly the same
from generation to generation, especially in very
small populations. A process called genetic
drift - random changes in allele frequencies -
occurs in all populations.
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Figure 23.7 Genetic drift
Genetic drift chance allele frequency change
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Genetic drift chance allele frequency change
In any population, but especially small
populations, allele frequencies change over time
by chance.
An allele frequency graph
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Genetic drift chance allele frequency change
1
0.8
If you follow allele frequency change in several
small populations, you get a different result
each time. One possible outcome is allele
fixation - loss of one allele.
0.6
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generations
An allele frequency graph
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Genetic drift chance allele frequency change
1
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N 10 beetles in population
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red
freqency b allele
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b b
A real experiment with the b locus in flour
beetles.
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generations (time)
1
brown
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b b
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frequency b allele
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N 50 beetles in population
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black
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b b
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generations (time)
Note effect of population size
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Figure 23.8 A special case of genetic drift The
bottleneck effect
Genetic drift chance allele frequency change
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Genetic drift chance allele frequency change
Some facts about genetic drift 1. It occurs the
same way if there are more than 2 alleles, it
just produces a more complicated-looking
graph. 2. It is most powerful in small
populations, but is inevitable even in large
populations. 3. If it goes on long enough, it is
possible for one allele to become the only allele
in the population. This allele is now said to be
fixed. 4. The opposite of fixation is allele
loss or allele extinction. New mutants run a
huge risk of going extinct, but some do increase,
and even become fixed, by chance.
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Mutations types rates effects
All variation (all alleles) must originate at
some point by mutation. A mutation is
essentially an error in DNA replication. We will
follow the book with a very simple classification
of mutations, but I add a few terms. Point
mutation - change in a single base. These can
either change an amino acid (missense mutation)
or have no effect (silent mutation - remember
that the genetic code is redundant). Changes in
gene number or order. The most important class
of these are gene duplications.
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Mutations types rates effects
Mutations are low probability events, one in a
million events, but one must remember how many
DNA replications there are in the development of
an organism, and that population sizes can be in
the billions. Thus, mutations are inescapable.
The probability of a mutation is called its
mutation rate.
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Mutations types rates effects
A mutation rate is calculated by dividing the
number of mutants in a defined study group,
during a set period of time. Here is an example
calculation involving achondroplasia (Fig. 14.15,
right).
From 242,257 births from hospital data in four
large cities in the US in one year, 7 infants had
mutant (not inherited) achondroplasia. Because
it is a dominant, mutation rate can be easily
calculated. Each birth represents two copies of
each gene, so the rate is 7/(2 x
242,257) 1.4 x 10-5 Gardner RJ. A new estimate
of the achondroplasia mutation rate. Clinical
Genetics 1977 Jan11(1)31-8. I simplified the
analysis slightly.
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Mutations types rates effects
A mutation rate is calculated by dividing the
number of mutants in a defined study group,
during a set period of time. Here is an example
calculation involving achondroplasia (Fig. 14.15,
right).
All known mutants are of the fibroblast growth
factor receptor 3 (FGFR3). Amazingly, all the
mutations were at the same site in the gene, and
almost all of the mutations were the same. 15 of
16 achondroplasia mutations in one study were
G-to-A changes at nucleotide 1,138, and an
additional mutation was a G-to-C change at the
same nucleotide. Both mutations resulted in the
substitution of arginine for glycine at amino
acid 380 of the protein. The mutation reduces
production of cartilage.
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Mutations types rates effects
The effects of mutations range from one extreme
to the other. Many are deleterious - they have a
negative effect on the organism (often only in
homozygous condition). Example the vestigial
mutation in Drosophila melanogaster - no wings,
cant fly away from predators! Many, many
mutations are severely deleterious, and produce
abnormal development when homozygous. The most
extreme deleterious mutations are lethal. Lethal
recessives are the ones of most importance for
population genetics. (Why?)
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Mutations types rates effects
The effects of mutations range from one extreme
to the next. A great many are what we will call
neutral - (or selectively neutral, as we will
see) - they have no effect on the organism.
Microsatellite alleles are an example.
The sAVA3 region of 100 bp has been amplified,
and the DNA fragments separated on a
electrophoretic gel (smaller runs faster).
Run direction
Pedigree for human family K21 variable for a CA
repeat.
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Mutations types rates effects
The effects of mutations range from one extreme
to the next. A small number are actually
beneficial to the organism. Mutations for
insecticide resistance are an example - if you
spray insecticide on on a population of
mosquitoes, and some survive because of a
mutation that makes them able to survive
insecticide treatment, then the mutation is
beneficial for the mosquitoes with this mutation
in their genotype. More about these in the next
lecture.
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Lecture 15 Darwin microevolution
Assigned Readings Ch. 22 23.1 - 23.3
Main Ideas in the Origin descent with
modification and natural selection phylogenetic
trees, fossil, fossil record, heritable
variation overproduction of offspring
environmental selection, use and disuse of
characters, acquired characters lack of
inheritance of acquired character, artificial
selection, cultivation cultivated varieties,
adaptation  Aspects of biology explained by
Darwinian evolution . adaptation in real time
(observable in human lifetime or recorded human
history) homology (anatomical, embryological,
molecular) biogeography and convergent evolution
Microevolution genetic drift, mutation, natural
selection (next lecture) Genetic drift chance
allele frequency change  Mutations missense,
silent, insertions and deletions, frameshifts,
deleterious, neutral, beneficial allele tree