Download Natural selection, and variation through mutation

BIOL 1010 Introduction
to Biology: The
Evolution and Diversity
of Life. Spring 2011
Sections A & B
Steve Thompson: [email protected]
http://www.bioinfo4u.net
1
Tuesday, January 11, 2011
Natural Selection and
Variation through Mutation
Or “survival of the fittest.” But what does
that really mean? Certainly not the biggest,
baddest ass around. Actually to be the fittest
in this context is to pass on more of your genes
to the next generation than your peers, i.e. to
have higher reproductive success.
And to start out with a bit of humor:
http://www.youtube.com/watch?v=nf v-Qn1M58I
The Banana — an atheist nightmare! Yeah, right.
2
Tuesday, January 11, 2011
Once more, what’s evolution?
Evolution is a change in the genetics of
populations over time.
It occurs in a population when some alleles
become more common, and other less
common, from one generation to the next.
A population is a group of interbreeding
organisms, and an allele is a version of a
gene. All populations have many alleles.
Many explanations have been proposed
for life’s diversity. Evolution explains it all!
3
Tuesday, January 11, 2011
A history of evolutionary thought
Buffon – one of first to openly suggest that closely related
species arose from a common ancestor and were changing.
Hutton – uniformitarianism – the processes of erosion and
sedimentation that act in modern times also occurred in
the past.
4
Tuesday, January 11, 2011
History, cont.
Cuvier – catastrophism – brief geological
catastrophes are responsible for most geological
formations.
Principle of superposition – lower layers of rocks
(and fossils) are older than those above them.
Fossils represented extinctions, but the organisms
are replaced by those from surrounding areas.
Lamarck – inheritance of acquired characteristics.
He was the first to suggest that animals could
change or become extinct in response to
interactions with their environment.
Lyell – renewed uniformitarianism – natural
processes are slow and steady.
5
Tuesday, January 11, 2011
This can be seen in places like
Arizona’s Grand Canyon
Also see: http://bio.fsu.edu/~stevet/Boating/GrandCanyon/GrandCanyon.html and
http://www.valdosta.edu/~stthompson/videos/Chapter13/GrandCanyon2010.m4v
6
Tuesday, January 11, 2011
And then along came Dar win . . .
Charles
Dar win – five
year voyage as
naturalist on
HMS Beagle.
Read Lyell and
trusted
uniformitarianism.
Tried to
reconstruct
the past from
contemporary
observations.
Noted
similarities
and
differences
among
organisms.
Month in
Galapagos
Islands was
the seed of
theory of
evolution by
natural
http://www.valdosta.edu/~stthompson/videos/Chapter13/Dar win.mpg from
selection.
http://www.evolution-of-life.com/en/observe/video/fiche/dar win-on-the-evolution-trail.html
7
Tuesday, January 11, 2011
One particularly striking obser vation was
convergent evolution.
This is the idea that many organisms have similar
characteristics, because they evolved in similar environmental
conditions, even though they are not closely related.
8
Tuesday, January 11, 2011
He returned to England and
started thinking . . .
The finches were
especially
interesting.
There were 14
different types,
yet all must have
originated from
one ancestor
that made it the
islands a very
long time ago, i.e.
“descent with
modification.”
9
Tuesday, January 11, 2011
He combined ideas from Malthus’ Essay on the
Principle of Population with this and what he had
seen in artificial selection (selective breeding) . . .
10
Tuesday, January 11, 2011
To come up with the concept
of natural selection:
Which is the differential reproductive
success of individuals with particular
genotypes (in modern lingo). Or as he
stated “preser vation of favourable
variations and the rejection of
injurious variations.”
See http://en.wikipedia.org/wiki/
Natural_selection
11
Tuesday, January 11, 2011
Dar win described his theory of evolution by natural selection in a brief sketch
in 1842, and in a longer analysis t wo years later, but he did not publish either.
He submitted his paper with one from Wallace to the Linnaean Society in 1858.
And then finally he published On the Origin of Species by Means of Natural
Selections, or Preser vation of Favored Races in the Struggle for Life in 1859.
12
Tuesday, January 11, 2011
Other beautiful cases of diversification by natural
selection can be seen in Madagascar and Lake Victoria.
Diversity and evolution of cichlid fishes
LEMURS!!
LEMURS!!
Diversity of cichlids (Family Cichlidae)
At least 100 species of lemurs
in Madagascar. Around 250
different species of Cichlid
fishes in Lake Victoria!
And, of course, the well known antibiotic resistance in
bacteria phenomenon.
13
Tuesday, January 11, 2011
Natural selection is but one mechanism
involved in evolution, but it’s certainly a
powerful one, particularly in “microevolution,”
i.e. relatively short-term changes in allele
frequencies within a population or species.
But where does this necessary variation
come from?
Primarily from mutation. This is the random
occurrence of changes in an organism’s DNA,
and in sexual organisms this has to be in the
germ line (egg or sperm) in order to be
inherited.
14
Tuesday, January 11, 2011
Other sources of
variation include . . .
migration bet ween populations
(gene flow),
shuffling of genes through sex
(independent reassortment and
recombination), and . . .
horizontal gene transfer and
hybridization.
15
Tuesday, January 11, 2011
Natural selection eliminates
certain phenotypes.
Mendel’s work although contemporaneous with Dar win’s,
was not well known, so the mechanisms of how all this
worked remained obscure. Now we now know that . . .
Gene pool – entire collection of genes and their alleles in a
population.
Proportion of alleles for each gene determines
characteristics for that population.
A change in allele frequency in a population is evolution.
Poorly adapted phenotypes are “weeded out.”
Adaptive phenotype in one set of of circumstances may be
a liability in others.
Constantly changing conditions means evolution never
stops.
16
Tuesday, January 11, 2011
Natural selection does
not have a goal!
It does not lead to more “perfect” organisms — it
can’t look ahead, nor predict what will work.
Every genome has a limited potential.
No gene pool contains every allele needed to
confront every possible change in the environment.
Disasters can indiscriminately wipe out the best
allele combinations just by chance — extinction is
the rule, not the exception.
Some harmful genetic traits are out of natural
selection’s reach (e.g. if they appear after
reproductive age).
17
Tuesday, January 11, 2011
Modes of natural selection:
Directional selection – one extreme
phenotype is fittest, others are selected
against. This is a type of positive selection.
Disruptive selection – t wo or more extreme
phenotypes are fitter than the intermediate
phenotype, a.k.a. diversifying selection. This
is also a type of positive selection.
Stabilizing selection – extreme phenotypes
are less fit than the optimal intermediate
phenotype – most common in stable,
unchanging environments, a.k.a. purifying or
negative selection. Results in conservation
of features.
18
Tuesday, January 11, 2011
These can be illustrated thus . . .
19
Tuesday, January 11, 2011
And a biggy —
balancing selection.
This is also known as balanced
polymorphism and is often maintained
through heterozygote advantage,
sometimes called overdominance.
It can also occur through frequencydependent selection where the fitness
of one phenotype depends on
frequency of other phenotypes in the
population.
20
Tuesday, January 11, 2011
The best known case is
sickle cell anemia.
Multiple alleles of a gene persist indefinitely in
the population in balanced polymorphism.
Why do seemingly harmful alleles persist?
Often it’s heterozygote advantage:
The heterozygote has greater fitness than
either homozygote.
Sickle cell disease and malaria — those with
both sickle cell alleles die, but those with only
one, i.e. are heterozygotes, are resistant to
malaria!
21
Tuesday, January 11, 2011
The distributions are congruent:
22
Tuesday, January 11, 2011
Several human diseases are
maintained in the population
because of heterozygote advantage:
23
Tuesday, January 11, 2011
That’s enough on natural
selection for now.
But we’re not done for the
day. We still need to go over
population genetics and the
neutral theory, so we’ll do
that right now.
24
Tuesday, January 11, 2011