Aug31_lecture2 - University of Arizona | Ecology and

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Transcript Aug31_lecture2 - University of Arizona | Ecology and

Lecture 2
A brief introduction to evolutionary
• brief history of evolutionary theory
• natural selection
• evolutionary thinking and some important
evolutionary themes (following the paper by
Stephen Stearns)
Adaptation, Constraints, Trade-offs, Conflict
A very brief history of evolution
• Evolutionary ideas go back long before Darwin
(Lamarck, Erasmus Darwin)
• Darwin was the first to present an overwhelming
case for “descent with modification”
• Critically, he also articulated a mechanism for
evolution: natural selection
“I have called this principle, by
which each slight variation, if
useful, is preserved, by the term
Natural Selection”.
Charles Darwin (1809-1882)
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Charles Darwin in 1854, five
years before publishing The
Origin of Species.
evolution by natural selection
Alfred Russel Wallace (while
suffering from a bout of malaria) hit
upon the same insight before
Darwin had published
On the Tendency of Varieties to
Depart Indefinitely From the
Original Type
evolution by natural selection
Alfred Russel Wallace (while suffering from a bout
of malaria) hit upon the same insight before
Darwin had published
They co-published a paper in 1858 but it wasn’t
until the publication of The Origin of Species
(1859) that the idea caught on
evolution by natural selection
Natural selection was eclipsed for several
decades because of misunderstandings about
Darwin, unaware of Gregor Mendel’s discoveries
about genetics, adopted incorrect ideas about
genetics: pangenesis, blending inheritance
In fact, genetics depends on particulate
evolution by natural selection
In 1900, Mendel’s findings were rediscovered.
But instead of leading directly to a positive reappraisal of Darwin’s ideas on natural selection,
early geneticists were originally opposed
The main problem was their focus on mutations of
large effect
JBS Haldane, RA Fisher, Sewall Wright were
population geneticists who synthesized genetics
and evolution: the modern synthesis
evolution by natural selection
Darwin started his argument for natural selection
with insights from pigeon-breeding (he spent a lot
of time drinking with animal breeders and became
a pigeon fancier himself)
“But man can and does select the variations given to
him by nature, and thus accumulate them in any
desired manner. He thus adapts animals and plants for
his own benefit or pleasure. He may do this
methodically, or he may do it unconsciously by
preserving the individuals most useful to him at the
time, without any thought of altering the breed.”
evolution by natural selection
“It is certain that he can largely influence the character
of a breed by selecting, in each successive generation,
individual differences so slight as to be quite
inappreciable by an uneducated eye. This process of
selection has been the great agency in the
production of the most distinct and useful domestic
breeds. That many of the breeds produced by man have
to a large extent the character of natural species, is
shown by the inextricable doubts whether very many of
them are varieties or aboriginal species.”
evolution by natural selection
A process much like artificial selection, used by
breeders of domesticated plants and animals to
select for desirable traits,also happens in nature:
“Why, if man can by patience select variations most
useful to himself, should nature fail in selecting
variations useful, under changing conditions of life,
to her living products?”
evolution by natural selection
A process much like artificial selection, used by
breeders of domesticated plants and animals to
select for desirable traits,also happens in nature:
1. Individuals within populations are variable
2. The variations among individuals are, at least in
part, passed on from parents to offspring.
3. In every generation, some individuals are more
successful at surviving and reproducing than
4. The survival and reproduction of individuals are
not random: those with the most favorable
variations are naturally selected
evolution by natural selection
“That many and grave objections may be advanced against
the theory of descent with modification through natural
selection, I do not deny. I have endeavoured to give to
them their full force. Nothing at first can appear
more difficult to believe than that the more complex
organs and instincts should have been perfected not
by means superior to, though analogous with,
human reason, but by the accumulation of
innumerable slight variations, each good for the
individual possessor.”
evolution by natural selection
“Nevertheless, this difficulty…cannot be considered real if
we admit the following propositions, namely, -- that
gradations in the perfection of any organ or instinct,
which we may consider, either do now exist or could
have existed, each good of its kind, -- that all organs
and instincts are, in ever so slight a degree, variable, -and, lastly, that there is a struggle for existence leading
to the preservation of each profitable deviation of
structure or instinct. The truth of these propositions
cannot, I think, be disputed.
evolution by natural selection
• Natural selection is a “blind
• It’s what?
evolution by natural selection
William Paley’s argument for a designer:
“In crossing a heath, suppose I pitched my foot against a
stone and were asked how the stone came to be there,
I might possibly answer that for anything I knew to the
contrary it had lain there forever; nor would it, perhaps,
be very easy to show the absurdity of this answer.”
evolution by natural selection
William Paley:
“But suppose I had found a watch upon the ground, and it
should be inquired how the watch happened to be in
that place, I should hardly think of the answer which I
had before given, that for anything I knew the watch
might have always been there.”
evolution by natural selection
William Paley:
“Yet why should not this answer serve for the watch as well
as for the stone? Why is it not as admissible in the
second case as in the first? For this reason, and for
no other, namely, that when we come to inspect
the watch, we perceive—what we could not
discover in the stone—that its several parts are
framed and put together for a purpose.”
evolution by natural selection
“Nothing at first can appear more difficult to believe than that
the more complex organs and instincts should have been
perfected not by means superior to, though analogous
with, human reason, but by the accumulation of
innumerable slight variations, each good for the individual
“Let us hope that what Mr. Darwin says is not true; but, if it is
true, let us hope that it will not become generally known."
Evolutionary thinking
The Stearns paper is the introduction to a book
about evolutionary medicine, hence the medical
“Evolutionary biology is a rich collection of welldeveloped alternative approaches to the
interpretation of biological diversity and
organismal design.”
Evolutionary thinking
Evolutionary topics:
• Adaptations
• Relationships and history
• Neutral versus selectively advantagous
• The study of conflicts and cooperation
• Maladaptation: Why am I not perfect?
Evolutionary thinking
Wikipedia: Evolution
The basic mechanisms that produce evolutionary change
are natural selection (which includes ecological, sexual, and kin
selection) and genetic drift;
these two mechanisms act on the genetic variation created
by mutation, genetic recombination and gene flow.
Natural selection is the process by which individual
organisms with favorable traits are more likely to survive and
reproduce. If those traits are heritable, they are passed to
succeeding generations, with the result that beneficial
heritable traits become more common in the next generation.
Given enough time, this passive process can result in varied
adaptations to changing environmental conditions.[6]
Evolutionary thinking
Different sorts of evolutionary biologists:
• Population geneticists (genes, alleles,
change or maintenance in variation)
• Evolutionary ecologists (design of
phenotypes for survival and reproduction,
life history, sexual selection, behaviour)
Evolutionary thinking
Different sorts of evolutionary biologists:
• Molecular evolutionists (history stamped
into genomes, patterns in DNA and the
processes underlying them)
• Systematists (phylogenies, relationships
among taxa
• Paleontologists (fossils, deep time, major
No clear boundaries, and many evolutionists
where several hats. All try to observe
patterns and infer process that underly
Evolutionary thinking
Different evolutionary approaches:
• Changes in gene frequencies (population
and quantitative genetics)
• Optimization approach
• Game theory approach
• Phylogenetic approach
natural selection and adaptation
In short:
Heritable variation
Differential survival and reproductive success
Leads to
Non-random survival and reproduction such that
favorable variation is naturally selected
natural selection and adaptation
The ultimate source of variation is mutation (in
DNA, it turns out)
Mutation is random
Selection is NOT!
Natural selection filters and preserves random
mutation-derived variation and is the antithesis of
randomness (it is, however, still blind)
natural selection and adaptation
“Selection extracts order from randomness”
31 genes, each with 26 alleles
This is just one of the 2631 possible “haplotypes”
natural selection and adaptation
Adaptation is both a process and a state
The process of adaptation = what happens over
successive generations of selection of heritable
variation in reproductive success
The state of adaptation = a particular trait that
does a job very well, just as though it were
designed by an engineer
E.g. opposable thumb, acute hearing, vision, sex
drive, camouflage, venom, crystallins in eyes
Constraints on adaptation
Natural selection does not produce perfection
Rather it is a “tinkerer” that produces “goodenough” solutions to context dependant problems.
Often this occurs through duplication and
Constraints on adaptation
“As natural selection acts by competition, it adapts the
inhabitants of each country only in relation to the
degree of perfection of their associates; so that we need
feel no surprise at the inhabitants of any one country,
although on the ordinary view supposed to have been
specially created and adapted for that country, being
beaten and supplanted by the naturalised productions
from another land.”
Constraints on adaptation
“Nor ought we to marvel if all the contrivances in nature be
not, as far as we can judge, absolutely perfect; and if
some of them be abhorrent to our ideas of fitness.”
Constraints on adaptation
Time is a major constraint on adaptation. It takes time
to generate variation and select for it
Absorbtion of milk sugar (lactose) by human adults
Normally lactase is effective until weaning age (about
4). If you can’t digest lactose you suffer flatulence,
intestinal cramps, diarrhea, nausea, vomitting
But in cultures where dairy has been used for thousands
of years, >90% of adults can digest lactose, like giant,
lumbering babies
Thousands of years, but probably not decades, are
sufficient for this selection to shape human evolution
Constraints on adaptation
Trade-offs are major constraints too
E.g. sex is dangerous for fruit flies. There is a trade-off
between survival and reproduction
Similarly, there is a trade-off between having a robust
immune system and suffering from asthma, or lupus, or
diabetes type I
Other trade-offs?
Constraints on adaptation
Historical constraints are also important:
“How strange it is that a bird, under the form of
woodpecker, should have been created to prey on
insects on the ground; that upland geese, which never
or rarely swim, should have been created with webbed
Constraints on adaptation
Historical constraints are also important:
The vertebrate eye has a basic flaw: the nerves and
blood vessels that feed it enter right in the middle of the
region of photosensitive cells.
You would fire any engineer who designed an optical
device in this way, but because of developmental
constraints deriving from much simpler, ancestral eyes,
that’s what we’re stuck with
If squid could talk, they would taunt us about our
poorly designed eyes.
Release from constraint: adaptive
Adaptive radiations can occur when new ecological
niches open up, or a new adaptation opens up new
possibilities (each beetles/flowering plants)
Natural selection can operate very rapidly in such
cases to generate new adaptations
Islands, such as the Galapagos, are classic areas for
observing evidence of adaptive radiation (eg
Darwin’s Finches)
But other favorable conditions also exist.
The Great Lakes of
Africa are home to
the most species
rich vertebrate
Hundreds of species
in each lake, with
relatively recent
common ancestors
A few million years
for the ~500 species
in Lake Malawi
12,000 years for
Lake Victoria
Evolutionary conflicts
Lots of conflicts arise in nature:
Predators and prey
Parents and offspring
Insects and plants
Fungi and crop plants they destroy
Chromosomes competing for transmission through
Evolutionary conflicts
Conflicts are where much of the action is in
evolutionary biology
“Conflicts occur when genes have different
patterns of transmission but interact, directly or
indirectly, in the organisms that carry them”
Perhaps the most obvious example is genes of
pathogens and genes of hosts
Endless cycles of damage and damage control
adaptations can lead to “evolutionary arm’s races”
Evolutionary conflicts
In one sense, these arms races lead to lots of
In another, they present a serious constraint on
adaptation and amount to running just to stay still.