Download Evolution 2 -- Natural Selection

Evolution 2
Natural Selection
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NATURAL SELECTION
1. In every species so many individuals are
born that they cannot all survive. Thus
there ensues a “struggle for survival” –
competition among the individuals to
capture resources and survive.
2. There exists variability in the traits these
individuals possess.
3. Traits are heritable – they “run in families”.
4. Some of the traits may make it easier for
the bearers to capture resources, giving
them an advantage in the struggle for
survival.
5. Over a number of generations those traits
should become more prevalent in the
population while the alternate traits are
lost.
While the fact that species have evolved is not debated among students of the subject,
the driving mechanism is.
Many people treat Darwin’s model of natural selection as given. Most textbooks,
including yours (in the reverse order), treat the topic of evolution by outlining the
“evidence” and then describing Darwinian natural selection in just exactly the way that
Darwin himself described it in 1859, and as we will do here.
Some authors then seem to hedge a little – to suggest that there may be other types of
variation that are selected by other “natural selectors” than Darwin would have
envisioned (or allowed, probably. He seems to have purposely ruled them out in a few
places in the Origin of Species without even realizing it!). This broadened definition of
“natural selection” is, I think, a mistake, particularly when it is just mentioned as a
sideline to the “real” mechanism.
So, we will work our way through the notion of natural selection and then we will look at
some of the problems in the model.
We begin with a return to your textbook authors’ first two lines of “evidence” for
evolution. Remember that what they are really are evidence for natural selection, not
evolution per se.
1a. – Observations of Natural Selection in action
Your book gives several examples, both
schematic and actual examples of classic
examples.
A. A virus (Myxoma) was introduced into
Australia to try to kill off the burgeoning
population of rabbits (which had also been
introduced, and were destroying farmland and
natural habitats). Almost immediately almost
all the rabbits were killed by the virus, and
everyone began congratulating themselves.
However, the ones it didn’t kill were naturally
resistant to the virus and they re-established
the population. This time it was a population
that was generally resistant to the virus, which
no longer killed many rabbits. Virus
resistance was a trait that is interpreted as
having evolved in the population. Certainly
the population had a gene pool that was
dramatically different before and after, and a
corresponding dominant phenotype in the
form of virus resistance. However, the virus
did not lead to the existence of the resistance,
it only selected for what was already there.
The rabbits did not, in other words, “evolve
resistance to the virus because of natural
selection”, as the book claims. It was already
there.
Hewitt
1b. – Observations of Natural Selection in action (cont)
Hewitt
B. The cartoon of the
birds eating off all the
insects except the
green one (which
they have a hard time
seeing against the
grass) suffers from
the same
misconception. The
green ones were
already present from
the git-go, and the
predation pressure
could only made
them more prevalent.
It could not make
them from scratch.
1c. – Observations of Natural Selection in action (Continued)
C. The peppered moth example is the
classic – perhaps the original example of
evolution by natural selection. In English
forests far from sources of pollution the
white form of the moth predominates
because it is camouflaged against the
lichens that grow on trees. When forests
close to London and Birmingham began to
suffer from polluted air, one of the first
problems was the loss of the lichen, leaving
the tree trunks black. In those regions the
black moths became the prevalent form
because they were better camouflaged
against the bare tree trunks. With laws that
have led to improvement in air quality the
populations have shifted back to the light
colored forms. This phenomenon has
happened in many moth species in Europe
and North America, but the same problem
holds in every case. Natural selection has
not created the dark colored moths, it has
only favored their spread in polluted areas.
(The first wild ones I ever saw were a black
and a white one, side by side on a white
bathroom wall in Cumberland Gap National
Park, KY – far from any polluted forest.
Hewitt
1d. – Observations of Natural Selection in action (Continued)
D. The cheetah example in the “check yourself box” makes clear what you are to infer about natural
selection from the previous examples – that it could create the adaptation that it then turns around and
selects. This example seems silly to me because there is plenty for a predator to eat on the African
savannas. If cheetahs started out too slow to catch gazelles then they probably would have just eaten
slower prey, not “tried harder” until some of them could catch a gazelle. Fast running in the population of
cheetahs might be maintained by stabilizing selection, but it is a pretty long leap of faith to assume that
it originated by natural selection.
Hewitt
1e. – Observations of Natural Selection in action (Concluded)
E. There are many similar examples of “natural selection” controlling populations’ gene pools. You can
probably find a dozen or more just by googling “examples of natural selection” (and wading through all the
peppered moths). Dawkins gives others in his books. All of them have the same flaw. We are left to
extrapolate back to some hypothetical point when the trait appeared because of the selection pressure.
But why should it? Which of these tweaks to the book’s example do you think is more likely?
Hewitt
Hewitt
(4) EXTINCTION
Hungry birds eat up all the insects
because they are so easy to spot.
Appearance of a green form because of natural selection.
(I don’t think either one is at all likely. When prey gets sparse predators usually shift their attention to more abundant things.)
2. –Artificial Selection – this refers to the techniques of animal and plant breeders, who have made
astonishingly diverse arrays of dogs, horses, cows, flowers, crops, and so on. Darwin himself bred pigeons.
He was drawn to this not only because of the process itself, which he saw as an analogy for natural selection,
but because of the obvious ability to create diversity. I think that Darwin misunderstood what the root cause
of the diversity was – how the traits came into being. We will compare the two processes.
ARTIFICIAL SELECTION
NATURAL SELECTION
1 & 2. In a litter of animals that a breeder is
raising some (but not all) may have a trait
that the breeder finds desirable.
1. In every species so many individuals are
born that they cannot all survive. Thus
there ensues a “struggle for survival” –
competition among the individuals to
capture resources and survive.
3. Traits are heritable – they “run in families”.
2. There exists variability in the traits these
individuals possess.
4. By allowing only those individuals that
have the desirable traits to breed the
breeder is, in effect granting an advantage
to them in the struggle for survival
(Actually, for reproduction, which is really
the point in both cases.)
3. Traits are heritable – they “run in families”.
5. Over a number of generations those
heritable traits may become “fixed” in the
population, creating a “breed” with specific
traits.
5. Over a number of generations those traits
should become more prevalent in the
population while the alternate traits are
lost.
4. Some of the traits may make it easier for
the bearers to capture resources, giving
them an advantage in the struggle for
survival.
Natural selection was, for Darwin, the mechanism that tied adaptation to all the diversity-related observations.
Consider one lineage on his diagram, delineated with the red line. As time passes (vertical axis – oldest at the
bottom, as in a superpositional sequence) the phenotypes (morphologies, forms, shapes, or appearances) of the
members changes (horizontal axis) because they are becoming better adapted to their environment, or to a
changing environment. The cause of this change, according to Darwin, was natural selection.
Notice that most of the lineages on the chart are sloped, reflecting Darwin’s hypothesis that the changes would be
gradual as slightly better forms replaced slightly less well adapted forms.
However, a few of the lineages (near the
middle) are drawn as long, straight,
vertical lines. This was to account for
“living fossils”, which were beginning
to be discovered even in Darwin’s time.
Periodically a population of the evolving species would
begin to be driven by some “selection pressure” in a
different direction and the original lineage would branch.
Then that lineage would branch, and that one, and so on
an so on. Pretty soon there’d be more lineages evolving
in different directions than you could shake a stick at.
They’d all be doing that under the control of natural
selection, creating diversity at an accelerating pace.
Let’s make a couple more things about the chart clear.
First, the x-axis is not supposed to be read as simply a 2
dimensional axis. It is meant to imply lots of
possibilities. When a branch (c) goes off parallel to
some ancestral lineage (a) it only means that it is going
in a different direction from its immediate parent (b), not
that it is evolving to look like a more distant one (a). The
x-axis is meant as a simple multidimensional axis.
c
a
b
Second, the ending of a lineage (at its upper end) in
many cases might represent extinction of that lineage or
it might simply mean that Darwin was running out of
room to illustrate more branches. Either one is possible.
It was clear in his time that a vast number of species
had become extinct, but remember that Darwin was
seeing natural selection as a way not just to improve
adaptedness (“fitness”), but also as a way to create
species diversity.
The rest of this slide show is a set of criticisms of natural selection as a mechanism for creating
diversity, but don’t be mistaken: natural selection is an elegant insight into the behavior of evolution.
Alfred Russell Wallace, just after Darwin, independently arrived at the same idea. T.H. Huxley,
Darwin’s friend and “bulldog” for evolution wrote, “how stupid of me not to have seen it first”. And I,
who had been “taught” about evolution for years didn’t really get natural selection until I read it in
Darwin’s own words, upon which I thought pretty much what all those folks thought: here is an
elegant solution to a complex problem.
Whether natural selection (in a strictly defined Darwinian sense) can or cannot be an adequate
driving mechanism for evolution, its discovery by Darwin (and Wallace) opened the door to
understanding what the mechanism might look like: a natural process selecting among naturally
occurring variations in body shape.
*****
In natural selection the variation to be selected is the ordinary individual-by-individual difference we
see in all populations. The selector is competition among those individuals to gather adequate
resources to survive long enough to reproduce. It is a serious mistake to say, as people often do,
that “the environment” is selecting. In Darwin’s sense of “selection” the environment is just a stage –
a place for selection to happen. Competition (because of overpopulation) is the selector.
But it turns out that there are even greater ranges of useful variation in populations than Darwin
realized. He may even have specifically ruled them out of his consideration because they did not fit
into his idea of natural selection. That greater range of variation cannot be selected by individual
competition, but by larger-scale natural processes. We are have only just begun to understand this
as the mechanisms of genetic control on development are being worked out.
Random genetic drift
Random genetic drift is a well understood phenomenon in natural
populations. In this case, genes mutate and phenotypes are changed
over time, not by any selection pressure, but by simple random
changes in the genotypes.
This is, in other words, evolutionary change that is not driven by
natural selection, which is, therefore, not necessary for the change to
occur.
Any hypothesis that is not necessary should be subjected to
particularly intense critical evaluation.
STABILIZING SELECTION
In theory, when competition is favoring
progressively “better” forms the
population’s genotypes and phenotypes will
change over time. This is natural selection.
But if the normal form of a species is
already optimal for capturing
resources then the same process will
favor that form over any variants.
This is called “stabilizing selection.
Stabilizing selection was
well known even in
Darwin’s day, though it
wasn’t called that. In fact,
some of Darwin’s early
critics pointed out that the
process he envisioned to
create new forms was
really about making sure
the forms “stayed true”.
The traditional story is that
in a stable environment,
stable species should
remain the same. On the
other hand, in a changing
environment then different
forms would be selected if
the proper variants arose.
It seems from examining
the fossil record in detail
that species do, in fact,
tend to maintain a stable
form for very long intervals
of time – ~5 my is generally
thought to be the average
duration of a species’
lifespan.
EQUILIBRIUM
In other words, what we see in
the fossil record is not a record
of gradual change – of sloping
line segments branching
smoothly into other line
segments. What we see
instead is mostly vertical lines –
long times without appreciable
change in form.
This looks very much like the
action of stabilizing selection.
In a sense, all living organisms
are “living fossils” it’s just that
the record for most of them
doesn’t go all the way back to
the Mesozoic or Paleozoic. On
average we’d expect them only
to extend to the Pliocene or
Miocene!
Nor do we see evidence for gradual
change between the vertical bars.
Instead, as we go back to the
beginning (lowest occurrence) of a
lineage we find it with other species
that are similar, but also adequately
distinct that there is no question of
them being different species. In
many cases we cannot even
reliably interpret which was the
ancestor of the descendant species.
The modern method of
classification does not even try!
The equilibrium of the vertical lines
is punctuated by rapid speciation
events. This is the pattern we see
in the fossil record, and Darwinian
natural selection, strictly interpreted,
is not adequate to explain it. If
selection is to occur at all, then
what is necessary is a larger scale
type of variation, selected by
something other than individual
competition.
Usually if a hypothesis is neither
necessary (random drift) nor
adequate (punctuated equilibrium)
then it is rejected. Natural selection
has an inexplicable staying power in
many minds.
PUNCTUATED EQUILIBRIUM
WHERE DID DARWIN GET THE IDEA OF GRADUAL EVOLUTION?
There are, I think, two parts to the
answer to that question. One was a
matter of data, the other a matter of
interpretation.
Darwin was formulating his ideas at
a time when it was not even yet
established that fossils are the most
reliable way to date rocks, and part
of the reason was that the fossil
record was not yet very well
understood – not enough collecting
had been done to show what it was
actually like. What Darwin had to
work with was a few scattered
fossils, represented by the red dots.
Quite reasonably he just “connected
the dots” (red lines).
The second part of the answer is
that, because his hypothesis of
natural selection predicted slow
natural change, that is the
interpretation he made of the data at
hand.
150+ years of further collecting has
created many more dots (green
dots), and these fit better on the
vertical lines on the chart than to the
sloped ones that Darwin drew.
THINK OF A LION
Is this what you
had in mind?
Without going too far into it, the changes that occur within the ordinary development
of a representative member of a species represent a huge amount of variation. How
those changes unfold is under the control of regulatory genes – genes that tell a
structure when to start (if it starts at all – think of your tail), how fast to grow, when to
stop growing, etc. (The genes that make the structure are called structural genes).
Any change in the mechanics of growth in a sub-population are likely to make for
substantial differences in the adult form. If those differences are adequately large, the
bearers will not compete with their cousins because they will be able to do something
completely different from what their ancestors did.
There are two hypothetical selectors in this case, and neither is competition:
1) The new developmental pathway must allow the growth of a functional individual.
If it can’t grow up and reproduce, it can’t establish a population with its traits.
2) There must be something for it to do – a way to make a living. If it starves because
its traits don’t allow it to feed itself, of if somebody else is already better at doing it,
then it cannot establish a population with its traits.
In those (probably) rare instances where it all works, then a new form arises because
of something other than Darwinian natural selection.