Evolution 2

Natural Selection

Unless otherwise noted* the artwork and photographs in this slide show are original and © by Burt Carter.Permission is granted to use them for non-commercial, non-profit educational purposes provided that credit is given for their origin.Permission is not granted for any commercial or for-profit use, including use at for-profit educational facilities.Other copyrighted material is used under the fair use clause of the copyright law of the United States.

*Scanned images are from course textbook: Hewitt et al., Integrated Science and are used under fair use clause of copyright law. (“Hewitt” on images.)

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.

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.

1a. – Observations of Natural Selection in action

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.

1b. – Observations of Natural Selection in action (cont)

Hewitt

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.

1c. – Observations of Natural Selection in action (Continued)

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?

(I don’t think either one is at all likely. When prey gets sparse predators usually shift their attention to more abundant things.)

Appearance of a green form because of natural selection.

Hungry birds eat up all the insects because they are so easy to spot.

(4) EXTINCTION

Hewitt Hewitt

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.

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.

ARTIFICIAL 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.

3. Traits are heritable – they “run in families”.

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.)

5. Over a number of generations those heritable traits may become “fixed” in the population, creating a “breed” with specific traits.

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.

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.

ca

b

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.

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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 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.

Random genetic drift

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!

PUNCTUATED EQUILIBRIUMNor 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.

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.