And now, class, it’s time for our science lesson. Today’s topic is Evolution. There are two main theories of the mechanism of evolution and I’d like to talk about both. First, there is Darwin’s original, Natural Selection.
I have a sign on my wall that says, “Take heart! Many great things have been done by people in poor mental health.”
A case in point are two Princeton scientists, Peter and Rosemary Grant. They are crazy in a very special way. They like to visit a small island in the Galapagos called Daphne Major.
I should tell you that the small islands of the Galapagos are small, dry, jagged, rocky, with little soil, little vegetation, and little life of any kind. Just the places you would not want to spend your summer vacation.
So, of course, the Grants have done just that. Every summer for the last 25 years.
Why? To measure finches and sift dirt.
Why (I hear you asking again)? Because that isolated, arid, nasty little island is the closest thing to a closed system that a biologist is ever likely to find.
The task the Grants set themselves was, on the face of it, completely lunatic. They decided to study the fourteen hundred or so ground finches on Daphne in as much detail as possible for as long as possible. Their species were considered hyper-variable and the Grants simply wanted to observe and document them over time to establish the birds’ baseline.
To do this, they counted and banded every finch on the island. They observed the matings, so that, in time, they knew the pedigree of every single finch. As each fledgling grew up, it, too, was caught and banded. In time, they could identify each bird by sight.
While banding, they weighed and measured each bird. Especially, they carefully measured the length, width and depth of the beaks. In theory, the three species each have a different beak size. In reality, each species presents a range of beak sizes which overlap each other at the edges. Still, the beak was the most reliable indicator of species.
The Grants also needed to know about the finches’ food supply, so they laid out meter square patches of ground around the island and meticulously sieved each one, separating out the seeds. Then they counted and measured each type of seed. Year after year.
Like the bird’s beaks, the seeds, too, broke into three types: large, thick walled seeds, medium seeds with medium walls and small, soft shelled seeds. In normal seasons, there were enough seeds to go around (this is called a lightly stressed environment) and the finches were not choosy. The big billed birds, if they found a bunch of small seeds, would happily eat them. The smaller birds, if they found a smaller and softer version of the big seeds, would do the same.
But in 1977, the Grants got a chance to see what happens when a lightly stressed environment is converted into a highly stressed one.
There was a drought. Only one inch of water fell the entire season.
The first effect was a total loss of new birds and a great die off of the mature ones. As food became harder and harder to find, the weaker of the finches began to die. Then more. Then more. Eventually, only one in seven survived.
The Grants continued their work, sadly noting each death and monitoring the gradual disappearance of the seeds the birds needed to keep them alive. They noted that the smaller, softer seeds nearly disappeared, then the medium sized. Finally, by the end of the season, only the largest, hardest seeds from the hardiest plants could be found.
But when they went home to Princeton and started to process their data, they discovered something they had missed in the day to day observations: as the smaller seed tally declined and the average seed became larger and tougher, the survival rate of the birds became highly dependent on the beak size, regardless of species. In other words, if one of the smallest species of bird happened to have an unusually large and strong beak (i.e., able to cope with the large and thick seeds), that bird had a greatly improved chance of survival.
The next year, when the weather returned to normal, the Grants returned to measure the effects of the drought in the new offspring. They found each species had seen an increase in average beak size, with the largest effect being detected in the smallest species.
What the Grants had achieved was stunning. By the insane thoroughness and exactitude of their measurements, they had managed to catch evolution in the act. For the first time, Natural Selection had been conclusively measured in the field.
But Natural Selection seems to cover only half of the problem of evolution. People have, for many years, pointed out an important problem with Darwin’s mechanism. It is easy to see how Natural Selection can select and favor some existing characteristic of a species over another. It is far less easy to see how Natural Selection could produce some completely new feature like, say, changing to erect posture, or fur switching to feathers.
Another theory was needed to cover the introduction of mutated features into species.
Spontaneous mutations do occur. In fact, under stable conditions, mutations occur at a steady, measurable rate. As environments become more highly stressed, the mutation rate increases dramatically.
But the existence of mutations alone does not solve the problem. In a large population, mutations, even favorable ones, tend to be selected against. That is, they are swamped (if not actually hunted and destroyed) by the “normals.” It is nature’s way of suppressing the deviants and maintaining stability.
So how do we get favorable mutations to stick around and become the new normal for a population?
The most popular theoretical solution to that problem lies in the theory called “Punctuated Equilibrium.” It is based on the idea that, on a geological time scale, most populations reach “equilibrium” and stay there until their life is “punctuated” by some sizable, long term event.
For example: imagine some geological event, say a volcanic explosion, or an ice age, or whatever, occurs. It is big, and it is long term. It could dry up the water, kill off half of the vegetation species, or any of hundreds of miserable consequences that produce a “highly stressed” environment. A highly stressed environment naturally selects against large groups of animals. After the initial massive die off, the surviving populations tend to break up into smaller, isolated groups. The reason is that such smaller groups impose less stress on their local habitat than a single, big group.
Inevitably, our imaginary small, isolated, highly stressed group finds itself producing an unusual number of mutations in its individuals. Most of the mutations are very bad for them and they die. Eventually, along comes some mutation in a single individual that gives that individual greatly enhanced ability to cope with the local stresses. That individual will live longer and be able to build up a family of descendants.
This family’s members, in turn, will have a substantial advantage when it comes to breeding. Many more of them survive, hence there will be more each season to breed. Given the enhanced survivability, members with this trait would eventually come to be the norm of this isolated group. By increasing the survivability of the group as a whole, the population should increase until it reaches a point where any further mutations are suppressed. Equilibrium has been restored and no further mutations are desirable. A new species has been created.
It should be noted that both of these processes, Natural Selection and Punctuated Equilibrium, modify the DNA of a species in such a way as to increase its survivability. One selects between existing versions of DNA while the other creates new ones.
Now, there is something odd about our DNA. While nature is generally strict about any excess baggage, stripping it off at the first opportunity, DNA is different. It carries a lot of old baggage in the form of currently unused code. The geneticists call it “junk” code.
Astonishingly, about 90% of our DNA is junk code.
The best guess is that well over 50% of that non-functional code consists of simple repetitions of working code. Lots of repetitions. There are whole long sections of our DNA that looks as if some xerox machine went slightly mad. These extra sections are not allowed to express proteins, so their existence has no effect except to lengthen the DNA.
That’s one kind of “junk” code. But some unknown percentage of it consists of perfectly valid code segments that are not duplicates. They exist nowhere else. We do not know what they are designed to do. We can see these code segments are quite prepared to become active genes, but they are shut off for some reason. [It is an interesting and terrifying question as to what human beings would look like if these mysterious sections were magically turned on and allowed to shape us.]
So only 10% of our DNA works to make us what we are. The excess code may do us no harm, but where did it come from?
The mechanisms of Natural Selection and Punctuated Equilibrium imply that the excess was created by stress events in our history. They are like the scars on a prize fighter. That, in turn, implies our DNA is a kind of record of the events that formed us as a species.
Here is a section that was produced when the great comet hit Yucatan. The impact that apparently drove the stake into the weakened heart of the dinosaurs also stressed the heck out of our mammalian ancestors.
There is the section where the climate changed again and drove us from the trees out onto the savannah, forcing us to stand on two legs.
So what does all this mean?
To me, it means this: it is true that when our parents got together in the act of love that produced us, they gave us life, this special life where we stand upright, hairless, with useless fangs and claws but a big brain, and capable of breeding in all seasons. But they gave us more than that. They gave us our heritage, in the most literal sense of the word. They gave us, in every cell of our body, the history of all the events of our lineage that combined to make us human.
Now that is a true gift — from the heart.