The Role of Chance in Evolution — explained in down-to-earth terms

People who argue against the theory of biological evolution often say: “Complex organisms cannot have been made by mere chance.” Well, that is actually true! Evolution doesn’t work with chance alone. If things were left to mere chance, no more interesting structures than crystals would emerge in nature. There is something else, something crucial that accompanies chance in evolution. It is called:

which, in a sense, is the opposite of chance. Talking about evolution and omitting “natural selection” is akin to talking about the history of scientific and technological progress in the 20th century and omitting any mention of the “United States”. We shouldn’t make the error to think that, according to evolution, living beings supposedly evolved by chance, and chance alone.

I won’t assume that the reader is familiar with how natural selection works against chance — or rather hand-in-hand with chance — to build complex structures, so I’ll explain how it’s done, below. But the reader will have to be content with my home-made, story-telling style. So, allow me please to give the podium to one among zillions of anonymous actors that have participated in our planet’s biological theater, to explain one of the ways natural selection can result in innovation.

Hello! You see what kind of thing I am, from my “picture” on the left. Specifically, I’m not just a frog, but a “tree frog”, and I’m about to tell you how my ancestors, also tree frogs, evolved from earlier ancestors, who used to spend most of their time in the marsh that still lies around here, and in which we lay our eggs that hatch and become our little ones.

A long, long time ago — I’m talking about nearly a million years ago — the marsh was not a marsh, but a great lake. Our ancestors were living most of the time in the waters of the lake. We amphibians are not like fish; we breathe air — at least after we turn from tadpoles to adults — like you, humans. But, depending on our species, we can be very well adapted to aquatic life. Our ancestors, unlike us, were aquatic frogs, spending most of their time in the ancient lake.

Then something started happening. Over thousands and thousands of years, the water in the great lake started decreasing in quantity. I don’t know exactly why this happened. Perhaps a river that used to be the main water-supplier of the lake started bringing less water from the mountains, due to some earthquake, or other, more gradual geologic changes, that altered its course. You’re familiar with this, right? Over millions of years, the face of the Earth can change completely. Mountains can emerge where there were none before, due to the movement of tectonic plates, but individuals like you and me live for such a fleeting moment in time that everything around us appears fixed: what’s a few months for a frog, or even 100 years for you people, compared to the millions of years required to observe a significant difference in land morphology? And yet, if one could run the movie of time in a fast forward fashion, one would see a constantly changing Earth, with mountains emerging, others being leveled, rivers changing course, even whole continents changing shape, colliding, emerging, being submerged. Within the scheme of greater continental change, my ancestor’s lake started being depleted of its water.

Now, if you look at any characteristic of yours, you’ll find that it presents some variation among individuals. For example, take height: some people are tall, others average, others short. Take skin color: some of you are dark brown, others very pale beige, nearly white, and all the in-between skin colors exist among you. Or, if you will, take a characteristic you don’t normally think about: for how long you can hold your breath. Some of you have larger lungs and can hold air longer than others, who have smaller lungs. If in addition to large lungs your body happens to require less oxygen per unit of time than most other people, you’d become a champion in the hold-your-breath competition, if there ever was one. Same for any characteristic of an individual, whether human, frog, or anything living: there is variation. This is the “chance” the creationists are talking about. This, and the changing environment. You haven’t seen the “natural selection” part yet. All in due course, don’t hurry. But, as far as this kind of chance is concerned, do you see anything unnatural in it? Is there something inexplicable in having variation in every characteristic, and in the changing environment?

So far so good. Now, just as “holding your breath for some time” is a characteristic of yours, likewise, for us frogs there is a more complex characteristic called “how much an individual frog depends on water to be present nearby”. I’m saying it’s more complex because it depends on many parameters of our physiology. Some of us can make do with slightly less water, some others need a bit more, and so on; and yet we are members of the same species. Same for our ancestors. When the lake started losing its water (over thousands of years, mind you), those ancestors of ours that depended less on water had better chances to survive and make little ones than those ancestor frogs who needed more water. This is a statistical thought, not so easy to grasp, but I’m sure that you humans have more wits to get it than us frogs. Here’s the tricky part: just because a frog needs a bit more water doesn’t necessarily imply that this frog can’t make many tadpoles with a mate. It could be that by a stroke of good luck, this frog will make lots and lots of little ones. But statistically, if you look at a large population of frogs, you’ll find out that you get more offspring from the part of the population that relies less on water, given the stressful conditions in the water-depleting lake.

Want an example of statistical thinking that sounds more familiar? Think of this: we all know that basketball players are generally selected among the tallest people, especially those who play in the NBA. A short person, or one of average height, is typically not expected to play in the NBA, let alone to slam dunk. Yet exceptions to statistical averages (basketball ↔ tall people) arise from time to time. There is the case of Anthony Jerome “Spud” Webb (photo, left), a 5 ft. 7 in. (1.70 m) tall man, who played basketball professionally from 1985 to 1998 in various NBA teams (Atlanta Hawks, Sacramento Kings, Orlando Magic), and in 1986 became the Slam Dunk Contest Champion. So, just because a person is short doesn’t necessarily imply that he or she cannot jump very high, even higher than most other people. But, of course, statistics do tell us some part of the truth: generally, short people cannot do what Spud Webb did in his career.

Ditto with us froggies. If what matters is not “how high you jump” but “how well you survive given the reduced water conditions” (and thus, how many little ones you can procreate under those conditions), then statistically it is more probable that those froggies with fewer needs in water will be favored. Not only will they have more descendants than the other frogs, but they will pass this characteristic (their lower dependence on water) to their little ones; who will in turn be selected (those with even less dependence will survive better, on average), and will pass their ability to their own little ones; and so on and so forth. This is natural selection. And the shrinking of the lake presented to our ancestor frogs is called an environmental pressure.

Now try to picture the overall situation: the water of the lake is being depleted over thousands of years. But within thousands of years, we frogs can have thousands of generations, because we are little things, we don’t live for nearly a century like you humans. As the water became less and less, our ancestors adapted to the new conditions, as described in the previous paragraph. It wasn’t an adaptation of a single individual we are talking about; it was an adaptation that took place generation after generation. The better-adapted frogs had their decreased water-dependence written into their DNA, because it is the slightly different DNA that causes the different dependence on water. And so, over thousands of years, we ended up with land-dwelling frogs whose DNA differed substantially from the DNA of their initial lake-dwelling ancestors. Their DNA differed so much that if a land-dwelling and a lake-dwelling frog could ever come together to mate, they would fail to have offspring. Or, they would have offspring which would be sterile, and thus a dead-end in gene propagation (genes are parts of the DNA). For this reason, because the two kinds of frogs would not be able to produce a viable population of descendants, we say that they belonged to two different species. So this is an important concept:

The definition of the concept of “species” in biology is more complicated, but for our purposes we can make do with the above. Of course, when I say that the ancestor lake-dwelling frogs and the later land-dwelling ones would not be able to conceive offspring after mating, I am referring to a hypothetical situation, because the older lake-dwellers ceased to exist by the time the new land-dwelling species emerged. One species died, and another one emerged. But the difference in their DNA’s would be real: if we could somehow transport an ancient female frog forward in time (for example, deep-freezing and preserving her) and revived her later, we would see that she would be unable to conceive when mating with a new land-dweller male frog. (Later, another fellow animal will give you an example in which it is not the case that one species disappears and another one emerges, but two species are created and they both continue to exist.)

All right, but I am a tree dweller, not a land dweller. How did my ilk end up on trees? Well, the land dwellers didn’t have an easy time on the ground. Once forced to spend most of their time on land, they had to confront different predators, who could find them easily now, whereas before, their ancestors were safer in the water. Those predators presented a new environmental pressure on poor little land-dwellers, ancestors of mine. By means of the same mechanism (always), which we called natural selection, some of them who were better at jumping on short bushes ran a lower risk to be found on land and make a good meal for a bird or snake. Generation after generation, those ancestors that turned out to be better at climbing higher and higher, hiding themselves in the foliage, fared better in survival rates. So we ended up being a yet different species, one that dwells on trees now, where we live relatively safely. None of the land dwellers survived; they went extinct, so that was a transitional species, as we say. There isn’t anything “wrong” with a transitional species; it’s just a species that lived for a relatively short time, because its environment wasn’t stable or safe enough for it, so the transitional species vanished, leaving one or more species as descendants who flourished in different environments. See what’s special with the transitional species? The ancient lake-dwellers might have lived in the lake for millions of years; we, tree-dwellers, might continue living on trees also for millions of years, if no environmental pressure strikes us meanwhile; but the transitional species lived only for a few thousand years, which is a blink of an eye in evolutionary terms. This concept will become very important when we consider the question of the “gaps” in the fossil record.

One last thing, which I didn’t mention explicitly yet, but you probably suspect: my species looks visibly different from both the lake-dwellers, and the land-dwellers. It’s not just some differences in our DNA molecules that are involved. Our different environments cause us to appear different (and our physical differences are encoded in our DNA’s). For example, my toes are quite separated from each other, and my soles are rubbery and sticky — suitable adaptations for climbing on trees. But the lake dwellers had their toes connected with a membrane, which made their legs look like oars, and of course they had no sticky substance on their soles. Also, the lakers had a more streamlined body, suitable for swimming in water, whereas my body is more stocky, having no strict requirement for water-navigation. The transitional land-dwellers had intermediate characteristics: they had a short membrane connecting their toes, which wasn’t as wide as their lake-dwelling ancestors’ one, and a more athletic body than mine, but less so than the lakers.

One question, dear froggie: What prevented your lake-dwelling ancestors from gradually occupying the land, and once there, conquer the trees, too? If variation exists among individuals, it exists always, it doesn’t wait for an environmental pressure — such as water depletion, or predators — to appear! So what is it that prevented the lake dwellers to gradually occupy the tree-living niche, even before the water started being depleted?

Oh, sexual reproduction! That, and the large population of froggies. You see, you have a large population which is aquatic, right? Suppose now an individual appears who is slightly less dependent on water, so this fellow can venture a bit longer away from the lake. But this chap will have to “marry” another froggie to have children, and this other froggie, the “spouse”, will belong to the general population. Because the population is large, chances are that the spouse will not be a land-champion. Some of their children might inherit the “good genes” (good toward less water-dependence) of one of their parents, but they will “marry” again with other froggies from the general population. So, the “good genes” of the first frog will diffuse and be lost in the large general population’s ocean of average genes. It is only when an environmental pressure eliminates the masses with the average genes that the champions in “less-water dependence” can find mostly other such champions to marry with, in a reduced-size population. Thus they’ll create descendants that will inherit the “good genes”.^(*)

That was the frog’s story. Do you see how chance would achieve nothing at all if there were no natural selection to help frogs first abandon the lake, and later take to the trees? This “help” is not random; it’s a result of changing environmental conditions. The species of frogs “responded” to environmental pressure by abandoning the shrinking lake, so they took to the trees. Keep in mind that this “response” didn’t occur by individual frogs, like the fellow who told us the above story. It was a response by an entire species; each individual frog was living for a very brief “moment” in time in the environment to which the frog was best adapted (lake, land, trees).

In any case, the response of the successive species could be predicted in principle by some savvy biologist who was seeing the depletion of the water of the lake coming. And so, please take note of the following observation:

which implies that to claim that evolution is based on “mere chance” is an awful misrepresentation, a caricature of evolution. Nope; evolution is based on natural selection, which works hand-in-hand with chance.

Want an example to see how statistical thinking allows predictability in spite of chance? Take a coin and toss it in the air 1000 times (if you have the patience; and if not, make a computer simulation of this experiment). Then, although each toss has a 50% chance to come either heads or tails, you can predict with a certainty of 99% that it will come heads between 459 and 541 times. Increase your tosses to 1,000,000 (one million; now you’ll definitely need a computer to simulate the experiment) and you can be 99% sure that you’ll get between 498,712 and 501,287 heads. Of course, since you are “only” 99% sure about that, it means that 1 out of every 100 repetitions of this million-fold coin-toss experiment you’ll get a number of heads that will be out of those limits. But then you can increase your certainty to something like 99.99%, in which case your heads (and tails) should be between 498,054 and 501,945. (Notice how little the interval was augmented.) Raise your certainty to 99.9999% if you will, and your interval for heads (and tails) changes to between 497,466 and 502,533. That’s elementary statistics, and shows how large numbers force chance to yield predictable results. In biological evolution, large numbers (vastly large, enormously large numbers) appear when we consider the number of individuals of a population that mate and have descendants over millions and billions of years.

Back to the frog’s story: it tells us that species come to “reflect”, in an abstract way, their environment. In our example, the scarcity of water was “reflected” in the diminished membranes between the toes of the frogs, in their less streamlined body, etc. It’s always like that:

So, it is entirely wrong to think that, supposedly, according to evolution living beings (such as the “first cell”) appeared suddenly, when atoms and molecules came together by mere chance.

Regarding the occurrence of the “first cell”, allow me to clarify one thing. Evolution does not make any claim about how the first living forms appeared. It only assures us that cells cannot have been the first living beings, because they are too complex, and must themselves be the end result of an earlier evolutionary stage. Indeed, from a biologist’s point of view, there cannot have been a “first” living being ever, because as we move back in time it is not possible to define whether a collection of molecules is “alive” or not; there is a gray area where “life” is not well-defined, so that “being alive” is not a black-or-white issue. An entity that undoubtedly led to an evolutionary process would be a self-replicating molecule, one that copied itself accurately enough so that we can say that there was an instance of self-replication, but one that did not achieve 100% fidelity in this process, otherwise we would have no variation (remember the frogs!) and natural selection would not work. The RNA and DNA are such examples of molecules, but they are still too complicated — they must themselves have been the end products of an evolutionary process. Whatever kinds of molecules eventually led to RNA and DNA are not around anymore; they vanished like our transitional frogs, and molecules do not leave traces (fossils) behind. In this article I have speculated about how self-replication itself could have emerged out of partial (inaccurate) copying, in a process that I would venture to call “evolving evolution”, but mine is mere theoretical speculation. The final, definitive answer will be given when chemists point to the specific substances (molecules) and reactions that possibly led to self-replication, after verifying their hypotheses in the lab; and this might take a long time, because it is mostly guesswork.

Excuse me for jumping in, but I was eavesdropping your conversation with the frog — or rather, the froggie’s monologue — and I couldn’t help thinking that you might find a horsie’s monologue interesting, too! You see, we horses are more familiar to you than those amphibians, and our story has some unique and interesting points to contribute to your understanding of evolution.

I want to tell you — briefly only, I won’t make you tired — about the relationship of us, horses, with our “cousins”, the donkeys, our “partners under the yoke”.

You know, of course, that horses and donkeys can mate, and our union produces the mule. All you need is a female horse and a male donkey, some good mood, and presto! You get your mule. Rarely, a male horse and a female donkey can also mate and produce the much rarer hinny.

But of course mules are sterile: they can’t mate amongst themselves and produce more mules. Thus, our definition of species, as given above, still holds for horses and donkeys, so we say that we horses and them donkeys are two different species. If mules could produce more mules, or individuals between mules and horses, or between mules and donkeys, then we wouldn’t have horses and donkeys as two different species, but an equine salad.^(*) We’d have one species, then. Are you following, dear human?

Horse (Equus caballus, zoo of Indianapolis, USA)	Mule (Island of Hydra, Greece)	Donkey (Equus asinus, zoo of Indianapolis, USA)

Good. My purpose is to bring to your attention this fact: that horses can mix with donkeys, even with a sterile descendant as a result. But dogs and cats can’t do that! Nor can dogs and bears, nor hippos and pigs. But lions can mix with tigers, did you know that? Yup! A male tiger and a female lion can give birth to an animal called a “tigon”, which has a mane like a lion if it is a male (but shorter), and stripes like a tiger. Likewise, a male lion and a female tiger can give birth to an animal called a “liger”, which is larger than both a lion and a tiger, for some reason that has to do with growth inhibition. (In fact, ligers are the largest cats in the world.) And both of those hybrids, tigons and ligers, are — you guessed it — sterile.

Why does this happen? Why a few pairs of species can mix and have hybrids, whereas most cannot?

It’s simple: because their DNA molecules have not distanced themselves so much yet to prevent a successful mating, and a little one to be born. But their DNA’s are distant enough for the little ones to be born sterile. Otherwise we’d have one species, not two.

Take for example us horses, and our cousins, the donkeys. We share a common ancestor who lived some 2.6 million years ago, in Asia. At that time there were no horses and donkeys living separately, but only that common ancestor of ours, who had actually come from North America into Asia crossing the Bering Strait, during a glaciation period when the sea level dropped and the Bering Strait became a land bridge.^(*) Those ancestors spread all the way into southern Africa. They formed also other pockets of populations in the central Asian steppes, in Europe, and in Arabia and North Africa. (This is no mere hand-waving, you humans have unearthed the fossil bones of our ancestors everywhere in the mentioned places.) Due to the geography of this vast region, the European and central Asian populations were separated from the Arabian and northern African populations, and those in turn from the southern African ones. This is called geographical separation, and is a major cause of evolution. Here’s what happened:

• The European and central Asian populations became the ancestors of us horses.
• The northern African populations became the ancestors of our cousin, the donkey.
• The southern African populations resulted in our other cousins, the three species of zebras.
• The Arabian populations became the ancestors of yet another cousin, the onager.
• And a further split of donkey-like cousins resulted in the dwellers of Tibet, the kiang.

Okay, but why did that happen? Why (and how) does geographical separation lead to the emergence of different species?

Consider the diagram on the right. Time runs from top to bottom (see left side of the figure), with our “now” situated somewhere at the middle of the overall height. At the top, above the dashed line, is a region of time when there was only one species of equine ancestors throughout the Old World. However, some parts of the population (those that would later become the horses and the donkeys) were geographically separated, and so their DNA’s were drifting apart. Still, if you could take a male from one part (say, the horse ancestors) and a female from the other part (the donkey ancestors), those two individuals would be able to mate (on average) and have a fertile baby. But such matings were not happening, because the two parts of the population were geographically separated.

Time went on. The two populations’ DNA’s continued mutating and diverging separately. So there came a time when DNA differences were such that horse and donkey descendants had some chances to be infertile. As time went by, those chances increased.

At present (the dashed “now”-line), the chances that a mule is fertile are really-really small; so small that most people think that mules never give birth to babies. And yet, occasionally it can happen. There have been 60 documented cases of female mules that gave birth to little ones when they mated with purebred horses or donkeys. The following report is from the Greek daily Eleutherotypia, on May 13, 2002 (I translate from Greek, below):

City of Drama, Greece:

One of the miracles of nature, the birth of a baby by a mule, occurred at Drama; the baby, unfortunately, was unable to survive.

The birth took place at the [...] farming unit of Prokopis Armen, 50, at the village of Taxiarches of Drama; his is the “feminizing” mule. When the man witnessed the rare phenomenon he notified the local vet, who confirmed the incident. However, after the birth — which was natural — the offspring sought the mother for milking. The problem was, the mule did not have udders to milk. Mr. Armen, together with other villagers, tried to keep the baby alive by giving it other kinds of food, which are used for older animals, but in vain. The newborn baby needed maternal milk, and after one day it died, filling with sorrow its mother, its owner, and the other villagers. They buried it at the outskirts of the village, after having taken all necessary health precautions. (Eleutherotypia, 5-13-2002)

As more time passes, well into the future, successful unions between horses and donkeys will become ever rarer, until at some point they will become impossible (second dashed line in the diagram, above).

We horses can even mix with zebras! Follow this link to see a picture of a “zorse”, at a safari park at Schloss Holte Stukenbrock, near the German–Netherlands border — she’s really amazing. She’s half white-horse, and half discolored-zebra! But she’s not unique: as you’ll learn in that page, horses and zebras are often crossbred in Africa. Zebras and donkeys can also mix.

What all this means is that we horses, donkeys, zebras, and the lesser-known onagers and kiangs, have not yet diverged enough from each other to completely prevent our hybridization. But you and chimpanzees, as far as I know, have already diverged enough, for I’ve never heard of a successful mating between a human and a chimp — “thank goodness”, I hear you thinking, because you’d be confronted with some serious moral issues if I am not mistaken. Your common ancestor with the chimpanzees dates back to some 6 or 7 million years, which is much longer than the 2.6 million years of the common ancestor of all living equines; so you and the chimps have had more time to diverge from each other than us, horses and kin.

So that was the horse’s story, who told us about a different cause that leads to evolutionary innovation: the geographical separation of individuals that initially belonged to the same species. The frog talked about a different cause: environmental pressure. There are more, but we can’t turn this page into a thorough treatise of evolution. We need to move on to another frequently raised issue, which some people believe is a problem for the theory of evolution. If you are interested to know what it is, click on the link below:

The fossil record is full of gaps, it doesn’t show species changing gradually to other species.

Feel free to contact the author if you have any comments or thoughts to share.

(^) The froggie makes it sound as if sexual reproduction is a bane: as if asexually-reproducing organisms would spread and occupy every nook and cranny in the environment. But, no, sexual reproduction has other advantages (for example, ensuring the variability of genes), which we don’t need to explain here. It is because of its advantages that sexual reproduction prevailed among larger living beings (especially animals) after it was “discovered” in evolution.

Note also that bacteria, which do not reproduce sexually, do exactly what the froggies of the lake couldn’t do: they spread and occupy every nook and cranny of their environment.

(^) Sometimes the opposite can happen: living beings create or modify the environment, which becomes suitable for occupation by different kinds of beings. For example, during the fist two billion years of life on Earth, the bacteria (the only forms of life in existence back then) modified the consistency of our planet’s atmosphere, making it richer in oxygen. This made it possible for larger cells to appear, whose size and metabolism required more oxygen — something that wasn’t possible before. Thus the “eukaryotic cells” appeared, and, with them, the tree of life radiated in unprecedented forms that would be impossible at the prior, bacterial stage.

(^) Reality is always more complex, so it is possible to get a little bit of that equine salad. But only a tiny bit of it. Read the article on hinnies to see the details.

(^) Others believe that the Strait was covered with ice. Either way, during glaciation periods the Bering Strait can be crossed by animals.