HOW THE GIRAFFE GOT ITS NECK
The idea of evolution was around long before Darwin,The Origin of Species, or when he started thinking about the “species problem,” it was around before he was born. What Darwin contributed wasn’t that evolution happened but that it was caused by natural selection. Before Darwin, one of the postulated mechanisms for evolution was the “inheritance of acquired characteristics.” This is often called Lamarckian evolution after Jean Baptiste de Lamarck (1744-1829), a prominent supporter and defender of the concept.
According to the theory, physical characteristics acquired during life by an organism can be passed on to its offspring. By using or not using a body part, thus modifying it, the change would accumulate over generations leading to the evolution of a new species.
The classic example is the long neck of giraffes. The original ancestor of giraffes was a short-necked animal, probably similar to the living okapi, which fed on the leaves and twigs of trees. According to Lamarck, these proto-giraffes would stretch their necks in trying to reach the leaves higher up in the trees. Over the course of their individual lives this would cause their necks to become slightly longer. Their offspring would be born with this slightly longer neck. They, in turn, would also stretch their necks slightly trying to reach even higher leaves. Repeated over a number of generations the short-necked proto-giraffes would evolve into the long-necked giraffe we see today.
Darwin would have explained the evolution of the giraffe’s long neck as an example of how natural selection working on a population with variable traits can create a different organism, adapted for a different econiche.
Within the population of short-necked proto-giraffes there were some with necks longer than average, also some with necks shorter than average. Also there was a similar variation in the length of the legs and body size (leg and neck length are in proportion to body size and also absolutely longer in proportion). Perhaps the traits were related and perhaps not. The individuals who were longer necked, those longer legged, and those who were bigger than the average had an advantage eating leaves from trees. They could reach leaves that the others couldn’t. They could eat the same leaves that the other could reach and leaves the other’s couldn’t. In times of drought or anything else that reduced the numbers of leaves on the trees (including large numbers of proto-giraffes), those who could feed on the higher leaves could obtain more food. They were more likely, on average, to be better nourished and to survive famine conditions. The shorter-necked proto-giraffes were less likely to either survive or produce healthy offspring who stood a good chance of surviving. Gould (Bully for Brontosaurus 1991:166) ) discusses this example and mentions that perhaps the long neck was not an adaptation for feeding but a selective advantage in sexual competition. Giraffes fight among themselves slamming their heads and necks against each other. Whether this fighting is for sexual competition or social dominance (which can also have sexual advantages), I don’t know. A giraffe with longer and heavier neck would presumably have an advantage. “Knowing” the way evolution works, longer necks could have been selected for both because they were advantageous for feeding and for inter-specific competition.
Over generations the average neck length, leg length, and body size would increase. These three characteristics, if not already caused by related genes, would increasingly tend to occur together(co-evolution of genes). That is, any proto-giraffe with all three traits would generally do better than one with only one of the traits.
The example of a giraffe evolving a longer neck either by stretching its neck to reach higher leaves in a tree, or long-necked giraffes surviving better because they could reach higher leaves than other animals is repeated in many books to demonstrate the difference between Lamarck’s Evolution by the Inheritance of Acquired Characteristics and Darwin’s Evolution by Natural Selection. It is often believed that either Lamarck used the giraffe to demonstrate his theory or Darwin used it show how his theory was different. I have an old biology textbook that calls it “the classic example (though now rather hackneyed).”S.J. Gould (Bully for Brontosaurus 1991:166) believes that this comes from “a few lines of speculation” that Lamarck wrote. And it was about the giraffe’s legs (and he was wrong. A giraffe’s front legs are not longer than the rear). Darwin used the giraffe’s tail and its use as a fly swatter. Further, as Gould notes, there is no evidence that the giraffe’s long neck is an adaptation to eat leaves on the tops of trees. The fact that male giraffes fight by swinging their necks against their opponents, “necking,” and the winner is usually the taller giraffe makes one wonder. But I digress. That wasn’t why I brought up the subject of giraffes and their necks.
I have come across an illustration using this example. It is a series of four drawings of a giraffe standing next to a tree. In the first drawing the giraffe has a short neck which gets progressively longer until in the fourth drawing it is as long as the present day’s giraffe. What really got my attention is that the rest of the body didn’t change. The legs on the short-necked giraffe were just as long as those on the long-necked giraffe and the back had the same sloping line.
The evolution of the long neck on a giraffe involved more, much more, than a simple lengthening of the neck. There were many other changes involved, changes that had to proceed in some sort of coordination with the longer neck. Just how do random mutations coordinate themselves? Must they all happen in one fell swoop?
The neck of the giraffe is quite a bit longer than yours or mine, but for all its length it still has the same number vertebrae (seven) as our necks do. The length is achieved by making the body of each vertebra longer in relation to its diameter. Ours are sort of short, squat and round. The giraffe’s vertebra are long, round cylinders. However that is only a small part of the changes involved in increasing the length go the neck (or maybe the height of the head above the ground).
The greater the distance the head is above the heart the harder the heart has to pump to get enough blood to that head, and then you have the weight of a large volume of venous blood coming back down that same neck to the heart. And the reverse when the head is just about as far below the heart when it is lowered to take a drink of water. A head that might have to be very suddenly and quickly raised to allow the giraffe to escape an ambush at the waterhole by a lion.
A giraffe does have high blood pressure, nearly twice that of us, and a very large heart. The lower part of the carotid artery is very elastic. It expands when the heart pushes a mass of blood into it and then contracts helping to push that pulse of blood further up. The jugular is thick-walled so it doesn’t expand under the weight of the descending blood and it has numerous valves to prevent a reverse flow of blood when the head is lowered. The reta mirabile around the brain has a “bypass” from the carotid to the vertebral artery (for the neck muscles). There is no connection of the vertebral artery to the reta mirabile. This prevents congestion in the fine blood vessels of the reta mirabile by the overflowing of the vertebral artery when the head is lowered. The reta mirabile is also elastic and will absorb some increase of blood when the head is lowered and hold it momentarily when the head is raised. This prevents giddiness. (Unlike one author who implies that the reta mirabile is a “special pressure-reducing mechanism . . . provided” solely for that reason and is only in giraffe, it serves other functions and is a basic part of mammalian anatomy.)
A giraffe’s heart beats very fast, especially for an animal of its size. The veins and arteries of the legs are buried in a long, bony furrow, this prevents their swelling and the skin is very tight, not very elastic, and thick (just like support hosiery). So it acts as a”g-suit” to counter the pressure of the blood.
Taylor also points out how a human breathing through an eight foot tube would die because he would never inhale new air, he would simply re-breathe the same air over and over. He says this problem is “intractable” and leaves us to wonder if giraffes don’t breathe.
Giraffes have very large lungs to actually move enough air to get oxygenated air to the lungs. Even so a large part of the inhaled air remains in the trachea and is not used, wasted. The blood has a large number of small red corpuscles, that take up more oxygen very quickly and giraffes have a slow the rate of respiration to more thoroughly use the inhaled air.
The steeply sloping back of the giraffe is not because its front legs are so much longer than the back legs, as so many have believed (including some who should have known better like Lamarck). The front and rear legs are nearly the same length and the spine is nearly level, parallel to the ground. The shoulders of a giraffe are not as high as you might think, the steep slope of the back is due to the very tall spines on the thoracic vertebrae. These spines provide greater surface for the attachment of the enlarged neck muscles and, like the tall pillars of a suspension bridge, a better angle for those muscles to move and support the neck.
So how did all these different adaptations (and more) occur together, in a coordinated system? If evolution is random they can’t have occurred together.
A number of people don’t believe in evolution because they do not comprehend how it can occur, others deny it is true because they say it cannot occur. Others think that there had to be some Intelligent Designer (God) directing the whole thing. The purpose of the following scenario is to present a way, a plausible but not necessarily the way, a short-necked animal (similar to the okapi) could have evolved into a long-necked giraffe with all the different adaptations they have that form an integrated system that works together without being caused by Divine Intervention, direction, or an implausible sequence of random mutations occurring simultaneously.
This is a “thought experiment”and we need to choose a stating point because you need to begin somewhere. We will start with a proximal beginning, the ultimate starting point is with The Big Bang, or whatever was the beginning of the Universe. However this would make things unmanageable, or at least really long. To make things manageable and more to the point we will begin much closer to the point of what we want to illustrate. This is similar to starting a story in the middle and there is already a number of factors (previous history) that already exist and constrain the future. Keeping things manageable is also why we tend to deal with a single gene (or trait). One of the points behind this thought experiment is to show how many genes can appear to cooperate and evolve in unison.
We start with three givens:
1) An animal similar to the okapi (Okapi johnstoni). The okapi is chosen because it is the closest relative to the giraffe and it is similar to known fossil ancestors of the giraffe. It does have a neck that is somewhat elongated and a short body. They currently live in dense forests of the Congo. In the scenario a population of these animals is expanding their range into the more open woodlands.
2) A selective advantage to these animals of a longer neck. This advantage need not be the ability to eat leaves off the tops of trees. After all, maybe giraffes eat the leaves off the tops of tress because that is where their heads are (and maybe they aren’t taller because most trees are not). Maybe males battle between themselves for dominance by “necking” and a longer neck is part of being successful and being dominant leads to greater reproductive success. To a point, the exact reason for the selective advantage doesn’t matter, only that longer necks are part of it.
3) That there be variation between individuals and that it be inheritable. There must be some variation between the individuals o the species and that some of this variation (e.g. neck length, length of leg, size of lungs, heart rate, elasticity of the skin, etc.) be due to genetic, and inheritable, differences. This can be the presence or absence of a particular gene or genes, or the traits that are caused by multiple genes, the percentage of particular genes. Say “tallness” is due to the ratio of “tall” alleles to “short” alleles, the more “tall” alleles an individual has in its genotype, the taller it can grow. Potentially a population can contain quite a lot of variation. There are some 800 breeds of dogs in existence. Most of these breeds are of recent origin. Even the oldest breeds are not much more than 10,000 years old. It does not seem very likely that all the “new” traits that distinguish the various breeds are the result of recent gene mutations. A very large part of the variation within and between the breeds was present in the original canine populations. Man has simply identified various traits and by interbreeding the selected dogs (artificial selection), concentrated the alleles so that the chosen traits breed true and consistently. Although once the genetic variation in a gene pool is exhausted, new variation can only arise through mutation.
At the risk of being accused of making up a “Just-so Story” or, worse yet, of having a plausible scenario taken as factual, I want to give here a scenario of how it might be possible for a number of adaptations to evolve, forming a “coordinated” system. It is not intended to be an accurate account of giraffe evolution or even intended to be how it really did happen. Rather, the scenario is only meant to be plausible, to be a demonstration of a way it could have happened, an analogy to the how it did occur and an answer to critics’ claims that evolution cannot work by “natural” means. Evolution and natural selection can function in a natural way without miraculous of extremely improbable events.
Mostly we construct scenarios as if only one gene at a time is evolving. This scenario deals with multiple traits and genes.
So, beginning with an okapi-like animal, an even-toed ungulate living in the forests of Africa and expanding out into more open woodlands that, for whatever reason, is under selective pressure for a longer neck. Males gain greater access to females by defeating their rivals in head-slamming contests is an adequate selective pressure. In the male social structure dominance is achieved by physical contests between the males and part of the outcome between two males is based on which one is taller. The higher a male ranks in the social hierarchy the greater his access to females and the greater the likelihood of him successfully passing on his genes.
Every population of living sexually reproducing organisms contains an amount of variation (unless they are clones) in its gene pool. Some traits/characters will be closely “fixed,” others will be more variable in their expression. Humans have two hands, each hand has four fingers with three joints and one thumb with two joints. The vast majority any way, so many that two hands with four fingers and a thumb on each is “normal.” I knew a man with two little fingers (one growing off the side of the other) on his right hand (and two little toes his right foot) and someone born with a thumb and only two fingers on each hand (I presume the feet were similar but do not know for certain). Others area born with abnormally long and thin fingers (arachnodactyly) and some with two fingers joined together (syndactyly). These variations are known but generally rather rare. Humans vary in adult height. There is no single height for an adult that nearly everyone reaches that can be considered normal in the same way that two hands is normal. There is a range of variation in adult height and if it is plotted on a Cartesian graph (adult height on one axis and number of individuals on the other axis) the distribution will form something like the standard bell curve. Most individuals will be close the average with fewer and fewer individuals toward the extreme ends of the curve. There may be 7 foot humans but they are very rare, rarer than 6 foot 4 inch humans who are rarer then 6 footers who are much more common but not as common as 5’4″, 5’6″, or 5’8″ humans. Height is due to the effects of both multiple genes and the environment. There is no single gene that controls how tall you grow. Height is determined by how fast you grow, how much of the energy resources consumed are committed to growth, how long the body grows, etc. Some alleles will increase the body’s ability to grow taller and others will reduce the body’s ability to grow. We can call these alleles “tall” genes and “short” genes if you remember that they probably have other effects than just on height and that the amount and quality of food consumed and at what stage of growth also determine stature. You might have the genes to potentially grow to seven feet but if you do not get enough food when you are growing you will never reach your potential. Humans shorter than the average have more than the average number of genes that act to keep adult stature short. Humans taller than the average have more than the average number of genes that act to increase growth. If those persons at one end of the curve (the tails) do not contribute their mix of genes (for whatever reason) to the next generation, the proportion of those genes that contribute to that extreme condition will decrease in the gene pool. This will shift the average toward the other end. This is what natural selection is about. It doesn’t even require a total failure to pass on those genes, only a reduction. If those people are one percent of the population but their offspring form just less than one percent of the next generation, the gene pool will slowly lose those alleles.
In a population of proto-giraffes the situation is similar. In the gene pool of these proto-giraffes there are a number of genes/alleles that influence neck length, each proto-giraffe has in its own genetic makeup these genes in a particular mix and therefore a neck of a certain length. I am not saying that the neck length could be one foot, two foot, or seven or eight feet. The variation need be only inches. There is also variation in the length of the legs, the thickness of the skull, size of the heart, elasticity of the veins and arteries, tightness of the skin, etc. A male proto-giraffe may have a neck of average length but a skull that is thicker than average. Because the head is heavier he can deal heavier blows (and the thicker skull can better withstand the impact) to his opponents and achieve a higher status than other average-neck-length-males. He has a combination of genes for short and long neck (they average out to medium) and genes for a thicker skull (more genes for a thick skull than thin). He mates with one of more females with their own combination of genes for short and long necks and thin or thick skulls. Their offspring gets genes from both. The mother is more likely to have had a slightly-longer-necked-than average father and so the offspring is more likely to have a longer neck genes and thicker skull genes than others of its generation. This combination of slightly longer neck and heavier skull is more advantageous then either alone. The proportion of these genes in the population’s gene pool begins to increase. Not only does the proportion of them increase but the proportion of them occurring in the same individual increases. Necks get longer and skulls get heavier throughout the population. Now even longer necks and heavier skulls are favored. A gene pool can contain a very large amount of variation. The gene pool of proto-giraffes may already contain enough variation in the population of six foot high animals to grow one (with all long-necked genes) that is 18 feet tall. That is conceivably possible, however it is not necessary.
The neck would lengthen and as it does so the need would arise for the means of getting the blood to the head, dealing with the added pressure, stronger muscles, adequate surface to anchor those muscles, and what we may call the side effects of an elevated head—the longer trachea, the great difference between the raised head and the lowered head, etc.
All the differences between an okapi and a giraffe that make the giraffe’s longer neck and elevated head possible had to have either already existed in the gene pool of the proto-giraffe or “appeared” (by mutation) in it as the proto-giraffe evolved into the giraffe. There is a limit to how long the neck can grow and the legs lengthen (remember it is not the length of the neck but the height of the skull above the ground that really matters) before all these ancillary/supporting changes also need to exist.
With longer necks and legs being more advantageous to the population, the average length of the necks and legs would increase until some point where the gene pool’s supply of long neck/leg genes is either exhausted of the taller giraffes begin to have problems with inadequate oxygen in their lings, low blood pressure, weak necks, swollen ankles, etc. The selective pressure for longer necks still exists, a longer-necked male giraffe would still be at an advantage over shorter-necked males if he had the stamina to run long enough and fast enough to escape his predators, if in swinging his neck up from drinking he didn’t pass out, if his circulatory system didn’t keep losing blood to seepage in his lower extremities. All these and other “ifs” had to be coordinated. The available variation in the genes of the length of the neck might be the limiting factor, but it might be the ability of the heart, or lung capacity, the height of the neural spines, the elasticity of the arteries/veins. All these factors are involved and the gene pool does not contain some variation involved with these. These form limiting factors, once the variation was exhausted, further selective pressure would have little effect except to maintain the change. Little effect until a mutation in any one of these genes permitted further increase until another limiting factor took effect. Then again things would halt until another advantageous mutation occurred. Evolution is not a gradual, smoothly continuous process, it will go in “fits and starts.” It can be gradual and smooth if all the needed genetic variation is already contained in the gene pool. If it isn’t, change has to await a useful mutation. Changes hits the end of available, stops, a useful mutation occurs, change resumes, uses of the new variability, stops, time passes, a useful mutation occurs, change resumes. ad infinitum. Faster, slower, irregularly, first a change in one thing, then something different. All the changes working over time (as long as the selective pressure remains) toward an end, that in looking back, appears to have been a coordinated and directed evolution to a predetermined goal. We believe (some of us anyway) that giraffes are as tall as they can be, evolution/natural selection has reached its “goal” with a 14 or 15 foot giraffe and stopped. It may be that an even longer neck is of no advantage any more in establishing dominance. Perhaps with a longer neck, a giraffe’s head would be above all the trees and there is not enough to eat. Maybe giraffes have reached the absolute physical limits for a mammal. Human basketball players can not get much taller. Our bones cannot withstand the stresses incurred by being taller, heavier, and active. Just maybe giraffes are as tall as they can get until an advantageous mutation occurs that permits them to exceed the current limits. Time will tell. Evolution hasn’t stopped.
Proto-giraffes were not trying to evolve into something new. No species is trying to evolve into a new species. Every organism is simply (well nothing is ever simple) struggling to be the species it is, just a little better than the others or, at least, no worse. By trying to maintain the status quo change happens. Evolution creates a new species.
When Darwin proposed his theory one of the problems with it that he recognized, was how a “favorable variation” (his term) could spread through a species. No one understood how “heredity” worked. It was apparent to people that children showed some traits of their parents. Others were aware that it was possible to breed animals for certain characteristics by selecting animals with the characteristics you wanted and breeding them (artificial selection). There was no knowledge of genetics or the principles of heredity.
Darwin had talked to various breeders, even doing some pigeon breeding himself. He had seen what humans had done to change various species and knew how they had done it. He had seen with his own eyes that species were changeable, variation normally existed between individuals of a species, and this variation was inheritable and by selective breeding certain traits could be emphasized.
Darwin’s critics pointed out that if a new trait, or favorable variation of an existing trait, appeared in one individual, that individual would have to mate with another who did not have the new trait. The new trait or variation would have to blend with the existing trait and it would effectively disappear. A proto-giraffe with a slightly longer neck would mate with one with an average neck, their offspring would mate have necks with a length in between that of their slightly longer-necked parent and the average-necked parent, so they argued. The offspring in turn would mate with average-necked proto-giraffe, resulting in offspring with necks even closer to average. Should any of these mate with a shorter-than-average-necked proto-giraffe the offspring would have average or even below average-length necks. Darwin had no effective answer to this, no one did. He could only say that he had seen breeds of animals developed by humans in which certain traits had been emphasized. Somehow it was possible for a new trait or favorable variation to increase in a population, become “fixed,” and bring about a significant alteration in the entire population.