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Here we come to the part of the theory of evolution that so many people donít understand. How, they ask, can a completely random process produce an improvement in a life form? They conceive evolution, as described by biologists, as resembling a tornado blasting through an auto junk yard near Detroit and assembling a shining new, fully functioning Rolls Royce before leaving the area. We recognize that scenario as an absurdity, so why do we continue to assert that the theory of evolution accurately portrays the history of life on Earth?
We start with the fact that every living organism begins as a single cell, from which it grows in accordance with instructions somehow encoded into a very long strand of DNA. Here we might make an analogy between the growth of an organism and the weaving of a complexly patterned tapestry on a Jacquard loom in accordance with instructions encoded on a belt of hole-punched cards.
Eventually the belts wear out and the weavers must replace them. Suppose that someone makes a mistake in the placement of the holes in the cards that make up on belt. The tapestry that comes out of the loom running on that belt will look different from the tapestries that the original belt produced. Year after year that process continues. After the elapse of many years the looms produce a tapestry that bears only slight resemblance to the original. We would say that the product has evolved.
In living beings DNA serves the function of the belts in the Jacquard loom. But DNA has a complexity and a fragility that go far beyond that of loom-guiding belts. Exposure to the high-energy radiation coming from space (in the form of cosmic rays) or from radioactive elements in the rocks in our environment can easily break the chemical bonds that hold together the parts of the DNA molecule. Normally those broken bonds reform themselves (because the pieces of the molecule donít have room to move) or get repaired by enzymes that evolved to maintain the integrity of the DNA molecule. On rare occasions, though, the bonds get repaired wrong or not at all, thereby leaving the DNA with a mutation that may affect how other molecules interact with the DNA and, thus, may affect the form taken by the creature that the DNA encodes.
Over the elapse of millenia such mutations accumulate and the population drifts further from conforming to the original stock. Either the mutations have no effect, either positive or negative, and thus evade selection or they follow changes in the environment that promote some mutations within the population and demote others.
We have, in the accumulation of mutations, a version of the paradox of the Ship of Theseus, the young man who, in Greek legend, slew the Minotaur and thereby freed the people of Athens from the barbaric overlordship of the king of Crete. According to Plutarch, "The ship wherein Theseus and the youth of Athens returned [from Crete] had thirty oars, and was preserved by the Athenians down even to the time of Demetrius Phalereus, for they took away the old planks as they decayed, putting in new and stronger timber in their place, insomuch that this ship became a standing example among the philosophers, for the logical question of things that grow; one side holding that the ship remained the same, and the other contending that it was not the same."
As the caretakers replace the parts of the ship as they decay does the ship remain the same or does it become a different ship? If we decide that the ship has not changed (after all, we canít see any difference between before and after), then what can we say if the caretakers make subtle changes in their replacements, such as using a different kind of wood or making the replacement parts with slightly different shapes and sizes, and eventually, after the elapse of some centuries, produce a ship that looks radically different from the original?
Can a population change over time and still be the same species? To answer that question we need a clear definition of a species. The one that seems to work best says that two different species cannot successfully interbreed. That statement means that if we have two populations of creatures and if a male taken from one population cannot produce viable offspring with a female taken from the other population, then the two populations represent two separate species.
In light of that definition we can see that the best way to understand how evolution works involves looking at how one species splits into two species. The Kaibab Squirrel gives us one nice example of evolution occurring due to geographic isolation.
Species: Sciurus aberti
Subspecies: Sciurus aberti kaibabensis
The Kaibab Squirrel is a tassel-eared squirrel that lives on the Kaibab Plateau in Northwestern Arizona in a habitat confined entirely to the ponderosa pine forests of the North Rim of Grand Canyon National Park and the northern section of Kaibab National Forest around the town of Jacob Lake, Arizona. In the past the Kaibab squirrel was given species status (Sciurus kaibabensis) but today naturalists consider it a subspecies of the Abert's squirrel (Sciurus aberti), which, with its several subspecies, has a much broader distribution and is found on the South Rim of the Grand Canyon, on the northern edge of the Coconino Plateau.
Both the Kaibab squirrel and the Abertís squirrel live in the ponderosa pine forests, where they build their nests out of twigs and pine needles. They eat acorns, fruit, and fungi, as well as the seeds, bark, and twigs of the trees where they make their home. The Kaibab squirrel's most significant source of food is the seeds found within ponderosa pine cones.
Originally only one species of squirrel inhabited the area. The squirrels occupied both rims of the canyon and the canyon itself; thus, whenever a mutation occurred, it could spread throughout the entire population through interbreeding. But then about 10,000 years ago, when the last ice age ended, conditions in the canyon changed in a way that no longer allowed the squirrels to get to the Colorado River, much less cross it. The canyon became a formidable barrier geographically separating the forests and squirrels on each rim. One population of squirrels had become two separate populations that could no longer interbreed.
When a mutation occurred in one population, it would spread throughout that population, but it could not reach the other. Over time different mutations accumulated in the two populations. The genetic code for the two populations became ever more different and ever more incompatible. Full incompatibility of the genetic codes means that a male from one population and a female from the other will not be able to interbreed successfully; that is, they will not be able to produce offspring. At that point the two populations have become two separate species.
But speciation doesnít happen abruptly. The production of non-viable offspring occurs on a continuum. As the genetic code of the parents becomes progressively incompatible, the processes that grow the offspring display more serious modes of failure.
In the first stage of failure the parents produce sterile offspring, such as mules. At this point the process of speciation has become irreversible.
In the second stage we see more stillbirths. The genetic code will still produce an offspring, but too many life processes go wrong for that offspring to survive.
Next more spontaneous abortions occur. The fetus begins to grow and develop, but when it reaches a certain stage of complexity the failures in properly expressing the genetic code kill the fetus and the mother expels it.
As the genetic codes of the two subspecies continue to change and the difference between them grows, spontaneous abortions come progressively earlier.
Finally there comes a failure to fertilize. The sperm simply wonít merge with the egg to produce even an unviable zygote.
Note that these stages all overlap each other, so we donít see a pure stage one, for example. In a given population of interbred animals both stillbirths and spontaneous abortions will occur, for example.
And we can actually catch speciation in the act of occurring. Mules provide the prime example of that act. Produced by the interbreeding of horses (Equus equus) and asses (Equus asinus), mules exist as an imperfect species.
We begin with a single population of equids spread across Western Asia and Eastern Europe. Those equids, ancestors of the modern horse, evolved in North America over the past 45 to 55 million years from a small multi-toed creature into the large, single-hooved (ungulate) animal of today. During the last ice age the equids crossed the land bridge between Alaska and Siberia and then spread across Asia and into Europe.
When the last ice age ended sea levels rose and the Mediterranean Basin filled with water, then the Black Sea filled up, reaching to the western end of the Caucasus Mountains. At the end of that process the equids had been divided into two populations that could not inter-mingle; thus, they began to evolve differences that would, over time, become incompatible.
As conditions in the world changed, mutation after mutation came into being and its manifestation was tested against the environment. Those animals that fit best into the environment survived and throve, while those that fit poorly did not reproduce their kind. Thus, the equids north of the Black Sea changed to range over the grasslands of the steppes and became the modern horse. South of the Black Sea, in the area we call the Middle East, the equids changed to accommodate themselves to a drier climate and became asses (donkeys).
Humans began to domesticate horses around 4000 BC, and their domestication is believed to have been widespread by 3000 BC. Likewise, people domesticated the ass (donkey), Equus africanus asinus, in the Fourth Millenium BC, likely in Egypt or Mesopotamia. Thus the stage was set for the creation of the mule.
Start with a horse:
Species: Equus ferus
Subspecies: Equus ferus caballus
Mate it with an ass (donkey):
Species: Equus africanus
Subspecies: Equus africanus asinus
And obtain a mule:
Species: Equus asinus x Equus caballus
Synonym: Equus mulus
A mule is the offspring of a jackass (a male donkey) and a mare (a female horse). Horses and donkeys are different species, with different numbers of chromosomes (Mules and Hinnies have 63 chromosomes, a mixture of the horse's 64 and the donkey's 62. The different structure and number usually prevents the chromosomes from pairing up properly and creating successful embryos, rendering most mules infertile.). Of the two hybrids between these two species, a mule is easier to obtain than a hinny (the offspring of a stallion (a male horse) and a jenny (a female donkey). All male mules and most female mules are infertile.
Note that the status of the mule as part of a failure to reproduce does not mean that it is a sickly animal. In some ways the mule is a better animal than either of its parent species. In that observation we see an example of hybrid vigor.
A mule has the size and ground-covering ability of a horse, but compared to a horse of similar size it is stronger and tends to require less food. It also inherits the even temper, patience, endurance and sure-footedness of the donkey father. Further mules exhibit a higher cognitive intelligence than do either of their parent species: they are highly intelligent and tend to be curious by nature.
So why donít horses and asses possess those superior traits? Shouldnít survival of the fittest have produced the best animals possible?
Many people miss the point that evolution does not produce perfection. In order to exist a species simply has to be good enough. True, within the species those who fit into their environment best reproduce themselves more prolifically than do those who fit more poorly, which is what we mean by survival of the fittest. But the best at something does not have to be perfect at it.
And, of course, we have the example of our own evolution from something that looked very much like a chimpanzee. Originally evolved to live in forest, our remote ancestors had to live on a progressively drier landscape, one that eventually became the African savannah. The territoriality of chimpanzees prevented the tribes living on the drying landscape from entering the surviving forest; thus, our ancestors had to adapt or go extinct. The dance of mutation and selection produced apes that could live on the savannah, the australopithecines, and then transformed those apes into humans.
Oh, the irony! The descendants of the winners in the struggle to stay in the forest now stand on the verge of extinction, pushed there by the descendants of the losers in that contest. It reminds me of one of the themes in H. G. Wellsí novel "The Time Machine", in which the descendants of the idle rich, the Eloi, live in indolent splendor in their palaces and the descendants of the working class, the hypercompetent Morlocks, use the Eloi in the same way we use turkeys. Hammered on the forge of a changing environment a species becomes something completely different.
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