Descent With Modification
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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.
Mutations are, of course, random. No one can anticipate where or how a given strand of DNA will be damaged by radiation or chemicals. A living organism is a finely tuned array of chemical processes, so mutations generally cause the organism to fail in some way. When such mutations occur in humans, we call them birth defects and theyíre almost always bad.
But almost is not the same as completely. On rare occasions a mutation makes the organism that displays it do something beneficial to itself, thereby ensuring that the mutation spreads throughout the population of that species of organism. In that way the whole species changes.
How do we know that a mutation is beneficial? Properly we must ask Beneficial relative to what? Here we encounter the process of natural selection, Natureís own breeding program.
No organism lives in a vacuum. It exists in an environment, land or water, filled with plants and animals. It lives its existence by interacting with that environment, primarily by obtaining food and to a lesser extent by avoiding becoming food for some other organism. A mutation is beneficial if it helps the organism achieve those goals or others related to surviving in its environment and reproducing itself.
To see how that works consider a simple example Ė the common potato.
Division*: Angiospermae (flowering plants)
Family: Solanaceae (nightshades)
*Division for plants is the equivalent of Phylum for animals.
As a species in Genus Solanum, the potato is related to the tomato (S. lycopersicum) and the eggplant (S. melongena). It is a perennial that produces berries as well as tubers. The berries contain the seeds, which provide for the usual gene-mixing reproduction of the plant, and the tubers enable the plant to reproduce by cloning itself. Because the potato is a member of the nightshade family, green parts of the plant Ė the leaves, the stems, the berries, and even the tubers if they are exposed to the sun Ė contain solanine, chaconine, and other glycoalkoloids, which make those plant parts toxic. And yet the tubers, if they havenít been greened, are safely edible. How did this otherwise useless plant get to be useful for us?
Potatoes as we know them originated in the Andean highlands of southern Peru and far northwestern Bolivia. From there the Quechua people spread them throughout their empire, whence they were then introduced to the rest of the world. Recent genetic studies have shown that S. tuberosum shares a common ancestor with S. brevicaule, which grows on land in western Amazonia stretching from central Peru to northern Argentina.
We can reasonably assume that the ancestor of the potato plant evolved in the tropical forest of western Amazonia. As a member of the nightshade family, that plant Ė call it S. precursor Ė was toxic and, thus, it did not get eaten by animals. That fact helped it survive in a land teeming with hungry animals. But the toxicity also extended to the plantís berries, which fact seems to have cancelled the evolutionary advantage of the plant having those little seed carriers.
When flowering plants evolved, beginning about 160 million years ago, they also evolved a unique method of spreading their seeds, thereby ensuring the survival and reproduction of their kind. A flowering plant sets its seed inside or on the surface of a skin that encloses a juicy, nutrient-rich pulp; thus, the plant produces a fruit or a berry. Attracted to the pulp, an animal eats the fruit and some time later, far from the original plant, the animal ejects the seeds in a little pile of moist fertilizer. Itís a very effective way to spread a plant to all available environments, which ensures the continuation of the species.
Bitter or toxic berries donít get eaten, so their seeds donít spread via the alimentary express. But berries tend to grow on vines, which spread along the ground, thereby putting the berries at some distance from their plantís root center. The seeds sprout where the berries fall and, thus, the plant spreads its progeny, slowly expanding its occupation of the landscape.
Potato plants donít send out vines, so the berries donít fall far from the plant. We may safely assume that S. precursor didnít send out vines, either. Instead of sending out vines, the potato (and its ancestors) sends out stolons, rhizome-like runners that grow under the soil. At their ends the stolons grow clones of the parent plant. When the flowers of those clones get pollinated, they produce seeds with the genetic variability that the plant needs to remain viable as a population. Thus, from one plant of what we have called S. precursor, we get a patch of plants, one that grows, shrinks and even, over a course of many years, slithers over the landscape in response to changing conditions.
Thereís another way in which S. precursor gets spread over the landscape. An animal wanders through a patch of the plants and steps on some of the berries. Wet seeds cling to the animalís foot and later, perhaps miles away, they fall off and germinate. Another patch of the plants grows and the process repeats. This may be the process by which S. precursor and its descendants migrated from the lowlands of Amazonia to the Andean highlands of Peru and Bolivia.
But S. precursor didnít make it to the highlands. It evolved in a relatively warm environment, so it could not survive cold winters. As the plantís offspring migrated higher into the mountains they came under gradually greater distress. Instead of the migration stopping at some point, the plant evolved and continued its migratory ascent, going where the animals took it.
The biggest and most obvious change appeared in the stolons. In S. precursor the stolons already stored a small amount of nutrients to support the initial growth of the clones that would emerge from them. Because of genetic variation, some plantsí stolons stored more than the average amount of nutrients and some plantsí stolons stored less. In an environment more stressful than the one in which they originally evolved, the plants that gave their clones a bigger boost would have the greater chance of surviving and reproducing themselves. Over time the plants with the fatter stolons would come to dominate the population in a given area.
Now mutation comes into play. Changes in the plantís DNA, that chemical card-belt in the Jacquard loom of life, change the physical manifestation of the plant. More precisely, changes in the DNA in one of the plantís seeds make the offspring plant different from its parent. Some changes, for example, would make the plant grow broader leaves than the parent did. Others would make the plant produce less solanine, making the plant less toxic than its parent. And yet others would make the plantís stolons store more nutrients to boost the plantís clones into existence.
With broader leaves, those little organic solar panels that plants evolved over half an eon ago, the plant can capture more solar energy to support the plantís life processes. If, however, the leaves overshadow each other, that benefit gets diminished. The survival of the plant comes down to a matter of balance: does the benefit derived from the change exceed the extra cost to the plant of manifesting the change? If the answer is No, then the plant has less ability to survive whatever stresses its environment imposes on it and is therefore less likely to reproduce itself: the genetic coding for the mutation eventually gets flushed out of the gene pool of this plant species. If the answer is Yes, the plant has an enhanced ability to survive in its environment and reproduce itself, so the mutation spreads throughout the gene pool over time.
A plant that produces less solanine than its parent did has reduced its cost of existence. But it also risks increased predation, especially from insects that have evolved some tolerance of solanine. Again, the survival of the plant depends on a proper balance between the cost of the mutation and the benefit. The ideal plant achieves that balance through the action of natural selection, which eliminates those plants that waste too much effort making poison and those plants that donít make enough poison. Thatís what we mean when we talk about "survival of the fittest"; the preferential survival and reproduction of the organisms that best fit their environment. Ultimately only those organisms favored by natural selection will pass on their genetic coding to descendant organisms, thereby shaping the whole species.
Because of the basic genetic variability that we find in any viable species, some members of S. precursor produced slightly fatter stolons than did others. Those plants boosted their clones into existence more quickly and more efficiently than did the plants with thinner stolons: the clones didnít rely as much on the parent plant for the means to grow. Where the plants came under thermal stress (i.e. cold) that feature gave the plant a reproductive advantage over plants that lacked it. That fact promoted mutations that made the plantís offspringís stolons even fatter.
In a given area where cold weather came in, the pudgy-stolon plants outnumbered the skinny-stolon plants. When they got pollinated and set their seeds, they were the more likely plants to get the DNA in their seeds mutated (by cosmic rays, nuclear radiation in the soil, or even by chemical insult). If the mutated seeds germinated, then natural selection, Natureís own breeding program, determined whether the mutated plants that grew from them survived long enough to reproduce their kind. Any mutation that made the plantsí stolons fatter with stored nutrients thus got preserved and spread throughout the population of plants.
Those mutated plants were able to migrate to higher altitudes in the mountains, to where the weather got even colder, and still survive and even thrive. In those circumstances the plants that fit their environment best were the ones with the fattest stolons. Subsequent mutations then produced plants that made even fatter stolons. Eventually the plants produced stolons so plump that they could produce the clones even after the parent plant had died from the cold. The stolons had become tubers, which will sprout when the weather and the soil become warm enough for the plants to survive. At that point S. precursor had become S. tuberosum and the tubers had become the first potatoes. Subsequent development of the potato then becomes a matter of botanical history.
Thus we see how the interplay between mutation and natural selection changes life forms and creates new species. In the present example only a small population of S. precursor evolved into S. tuberosum. But S. precursor continues to exist in its original environment. We have a true separation of species when pollen from one of the species will not fertilize the seeds of the other species.
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