by Shane L. Larson
At a conference recently a colleague and I were having dinner, and talking about the woes of the world, a not altogether uncommon topic incentivized by some good wine and an excellent tutto mare. As it turns out, my colleague is German, an excellent laser physicist, and was telling me about the German equivalent of a “silver bullet” cure for everything. The German’s call this an eierlegende wollmilchsau, which translates into “egg laying wool milk pig” — an animal that provides everything!
Such a creature is, of course, non-existent, though some animals come close. Cows are farmed for their meat, their milk, and the leather from their hides. Chickens provide eggs and buffalo style chicken wings. We don’t have a eierlegende wollmilchsau, but the contemplation of how we might get one begs a lovely question — where did all of the animals ennumerated in “Old MacDonald” come from?
Were all of the creatures seen on modern farms once wild and vicious? Did there used to be vast herds of chickens covering the plains of North America? Did black and white jersey cows used to scrape tooth and nail for survival in a world full of saber-tooth tigers and cave bears but now enjoy an idyllic farm life where humans provide them with food and water? No, none of the creatures of the farm existed long ago in their present forms. Almost all of them were created by us.
In the distant past, one of our ancestors decided that domestication of some animal species would be good. The reasoning probably went on the order of, “It’s hard work chasing this stupid water buffalo all over creation! When I finally kill it so we can eat it, I have to carry the stupid thing all the way back to camp which makes me even more tired! If I trap him between all these trees we knocked over (let’s call that a “fence”; neat idea,huh?) and throw some grass in there for it to eat, I can kill it at my leisure some Saturday morning next fall before the snow flies, and we’ll have steaks all winter!”
After doing this for a couple of years, a friend in the cave next door said, “I’m tired of chasing and catching a water buffalo with you every year. What if we catch a boy and a girl and have them make new baby buffaloes here each year? Then in the spring we can sit around and brew stump beer instead of chasing buffaloes.” That was a good idea, so they built a bigger fence and started breeding buffaloes.
Once breeding started, the nascent farmers could be selective about how they bred their captive buffaloes. They only bred animals that were large, to produce more meat, or they only bred animals with black hides because they liked the quality of the leather that resulted. Over time, selective breeding strengthens the gene that produces each of these traits, and a new species, engineered by us, is born. This process of selective breeding is known as artificial selection, and is responsible for all the domesticated plants and animals we know. It is still widely practiced today, producing seedless oranges, high yield grains, and enormous Thanksgiving turkeys.
The genetic process of selection was first understood and described by Gregor Johann Mendel. Mendel was an Austrian scientist and an Augustinian friar. He came to understand how genetic traits are passed from parents to children through experiments with the hybridization of pea plants. Between 1856 and 1863 he cultivated some 29,000 plants, carefully documenting hereditary traits and how they passed from one generation to the next. His results were not immediately appreciated by the scientific community, and languished for nearly 40 years until they were “rediscovered” in the early 1900s. Today, his work is the core of classical genetics, and explains the great success of sexual reproduction (reproduction with genetic exchange) as a strategy for maintaining genetic diversity. Mendel’s Laws can be summarized as:
- Every trait an organism has derives from heredity units, which Mendel called “factors” and modern biologists call “genes.” These factors are built from elemental pairs, with each element coming from one of the cell’s two parents. Each factor is identified as being either dominant or recessive. During the formation of sex cells, which are combined from each of the two parents, each sex cell only receives one of the elements from the pair the factors are built from. Half the hereditary information comes from one parent’s sex cells, and the other half of the hereditary information comes from the other parent’s sex cells. This is the Law of Segregation (Mendel’s First Law).
- Every trait an organism has is passed on from parents to offspring independently of other traits. If you are a cat with a brown-haired long tailed father and a white haired short tailed mother, your genetic adoption of a particular fur color is independent of your adoption of a tail length in your genetic makeup. This is the Law of Independent Assortment (Mendel’s Second Law, or the “inheritance law”).
Mendel came to understand his laws of genetic inheritance by breeding peas and looking at flower colors of the offspring, either purple or white. The flower color of pea plants, like many other traits in organisms, are inherited; they breed true from parent to offspring. Mendel’s Laws explain the outcome of that breeding. Consider eye color in humans. The genetic expression of eye color is dominated by a signature for brown eyes, but recessively expressed by a signature for blue eyes. Let’s call the element for brown eyes “B” (the dominant genetic expression) and the element for blue eyes “b” (the recessive genetic expression). If a genetic pair has any element of the brown element B, then your eyes are brown — that is what we mean by “dominant.” In order to have blue eyes, both your genetic elements must be the blue element b.
Mendel expressed this heredity in the context of a table containing two rows and two columns. The headers for the columns are the genetic makeup of your mother, and the headers for the rows are the genetic makeup of your father. The entries of the table then are all the possible mixtures of genetic information you receive from your parents together.
Let’s do an example of how this works. I have brown eyes, and my wife has blue eyes. My daughter also has blue eyes. By what genetic pathway is that possible? For my daughter to have blue eyes, she must receive a “b” factor from both of her parents, giving her bb genes. Since my wife’s eyes are already blue, that means she automatically gives one of the “b” factors because her cells provided a bb starting point. That means the other b factor came from me; but I have brown eyes! That means I must have a recessive b trait, or my genetic makeup is Bb. This is illustrated in the figure below (a Mendelian inheritance table).
As it turns out, that makes perfect sense. My father has brown eyes, and my mother has blue eyes. I must have picked up a “b” trait from my mother, and a “B” trait from my father. But what is my father’s makeup? As it turns out, my brother has blue eyes, meaning he has bb genes, and so he must have picked up a b factor from my father, meaning my father also has a recessive blue gene for eye color. AND SO ON…
There are six possible starting points: (1) Both parents have BB brown eyes. (2) Both parents have bb blue eyes. (3) One parent has BB brown eyes (dominant brown genes) and one parent has Bb brown eyes (dominant brown genes, with a recessive blue trait). (4) One parent has BB brown eyes, and one parent has bb blue eyes. (5) Both parents have Bb brown eyes (dominant brown genes, with recessive blue traits). (6) One parent has Bb brown eyes, and one parent has bb blue eyes. The six possible tables are shown below. Try to figure out what your family eye color heredity is!
This discussion started with the domestication of animals. An organism’s traits breed from their parental genetics, and the outcomes depend on the dominance of the genes. We attempt to strengthen genetic expression when we domesticate animals by altering the statistics by which certain genetic factors are expressed. Suppose my cattle population has two fur colors — dominant black fur (F) and recessive brown fur (f) — and I (erroneously) get it in my head that chocolate milk comes from brown cows. I’d like to strengthen the occurrence of brown cows in my herd. What do I do? In a normal population of my cows, there are three genetic ways to have a black cow (FF, Ff and fF) and only one way to have a brown cow (ff). If I want to increase the number of brown cows then I should breed brown boy cows (ff) with brown girl cows (ff) and the result will be brown baby cows (ff is the only outcome)! The keys to artificial selection, are enforced selection of genetic traits, and time.
If I do this long enough, eventually I will encounter an anomaly. I’ll get a white cow, or a tawny cow. If the Cosmos really loves me, I’ll get a panda cow (these are called belted galloways, http://en.wikipedia.org/wiki/Belted_Galloway)! What’s going on? If I had all ff cows to start with, how did I end up with this completely new color? The answer is genetic mutation. Nature is a prolific author in the writing of genetic codes. Most of what goes on in the nucleus of your cells is the transcription of genetic information — reading and copying the long sequence of codes that make up the genes that give you your traits. Occasionally, errors are made, either because a random error is made or because an external event, like a cosmic ray or high energy photon, causes some damage, leading to an error. Genetic errors are like spelling errors; sometimes they produce complete nonsense, but sometimes they make something new that does make sense. Consider the word firm. A spontaneous letter mutation might change the i to a b, but fbrm spells complete nonsense! But a spontaneous change of the i to a spells farm which does make sense!
Spontaneous mutations happen in genetics all the time. When they don’t make sense, nothing happens. Sometimes they do make sense, and something does happen — a kitten is born with stripes instead of spots; a baby duck has two green feathers instead of three. As a farmer, I can choose to what to do about such mutations. I could choose to try and strengthen such mutations, or I could try to quell such mutations in my breeding stock. Wheat that produces smaller yields is not planted in favor of wheat that produces double yields. Radishes with bitter after tastes are not replanted and replaced with better tasting stock. Apples that go rancid a week after picked are replaced by apples that store easily for a month or more.
What does Nature do with mutations? Nature lets the mutations run rampant. Suppose some 8 million years ago, you were a spotted ungulate (hoofed animal) living in the forests of what is today Africa. You and your mate have lots of baby ungulates, and one day one of your progeny has a slightly longer neck than you. What happens? The slightly longer neck doesn’t seem to hurt the little tike, so life goes on. She can reach higher branches than her siblings, so gets a few more of those ruffled fern leaves that you let the kids eat for a treat, but that seems to be the extent of it. Some of her offspring have long necks too, and so this becomes a trait in the population. Unbeknownst to you (the ungulates haven’t developed science yet) the savannah around your forest home is expanding, and as it does the low-lying leafy plants that your kind feeds on becomes more scarce. Those ungulates which have shorter necks starve more often, and those with longer necks survive more often. The surviving population is comprised of more long-necks, and so more of the baby ungulates are long-necks. Pretty soon, there are a lot of long-necked ungulates around. So many, in fact, that 8 million years later, they have a name. We call them “giraffes.”
This process is called “natural selection,” and it is the natural counterpart of the “artificial selection” we discussed earlier. It is Nature’s way of farming, and it is responsible for all the diversity we see in the biological world. The keys to natural selection, are random mutation of genetic traits, and time.
Will Nature ever produce an eierlegende wollmilchsau? Probably not; there doesn’t seem to be any obvious reason why an egg laying wool milk pig will have a survival advantage over an ordinary pig. But if you really wanted one, you could make one. By selecting the traits you want, and breeding them true in your population of domesticated farm animals.