Home: Scrying: For the love of species V: Selection and gourmet cuisine
Previously in this series, I briefly discussed how natural selection leads to speciation, but I didn’t go too deeply into the details. It is the details that count, however.
I learned this as a young chef, trying to impress my friends with a gourmet meal. My family had been making beef bourguignonne for generations, and I knew the stew was certain to be a hit. I called my mother to get the recipe, but she was out running errands. My little brother was still living at home at the time, and was happy to pull the recipe card out and read it to me over the phone. Unfortunately, it was just like that childhood game of “telephone”, and the ingredients became slightly scrambled in the transmission. Instead of adding 2 tablespoons of flour to thicken the broth, I added 2 cups. Needless to say, my stew didn’t make it. (If anyone is interested, I’ve posted my corrected version of the recipe here.)
DNA is simply a recipe book for life; each codon, like each line, lists an ingredient. Sometimes, when the recipe is copied, an ingredient gets changed.
So, as I found out, a small change to the recipe can make a huge difference. Instead of making the intended product, you might end up with a sticky, soupy mess. Of course, sometimes you end up with something better (like chocolate chip cookies or sandwiches.)
Natural selection works the same way. DNA, simply a recipe book for the smorgasbord of proteins in a living being, is an essential component of life. Changes in the amino acids, the ingredients in the recipe, are the basis for the variety of differences, or flavors, amongst species. (Why do I smell rosemary?) A single substitution for an amino acid is called a point mutation. Many point mutations can go unnoticed; if they happen in an intron, an unused bit of code, they will make no difference. If they happen to create a variation in a necessary protein, however, a point mutation can mean the difference between life and death.
A point mutation can be harmful, and if affecting the organism before reproduction, will not exist very long in the population. If the mutation turns out to be beneficial, even in a roundabout manner as in the case of sickle-cell anemia and malarial resistance, the mutation may persist in the population. In this manner, point mutations are a basic agent of evolutionary change, whether selected for or against by nature. If enough point mutations build up within the DNA of a portion of a population, they may become distinct enough to isolate themselves from another section of the same population. Eventually, point mutations lead to speciation.
Selection itself is classified into different forms and processes. Before I go too deeply into the details, I must reiterate this warning: The following processes doesn’t work for all organisms. These definitions apply easily to those of us that reproduce sexually, but ignores the rest. Today, I’m writing the next section of my biology assignment: Separately define species for protists, bacteria, and viruses. (I’ll probably post that one later next week.) Since they all do it a little differently, none of the following applies. Nonetheless, this is in the science books, and generally applies to eukaryotes, so here it is:
Forms of Selection
There are three types of selective forces, directional, disruptive, and stabilizing. In directional selection, as the name suggests, the entire population will evolve in one direction. One distinct phenotype offers an advantage in the changing environment, and the entire population is pressured to have it. Those which carry the trait tend to survive, while those lacking the specific allele are not as likely to reproduce. Eventually, the entire population, or most of it, will display the extreme phenotype. For instance, a disease changes the leaves of a tree from green to red. Originally, lizards living on the tree tended to be all green in order to camouflage, with a few red outliers. Directional selection will eventually make red the dominant trait on that particular tree.
Disruptive selection occurs when one trait or another is favored, but not both. Imagine a population of lizards which happens to include red lizards, green lizards, and green and red striped lizards. If the red lizards can hide from predators in the mud, and the green lizards can hide on leaves, both types of lizards will be successful. Their stripy cousins, on the other hand, are unable to hide in either the trees or the mud, standing out in either case, and are not as likely to survive. Eventually, the population will include red lizards and green lizards, but no longer an in-between.
Stabilizing selection is the opposite of disruptive selection. Rather than the intermediate form being unsuccessful, the extreme forms are selected against. If the stripy lizards in the last example happened to taste more revolting and bitter than their solid-colored cousins, they would be more likely survive. Eventually, the population would include hardly any lizards without bitter stripes.
Prezygotic and Postzygotic Selective Processes
Processes leading to speciation can be divided into two categories, prezygotic (events occurring before mating) or postzygotic (events occurring after mating.)
Examples of prezygotic processes leading to isolation can involve habitat, mechanics, time, or behavior. Habitat isolation occurs when members of a population are divided as they move into different niches in the same environment. For instance, one group climbs a tree while the other remains on the ground. Lions and tigers are a good example of this, as they once covered the same territory. They became isolated as lions remained on the open savanna and tigers kept to the jungles. Eventually, they became distinct enough from one another to be unable to produce fertile offspring.
Mechanical isolation is a technical way of saying, “the parts no longer fit.” When structural differences between two groups, for instance size, become so great that reproduction can no longer occur, the two are effectively divided from one another. Temporal isolation occurs when reproduction occurs at different times of day or during different seasons. This type of isolation divides many plant species: flowers that bloom in the night or day, or grasses maturing at different times of the year.
The final type of prezygotic isolation is behavioral. If one group shows substantial preference (or a fetish, if you prefer) for one type of behavior, they will tend to ignore the other group for potential mates. One group of birds may shake their tail in a fashion that is alluring to others, while the less talented birds are forced to mate amongst themselves. Eventually, two distinct species will occur, one that can perform an elaborate dance ritual, and another which relies on less dramatic methods of attraction.
Postzygotic isolation methods are those which occur after mating, preventing continuation of the species. The most dramatic example of this is zygote mortality. The hybrid zygote is simply too deformed to survive proper development, let alone reproduce. Postzygotic isolation also occurs if the offspring survives, but is unable to reproduce. This type is referred to as hybrid sterility.
Other types of postzygotic selective processes include gamete isolation (which occurs if the sperm is unable to fertilize the egg) or the birth of offspring with incomplete chromosomes or abnormally developed sexual organs.
Next in this series: Extinctions and the Fossil Record