Evolution 101

Saturday, February 25, 2006

Random or Nonrandom?

GordonOKC asks: Richard Dawkins is emphatic when stating evolution is "nonrandom". Can you please elaborate?

Sure, no problem. For those of you that don’t know, Richard Dawkins is a zoologist at the University of Oxford in England, serving as Professor of Public Understanding of Science. He’s best known for his books, starting with “The Selfish Gene” in 1976, which is an excellent work of popular science. I highly recommend that book and any others that he’s written, by the way. He’s also served as a vigorous critic of creationism and intelligent design, and it could be argued that he’s now one of the most prominent living critcs.

So why would Dawkins be so emphatic about evolution being nonrandom? Well, because one of the ideas about evolutionary theory that’s often promoted by creationists is that evolution is a completely random process. The creationist would say, if evolution is true, then all organisms have developed randomly. This is usually followed by some quotation of statistics, implying that it would be more likely for a 747 to assemble itself out of a junkyard than for a single protein molecule to develop with random processes. This creationist argument is not an accurate description of evolution, though- it uses what’s called a “strawman.” This means that, instead of engaging an opponents actual argument, you construct a weaker version of their argument, called a strawman, and then argue against that. The creationist argument that characterizes evolution as a completely random process is just such a strawman.

Well, if the creationist assertion that evolution is a completely random process is wrong, that explains why Dawkins would be emphatic in stating that it’s nonrandom. But that’s not the whole story. Evolution is a nonrandom process, but it’s also propelled by random mechanisms. What do I mean by this?

There are two basic forces that function within evolutionary theory. One is selection. This can be natural, as in the evolution of finch species in the Galapogos Islands, or it can be artificial, as in the evolution of animal and plant species because of human culture. Neither of these is random- the selective force of the environment is the deciding factor on whether one population will be successful or not. The other basic force is mutation. This force is random, at least to a certain extent.

Mutations are mistakes. Every organism contains DNA that is copied every time a cell divides. DNA is a strand of four different nucleotides, adenine, cytosine, thymine, and guanine. These can be written as a string of letters represented by the first letter of their name, A,C,T, and G. A strand of DNA can thus be represented by a string of those four letters, in any combination.

When a strand of DNA is copied, there can be errors. These errors can be caused by a number of factors, including radiation, certain chemicals, or viruses. Radiation, especially ultraviolet radiation, tends to affect adjacent thymine bases, so it’s not completely random, but it’s very close. But there is also a base rate of mutation that occurs randomly but at a measurable average rate, that results in one base being switched with another during copying. In humans, this rate is at about 1 mistake per 100 million base pairs every generation. This is about 175 total mutations per individual.

Now, most mutations don’t affect an organism’s ability to live and reproduce. These kinds of mutations are called silent mutations. Since they’re not selected for or against, they accumulate at a regular rate in populations. But some mutations can affect genes either positively or negatively, with the result of having a positive or negative effect on the ability of an organism to survive in its environment, what we would call an organism’s fitness.

Now here’s where the random mechanism of mutation hits the nonrandom force of selection. In any given population, individual organisms will have a wide range of mutations. A small percentage of those individuals will have mutations that decrease their fitness, and they will be selected against, with the result being that their genes are taken out of the population’s genome. Another small percentage of those individuals will have mutations that increase their fitness, and they will be selected for, with the result being that their genes are increased in the population’s genome. So over time, we see that a population will become increasingly adapted to its environment, because positive mutations are selected for and negative mutations are selected against. The random mechanism of mutation is utilized by the nonrandom force of selection to drive evolution forward.

We can see examples of this nonrandom selection in the phenomenon known as convergent evolution. Convergent evolution refers to instances where similar physical characteristics have evolved in two different organisms that do not have a close evolutionary relationship. For example, the evolution of wings in both birds and bats. The function of wings is the same in both birds and bats, but the structures of both instances of the appendage show wide differences. For example, the surface area of a bird’s wing is made up of feathers that attach to the entire length of the arm, while in a bat’s wing it is made up of membrane stretched between individual digits. In addition to these obvious structural differences, bats and birds have different ancestries. Bats are mammals, and so belong to the synapsid lineage which parted ways from the reptiles lineage, to which birds belong.

Another good example of convergent evolution can be seen within the mammal lineage itself. The first divergence in the mammal lineage was between monotremes, which are mammals that lay eggs, marsupials, which are mammals that carry their young in pouches, and placentals, which give birth to fully-formed live young. Throughout most of the world, placental mammals have become the dominant groups, but the only place where marsupials have remained was Australia, at least until it was colonized by man. Marsupial mammals evolved to fill the same niches that placental mammals did- including carnivores. The thylacine, or Tasmanian tiger or Tasmanian wolf, looked incredibly similar to the placental wolf, but it was more closely related to the kangaroo or the koala bear. But the same nonrandom selective forces of the environment that shaped the evolution of the placental wolf also shaped the evolution of the Tasmanian wolf, and so they developed very similar physical characteristics.

So, to review, evolution is both random and nonrandom. The instances of individual mutations are random events, and provide a population with a certain amount of variation. This variation is the basis on which the nonrandom selective force of the natural environment directs evolutionary change.


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