Political Cortex: Brain Food for the Body Politic

Darwin's theory grew out of an era when considerable careful observations from around the world were beginning to be formulated into careful scientific ideas. Not all ideas from this era were equally scientific, nor equally valid. Charles Darwin's theory was formulated based on a huge amount of observation both personally made by Darwin and made by correspondents he wrote to from all over the world. It took many years for Darwin to put his ideas into words and his book, Origin of Species, spends a great deal of time addressing criticisms of the theory of Evolution. When Darwin formulated his theory, the Mendelian rules of genetics were unknown, and DNA wasn't even conceived of. So, in essence, the mechanisms and rules that govern evolution were unknown. Darwin defined the patterns of how living things changed and competed, and it was only later that those mechanisms were discovered, giving the statistical and molecular context for Darwin's theory. Those later discoveries have only strengthened Darwin's theory, never contradicting his ideas.

The most fundamental basis for the theory of evolution is the very simple and very evident observation that individuals within a species vary from one another. This may seem so obvious that it seems silly to state it, but it really is the foundation of Darwin's theory and he spends an entire chapter of his book demonstrating variability within species in nature. We now know that this variation is due to genetic differences, differences in the DNA sequence, among individuals. Darwin did not know this. He simply observed that in every species he had any information on, individuals showed clear differences in appearance, in abilities, in behavior and in internal structures. Simple differences like human skin color or our differences in eyesight are examples of this. What is important about individual variation is that such variations can make an individual better or worse able to survive and produce children. Since producing children is what contributes to the next generation, differences in an organism's chances of surviving and reproducing can determine whether or not that individual organism contributes to the next generation.

Throughout his book, Darwin compares what happens in nature to the simpler situation of human breeding of domestic animals. Humans select for larger fruit size, higher milk or wool production, less aggressive bulls, etc. and that selection is based on individual differences among individual animals or plants. Over time, our selection of certain traits can make that trait predominant in the animals we have domesticated. As Darwin summarizes it:

"No one supposes that all the individuals of the same species are cast in the very same mould. These individual differences are highly important for us, as they afford materials for natural selection to accumulate, in the same manner as man can accumulate in any given direction individual differences in his domesticated productions."

In other words, in the same way that humans select for specific traits within the diversity of a population of domesticated plants or animals, natural selection will do the same thing in nature. This natural selection is little different than what humans do when they select particular traits to breed for in our show dogs, our cattle, our wheat plants, etc.

What is natural selection? Clearly a plant or animal that can withstand cold better than another, say one rabbit has a thicker coat of fur than another, it is more likely to survive a cold winter or in colder climates than a plant or animal less able to withstand cold. The thicker furred rabbit would be better adapted to the colder conditions, so would be more likely to survive and reproduce. That is natural selection. Individual traits that favor survival in a particular environment will improve the chances that the genes (to use modern terms) of that individual get passed on to the next generation. Animals that are poorly adapted die before they can produce many offspring, and hence their genes do not remain in the population. Hence a trait that is bad for an individual's survival is unlikely to survive for many generations in a population. That is natural selection: the constraints the environment puts on populations of organisms, favoring the survival and reproduction of some individuals over other individuals.

Darwin came up with this after considering Malthus' theory of population growth. Malthus postulated that populations tend to expand exponentially (geometrically, meaning 2 become 4 become 16 become 256, etc.) while food production expands linearly (2 becomes 4, becomes 8, become 16, etc.). So, Malthus states that over time, populations will expand beyond their ability to find food and hence undergo periods of severe decline due to disease and starvation.

Darwin thought about this and realized that this process of rapid population growth and crash means that there is a selective pressure on a population that favors individuals that can survive the cycles of boom and bust. He recognized that several environmental factors would affect this including competition with other species and within a species. Hence, the basic Malthusian population dynamics would create a situation whereby each individual would be in constant competition with all other individuals of all other species in a given environment for survival. Or, as Darwin put it:

"If during the long course of ages and under varying conditions of life, organic beings vary at all in the several parts of their organization, and I think his cannot be disputed; if there be, owing to the high geometrical powers of increase [reproduction] of each species, at some age, season, or year, a severe struggle for life, and this certainly cannot be disputed; then, considering the infinite complexity of existence, causing an infinite diversity in structure, constitution, and habits, to be advantageous to them, I think it would be a most extraordinary fact if no variation ever had occurred useful to each being's own welfare, in the same way as so many variations have occurred useful to man. But if variations useful to any organic being do occur, assuredly individuals thus characterized will have the best chance of being preserved in the struggle for life; and from the strong principle of inheritance they will tend to produce offspring similarly characterized. This principle of preservation, I have called, for the sake of brevity, Natural Selection."

Variation occurs, competition occurs, so variation that improves an individual's chances of surviving the competition is selected for and passed on to offspring. That is natural selection. Again, Darwin hypothesized this BEFORE anyone knew about Mendelian genetics or DNA, which are what provide the mechanism for variation and selection.

There is another layer of selection that tends to get ignored when Darwin's theories are considered, and yet Darwin considered it just as important as natural selection. That layer is sexual selection. Natural selection is the selective pressure for certain traits over others due to the fierce competition for survival among all organisms in a given environment. Sexual selection is simply the tendency for certain traits to be favored by other members of a species in a mate. These often are NOT traits that are beneficial in natural selection and can sometimes seem bizarre. Selection by peahens for peacocks with particularly large and dazzling tail feathers is actually selecting for a tail structure that makes the peacock MORE vulnerable to being killed by predators. And yet, the fact that peahens favor it is still a selective pressure for a large and dazzling tail. Many such examples of sexual selection can be seen in nature, as well as in any pick up bar in any city on earth. If a trait, for whatever reason, is favored by the opposite sex, then individuals with that trait will be more likely to reproduce and so pass on the genes for that trait. It is the COMBINATION of natural and sexual selection, acting on individuals of a species that vary from one another, that drive evolution.

As a population changes, it may come to differ from other populations of the same species. This is a well-known phenomenon whereby multiple varieties can exist of the same species. Our domestic dog, cat, cattle, etc. breeds are examples of this. Such varieties can easily be crossed back together to lose their distinct characteristics, but the tendency to have those characteristics will remain. For example, crossing two breeds of dogs will give you a mutt, but that mutt will still have some of the genes of the pure breeds and it's offspring will show similarities to the pure breeds. Varieties occur in nature, often when different populations of a species are separated by a physical barrier: a river, lake, mountain, etc. Darwin spends a great deal of time outlining the observations concerning varieties and how most of those same observations could be applied to species as well. He showed how over time the differences between isolated varieties would increase, producing sub-species and, eventually, new species. This is the final part of Darwin's theory. Variation within a species, acted on by natural and sexual selection, lead to geographical divergences within a species into varieties, then sub-species and finally species. He discusses how the differences between varieties and species can be controversial, leading one scientist to declare two populations mere varieties of one species, and another scientist to declare them two separate species. He suggests that varieties and species are merely opposite ends of one process. As I will discuss in my next essay, this is an observed fact in some cases.

Darwin argues that the way different species are distributed around the world and how they differ from each other is easily explainable by his theory of evolution, where different species branch off of a common progenitor species, as opposed to the then commonly believed theory that each species was separately and independently created. He spends many chapters simply showing how the observed distribution and structures of living things on earth today is more consistent with his theory than with the theory of independent creation of each and every species. For example:

"We have seen that the members of the same class [of organisms], independently of their habits of life, resemble each other in the general plan of their organization...What can be more curious than that the hand of a man, formed for grasping, that of a mole for digging, the leg of the horse, the paddle of the porpoise, and the wing of the bat, should all be constructed on the same pattern, and should include the same bones, in the same relative positions?

...

"Nothing can be more hopeless than to attempt to explain this similarity of pattern in members of the same class, by utility or by the doctrine of final causes...On the ordinary view of the independent creation of each being, we can only say that so it is;--that its has so pleased the Creator to construct each animal and plant.

The explanation is manifest on the theory of natural selection of successive slight modifications,--each modification bring profitable in some way to the modified form...In changes of this nature there will be little or no tendency to modify the original pattern or to transpose parts."

In other words, the similarity of the structure and pattern of the bones of human hands, horse legs, whale fins and bat wings doesn't make sense in terms of similarity of function, because these limbs are used for very different things. It seems rather sloppy of a Creator to use this same pattern for all these limb types if independent creation is believed. Independent creation would logically have each limb separately designed for its particular purpose and there is no reason for the identical pattern of bones, including cases where some bones are no longer useful but still remain as vestigial elements in the limb. However, evolution would say that they all have the same pattern because they all were formed from the limbs of a common ancestor by gradual changes over millions of years by natural selection. It is the most logical explanation for all such similarities in structure across classifications of organisms. Now that we understand genes and DNA, we find an even better fit between reality and Darwin's theory.

We now know that the origin of variation is mutation in the DNA. DNA is a molecule with, in essence, a 4-letter alphabet and "words" made up of 3 letters that tell the cell what amino acid goes where in a give protein. DNA also tells the cell under what conditions a protein should be turned on or off. Together, this is what determines what each individual organism looks like, can do and even, to a large degree, how it behaves.

Mutations occur from two sources. Both are, in essence, random. One source is the fact that the molecular machinery for replicating our DNA sometimes makes errors. Most of these errors are edited out. But sometimes the editing machinery fails and a real mutation occurs. The second source is from the environment: many chemicals (natural and artificial) as well as UV radiation cause mutations in our DNA. Again, often these mutations are repaired, but sometimes they slip through.

Mutations in DNA can be silent (have no effect), can alter the structure of a protein, or can alter the way a protein is turned on and off. When a change is made, it often impairs the function of the protein, but occasionally simply alters it in a neutral way or actually improves it. These mutations that lead to changes in the way a protein works or is regulated are the source of variations among individuals. Again, it is by and large a random process.

Natural selection gives the direction to evolution. Changes in a species due only to random mutations with no real selection is called genetic drift, and it can happen in isolated situations. But in general, species change due to natural and sexual selection, directing the changes towards an improved survival or reproductive potential. Changes that produce new varieties and new sub-species are observed all the time in domesticated animals, in nature and in the laboratory. It is the change that leads to the origin of NEW species that is harder to see, though I will show in my next essay that it is indeed observable.

The lack of precision that existed during Darwin's day in determining varieties from sub-species to species has been improved thanks to DNA. We can compare the DNA sequences of two groups and tell exactly how closely related they are. This comparison, on a smaller scale, is what we do when we do modern paternity tests or DNA fingerprinting in criminal cases. On a larger scale, this kind of analysis has shown that the wooly mammoth is very closely related to the modern Asian elephant. In essence, the Asian elephant and mammoth are sister species and both are close cousins to the African elephant. The same DNA analysis that can give us DNA fingerprinting can, with greater care, give us relationships among species including an estimate of how long it has been since those two species diverged. These kinds of relationships on the level of DNA are far harder to explain if each species was created independently than if modern species evolved from common ancestor species the way Darwin hypothesized.

So in the more than 100 years since Darwin published Origin of Species, scientific discoveries that Darwin could never have imagined have done nothing but bolster his theory of evolution. It is a theory that was tenuous, though carefully thought out and reasoned, at the time. Further observations at that time all fit well with Darwin's theory. Mendel's laws of genetics fit very well with Darwin's theory and could almost have been devised by Darwin himself since he had done some very similar experiments, but just hadn't done them as extensively as Mendel. Finally, the discovery and understanding of DNA and the entire field of molecular biology, something that Darwin could never have even imagined, fit perfectly into the theory of evolution and in many ways explain aspects of evolution that seemed mysterious to Darwin himself.

The theory of evolution has itself evolved since Darwin's time. Darwin was convinced that evolution happened only on very long time scales in very slow increments. He believed the evolution of new species as well as the extinction of species would be almost imperceptible events, needing immense time spans. What he didn't know was that evolution can get boosts, not of mutation rates, necessarily, but in the nature of competition, when mass extinctions occur due to meteor or comet impacts, supervolcanoes, or rapid climate change. Such occurrences can lead to extinctions within a very short period of time...within months or years, rather than millions of years. When that happens, the survivors face a greatly reduced population as well as a greatly altered environment, leading to rapid evolution of new species. This is called punctuated equilibrium and is a modern modification of Darwinian evolution. Some people find punctuated equilibrium hard to swallow, but we DO know that mass extinctions occur. We see it in the fossil record and we find the impact craters that correspond in time to some of those mass extinctions. Punctuated equilibrium fits these observations better than any other theory.

Despite this and other modern modifications, Darwinian evolution is one of the most robust theories in human history. New observations, in molecular and organismal and ecological biology, almost across the board fit Darwin's theory. Nothing has contradicted it. Many of the problems with his theory were pointed out by Darwin himself and most of them have either been addressed or at least reduced as problems. None of these problems have ever been shown to be fatal to any part of Darwin's basic theory. In my next essay I will address a couple of the "objections" raised to Darwin's theory. Most of the ones raised today are ones raised during Darwin's times and addressed partly by Darwin and his supporters. Many have only further been addressed by modern research. That is what I will discuss next.

All quotes are from Charles Darwin's Origin of Species, Everyman's Library 2003 edition.