The Science of Evolution

Part 2

by Ardea Skybreak

Published on June 28, 2005

The Evidence of Evolution in Action Is All Around Us

In the previous installment of this series (available online at we reflected on how significant it is that life-forms on this planet have not stood still but have actually evolved (changed) over millions and billions of years. We recalled how, for most of human history, people really had no way of understanding scientifically how all the plants and animals, including people, had come to be, so people all over the world had made up all sorts of imaginative stories about mysterious supernatural forces they imagined might be responsible for all this (these stories are the many "origin" and "creation" myths, which became part of the different religions of the world). It was not until comparatively recently (a mere 140 years ago or so) that the great naturalist Charles Darwin figured out how life had evolved through purely natural means, over hundreds of millions of years. The discovery of evolution by means of natural selection--which thousands of scientists since Darwin's time have been able to test and verify over and over again --was one of the greatest scientific discoveries of all time. Like the Copernican discovery that the earth is not in fact the center of the universe and that the earth orbits around the sun, rather than the other way around, the discovery of evolution completely revolutionized the way people think about the history of our planet and all its life-forms, including people. From that point on, the science of evolution provided a foundation upon which all modern science continues to build.

Last time we reviewed a few basics about how evolutionary change takes place, focusing particularly on explaining that fundamental mechanism of evolutionary change which is known as Darwinian natural selection (and we also briefly discussed some additional mechanisms which contribute to evolutionary change, including genetic drift and founder effects.) In later installments of this series, we will further explore the evidence concerning the really large-scale ("macro-evolutionary") changes which have taken place over the course of the past 3 1/2 billion years of life's history on this planet (such as how we really know that mammals were descended from reptilian ancestors, or that whales were descended from a four-legged terrestrial mammal, or that our own distant ancestors were also the ancestors of the modern-day chimpanzees and gorillas, our closest living relatives). We will see that evidence for many of the kinds of major evolutionary changes which took place in the past hundreds of millions of years can be found in such things as the patterns of similarities and differences between species (in both the fossil record and in living species) and in the many particularities of plants and animals which only make sense if they are in fact consequences of evolution.

In short, there is lots of concrete evidence about the large-scale evolutionary changes which marked life's history for the past 3 1/2 billion years. But evolution is not just something that happened in the past: it is a process that is happening all the time, in all living things. So this installment is going to start off by bringing out a few examples of the kind of evolutionary change that we can easily observe taking place all around us. These are the kinds of evolutionary changes that are taking place within populations and species of plants or animals and that are sometimes referred to as "microevolution," to distinguish them from what is sometimes called "macroevolution," the larger-scale patterns of evolution above the species level, involving such things as the emergence and successive branchings of whole larger taxa and lineages (groups of groups), spread out over millions and hundreds of millions of years.1

Evolution in Action Today

So can we actually see evolution in action today ? Absolutely.

As a fundamental characteristic of living matter, evolution is always going on. Many people don't realize this, because they are confused about what evolution is (and what it is not) and so they don't know what to look for. It is important to realize that, even though the "raw material" of evolutionary change can be found in the inheritable characteristics of variable individuals , overall evolutionary change is something that is best observed at the level of whole populations and whole species of plants and animals, and over many generations .

Let's just go over a few well-known examples of smaller-scale evolutionary changes which happen so quickly, relative to human lifespans, that we can actually see them happening.


Rapid evolutionary change observed in a population of moths

If you ever studied evolution in school, you probably have heard of the moth species Biston betularia,also known as the peppered moth. This particular species of moth has been studied in England over many successive generations and provides an excellent example of evolution by natural selection which can be directly observed.2

Until the middle of the 19th century, almost all B. betularia moths were very light grey in color. Up until that time, the bark of the local trees was also light in color, so when the moths rested on the tree trunks during the daytime, they tended to "blend into" the background. Because of this, local birds looking for insects to eat probably tended to miss a lot of these light-colored moths. But then an odd thing happened: with the increase in the development of industry, starting in the late 19th century, the air became increasingly polluted with black dirt and soot from the local factories; and, as a consequence of this, the bark of the local trees became much darker. This in itself was not so surprising. But what was really interesting was that the local moth populations also became much darker! Pretty soon people noticed that the light-colored moths had been almost entirely replaced by moths with black wings. What people were witnessing here was evolution in action--a classic example of the kind of commonplace evolutionary changes that happen through natural selection , in this case leading to a striking adaptation of the moth populations to their changing environment.

This is what happened:

The original population of moths was made up of varied individuals . Most of them were light-colored, but within the total population there were a few dark colored variants. Prior to industrialization, the light-colored moths greatly outnumbered the darker moths; dark-colored moths would really show up against the light bark of the trees and, on average, they would get picked off by birds more often than the better camouflaged light-colored moths. So, in that particular environment, the light-colored individuals had what biologists call " a differential reproductive advantage" relative to the dark-colored moths. This simply means that, since the light-colored moths tended not to get eaten as often as the dark-colored moths, they were, on average, better able to survive and therefore were more likely to produce descendants. Since the genetic variability for wing-color happens to be an inheritable characteristic, the surviving moths passed on this characteristic to their descendants, who passed it on to their descendants, and so on. For as long as the tree bark remained light-colored, light-colored moths had a "selective advantage" and the moth populations were almost exclusively made up of light-colored variants. There were still occasional dark variants around, but they were very rare.

This situation began to change dramatically when the environment changed and the trees became black with soot: now those very rare individuals who just happened to have dark wings (because the genetic information for black wings had not been completely "lost" from the total genetic pool of the whole population) were the ones that had a significant reproductive advantage--they were now the ones that birds most often "missed", and so they were the ones that, on average, were most often able to survive and produce descendants. In this way, over a number of generations, the numbers of dark moths increased and became a bigger and bigger proportion of the total population . It got to the point where pretty much the only moths you would have been likely to see on a given day were the ones with dark wings.

The moth population had evolved !

It is also interesting to note that in more recent years this overall evolutionary trend has apparently reversed itself in some localities: as the quality of the air has improved, and the amount of industrial soot in the air has been reduced, the tree trunks have once again become lighter in color. In step with this environmental change, the populations of moths have once again evolved, but this time back in the direction of being mainly made up of the light-colored types. This change was possible because the genetic information for light-colored wings was still present in the total genetic pool of the population: even in the days when the population was made up almost entirely of dark moths, there were always a few light-colored moths around. They were rare, but the fact that a few such individuals were still around at each generation was enough to provide the genetic basis for the proportion of light- colored individuals in the total population to begin to increase once again when the environment changed and the dark colored moths once again became more visible to the bird predators. Withevery passing generation , the dark moths on average produced fewer descendants, and the light moths on average once again produced more descendants. This classic example of natural selection is all it took for the moth population to evolve once again. [See "The Moth Flap"]

Where would we be without fruit flies?

Since evolutionary change only happens over a series of generations , it is always helpful to be able to study plants or animals which produce new generations very quickly . This is why various species of tiny fruit flies ( Drosophila ) became so famous in biology: they are easy to raise in a controlled laboratory environment, they reproduce many new generations in just a few months, and it even turns out that their DNA is relatively easy to work with. In the first half of the 20th century, scientists like the geneticist Theodosius Dobzhansky observed populations of fruit flies in the lab and in the wild. Dobzhansky conducted many experiments, such as artificially "selecting" flies for certain features like eye color, wing shape, number or position of legs, and so on; and he observed how the proportions of individuals with different features shifted over the generations under various conditions. These kinds of studies made it possible to better work out the principles of inheritance and to figure out what exactly is going on at the genetic level when populations are evolving. In many cases it was possible to closely correlate (clearly connect) the shifts taking place in the proportions of individuals in a population having certain distinct features (a certain eye color, for instance) with the changes occurring from generation to generation in the underlying gene frequencies. For example, an increase in the "proportion of flies with red eyes" in a population could be shown to be directly connected with an increase in the "frequency of the allele (gene variant) that `codes' for red eyes" in the total genetic pool of that same population.3

Darwin had figured out how natural selection works on populations of varied individuals to cause evolutionary change, and he knew that only inheritable characteristics were relevant for evolution by natural selection over successive generations. But he never understood what the source of that individual inheritable variation really was, because in those days nobody knew yet about genes and DNA or even how inheritable characteristics get transmitted from one generation to the next.4

In fact it was the combination of Darwin's theory of evolution with the advances made in the new field of genetics that made possible the "modern synthesis" developed in the first half of the 20th century--a much more complete and comprehensive understanding of the underlying dynamics of evolutionary change.

Our deepening understanding of how evolution works now allows us to do all sorts of new things, such as combining evidence derived from different sources (for example, evidence from the fossil record and evidence from a molecular analysis of the DNA of living species) to more accurately reconstruct past evolutionary pathways and to identify at what points on the evolutionary tree of life different ancestor and descendant lineages branched off from each other. Combining an understanding of the basic principles of evolution with the basic principles of genetic inheritance has also made it possible to do such things as improve treatments of many diseases; make bacteria produce medicines (such as insulin) for use by human beings; and develop new strains of domesticated plants or animals in agriculture.

None of this would be possible if evolution weren't true and if we didn't have some understanding of basic evolutionary mechanisms and principles. The whole new field of "genetic engineering" is a case in point. There is nothing "inherently evil" in the idea of human beings seeking to transform living matter for the benefit of humanity--this is, after all, what plant and animal breeders have been doing for centuries, through more traditional techniques of artificial selection, and it is a fact that human beings cannot live without in some way consuming and transforming other species of living plants and animals. However, many people are justifiably concerned that the whole new field of genetic engineering may lead to an unprecedented degree of careless and poorly thought-through genetic and evolutionary "tinkering." Left in the wrong hands (keyed to private profits, and/or pressed to serve reactionary social objectives, as was the case with the development of the ability to "tinker" with nuclear energy, for instance) advances in the field of genetic engineering could well have some very bad consequences for both human health and the health and relative stability of entire ecosystems. Genetic engineering is a subject for a whole other discussion best left for another day. But I raise it here in passing to once again make the point that, if evolution were not true (as the Creationists would like us to believe), if evolution were not a real phenomenon, then fields like genetic engineering (and both the good and bad that they are doing and can potentially do) would not even exist!

The Creationists say they "don't believe in evolution," but they certainly do live in a world where people constantly use the rules of evolution to transform their world, for better or for worse.

One species splitting into two on opposite sides of the Grand Canyon:

A bit later in this series I will be discussing at greater length what scientists now understand about the evolutionary processes whereby a whole new species of plant or animal can come into being as a modification and departure from its immediately preceding ancestors--a process known as " speciation ." Since my purpose in this installment is mainly to give readers a bit of a feel for the kinds of evolutionary change which are constantly going on and which we can see happening all around us, let me give just one example of a case where two populations have actually been observed (over a number of generations) as they have begun to diverge into separate species.

Any time you have a situation where different populations belonging to the same species become reproductively isolated from each other (for many generations), there is the potential for speciation to take place. In everyday life, complete speciation doesn't take place all that often, because what often happens is that two populations which have been reproductively cut off from each other (by some kind of physical barrier, for example) often end up getting back together after a while (for instance, if the barrier disappears or is removed) and so they once again begin to mate with each other and mix up their genetic material in one single big genetic pool, which prevents the speciation process from going forward. But sometimes different populations of one species remain reproductively isolated long enough that speciation can actually be completed (to the point where individuals of the two populations would not be able to successfully mate and reproduce any more, even if they once again shared the same locality).

The case of the Kaibab and Abert's squirrels that live in the area of the Grand Canyon shows what happens when one species begins to diverge (split) into two. In the fairly recent past, the ancestors of these two types of squirrels were one single species , made up of individuals that all looked pretty much the same. But in one area, two populations of these squirrels found themselves separated from each other, just because they happened to end up on opposite sides of a significant physical barrier--the Grand Canyon! Because they are not able to easily cross the Canyon, individuals in the two populations have become effectively reproductively isolated : they are unable to get together to mate and produce offspring. And because of this reproductive isolation, the two populations no longer share the same total pool of genetic information. Each population is missing some of the genetic information available in the other, which would have been available in the total genetic pool of the larger population of the original species. To use an analogy, it's a little bit as if each of the smaller populations had accidentally ended up with only a part of the alphabet (rather than the whole alphabet), and on top of that the two populations had ended up with different parts of the whole alphabet. So not only is each population "missing" some genetic information, but each is also "reshuffling" the overall genetic information available to it (its part of the alphabet) somewhat differently at each generation. This has important consequences: As each reproductively isolated population produces generation after generation without the ability to tap into the gene pool of the other population,it begins to accumulate different kinds and degrees of changes in its total genetic variability.

This is exactly what happened with the two populations of Grand Canyon squirrels: They are all still recognizable as squirrels, and they still have many features in common (they eat the same kinds of food, for instance), but the proportions of certain gene frequencies in each population has changed. The normal processes of random mutation and genetic recombination (which take place whenever organisms reproduce) have sorted themselves out somewhat differently in the two populations, which is what you'd expect if they weren't working with the same total gene pool to begin with. As a result, the two populations have been diverging into two different species and they are becoming increasingly different in appearance.5

For instance, the Abert's squirrels, which live only on the South Rim of the Grand Canyon, all have a grey body, a reddish back, and a dark tail. The Kaibab squirrels, which live only on the North Rim, all the way across on the other side of the Canyon, are grey with a white tail.

As long as they are unable to regularly interbreed and "mix up" their total genetic variation into one larger genetic pool, the two reproductively isolated populations will continue to accumulate different changes. They have already become different in appearance, and it is quite possible that as they continue to evolve separately in their different localities they will eventually exhibit even more significant differences, including in certain behaviors and ways of interacting with their environments (they might begin to eat different kinds of foods, for instance), simply due to the fact that the automatic reshuffling and sorting out of genetic variability which occurs from one generation to the next is now taking place on a somewhat different genetic basis in the two populations. And these differences will keep getting accentuated the longer the two populations remain reproductively separate .6

Through the normal processes of random mutation and genetic reshuffling occurring at each new generation, whole new characteristics which have never before appeared in either population will keep showing up, but they will be different in the two populations and will therefore contribute to pulling them further and further apart.7

The Kaibab and Abert's squirrels' divergence into two separate species over a number of successive generations is yet another example of evolution taking place in the world around us-- a change that people can actually go out and directly observe taking place over a reasonable time- span. We may not have been able to be there when the first terrestrial vertebrates (ancestors of today's frogs and salamanders) evolved from those odd fishes which had evolved a primitive air breathing "lung" some 400 million years ago; or when the first birds evolved from the feathered relatives of the dinosaurs some 200 million years ago; but that doesn't mean we can't witness the frequent emergence of genuine evolutionary novelties on a smaller scale--and even the first steps in the occasional emergence of whole new species--taking place all around us.

The really major evolutionary transitions in the history of life were generally built up step- wise, and were in any case spread out over millions or hundreds of millions of years. But the basic underlying principles and mechanisms involved would have included much of the same kinds of processes and changes we see taking place on a smaller scale in the world around us every day.

Just don't expect to ever see an individual plant or animal "instantly" transform itself before your eyes, or one species instantly turn into another species, as if by some kind of magic trick--that never happens, and that's not what evolution is about.

We know, for instance, that the ancestor of modern whales was actually a four-legged ungulate (a type of hoofed mammal) that lived on land, not in the ocean. We know this both from the fossil record and from the evidence of the degree of genetic relatedness of whales to certain lines of terrestrial mammals even today. But we also know that this whole process took place through a whole series of transitional species , each one representing an evolutionary "modification" of its immediately preceding ancestor. The Creationists like to claim that there are no "intermediate" fossils in the fossil record and that this shows that evolution by "descent with modification" is just something that evolutionists made up. Well, actually, there are quite a few good intermediate fossils (and we'll come back to that whole question in a later installment of this series). A case in point is the series of fossils of related species which link that four-legged ungulate ancestor to its later whale descendants through a dozen or so transitional species. These fossils reveal what in this case is a pretty clear-cut sequence of step-wise modifications, from a body shape more suited to life on land to one suited for life in the sea--all taking place over a period of 25 million years or so.8 


Evolution in action--the distribution of sickle cell genes in human populations

Another well-known example of evolution which can be directly observed in the world around us right now is the evolution of sickle-cell hemoglobin. Hemoglobin is a blood protein which helps blood carry oxygen to the lungs. There are two different variants (slightly different varieties) of the hemoglobin gene in humans. Let's call them the A allele and the S allele (an allele is just an alternate form of a gene). A human being inherits one hemoglobin allele from each parent, so each individual will have either the AA, AS, or SS combination of hemoglobin alleles. People born with the SS combination (who have 2 copies of the S or "sickle cell" allele) are in trouble, because this combination causes sickle cell disease, a serious form of anemia which can be lethal. This is a public health issue that needs special attention because populations of Africans and people of African descent (including African- Americans) have a higher proportion of sickle cell alleles than other populations, and so it is important to pay extra attention to screening for sickle cell and providing treatment in populations of African ancestry in particular.

What could possibly explain this evolutionary quirk which makes people of African descent more susceptible to sickle cell disease and causes them such extra grief and hardship? Is it that Black people are inherently more "unhealthy" than Europeans? Is it that Black people have been singled out by God for special punishment? Not at all. Evolution supplies a clear-cut and simple answer: people who are heterozygotes for sickle cell (which simply means that they are born with the AS combination, having just one of the sickle cell alleles) actually have an advantage (a health benefit) in those parts of the world where another serious disease--malaria--is commonplace (as it is in large parts of Africa). It turns out that having just one copy of the sickle cell gene actually protects you against malaria ! It is for this reason that the allele for sickle cell would have been preserved in the course of the evolutionary history of human beings, rather than eliminated by natural selection. In parts of the world where there is a lot of malaria, individuals born with one copy of the sickle cell allele would have had a better chance of surviving long enough to bear children than people who didn't have any sickle cell alleles at all and who might die from malaria. The survivors with the one sickle cell gene would have passed it on to their children, who would have passed it on to their children...and each individual who inherited just one sickle cell allele would have had a "selective advantage" in surviving malaria. Sadly, a certain number of people in each generation would have been unlucky enough to inherit two copies of the sickle cell gene (one from each parent) and these individuals would likely get severe sickle cell disease and die without leaving any children at all. If a god were responsible for this suffering and death it would be an outrage! Why would a god give people malaria or sickle cell disease in the first place? And why would an all-powerful Creator have to come up with such a convoluted and imperfect scheme to protect people against malaria? Surely an all-powerful god could do better than that.

Natural selection, however, does not produce "perfect" solutions. It is a "blind" natural process, which makes no judgments and takes place automatically, without the involvement of any outside consciousness or the guiding hand of any "intelligent designer." Evolution by natural selection is neither inherently "good" nor inherently "evil." It just is . If the only thing the allele for sickle cell did was cause a fatal disease, natural selection would, most likely, have eliminated it entirely from human populations at some point. This is because, if sickle cell genes had no beneficial side effects at all and only caused disease, individuals having no sickle cell alleles at all would likely, on average, produce more descendants than people with even one such allele. But because one single sickle cell allele, in combination with a non-sickle cell allele, actually allows people in areas where malaria is a problem to be protected from that disease (and therefore likely live longer and have a better chance of leaving healthy descendants), natural selection will tend to "conserve" the sickle cell allele from generation to generation in human populations who live in those regions. And this will happen even though this will be at the cost of a number of individuals who end up inheriting two copies of the gene and getting that severe sickle cell disease. Again, natural selection is not making any judgments, and it is not consciously favoring some humans over others: it is simply an unconscious process which sorts out the relative proportions of different inheritable alleles in a population of varied individuals, as a simple by-product (nothing more) of how many descendants different individuals are ultimately able to contribute to the next generation.

Think about this: if you didn't understand this, if you didn't understand how evolution actually works, you might mistakenly believe that Black people were somehow being mysteriously singled out for some kind of special punishment since people of African descent tend to get more sickle cell disease than Europeans, for instance. This is just one more example of why it's so important for everyone to: a) understand how we know for sure evolution is a fact of life, b) learn the basics of how evolution works, and c) strongly oppose any religious fundamentalists who try to keep people from learning these facts.


A few words on Creationists

Despite all the evidence to support evolution, Creationists continue to deny life has evolved. Fundamentalist Christians like Jerry Falwell attend White House dinners and counsel Presidents,

at least two of the Justices on the current Supreme Court would, if it were just up to them, have imposed the teaching of anti-evolution Creationism in science classrooms, and when the current President was asked if he believed in evolution he supposedly answered that "the verdict is still out"!!


If Creationists ran the university science departments, research institutions and hospital wards, what chance would we have of effectively countering the evolution of drug-resistance in bacteria and viruses?

If Creationists ran agricultural research stations, what chance would we have of counteracting the evolution of pesticide- resistant "super-pests" which can severely limit and disrupt the production of food?

If Creationists had the power and authority to dictate policy and curtail the work of evolution-trained ecologists, population geneticists and conservationists, what hope would we have of being able to reduce and prevent critical animal and plant extinctions, of recognizing the essential value of biological diversity, and of being able to conserve critical ecosystems?

Creationists have a reactionary social and political agenda which requires them to attempt by every means possible to replace the scientific teaching of evolution with the unscientific teaching of Biblical creationism. So think about the implications of this the next time you hear people at your local public school arguing that "creation-science" should perhaps be given " equal time " with evolution and taught in the science classrooms; or the next time you open your kid's biology textbook only to discover that the editors have inserted a "disclaimer" stating that evolution is "just one of a number of competing theories" and students are therefore free to believe in anti- evolution creationism if they prefer; or the next time an entire state like Kansas, or now Ohio, becomes a backward laughing stock by actually debating whether state laws should be passed to order schools to teach creationism; and think about this the next time this battle goes all the way up to the Supreme Court, and maybe they can scrounge up a few more reactionary and ignorant Justices to dissent from all reason and actually order the teaching of creationism.

All ideas are not equally true : some ideas much more closely correspond to the way things really are in the world than other ideas. Promoting ideas which have long been proven to be false (such as creationism, in opposition to evolution) has genuine harmful social implications in the real world.

So no, it's not OK to teach creationism as if it were an acceptable "alternate" scientific theory, "just so all points of view can be heard." Not when the truth of evolution is so well established.

So please, think about it . Think about it the next time someone you know is tempted to say things like "maybe things would be better if kids were exposed to more religion in the schools." Or "maybe it wouldn't do any harm to give Biblical creation stories equal time with the theory of evolution in the science classrooms--the kids should be able to decide, right?" Wrong! Should the flat-earth theory be given equal time? Should science teachers be forced to teach it as a valid scientific theory? Should children be "free to decide" whether the earth is flat or round?

Think about it.


Future installments of this series will discuss such things as adaptation and co-evolution between species; more on what we know about the process of speciation (how whole new species of plants or animals come into being) and where we can look to find trace evidence of the macroevolutionary changes which took place over millions of years; how the different categories of evidence from evolution combine to provide strong proof of evolution, but are completely inconsistent with the idea of a super-natural Creator; what we know about human evolution and our relationship to the apes; more on some of the absurd arguments and unscientific methods of the Creationists; How Intelligent Design theories are still just Creationism and just an updated rehash of old unscientific ideas long proven to be wrong.



1.    It is important to realize that there is lots of concrete evidence for both microevolutionary changes and macroevolutionary changes. In addition, there is not some kind of absolute division between the kind of evolution which takes place at or below the species level and the kind of evolution which takes place at the level of whole larger lineages of species. Many of the same evolutionary processes are involved in both, including natural selection operating on populations of variable individuals. Macroevolutionary and microevolutionary changes are features of different levels and different time scales. We are able to indirectly reconstruct many aspects of the macroevolutionary changes of the distant past by studying the trace evidence of those changes which are preserved in such things as fossils, or the patterns of distribution and degrees of similarities and differences between living species; we are often able to gather much more direct evidence of microevolutionary changes as they are taking place, since they are going on all the time in populations of present-day plants and animals, within more limited time spans which often make possible direct human observation.

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2.    Moths are similar in many ways to their close relatives the butterflies, but unlike butterflies they are active mainly at night.

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3.    It is important to realize that particular genes, contained inside individuals, are not in and of themselves direct targets of selection--it is whole individuals that do or do not successfully reproduce, contributing offspring with varied features to the next generation. And it is these whole individuals (each with a complex mix of features which cannot be simply reduced to the individual's underlying genetic make-up) who are targets of the "sorting out process" of selection. In a given environment, those individuals who end up contributing more descendants to the next generation will obviously end up having contributed more genes overall to the next generations than those individuals who did not produce as many descendants. But the underlying genes themselves do not directly "cause" evolutionary shifts in populations; these shifts occur when the proportion in a population of whole individuals with certain characteristics changes (through selection and related factors), and these shifts are then reflected in changes in certain gene frequencies (some will increase, some will decrease) in the total population. This, in turn, will affect the total amount of genetic variability available as raw material for ongoing evolutionary changes.

Keep in mind also that it is not often, in any species, that a single gene (such as the allele that codes for red eyes in fruit flies) can be so neatly tied to a single characteristic. Geneticists have known for some time now that most of the characteristics of individuals that are inheritable (the only ones relevant for evolution) are affected by a number of different interacting genes, acting in concert in some complex and not well understood ways; in addition, most genes have influences and effects on more than one characteristic at the level of the whole individual organism. Furthermore, many of the characteristics of whole organisms are shaped by complex interactions of these whole organisms with their outside environments and therefore cannot be reduced to the effects of underlying genes. (It is worth remembering that genes are just sections of DNA that make different kinds of proteins in the body). Nevertheless, it is the case that the underlying genetic variability that exists within any population of plants or animals (and which codes for those features which can be passed on to descendants) serves as the raw material through which much overall evolutionary change is realized.

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4.    It would take the discovery of the basic principles of inheritance (beginning with the work of the 19th century monk Gregor Mendel, and qualitatively extended in the first half of the 20th century), plus the discovery of the structure of genes and DNA, to fill in the missing pieces of the puzzle.

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5.    Organisms are said to belong to two different species if they cannot interbreed and produce offspring which can in turn survive and reproduce.

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6.    There are a number of different factors which can serve to keep two populations reproductively isolated (separated) from each other besides physical barriers in the environment. If full speciation (separation into two different species) has taken place, individuals of the two populations will no longer be able to mate with each other and produce new generations even if they somehow end up sharing the same environment again at some later time. For instance, if two even closely related species have remained reproductively isolated longenough , their different behaviors and appearance might cause them not to recognize each other as potential mates; or they might no longer be active at the same time of day, or reproduce at the same time of year; or they might be able to mate, but DNA incompatibility might cause them to produce only unhealthy or sterile offspring. All of these kinds of factors can ensure ongoing reproductive isolation even if members of the two species end up co-existing in the same locality.

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7.    Many people are very interested in understanding better under what conditions true evolutionary "novelties"-- characteristics which have never before existed--are most likely to emerge. There is evidence to suggest that such brand-new features are especially likely to appear in populations of plants or animals that are particularly small . It has been shown experimentally (including, once again, in fruit flies!) that smaller populations tend to accumulate novel features more readily than larger ones. It may be that the overall reduction in total genetic variability (that is, the relative "genetic impoverishment") of small isolated populations serves to "relax" some of the developmentally constraining (limiting) factors associated with a more diverse overall genetic mix (as would be present in larger populations), and this may more readily allow radical evolutionary departures from the relatively more static (more unchanging) features which often seem to characterize larger populations or groups of populations. This is a phenomenon which has been explored in particular by the biologist Ernst Mayr, who has contributed perhaps more than anyone to the modern understanding of speciation. The relative ability of smaller populations to depart from the conditions of relative stasis (relative "steady-state") of larger populations of the same species is a phenomenon occurring at the population level, but it would seem to interpenetrate in some interesting ways with models of punctuated equilibria proposed by Stephen Jay Gould and Niles Eldredge to explain the patterns of larger scale evolution at the level of higher taxa (larger plant or animal lineages). We will return to this point later in this series.

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8.    It should perhaps be noted here that even the Gould and Eldredge theory of "punctuated equilibria"--which proposes that many of the major evolutionary transitions and periods of major evolutionary diversification in the history of life may actually have been realized through relatively rapid and concentrated bursts of evolutionary change--still recognizes that it is only in a relative sense that such changes could be characterized as taking place in a short period of time, or in an accelerated way: these changes would still have been spread out over what-- from a human perspective--would have been almost inconceivably long periods of time! There is also general agreement about the fact that, regardless of whatever other factors might have (to a greater or lesser degree) affected the overall tempo and pacing of these major changes, the well-known mechanisms of basic Darwinian evolution at the population and species levels would have been part of the "mix."

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The original article was appeared in English and Spanish on Revolutionary Worker Online