by Ardea Skybreak
Revolutionary Worker #1163, August 18, 2002, posted at http://rwor.org
The diversity of life-forms on this planet is practically mind-boggling: species of plants and animals previously unknown to science are still being discovered (especially in the canopy of rain forest trees and in the deepest layers of the oceans), and rough estimates place the total number of species on earth as somewhere in the order of 10 million! And that's just the living species: the vast majority of the species which lived in earlier geological eras are now extinct. In fact, it appears that more than 90% of all the species that ever lived on this planet are no longer with us.
But why have there been so many different kinds of species in the past history of this planet? And why are there so many different species of plants, animals, bacteria and fungi inhabiting the earth today? In other words, why is life so diversified ? The basic answer to these questions is that the increasing diversification of life is a simple by-product of the evolution of life. The fact that life on this planet is so diverse has nothing to do with any kind of god or other supernatural force; it is not necessary to imagine some kind of out-of-control or perpetually dissatisfied Creator who just can't seem to stop "designing" more and more new creatures even though this planet already holds something like 10,000 different species of birds and at least 27,000 different species of fish, for instance! We don't need to invoke such silliness, because the science of evolution provides a much more sensible explanation for why there is so much diversity. There is plenty of both direct and indirect evidence that all this diversity is due to some fairly straightforward natural phenomena, taking place over very long periods of time.
As we will be discussing in this installment, the diversity of species on this planet has everything to do with the fact that: 1) all populations of living things are constantly evolving (accumulating inheritable changes and modifications over the generations); 2) all populations are constantly evolving in interaction with the local environments they occupy, which are themselves always changing and presenting species with new evolutionary pressures and "challenges"; and 3) every now and then the quantity or quality of evolutionary modifications which have accumulated over generations in one single population of a more wide-ranging species are significant enough (in terms of resulting in sufficiently important changes in the anatomy, development, or behaviors which characterize the individuals of that population) that this population "splinters off" ("splits," "diverges," "buds off") from the rest of its "parent" species and actually becomes a brand new separate species. And since evolutionary change never stops happening, this new species is likely at some point to splinter off yet again into one or more even more different "daughter" species of its own.
This same basic process has been repeating itself over and over again for hundreds of millions of years, with new features appearing at each juncture where a new species evolves. And the longer this process goes on, the more the descendant species will become different from their more remote ancestors, even while they retain some of the features of those ancestors.
As we discussed in previous installments of this series, there is absolutely no doubt that all the different species of life on earth originated as evolutionary modifications of pre-existing ancestor species.This means that all living species are related to each other, through a succession of shared ancestors, going all the way back to those very first primitive life-forms which emerged out of earth's chemical "soup" some three and a half billion years ago. And this means that our own human species is related, by different degrees of kinship, to all other species on the planet , including the pigeon on the sidewalk, the maple tree in the park, or the neighborhood dog--we are all the product of a long process of accumulating evolutionary modifications from common ancestor species, though obviously evolution caused life to diversify in many different directions and along many different pathways at different junctures in the three and a half billion years of our collective history!
As we will discuss more in another installment of this series, tracing our own human ancestry would be like going back in a time-machine. It would be like following a line back through many major evolutionary splits and junctures, through a succession of many different kinds of ancestors, who were also the shared ancestors of whole other evolutionary lines. First, we would go back in time through a series of different hominid (human-like) ancestors, who diverged more and more from our more ape-like ancestors of 4 to 10 million years ago, thanks to the evolution of upright-walking, increasingly large brains and the capacity for advanced language and social coordination. Our close cousins the modern chimpanzees and gorillas evolved out of the same ancient ancestor species, but followed significantly different evolutionary paths.
Going back further still, through many more evolutionary splits and junctures and a great many different species, we would trace our ancestry back to the very first mammals (warm-blooded animals that evolved the capacity to give birth to live young nurtured with milk from mammary glands). Keep going back further still through our "family album" and you eventually get back to the very first reptiles (whose tougher skin and amniotic eggs kept them from drying out away from water and allowed them to colonize land without having to periodically return to water like their amphibian ancestors). Follow things back to the first amphibians (ancestors of modern frogs, toads and salamanders) who evolved out of more fish-like ancestors, and had the capacity, for the first time in the history of life on earth, to crawl out of the water on stumpy legs and breathe air into lungs. Keep going back in time, through the ancestors of the first bony fishes, to the very first marine vertebrates (the first animals to evolve a backbone), and back further still to the first marine invertebrates (which lacked a backbone but had evolved the first complex multicellular bodies), and eventually you'll arrive all the way back to our very oldest ancestors, the bacteria-like creatures which were the first to evolve the capacity to surround little packets of DNA molecules with some kind of cell wall or membrane and which had themselves evolved out of some self-assembling protein strands present in earth's early "chemical soup."
Keep in mind that the human species is not some kind of ultimate end-point or uppermost culmination of the whole three and a half billion years of life's evolution. We do have many very special qualities which distinguish us from all other species, including our unprecedented capacity to consciously transform our external world and to transmit accumulated knowledge through non-genetic cultural evolution. But there are other evolutionary lines which have been successful at maintaining themselves over hundreds of millions of years, such as the bacteria, which are quantitatively the best represented organisms on the planet! As for the many evolutionary lines which went off in completely different directions, it is clear that there are many which ultimately resulted in evolutionary dead-ends, but there are also many others which continued to repeatedly branch off into a succession of successful life-forms, many of which persist on our planet today.
[See "Were We Bound To Evolve? --
The Role of Random and Non-Random Factors in Evolution"]
So the human species is just one of the millions of current-day species that happens to sit at the current tip of one of the "twigs" of the branching bush of life. The many other evolutionary branches and twigs resulted in the many other modern-day species. For instance, in addition to the 27,000 different species of fish and the 10,000 different species of birds, there are 4,000 different species of amphibians, more than 7,000 different species of reptiles, and close to 5,000 different species of mammals. And there are also millions of additional living species, including among the flowering and non-flowering plants, the bacteria and other microorganisms, the shellfish, the insects, etc. In fact the largest single group of animals on earth today are the beetles (the sub-group of the insects which includes such familiar species as cockroaches and ladybugs): there are about half a million different species of beetles on the planet today!1
How Do We Know Something Really Happened
If We Weren't There To See It?
As we will review further in a later installment of this series, there are actually many different "categories of evidence" which combine to confirm the basic picture of how life has evolved on this planet. This includes evidence from the fossil record (which shows a series of successive changes in plant or animal lines spread out over millions of years) and evidence from molecular biology (which backs up the fossil record and which reveals the degree to which different evolutionary lines are more closely or distantly related according to the degrees of similarity or difference of their DNA). Such mutually reinforcing categories of evidence are reinforced further still by evidence stemming from developmental biology and embryology, and even from the patterns of distribution of species around the globe. All these different kinds of evidence, taken together, leave absolutely zero doubt that all living species on this planet are related to each other and are the products of descent with modification from a series of common ancestors .
The Creationists often like to complain that "nobody was around" to "see" evolution taking place over hundreds of millions of years, so it's just an unproven story. But the fact that we weren't around to witness past events doesn't mean they didn't take place, and it doesn't mean we don't have ways of figuring out what happened. Think about it: evolutionists figure out how life-forms repeatedly changed and diversified over time; astronomers and cosmologists figure out how galaxies and solar systems came into being billions of years ago and how they too change over time; historians and anthropologists figure out how humans organized their societies thousands or tens of thousands of years ago; molecular scientists and particle physicists figure out the characteristics of chemical bonds and the interactions of sub- atomic particles they cannot directly "see"; linguists figure out how current human languages evolved through a series of step-wise cultural modifications of much older languages which were spoken by people who have long ceased to exist. We weren't there to directly "see" any of these changes happening, but we do have techniques which we can use to figure out a lot of what happened in the past.
In all these so-called "historical sciences," there are scientific methods which make it possible for us to uncover the left-over marks of the past--those things which "carry over" through time and which are still present in modern-day systems and entities. Things such as the anatomical similarities of body structures which link a modern species to an ancient fossil ancestor, or things such as the similarities in grammar and vocabulary of the French, Spanish and Italian languages which mark them as closely related to each other and all derived from their ancient Latin ancestor language. All the historical sciences contribute to building up human knowledge through investigations of such historical ties and through the process of historical inference, deducing from the actual concrete evidence comprehensive theories which have the power to consistently explain a number of different related processes and phenomena. Historical scientists come to achieve great confidence in their theories (and widespread consensus, as is the case with the theory of evolution) whenever they detect clear patterns of consilience of evidence (which simply means that lots of different streams of evidence, coming from many different directions, all point to the same conclusions and reinforce our understanding of something, such as when the molecular evidence and the evidence from the fossil record both agree on when two evolutionary lines diverged in the past).
Furthermore, in the historical sciences (including the science of evolution) scientific methods are used to make predictions which can then be tested:predictions both about what we should be able to find, and also predictions about what we should not be able to find, if a particular theory about the past is true. And scientists do actually go out and test these predictions in the real world. As just one example, we can predict that if the theory of evolution is true, we should be able to find step-wise progressions of certain anatomical modifications in series of fossils arranged by age (and we do find this); and we can also predict that if the theory of evolution is true we should not be able to find something like a human fossil embedded in a rock layer containing dinosaurs, since everything we understand about how evolution works tells us that humans evolved long after the dinosaurs had become extinct (and in fact fossils of dinosaurs and of human ancestors are never found in the same rock layers). So, unlike "religious beliefs," scientific predictions (including predictions made about the processes involved in evolution) are actually testable and verifiable.This, probably more than anything else, is why there is such a strong consensus among scientists the world over concerning the basic facts and principles of evolution.
Like any good scientific theory, the theory of evolution is "falsifiable"--which simply means that it is possible to conceive of any number of different ways it could be proven to be false (and therefore rejected and discarded) if certain kinds of evidence (evidence fundamentally incompatible with the theory) were ever found. Any scientist will tell you that it is easy to make a list of things which, if they were ever found --whether in the fossil record, in the DNA of organisms, in the anatomy and patterns of development of living plants and animals, or even in the patterns of distribution of species on earth--would leave scientists no choice but to reject the theory of evolution as false. But in the 140 years since Darwin first proposed the basic theory of evolution, there have been innumerable scientific studies and experiments which have supported the theory of evolution, but there has never been a single shred of concrete evidence, in any field, which could cause scientists to doubt or call into question the basic facts and fundamental principles of evolution. Not one. No wonder so many scientists consider that the evolution is one of the "very best supported theories in all of science!"
Speciation and the Diversification of Life Are Rooted in the Same Basic Phenomena, Including Natural Selection
The science of evolution has allowed us to understand that the process of speciation (or how brand new species evolve out of pre-existing ancestor species) is very interconnected with the question of why there ended up being so many different kinds of species on this planet. In fact (as we will see), understanding speciation is key to understanding diversification:understanding how and why new "daughter" species can emerge out of a "parent" species (speciation) also allows us to better understand what "drove" life-forms to further subdivide (frequently and repeatedly!) into so many different plant and animal species. The science of evolution can answer, for instance, such intriguing questions as why, since bacteria have proven so successful at maintaining themselves on this planet, life didn't simply "stick" with bacteria.
Living Things Can't Help But Evolve
First of all, it is important to remember that very crucial "fact of life" we discussed in previous installments of this series--that a fundamental characteristic of all living populations of organisms is that they continually evolve (change). They do so automatically, as part and parcel of the basic process of living--unconsciously, and without any supernatural force or direction needing to be involved in any way. In previous installments we gave a number of examples of how we can actually directly observe evolutionary change taking place over a number of generations in living populations of plants and animals, both in the lab and in the wild. We saw that all that is needed for evolutionary change to take place is for the following ingredients to be present: a reproducing population (reproductive community) made up of a collection of varied individuals;and a mechanism ( such as inheritance of DNA-based genetic variation ) for at least some of that individual variation to be transmitted (passed on) to succeeding generations (descendants). Anytime you have those ingredients, evolution will automatically take place--because at each successive generation, those individuals in a given local environment which have inherited from their parents certain features which happen to give them a reproductive "edge" (relative to individuals who don't have those features) will on average be able to contribute more descendants to the next generation, who will in turn contribute more descendants to the next generation, and so on and so forth. In this way, those particular features will tend to spread throughout a whole population. This is what it means to say that a population has undergone evolutionary change.
This is evolution "by natural selection" and, as we discussed previously, it happens all around us, all the time. Keep in mind that evolutionary change does not happen in individuals, and it never happens "instantly": it only happens in populations (made up of lots of varied individuals), and it only happens over many successive generations. The fact that natural populations evolve by natural selection is an extremely well-documented fact in science (see examples in previous installments of this series, and see the accompanying box on the evolution of pesticide resistance in insects for yet another example of how we can tell that living populations are always evolving).
[See "Evolution of Pesticide Resistance in a Population of Insects"]
By now, scientists have collected so much concrete evidence of evolution that most people who are at all familiar with basic scientific understanding and principles accept as proven fact the kind of evolution by natural selection that can be commonly seen to be taking place among populations of any living plant or animal species. In fact, the evidence is so strong that even the Catholic Pope recently recognized evolution to be true. And even some of the so-called "scientific Creationists" aligned with Christian fundamentalism (who are definitely religious but not at all scientific!) have at times been willing to recognize that relatively minor evolutionary changes can take place within species. But they still argue that everyone should just accept on what amounts to blind faith that--at least "in the beginning"--god created all the initial different "kinds" of plants and animals separately,because that is what it says in the Bible. They argue that god probably just decided to "allow" evolution to take place "within" these separately created "kinds" after the initial act of Creation. Of course, the Bible makes no mention whatsoever of evolution of any kind, since evolution was not something people knew anything about at the time the Bible was written by human authors. But what is happening is that at least some modern-day followers of the Bible are somewhat desperately trying to find some way of reconciling their religious beliefs with the rather obvious and undeniable advances in scientific knowledge which have occurred since the time when the Bible was written.
Meanwhile, the craziest of the Creationists and "literalist" interpreters of the Bible (as "the Word of God") refuse to budge even one inch on the question, carrying signs that claim evolutionists will all burn in hell, trying to get laws passed to force adherence to the Bible and the teaching of religious Creationism in the public schools, and trying, not so unsuccessfully, to weasel their way into positions of power and influence, from local school boards to presidential dining rooms.
One thing all the different types of Creationists refuse to recognize is the simple fact that there is plenty of concrete scientific evidence that proves not only that evolution is constantly going on within living populations of plants and animals, but also that brand new species of plants and animals really do come into being, not out of thin air or through divine intervention, but as derived evolutionary modifications of previously existing ancestor species . All the evidence points to the fact that there are not now, nor have there ever been, any "separately created kinds."2
Unfortunately, there continue to be a lot of misconceptions about what it means that a new species emerges out of a preceding ancestor species.
For the purposes of this installment, it may be helpful to remember that the term microevolution generally refers to evolutionary changes taking place within populations and species , and the term macroevolution refers to the major evolutionary trends which have marked higher order splits and junctures at or above the species level and taking place over the whole long history of evolution on earth. The phenomenon of speciation (the emergence of a new species out of an ancestor species) is the focus of this installment, because it is in a sense the key "bridge" linking the kind of evolutionary changes which we can observe happening all the time within populations and the kind of qualitative splits and ruptures which mark the start of a whole new evolutionary line , such as a new Family or Order , and which begin with particular speciation events, or with rapid "bursts" of such events.3
Speciation events (the birth of new species) sometimes takes place against a backdrop of species extinctions (the death of old species), since these are also part of the ongoing process of evolutionary change, including on the larger (macroevolutionary) scales. In short, understanding both how new species come to be and how and why species may go extinct is crucial to understanding the history of the evolution of life on earth as it has actually unfolded over billions of years, and as it continues to unfold even today.
It is also important to realize that there is not some big artificial wall separating micro- evolutionary change from macro-evolutionary change. Macro-evolutionary change has some additional features and characteristics which pertain to larger-scale "trends" (our knowledge of which keeps expanding) but it encompasses (and is rooted in)all the well-known mechanisms of micro- evolutionary change, including natural selection.
All this should become clearer as we go along. But first, let's get rid of some basic confusion: the emergence of a new species does not mean that a cat can turn into a dog, or that a fish can turn into a parrot; it does not mean that you can hold a lizard in your hands and see it turn into a bird; and, even though humans and the modern great apes (chimpanzees and gorillas) really are close cousins by evolutionary standards (and we even share more than 99% of our DNA!), that doesn't mean that one day long ago some kind of chimpanzee gave birth to a human baby. This is not how evolution works!
This is why I put so much stress on making sure we all keep in mind that, while individuals reproduce, it is whole populations that evolve, and also that they can only do so, step-wise, over many , many generations. Major evolutionary change is never "instantaneous."
How Do Entirely New Species Come Into Being?
To understand how a new species can emerge out of an ancestor species, you first have to really understand what a species is. In modern biology, a species of animal or plant is usually defined as a collection of populations which can successfully interbreed (mate with each other) and produce viable fertile offspring (young which will be able to survive and in turn reproduce). To qualify as a species, a group of organisms has to be "reproductively isolated" from all other species. As we will see, there are many different "isolating mechanisms" which can lead different populations of even closely related organisms to become "reproductively incompatible" with each other, even if they live in the same geographic area and even if they sometimes still "look" (to our eyes at least) almost the same. Two closely related species of frogs, for instance, might look almost alike and live in the same area but still be reproductively incompatible simply because they have evolved different mating calls and so don't even recognize each other as potential mates. "Reproductive compatibility" (or the capacity for successful interbreeding) is what ends up grouping populations of organisms together into one single species, and it is also what ultimately separates one species from another.
In the real world, populations of living organisms belonging to the same species often vary slightly, relative to each other, across a range of geographic distribution. For instance, some species of birds are made up of different populations with slightly different color patterns in the Eastern and Western United States; and populations in the Mid-West, where the Eastern and Western varieties overlap and hybridize ("cross-breed" with each other), are often intermediate in appearance. Despite these superficial differences, the different populations of such a bird species are still part of one single species, because they have not lost the ability to interbreed and produce viable and fertile offspring. In such cases, the different populations are simply considered different "geographic races" or "subspecies" (as opposed to different species).
This, of course, is also why biologists understand that all human beings all around the world are all part of a single species. We may have minor superficial differences in things like skin color or hair types, but we have no consistent and fundamental differences, we all share a common gene pool, and we are all perfectly capable of successfully interbreeding with each other. Racist theories of racial superiority or inferiority, which confuse some people into thinking that different human "races" are somehow different "kinds," have absolutely no basis in scientific fact! We are truly one single intermingling species, spread throughout the globe.
Organisms can only be said to belong to different species if they are sufficiently reproductively isolated from each other and would therefore no longer be capable of successfully interbreeding even if they shared the same locality.4
Understanding some of the many ways in which one population of organisms can become reproductively isolated from its parent population is key to understanding how a brand new species can come into being. This is because speciation often occurs when a population which has become reproductively isolated from its parent population ends up accumulating enough genetic differences (through the sorting out processes of natural selection and related phenomena) that it loses its ability to interbreed with the original parent population.
Once reproductive isolation has been maintained for a long enough period of time (numbers of generations) and full speciation has occurred, the populations of the two now different species will both continue to evolve in the usual ways, including by natural selection and in interaction with their particular environments. But now they will do so separately (on the basis of separate and somewhat different gene pools), and for this reason they will accumulate ever greater differences over time, pulling them further and further apart. At every generation, both in the original parent species and in the new daughter species, the usual random processes of mutation and genetic recombinations will re-shuffle the (now somewhat different) genetic deck of cards, and this will bring forward some individuals with new features which may or may not prove reproductively advantageous in a particular environment, but which in any case won't be exactly the same in the two species since they are no longer mixing their gene pools at each generation. This, combined with the fact that even slight differences in the environments they occupy can also exert somewhat different selective pressures on populations of the old and the new species (for example, "favoring" the development of short and stout bird beaks where tough hard seeds are available, or of thinner, tube-like beaks in environments where nectar-filled flowers offer a good alternative food source) will tend to make the populations of the old and new species grow further and further apart (becoming more and more different in such things as anatomy, development, or behaviors) the more time goes on.
In addition, a parent species may end up "budding off" some "daughter species" more than once in the course of its history (though perhaps at different times and in relation to different sub-environments), and many of these daughter species are also likely to, in turn, "bud off" additional (and even more different!) daughter species of their own. All of this contributes to pulling entire ancestor and descendant lineages (lines) further and further apart over time. That's why life is so diversified!
A species which has relatively recently "budded off" from its parent species is likely to be recognizable as a "closely related" species for some time. But if the speciation process keeps getting repeated over and over again, and over many millions of years , the shared ancestry of different organisms may well become less immediately obvious. For instance, you would not automatically think that modern-day whales are descended from a four-legged mammal ancestor that walked on land, but in fact we now know whales are descended from just such a creature! We can see this because people have uncovered a whole series of fossils of intermediate species (as well as some additional molecular DNA evidence) which reveal the seemingly odd family connection between an original land-based ancestor and the modern whales.
There is actually a region of the dry Sahara desert of northern Africa (which used to be covered by an ocean millions of years ago) which today is known as the "Valley of the Whales" because its sands contain an incredible number of well-preserved fossilized skeletons of a whale species which swam in the area 40 million years ago. This whale species has some features which are similar to those of modern whales, but it also has vestigial (remnant) toes, legs, and pelvis ! These features link it (through a number of other intermediate steps) to a land-based mammal which lived in the area some 10 million years earlier. And it would take another 15 million years or so (and many more instances of daughter species budding off from parent species) for those remaining leg and toe bones to end up fully converted into the kind of modern flippers we see on whales today. When you look at a number of fossils in the series, you can see the step-wise nature of the modifications which were favored by natural selection at different junctures. But it's not as if the whales' oldest land ancestor "suddenly" lost four legs and re- appeared the very next day with flippers instead! There were quite a few intermediate steps, and these anatomical transformations (which were probably also accompanied by significant changes in behaviors and habitat utilization) happened over a great many whale generations!
At every evolutionary juncture in the history of life, the story would likely have been the same: a population occupying a local environment would have, for one reason or another, become reproductively isolated from a larger parent population of the species. The reproductively isolated sub-population would in time have produced and consolidated different anatomical, developmental or behavioral features (relative to the "parent" population) as natural selection and related phenomena operated on a somewhat altered base of genetic variation in the two populations to produce different inheritable traits, and as the two populations encountered somewhat different sets of selection pressures in relation to the particularities of the environment with which they interacted.
As the ongoing genetic reshuffling and recombination continued to occur in the new species, new features would periodically appear which had never before existed in the ancestral population. Geneticists often like to remind people that "mutants are not monsters": mutations are simply fairly commonplace "copying errors" which occur in the course of DNA replication when organisms reproduce. They can have large or small effects on the anatomical, developmental, or behavioral traits of individual organisms who inherit these mutations, or they can even produce no noticeable effects at all (so-called "neutral" mutations which nevertheless contribute to overall genetic variability in a population and which may contribute at some later point to the development of some evolutionary modifications). Both mutations and the more routine genetic recombinations which occur at every generation in sexually reproducing organisms can produce traits which, in a given local environment, put the individual at a reproductive disadvantage relative to others in the population. In such a case, natural selection will tend to fairly quickly eliminate this variation from the population. But if a new trait emerges that provides individuals having this feature with a distinct reproductive advantage in a given environment (which is more likely to occur when the environment is itself changing or the daughter species has moved into a new environment), then natural selection can quickly spread this new feature over the generations to greater and greater numbers of descendants.
It is also important to understand that when reproductively isolated populations "bud off" into new species and accumulate really new and significant evolutionary "innovations" (evolutionary features which did not exist in their ancestors) they can sometimes expand very rapidly and spread into whole new habitats , where they can often make use of a whole range of environmental resources that had never before been available to the ancestor species. In such circumstances, new species may diversify even further , and often relatively rapidly, subdividing into a number of additional daughter species which may have evolved a number of particular "specializations" enabling them to become even more finely-tuned (adapted) to specific components of their new environments and exploit somewhat different types of food, nesting sites, or other such resources.
This kind of rapid expansion and "adaptive radiation " has been observed to occur when so-called "founder populations" of just a handful of individual organisms manage to colonize islands or other relatively "open" habitats where there are as yet no closely related species, and which may be entirely lacking the particular competitor and/or predator species that the ancestor species had to contend with.
This is what happened in the case of a family of birds known as the Hawaiian honeycreepers, for instance. A single ancestor species of these birds migrated to these islands in the past but then rather quickly diversified (through repeated speciations) into a great number of closely related species which differ greatly in the shape and size of their beaks. These differences represent different evolutionary modifications (and in this case clear adaptations) in relation to an assortment of available food sources. Studies have shown that even small anatomical modifications in beak size and shape can greatly affect what kind of food birds are able to exploit. Among the many closely related species of Hawaiian honeycreepers, there are species with short thin beaks which eat mainly insects, species with thick stout beaks which eat mainly fruits and hard seeds, and species with long thin beaks which suck nectar out of flowers. And there are also a number of species with features that are intermediate between these types. All these different species are closely related, and are descended from that one single honeycreeper ancestor species which made it to the Islands.
In the larger-scale history of life on earth this kind of rapid "adaptive radiation" is also the kind of thing that happened when, for instance, a major evolutionary innovation such as the emergence of feathers and lightweight hollow bones suitable for flight "opened up" a whole new environment (the previously unpopulated skies) to what quickly became a veritable explosion of bird species. Major innovations in one line of descent of organisms can also have profound ripple effects on other lines. For instance, biologists generally agree that a tremendous increase in the variety of insect, bird and mammal species in the Cretaceous period, some 140 million years ago, was in many ways tied into the rapidly increasing diversification of flowering plants, a relatively recent evolutionary innovation.
Imagine also the tremendous evolutionary "openings" which were likely encountered by the first species of bony fishes to give rise to the first primitive amphibians some 400 million years ago! These were the very first four-legged creatures to crawl out of the waters and expand onto land. They emerged as only relative slight modifications of some bony fishes which had evolved a couple of anatomical and developmental oddities such as a lung-like air bladder and stumpy leg-like fins. Both of these features would ultimately allow some of their descendants to at least occasionally spend time out of the water. We know from the fossil record that these evolutionary modifications did in fact occur among a group of bony fishes known as the lobe-finned fish (whose living descendants include the very primitive coelacanths and various species of lung-fishes).
We can only speculate about exactly what reproductive advantages the evolution of air bladders and bony finger or leg-like fins conferred on individuals having these features: perhaps the air bladders allowed some fish to survive periods when pools of water occasionally dried out (as is the case in living lung- fishes), and perhaps modification in the bones of their fins at first just enabled some species of fish to better escape predators or chase after other fish, and then later these emerging structures were co-opted for a new use, to get around on dry land. In any case, fossil skeletons of the very first land-dwelling four-legged animals (the most primitive salamander-like amphibians, which looked a lot like fish with legs!) show extremely close similarities to the skeletons of those water-dwelling lobe-finned fish. And what is truly amazing is that the basic pattern of bones which make up the limb structure of all the later four- legged land-dwellers can already be seen in the underlying structure of the bony fins of those ancient fish ancestors!
No other animals had ever before crawled out onto land, so opportunities for fairly rapid diversification and specializations in relation to different sub-features of local land environments must have abounded. In fact, the fossil record reveals that amphibians would diversify greatly for about 100 million years and also bud off the first reptiles, who would in turn diversify greatly and bud off the first mammals and birds. But each one of these truly major departures in the history of life would have had its origins in simple speciations--the appearance of an increasingly divergent "daughter" species which became reproductively isolated from its parent species, and then began to separately accumulate evolutionary modifications.
[To be continued in the next issue]
1 The sheer quantity and diversity of beetle species led the 19th-century scientist and proponent of Darwinian evolution T.H. Huxley to humorously (and sarcastically) quip that, if god had created all of nature, he must have had "an inordinate fondness for beetles"!
[Return to article]
2 One of the things which reveals that Creationists are fundamentally non- scientific is the way they tend to view different plant and animal species as completely segregated and unchanging "kinds" in the first place. In reality, every biologist understands that the boundaries separating living organisms are relative (and not absolute), that their features tend to blend and blur into each other (instead of being rigidly compartmentalized), and that no life-form is ever absolutely "unchanging." By contrast, the Greek philosopher Plato, who lived about 2,400 years ago, believed that all things have a fundamental and immutable (unchanging) "essence"; this view became widespread in his day and remained popular until human beings began to develop the kind of modern scientific methods and outlooks which have allowed us to discover that, in reality, all forms of matter --from atoms, to life-forms, to galaxies-- are always undergoing change . In fact, modern science in all the different fields--and so much of how we use modern science to transform our material world--is based on that very understanding! At the dawn of the 21st century, isn't it about time to let go--once and for all--of outdated, static and "essentialist" views of things?
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3For those of you who are not very familiar with the biological system of the "group within group" taxonomic classification of plants and animals according to their degrees of similarities and relatedness, the following example may be helpful: an individual wolf belongs to a reproductive population of wolves in a given region; all the different populations of wolves scattered throughout the range of the wolves make up an entire wolf species. All the different but closely related wolf species (such as the Red Wolf, the Mexican Wolf, etc.) get grouped into the wolf genus ; all the wolves are then also placed, on the basis of shared features, in the Family of the Canidae (which includes dogs and foxes, but not, for instance, cats, which belong to a different Family); the wolf Family is then grouped along with some other Families (still on the basis of a set of shared similarities) into the Order of Carnivores (meateaters) and the Class of the Mammals (animals which have hair, give birth to live young, produce maternal milk, etc.). So, for instance, wolves would be grouped with other Carnivorous Mammals, such as dogs, foxes, bears, seals and weasels (and there is both fossil and molecular evidence that these animals really do share a common ancestry). Beyond that, our wolf also belongs to the Phylum Chordata (all animals having a backbone) and finally, and most obviously, to the Kingdom of Animals (distinguishing them from organisms belonging to the Kingdoms of the plants, the fungi, the algae-like protists or the bacteria-like prokaryotes, which all represented very different evolutionary pathways in the history of life). In real life, the boundaries between species or higher lineages are not always perfectly clear, and species sometimes have to be reclassified into new groups as we learn more about them. But, by and large, the system of biological classification assigns organisms to each category based on sets of real similarities and differences which actually reflect patterns of inheritance from common ancestors, and which distinguish them from evolutionary lines that went off in significantly different directions.
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4You may be familiar with the classic example of the horse and the donkey:because they are closely related, you can actually get a horse and a donkey to mate, and they can even produce an offspring capable of surviving which has some characteristics of both parents and which we call the mule . But a mule is what is called a "sterile hybrid": it can live and grow, but it is unable to reproduce. To be considered to belong to the same species, organisms have to pass the so-called "species test": they have to be able to mate, and to produce viable offspring (offspring capable of surviving); and these offspring in turn also have to be able to reproduce. Some species are separated by pre-mating mechanisms (they are unable to mate in the first place for any number of reasons) and sometimes they are separated by post-mating mechanisms (they can mate but their hybrids are deformed and die, or survive but can't reproduce). Applying the "species test," we can see that horses and donkeys belong to different (even though closely related) species.
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