The Science of Evolution
by Ardea Skybreak
Published on June 28, 2005
A Few Words about Adaptation
Many evolutionary changes represent adaptations of organisms to changing external environments, though not all evolutionary change is adaptation.1
Adaptation is a process which takes place over time. It refers to the development of what often appears to be an amazingly tight "fit" between certain characteristics of organisms and the environments in which they live--a kind of "fine tuning" of species to environments which frequently results from natural selection operating over many successive generations. Examples of adaptations in nature are as numerous as they are wonderful. Camouflage is a good example. Many mammals in the Arctic (such as polar bears, arctic foxes, and so on) are camouflaged by their white fur which makes them harder for predators to see against the background of white snow; some insect species are difficult for bird predators to find because they have evolved to look almost exactly like plant leaves, or twigs, or pieces of bark!
The evolution of organisms which derive a reproductive advantage by being "mimics" of other species is another example of the evolution of adaptations: For instance, some insects have black and yellow bands that make them look like bees and wasps, even though they don't really have any venom and would be perfectly tasty for birds or other predators to eat--birds are "fooled" into leaving them alone! Remember, there is no consciousness involved with any of this--the insect bee-mimics don't know they are fooling the birds. It's just that individual insects that happened to have these kinds of markings on average did better at surviving and producing descendants than those in the same species that didn't have such markings. So, over a number of generations, natural selection led to the bee-like markings spreading to all (or almost all) the individuals in the population. There are many such species of mimics among different kinds of organisms: many insects that lack venom have the markings of bees and wasps; non-venomous king snakes look almost exactly like the very venomous coral snakes, etc.
But then, if "mimicking" poisonous animals can provide such a reproductive advantage, why haven't all the insect species evolved the black and yellow "warning colorations" of bees and wasps, for instance? This is an important question to reflect on. The answer is basically that evolution can only work with whatever raw material (genetic variability) happens to be at hand-- there is no guarantee that any given population will have inherited from the preceding generation the kind of genetic information required for certain particular features to emerge, no matter how advantageous those features might turn out to be in the abstract. For instance, it might objectively be advantageous for human beings to be able to fly by flapping their arms, but humans just don't have the basis in their underlying genetic "kit" for that particular capability to emerge. So there's no way natural selection could lead to the evolution of flight in humans, even if it could somehow be shown that it would benefit human survival and reproduction. The biologist Doug Futuyma (who has written excellent college textbooks on evolution, as well as a very good book for broader consumption-- Science on Trial, the Case for Evolution ) has an even better example: he makes the point that it would no doubt be very advantageous for animals to be capable of photosynthesis (the method by which plants derive energy from the sun to produce carbohydrates) if only because it would always provide an extra source of reliable nourishment when regular food is scarce. So, if there were any possible way that an ability to photosynthesize could emerge in any animal population, it would likely be heavily "selected for" by natural selection, and would spread rapidly across the generations. But no matter how advantageous this might be, it will never happen! Again, this is because no animals have ever inherited from any of their animal ancestors any of the genetic information required to make the machinery of photosynthesis, which evolved only in plants.
So, in an animal line, no amount of genetic reshuffling and recombination of genetic material inherited from an immediately preceding generation could bring forth a feature for which the basic building blocks don't even exist--again, no matter how advantageous it might be! This is yet another illustration of the important point that evolution can only work with what is already available--the genetic variability inherited from previous generations--and that evolutionary pathways (including the emergence of true evolutionary "novelties") are therefore significantly constrained (both channeled and limited ) by past history.
Still, even with such built-in limitations, evolution has led to an amazing diversity of form and function throughout the plant and animal world. Consider all the different shapes and colors of flowers. Why isn't there just one single shape for all flowers? Why isn't there just one single color? Why so much diversity? The answer has to do, at least in part, with the historical process of co-evolution between different species. Take the case of plants and pollinators for instance. Some plants don't make any real noticeable flowers and reproduce just by releasing pollen (plant sperm cells) into the wind, where it then drifts along and sometimes happens to land on female plant ovaries to end up producing a new seed. But in many species of plants, evolution has developed a mechanism that ensures that successful pollination is less left up to chance: the plant produces showy flowers, which may also have a distinct scent, and which contain not only plant reproductive organs but also an additional pot of honey (plant nectar). The showy flower petals and the sweet nectar are of no direct "use" to the plants themselves, but they attract animal pollinators --members of animal species who come to recognize the flower and to remember that it is in effect a "flag" advertising a source of food--the nectar. Pollinator species include many species of flying insects (such as honeybees), some species of birds (such as hummingbirds) and even a few species of bats and monkeys. In all cases the story is basically the same: the pollinators come to the flower to suck up the sweet nectar, and in doing so they accidentally pick up pollen (plant sperm cells) on their bodies at the same time. Then, acting as much more consistent and reliable pollinators than the wind, they move on to another flower of the same plant species to get some more nectar; that's when the pollen from the first flower gets rubbed off their bodies and enters into the second flower's ovaries, where it will serve to produce new seeds. The plant species has basically evolved a way to "use" the animal species to ensure its own more successful reproduction, while the animal species has co-evolved a way to ensure itself a reliable source of food. Both species benefit from this symbiotic (mutually beneficial) relationship, though of course neither the pollinator nor the pollinated is in any way conscious of this process.
What we are seeing is simply the result of a process of natural selection, taking place over many generations and considerable periods of time. These particular processes obviously did not take place in all evolutionary lineages (there are still many plant species that are pollinated simply by the wind, and there are many animal species which do not serve as plant pollinators) and, in any case, evolution was not "bound" to unfold in those particular directions. But among at least some of the ancestors of modern-day pollinators, individuals who went to flowers to get nectar must have reproduced more than those that didn't; so, as long as the ability to detect and exploit such a fine food resource was inheritable, it would have spread to more and more descendants. And among some of the ancestors of today's plants, individuals which happened to produce flowers with a pot of nectar and a showy form of advertisement (bright colors, sweet smells, etc.) which increased the odds of successful pollination must have had a distinct reproductive advantage over those that didn't have these features. Through simple natural selection, those features would have spread to more and more descendants as well.
This completely unconscious process (which requires no outside "designer" whatsoever) can be observed over and over in nature. And even though we can't usually be present to see a new species come into being (many of the plants and animals we observe today are the products of millions of years of past evolution and adaptation to changing environments), we can conduct experiments to manipulate (transform) nature and actually observe--and even measure --whether or not (and to what extent) certain features of an organism, which we suspect might provide them with a reproductive "edge," actually do provide that advantage.
Here's just one of thousands of examples of such experiments: Quite a few plant species, especially in the tropics, produce little nectar pots on their leaves or stems which seem of no direct use to the plant itself (and which are energetically "expensive" for a plant to produce) but which attract ants which crawl all over the plants, visiting the little nectar pots and sucking up this rich food source. The advantage to the ant species is obvious (a reliable source of food), but is there really an advantage to the plant species? Field experiments have shown that, if you prevent ants from getting to plants of these species, the plants will tend to get devoured by leaf-eating insects. But when ants are continually "patrolling" up and down the leaves and stems of the so-called "ant- plants," the leaf-eating insects are largely driven away, and the plants stay much healthier and are better able to grow and produce seeds. So the evolution by natural selection of this unconscious co- relationship between ants and plants has given populations of both a measurable "reproductive edge"; and the proof is there for all to see.
Going back to our pollinator example: the co-evolved adaptations of plant species and their pollinator species is often so "tight" that biologists can tell what kind of pollinator will visit a plant species just by checking out the shape, color, and smell of the flowers: for instance, red flowers with a long, thin, tube-like shape are pollinated by hummingbirds, which are attracted to red and suck out nectar with long thin beaks; brightly colored flowers with flatter parts and sweet smells are usually pollinated by bees and similar insects which are able to detect both the colors and the scents of these flowers. Colorless white flowers with strong sweetish smells are usually pollinated at night (when colors are irrelevant) by night-flying animals like moths or bats. And then there are those thankfully rare species of plants which produce rather colorless flowers with a horrible "rotten meat" smell--those are the ones that are pollinated by some species of flies!
Much of evolutionary change by natural selection seems objectively "driven," in a sense, by the simple fact that the natural world is not one of infinite resources and that living organisms must objectively compete in one way or another for these limited resources. Both intra-specific competition (competition between individuals of the same species) and inter-specific competition (competition between individuals of different species) contributes to evolutionary change by natural selection. For instance, individuals of the same species often compete with each other for such finite (limited) resources as food, water, mates, territories, nesting sites, pollinators, and so on. Individuals who happen to have genetically transmittable features which make them relatively more successful than others at obtaining such limited resources, will likely "do better" at surviving and reproducing than individuals who don't have these features. Unless some other factors intervene to channel overall evolutionary changes in some other directions (or even lead to a species' extinction), those features will then tend to spread over the generations by natural selection and may come to characterize a whole population or even a whole species.
Individuals belonging to different species but occupying the same habitat may also objectively compete with each other for finite resources, such as food or water (for example, when members of different plant species compete for energy-producing sunlight in the understory layer of a forest). In addition, members of many different species may also interact with each other as potential predators or potential prey. In fact, the great variety of predator-prey interactions which take place between members of different species has likely played a very important role in objectively "driving" (or channeling) a great deal of evolutionary change by natural selection (including species diversification) throughout the history of life. Evolutionary modifications that make predators more efficient at capturing prey, or prey more efficient at avoiding predators, often represent some of the most clear-cut adaptations of organisms to their changing environments.2
Evolutionary ecologists and population geneticists working together have provided a great deal of evidence of evolution in action in whole ecosystems--recording evidence of the ways different populations use limited resources, the effects of intra-specific and inter-specific competition and predator-prey interactions, the direction of evolutionary changes taking place in different populations in response to changes in physical or biotic environments (droughts; floods; increases or decreases in competitor or prey species), and so on. All these kinds of things can be assessed experimentally and the effects of various variables on what scientists call "reproductive fitness" can actually be measured. Evolutionary changes observed to be taking place at the level of populations can also be correlated with (linked to) changes in underlying genetic variation at the molecular level: for instance certain DNA alleles can be shown to have increased or decreased in frequency in a given population in line with certain observable changes in the appearance and/or mode of existence of that population, and all this can in turn be compared to what is going on in a different population of the same species which may be interacting with a somewhat different set of environmental factors.
It is important to remember also that evolutionary change doesn't happen in a pre-set direction. In fact, the direction of a local evolutionary change can actually get reversed in a population if some feature which gave individuals a reproductive advantage in one situation is no longer an advantage in a changed environment: for instance, in a hot dry desert, adaptations which help to conserve water (like the needles of cactus plants) are very common, but species with such specialized adaptations would likely evolve new features (or alternatively go extinct) if large-scale climate changes transformed the desert into a lush tropical wetland. Again, evolutionary change happens over time in relation to local environments which are themselves always changing --so there is no one direction, no single track to "progress," no pre-set purpose and no final end-point.
There are, however, so many spectacular examples of "finely tuned" adaptations between organisms and their particular environments (including other species) that it is not surprising that many people think these wonders must have been produced by some all-knowing "designer" (god). But, actually, when you look into the matter further and with a scientific approach, you learn that all the wonders of nature and all its diversity can in fact be produced through modifications of pre-existing living material, through step-wise processes spread out over long enough periods of time. And, as we will discuss a bit later in this series , you also find out that evolution is far from a "perfecting" or "optimizing" mechanism: there is plenty of evidence of things which are evolutionary quirks, vestigial features (no longer needed "carry-overs" from distant ancestors), wasteful tidbits, and even detrimental "imperfections"--all of which would make absolutely no sense if an all-powerful and all-knowing god had "designed" nature. These things do make sense, however, if all life evolved from a common ancestor and then diversified through natural selection and related mechanisms into countless different forms--but only on the basis of whatever raw material (genetic variability) one generation inherits from the immediately preceding one. This raw material is constrained (channeled and restricted) by past historical development, and is therefore not capable of producing limitless or "ideal" modifications.
In a later installment of this series, we will discuss more fully why the actual evidence of evolution disproves the idea of a supreme "intelligent designer."
1. As we mentioned earlier in this series, there are processes other than natural selection which can contribute to the evolution of populations of organisms, including accidental genetic drift, founder effects and neutral mutations. All these can contribute to a population evolving merely by causing shifts in the relative frequencies of certain genes from one generation to the next. These sorts of shifts in gene frequency are not the result of natural selection. Natural selection is, however, responsible for that fine-tuning process we call adaptation. At the same time, it is important to keep in mind that you can't just look at a feature of an organism and assume it represents an adaptation. Many features of organisms are not adaptive, and some of them may just be the result of developmental constraints (limits) or simple by-products attached to the development of another feature which may itself be adaptive and the result of natural selection.
2. It is important to remember once again that adaptations are not the only kind of evolutionary change. For instance, a population might evolve significant differences relative to the ancestral population from which it is descended when a small number of individuals find themselves cut off from the larger population when they migrate into some isolated geographic pocket such as an island, a hidden valley, the other side of a ridge or some other barrier. Such a small sub-population in its more isolated pocket just doesn't have as much genetic variation in its total gene pool as the larger population it came from. This can make for a loss of some of the raw material evolution can work with (relative "genetic impoverishment"), but it can also create conditions for the emergence of evolutionary "novelties" which may be more likely to emerge accidentally in smaller populations. Phenomena such as genetic drift and founder effects are important supplements to standard Darwinian natural selection in contributing to evolutionary change.
The original article was published in English and Spanish on Revolutionary Worker Online