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Why Evolution Matters

by Stephen R. Palumbi, Harvard University

Society for the Study of Evolution
NABT Convention, Reno, NV, Nov.4, 1998

Why Evolution Matters

by Stephen R. Palumbi, Harvard University

Society for the Study of Evolution
NABT Convention, Reno, NV, Nov.4, 1998

There are abundant popular views about evolution. It isn't that most people wake up and stumble towards the coffee maker thinking, "Oh, man, what am I going to do about evolution today?" It is instead that evolution as a subject has become enough of a cultural common currency that most people feel comfortable having an opinion.

Among biological sciences, this is actually a somewhat enviable position--herpetologists (who study reptiles and amphibians) would give a year's supply of snake skins for the popular attention given evolution. But such attention has its bad points as well as good and does as much to complicate the understanding of evolution as it does to raise interest in the subject. Although many people have a deep understanding of evolution, several major misconceptions about evolution are so common as to seem like dogma. These misconceptions simultaneously raise a storm of controversy about evolution and prevent understanding of an increasingly important biological crisis riding the crest of a human-made evolutionary wave.

 

Evolution and the Origin of Species

A critical misconception is that "evolution" and "the origin of species" are exactly the same thing. Most discussions (or law suits) on the " theory of evolution" are really about the "theory of the origin of species due to evolution by natural selection." In fact, evolution by natural selection is a biological process that is abundantly documented by observations and experimental evidence. It is as much a scientific reality as nuclear fusion (and is easier and safer for the average person to observe).

Evolution by natural selection can be seen in many experimental situations. For example, fresh water guppies are eaten by larger predatory fish that tend to consume more brightly colored males than duller males. When predators are abundant, the result is an evolutionary shift, in just a generation or two, towards males with drab colors. But females prefer to mate with bright males (in terms of hue), so in the race for mates, colorful males dominate. These two types of selection, driven by predators and choosy mates, result in evolutionary shifts in color when ecological conditions change. When predators are common, dull colors predominate. In the absence of predators, gaudy males dance the stream beds to attract discerning mates.

Another example is the beak size of seed-eating finches living on dry islands in the Galapagos archipelago. During particularly dry years, plants do not produce many seeds. Finches live mainly on a diet of seeds and tend to consume smaller, softer seeds first. As the small seed supply is consumed, finches are left with the chore of cracking into harder, larger seeds. Birds with the smallest beaks can not do this well. As a result, during drought years birds with small beaks may well starve, leaving populations dominated by larger members of a species. Years with unusual weather thus lead to populations of unusual finches.

To such examples of natural selection can be added many examples of artificial selection conducted in laboratory settings or by selection for particular characteristics by plant and animal breeders. In virtually every experiment in which a sufficiently large and variable population is culled by artificial selection, an evolutionary response is observed. This response has been used countless times in the domestication of animals and plants. Long before Darwin recognized this as an evolutionary response, it had been used to shape the nature of agriculture. To be fair, there are limits to this response and sometimes there are unforeseen consequences. One fruit fly biologist tried to select for resistance to high temperatures by eliminating flies that passed out and fell off a heated cylinder. Instead, he ended up selecting for flies that passed out but hung tenaciously and unconsciously onto the cylinder wall. Nevertheless, evolution by natural or artificial selection is a biological phenomenon that's easily and rigorously demonstrated. One area of active research is how evolution by natural selection leads to the formation of new species. Darwin envisioned that evolutionary "adaptation" in changing environments could lead to divergence of new species from their ancestors. Other mechanisms of species formation have also been envisioned (Mayr, etc.), and processes by which particular species form are often hotly debated. This is because speciation is not as easily observable or experimentally demonstrable as is simple evolutionary change.

This is not to say that speciation is an inexplicable process. Indeed, the dominant view of species evolution, the gradual divergence of populations in separate geographic locales, has been observed experimentally. However, successful experiments in species formation are rather rare They take a long time and result in small shifts in morphology or behavior compared to species observable in the wild.

In a few cases, mechanisms of species formation have been observed. Abrupt shifts in behavior can result in species boundaries. Tephridid fruit flies lay their eggs on ripening fruit, and often females have highly discriminating tastes about which fruits are the best for their larvae. In the late 1800s, flies that lived on Hawthorn fruit in North America developed an egg-laying preference for apples introduced for agriculture. The shift in preference is heritable - daughters of apple-loving flies also lay eggs on apples - and so a new race or species became established. How different are these species? It takes an expert to tell them apart. But the potential now exists for these two types of flies to evolve separately, gradually diverging in defining characteristics, some important, some not, some visible, some depending on deep physiological or biochemical differences.

Because speciation is so difficult to study, understanding it has long been a primary evolutionary quest. It is a subject rich in experimental possibilities and theoretical convolutions, and it draws heavily on what we know about evolution by natural selection. But "evolution" and "speciation" are not the same, and uncertainties about the how the latter occurs do not mar our increasing understanding of the former. In addition, the impact of humans on the biological world around us does not hinge on species formation. Instead it hinges on evolutionary shifts in diseases and insect pests and the plants we raise for food. So we must move on to consider the second common misconception about evolution - the one that matters most - the speed of evolutionary change.

 

The Speed of Evolution

The dinosaurs evolved and ruled the earth for over 100 million years, succumbing finally in the
aftermath of an asteroid strike 65 million years ago. Over tens of millions of years, different forms arose and went extinct. The plastic stable of species familiar in toy chests of most 5 year-olds (Triceratops, Stegosaurus, Apatosaurus, Tyrannosaurus) slowly came and went on the evolutionary stage. And so evolution seems a majestic and slow process, minutely ticking away during vast stretches of time. Infinitesimally - different generations are strung together long enough so that, while the mountains wear away and the very continents plow the seas, species evolve new forms.

Darwin thought this way too. Evolution by natural selection was born as an idea in the early decades of the 19th century, when the age of the earth was considered to be immense--so large that tiny, random variations could slowly be selected for during the struggle for survival. In fact, a debilitating challenge to the theory of evolution by natural selection came from calculations of physicists who concluded in the late 1800s that the Earth was cooling so fast that life could be no older than 20-40 million years. Darwin was dismayed by this time frame - too short for his view of slow evolution to play out - and died before the discovery of heat-producing radioactive decay could over-turn these objections.

To find examples of evolution, Darwin and his predecessors tended to scan large tracts of time. They saw in the stately change of the fossil record or in the fine-tuned adaptation of precise biological machinery the signature of evolution over the millennia.

So, too, modern accounts of evolution often emphasize missing-link fossils from long ago and the happenings (especially for the Hollywood fossils--the dinosaurs) of tens of millions of years in the past. Yet biologists have long noticed rapid evolutionary change, both within the fossil record and in modern plants and animals. Sometimes studied for their value as exceptions to the slow evolutionary rule, sometimes studied for purely practical reasons, rapid evolution is increasingly well-known. We no longer need to dig into Earth's rocky past to uncover evolutionary events, but we can turn to the modern world around us. And once we begin to look for active evolution, we can find it all around us. In the signature of every harsh winter, every drought summer, every invasion of a new pest and every dose of antibiotics, there is an evolutionary twist, a shift toward a new way of living. Perhaps the shift will be short-lived like this year's rain hydrating last year's drought, as one evolutionary shift cancels out the previous. But also perhaps the chance evolution of a resistant bacterial strain could create a plague or require a billion-dollar search for a novel antibiotic.

To understand this speed requires an understanding of evolution's engine - the interlocking gears and power train that drives change across the generations. We need to understand what connects the engine's parts and why they work together. We need to understand why sometimes the engine runs quickly and sometimes slowly - why it races and why it stalls. And once we do understand it's function we can apply that intuition to the world around us and to the evolutionary events that transpire every day. The power of evolutionary science to explain and the power to predict will then let us organize our effect on the biological world to take inevitable evolutionary change into account. This change is all around us already, but we cast it as an adversary - a facet of the biological world to tame. Tame it we can, but not without understanding, and not all the time.

 

If Evolution is Fast

If evolution is fast then humans are the most crucial evolutionary force ever unleashed on the planet. This is because we change the world rapidly and repeatedly. We create new environments by the way we live, we create new biological hurdles by the way we protect our crops or cure our diseases. We change what the best strategies are for successful reproduction of other species by choosing when to hunt for them or harvest them or when to disturb the environments they live in. We also have created a world-wide transportation web that is virtually instantaneous compared with "natural" means of movement for most species. Like the asteroid strike that spelled doom for the last of the dinosaurs, we have dramatically altered the biological stage. The landscape and seascape have changed more radically in the past fifty years than virtually any time in the past. And it does not take an intelligent species to respond to the new human world - it does not take planning and forethought, committees and blueprints. All it takes is selection of individuals better adapted to these new environments. All it takes is for the progeny of these selected individuals to inherit these new abilities.

 

Evolution of antibiotic resistance

Remember the pediatrician's stain-resistant office, and the shrill sounds of crying. You have a baby, and this means that bodily secretions are now acceptable dinner conversation. But when a cold strikes in those tiny airways, the clog of all those secretions overwhelms even the most stoic parent. What fate awaits the pediatrician's decision? What antibiotic will she give baby for the cold? The increasingly likely answer - none.

In small children, a large fraction of chest and head colds are viral. A good pediatrician will not always prescribe antibiotics for colds - but often waits for them to clear naturally. The reason is an evolutionary one - too frequent use of antibiotics selects for resistance. And when a bacterial infection does strike, it would be better if the bacteria hadn't already evolved to eat antibiotics. This ushers in the new medicine, a set of procedures that assumes that evolution will happen, and will happen fast. Part of the current strategy requires withholding medications when not needed, and part of the current problem remains incorrect patient use of the medications that cure.

It hasn't always been this way. The first antibiotics were hailed as the wonder drugs of the 20th century (despite having been described in the Bible - then discovered and forgotten by Pasteur and Belgian scientists). Finally rediscovered by Alexander Flemming in 1928 and used extensively in World War II, penicillin was crystalline death to most infections. But by 1947 the first strong resistance emerged. Since then, the arms race has been fierce, expensive, continuous, and usually won by bacteria. A zodiac of different drugs have been invented by humans and Houdini'ed by bacterial escape artists. Some of the best ones are now reserved as the drugs of last resort -withheld from all but the worst infections. The evolutionary engine dictates this strategy, as well as the emergence of evolutionary medicine as a clever treatment method. Thus, medicine can no longer afford to ignore the fact of evolution and instead has begun to define treatment protocols that 1) assume evolution will occur, and 2) limit the opportunity for evolution as much as possible.

 

Other examples

There are many other examples of rapid evolution in the biological world around us: HIV viruses
evolving within a single person during the course of AIDS, evolution of small body size and new reproductive tactics in over-fished salmon, the evolution of insects to insecticides, including those genetically engineered into crop plants. These examples provide a source of insightful information about the evolutionary process, and show how important evolution has become to our modern society. These examples can be very personal, deal with daily lives of many of us, and thus may be a powerful tool in teaching evolution at many levels.