In 1859, Darwin's treatise, On the Origin of Species by Means of Natural Selection, or the Preservation of Favored Races in the Struggle for Life, was published to great effect. This book presented a historical and naturalistic hypothesis for the perfection we see in nature all around us. Whether explaining colorful flowers attracting busy pollinators, or sleek antelopes evading hungry carnivores, Darwin's idea of natural selection over an immense period of time provided a reasonable alternative to specific deistic design.
In 1802, the Reverend William Paley had published Natural Theology: Or Evidence of the Existence and Attributes of the Deity, Collected from the Appearance of Nature. If we walk across a moorland and come upon a watch, Paley argued, we could easily see—from its complex inner workings—that this watch had been designed by the maker of the watch. And so, surveying a wide swath of natural history, Paley argued that the work of God's design was clearly evident all around us. Paley's book went through many editions; it was hugely popular and profoundly influential. Here was a richly documented treatise on what we now call Intelligent Design!
Nevertheless, and even if we were all to agree on an intelligent and prescient deity having created our world, an underlying question remains: how did the world around us actually come into being? Did God use his almighty powers to have the world's many wonders simply leap into existence from nothingness? Or did our God set the universe in dynamic motion (Big Bang!), decree the basic laws of nature, and then let the sweep of time fashion the world in which we live? Advocates of intelligent design have the same problem today; did new designs simply leap into existence out of nothingness, thanks to divine magic, or might they have come into being from simpler precursors through the workings of Natural Law over time? Here is where Darwin's book made its primary impact. By implying the operation of the same natural laws we see around us, Darwin's bold hypothesis opened the question of biological design to scientific inquiry.
Surely, if God used magic to fashion the things around us, we'd have little chance of understanding our world. We humans aren't much good at magic. But if much of the world has developed slowly over time by natural processes, then—like so many detectives—we can begin looking for the evidence of our origins. Darwin transformed the design question from one of Divine Mystery into one of pragmatic scientific inquiry.1 Darwin coupled variation-within-species with selective survival and imagined these dynamics operating through deep time.2 In fact, it was only with the realization that the Earth had a very long geological history that Darwin was able to postulate selection operating slowly over thousands of generations. His insight opened a new path for examining the historical development of the living world.
Unfortunately, there's a persisting problem with Darwin's book. On the Origin of Species, in fact, had very little to say about the origin of species. There are many pages about natural selection, variation in populations, breeding fancy pigeons, and lots of other stuff, but the differentiation of a population to become two new and distinct species gets only a few paragraphs. Even today, the creation of two species, where there was only one species before, is a controversial area in biological inquiry. We do know there's got to be lots of speciation on this planet. How else might we get to 380,000 species of beetles?
Let's confine our discussion to the question of how species actually become differentiated over short periods of time. If we look at the fossil record, there are places where that record is detailed enough to show how a single species has actually changed over many thousands of generations. The earlier representatives of this lineage may look quite different from those later formed and be given different species names. These are called chronospecies, but because the fossil record is highly fragmented, and there is always the possibility of a related species confounding things by wandering in from an outside territory, we won't trouble ourselves with speciation over evolutionary time. Instead, let's consider the factors that can cause species to split apart in contemporary landscapes, in what is called ecological time. But first, let's reconsider the species concept itself.
WHAT, EXACTLY, IS A SPECIES?
We have already touched on a species definition, but let's look at this concept more closely. If we go to Ernst Mayr's classic definition, we are told that species are “groups of actually or potentially interbreeding natural populations which are reproductively isolated from each other.” What this clearly means is that species are on their own evolutionary track; they are not able to pick up genetic information from other closely related species. This concept demands that interbreeding and the exchange of genetic information occurs only within each species. From an abstract point of view this is fine: each species is an independent entity moving through evolutionary time.3
Unfortunately, Mayr's definition has some troublesome real-world problems. How do we know that the species we designate are not interchanging genes once in a while? We can't see or monitor “gene flow” in most wild populations. And the word potentially gives us botanists a big problem. Many plants that are able to interbreed when grown in a greenhouse do not appear to interbreed in the wild. Are these real species, or not? Botanists have tended to base their species concepts on what seems to be going on in the natural world. The word potentially was taken more seriously in zoology, where closely similar populations were often called subspecies—despite a lack of intermediates and evidence of gene exchange. Nowadays, with sophisticated DNA comparisons, ornithologists are finding that many “subspecies” are in fact genetically separate species. A recent study of two bird genera in Southeast Asia with ten accepted species and many subspecies has been transformed into sixty-one species! And it gets worse. The famous finches of the Galapagos islands—“Darwin's finches”—are beginning to look more like a dynamic hybrid mixture than an array of separate species with different lifestyles. Careful observation over many years, and through several El Niño cycles, points to a much more complex picture regarding Darwin's finches than was at first imagined.4
The situation in mammals is also troublesome. In 1982, Ian Tattersall revised the lemurs of Madagascar and recognized 36 species; by 2013, the listing of lemur species had escalated to 101 recognized species.5 How much of this is based on new discoveries and better analyses, and how much of it is changing fashions in taxonomic practice? Clearly, the species problem is a continuing challenge.
A more fundamental concern is that we know very little regarding how genes move within and between populations of plants or animals living in the wild. Things are beginning to improve, thanks to new techniques of gene analysis. We had a pleasant surprise recently when Mary Ashley of the University of Illinois at Chicago and her students analyzed the paternity of acorns among bur oak (Quercus macrocarpa) trees here in Illinois. (Yes, DNA data can reveal acorn paternity!) Some of these bur oaks grow in stands isolated by open fields all around them—not an unusual situation in Illinois, where “oak groves” were often found surrounded by fire-swept prairies. With careful genetic analysis, each tree of an isolated oak stand was identified by its unique DNA profile, and it was revealed that a good percentage of the acorns produced in that stand had been “fathered” by pollen from distant trees outside the isolated stand!6 This was a surprise; most everyone had thought that virtually all the acorns within the stand would have been pollinated by trees within the same stand. In this case, wind pollination was much more effective over longer distances than we had thought. And, clearly, gene flow was more effective over longer distances as well.
In the case of animals, documenting gene exchange is also a problem. We are reminded of this when we find dead wildlife along our countryside highways, often during the breeding season when they wander more widely and get run over more often. Not just the animals, but the genes are doing a lot of “travelling” at this time. In addition, we do not know how effectively the northernmost populations are sharing genes with southern members of their same species. How then have we described more than 380,000 species of beetle species?
RECOGNIZING SPECIES
The classical approach to species recognition has been a pragmatic one: do they look different, do they behave differently, do they live in different habitats? Plant and animal species have been described on the basis of how they differ from their closest relatives. Even when they live together in the same habitat, can they be clearly separated by their morphological or behavioral features? If so, we call them different species. Thus, the “gaps” or differences between closely related species help us distinguish them.7 Fundamentally, we assume that species look or behave differently because they are not exchanging genetic information with each other. Continuing gene exchange should cause a blurring of distinctions between different forms, and such varied forms cannot be considered different species. By using a combination of characters to separate two species, and finding that these differences are not bridged by intermediate individuals, we can conclude we are dealing with two different species.
Our own species is a fine example. Humans differ from each other within every population. In addition, there are continental patterns that are easy to see and supported by DNA surveys. While some regional groups of humans may be quite distinctive, we find intermediates with other groups at their borders, and all have the capacity to interbreed. Taxonomically, Homo sapiens is a single polymorphic species. (Though many anthropologists claim that “human races” are social constructs, the geography of DNA tells us otherwise.)
More significant with regard to the human species, there is no problem distinguishing any human with either of our nearest living relatives: the two species of chimpanzees. The variability of our own species is important; it reflects how human populations have adapted to local conditions over thousands of generations. Our varied ethnic differences, though minor, clearly reflect gene exchange over shorter and longer distances through thousands of years. However, there is no such thing as an animal intermediate between humans and chimps. There is a huge gap between both our physical and mental characteristics when we compare ourselves to chimps. Paleontologists estimate that this gap reflects a separation of more than five million years. Similar patterns can be found in many other animal and plant species. Each species may harbor considerable within-species variability, but with careful study we should be able to separate and distinguish them from their closest relatives.
When classifying plants and animals we are following in the footsteps of Carl Linnaeus, who initiated our system of binomial nomenclature. Simple, and used by scientists all around the world, the Latin binomial designates each and every species with two names. We call ourselves Homo sapiens. Homo is the name of the genus that includes living humans and a few extinct fossil representatives (such as Homo neanderthalensis). The sapiens part of our name is our specific epithet, our species name. Together, genus and species names form the binomial. The scientific name is italicized because it is in a foreign language: Latin. More important, species are the bottom rung of a hierarchy of Linnaean ranks. The genus is made up of species more closely related to each other than to species of other genera. Similarly, families are made up of genera more similar to each other than genera of other families. And so on up the line through orders, classes, and finally phyla (called divisions in plants). Though somewhat arbitrary, this hierarchy of ever larger groupings has proven immensely useful.
The binomial species name is, in fact, hugely informative. Having learned the name of the genus for the plant or animal you identify opens up an immense storehouse of knowledge. The genus will tell you the family, the order and the class to which your specimen belongs. Each of these ranks provides a broader array of features. In the case of Homo there is only one living species, but the genus name tells you that you are dealing with a primate, a mammal, and a vertebrate. All those categories carry information regarding the nature of the species. Our taxonomic system of ever larger ranks is a remarkably efficient way of organizing information about the world's living diversity. It is a logical system of nested sets. But let's get back to recognizing species.
Unfortunately, problems in recognition and separation are compounded when our so-called “species” seem to be exchanging genes. Eastern North American oaks (genus Quercus) are an annoying example. Finding a tree that looks like a hybrid between a bur oak (Q. macrocarpa) and a swamp white oak (Q. bicolor) isn't that uncommon. But their differences remain quite constant over much of eastern North America, and we continue to use these names as species. Are the hybrids contributing to gene flow? Perhaps, but hybrids may not be as successful in producing new offspring as are their parents. Because they are consistent over wide areas, we keep using our species concepts in oaks because they are useful in describing our native flora.
Another major problem in the species business, and one we'll gloss over, is that of synonomy—having more than one name for the same species. It stands to reason that taxonomist A in Austria and taxonomist B in Bolivia may discover the same species, whether in the jungle or in a museum collection. Each may realize that this species has not been described, and each may proceed to provide an appropriate scientific description and name. Publishing in different journals and at different times, there will now be two names for what may be the same species. But wait a minute! Careful comparisons of the type specimens (the specimens used to validate the names) may show minor differences. Are there two species here, or are these only slightly different variants of a single variable population? More collections and a study of their distribution should answer this question. But the problem remains: individual species may be burdened with many synonymous names, and this makes estimating actual species numbers a difficult task. Today, with the ascendancy of molecular biology, fewer taxonomists remain to get the names straightened out; as a result, many plants and animals have yet to be properly classified.
Coming back to Ernst Mayr's species definition, its importance is a philosophical one. The Biological Species Concept defines an independent lineage of plants or animals—independent because it cannot swap genes with other species. Such independence makes evolution a lot easier to explore. Best of all, the biological species concept works well for a high percentage of plants and animals. More significantly, Mayr's biological species concept informs our discussion of how new species form. By definition, the creation of new species—speciation—must be based on a cessation of gene flow between the new species and its parent species.
SPECIATION, PART 1: GEOGRAPHIC ISOLATION
The question of speciation—the formation of new species—revolves around the question of how gene exchange between populations can be terminated, and how that termination is rendered permanent. Larger scale geographical separation, surely, is the most obvious way to bring about genetic separation. If a rising mountain chain splits the range of a widespread species into two, and the separated populations are no longer able to interbreed, then slowly, over time, both will change. Each will pick up mutations not duplicated in the other, and each will adapt to the local circumstances within which they live. Both a “random walk” of new mutations and continuing adaptation to local conditions will cause the separated populations to go their separate ways. This process should, over time, result in genetic factors that prevent gene exchange, when and if the two separated populations have the opportunity to reunite later.
One can imagine the ancestors of horses and donkeys having become separated over many miles. Horses living in the steppes of Central Asia became expert at running over broad, relatively flat grasslands. Living in the rocky thorn bush of northeastern Africa and the Middle East, the ancestors of donkeys became less speedy but more sure-footed. Now, when mated, these two species form a very useful animal: the mule. Combining the greater size and strength of horses with the sure-footedness of donkeys, mules are very efficient beasts of burden. But there's a problem: they are sterile. The chromosomes of horses and donkeys do not line up properly in meiosis, and the sex cells of mules are not fully compatible. As a consequence, mules cannot reproduce! Adapting to differing environments, they became two species that are no longer capable of sharing genetic information—though they make a terrific hybrid. That's how geographic speciation is supposed to work.
Islands, surrounded by the sea, give us many instances of speciation by geographic isolation. Hawaii is the world's most isolated archipelago. Virtually all the native plants of Hawaii came from ancestors with minute wind-blown seeds, or they had fleshy fruit with seeds easily carried long distances by the birds that swallowed them. Today, over 95 percent of Hawaii's native plants are found nowhere else—that is, they are endemic to Hawaii. Likewise, the animals of distant islands had to have special attributes, allowing their ancestors to make the long journey over water. Arriving on an isolated island often presents new possibilities. A species of fruit fly (Drosophila) arrived in the Hawaiian Islands long ago; today these islands harbor almost a thousand species. The new colonists found unoccupied habitats free of their usual competitors, and diversified grandly. Lacking predation on an isolated island, some animals have become larger. Little turtles that rafted out to oceanic islands left their enemies behind, and they slowly became larger. The giant tortoises of the Galapagos are an example. But islands are often small, with not a lot to eat. Thousands of years ago, small elephants lived on islands in the Mediterranean, and in Indonesia as well. These small islands lacked larger predators and had a more limited food supply, resulting in selection for smaller, pony-sized elephants!
Another interesting pattern is the loss of flight among insects on small islands. Clearly, if you fly up off the ground and the wind blows you out to sea, you're not going to be contributing genes to future populations. On small windy islands, insects that cannot fly have a reproductive advantage, and they become the norm. Among the endemic species of carabid beetles in the Hawaiian Islands, 20 are fully winged but 184 have lost the ability to fly.
A fine example of “island isolation” has been documented among ferns growing on oceanic islands around South America. In this case the endemic island ferns—those found nowhere else—were shown to be related to rare ferns on the mainland. Apparently, it was easy to have common fern species arrive and become established on the island. But with continuing gene flow due to additional spores coming in from the mainland, these island plants “kept in touch” with their mainland conspecifics. With continuing genetic input, these island fern populations continued looking like their mainland counterparts. Since ferns can travel in the form of minute spores, they spread widely via the wind. New spores landing on the island will form new plants, and these will develop the sex cells that can interbreed with local ferns, maintaining genetic continuity with the mainland.
A different scenario played out when spores from ferns that were rare on the mainland arrived on the island. In this instance, it was less likely that the island fern would be joined by new immigrants from its mainland species. This new fern population was—in effect—more isolated on the island than populations of common species, and more likely to become distinctive. That's why the endemic island species proved to be related to rare mainland species, and not to the common ones.8 Similarly, spiders of the Galapagos Islands are mostly the same species as those found in neighboring South America, while the spiders of the more distant Hawaiian islands include a great many endemic species.
Beginning in the 1970s, the idea of geographic isolation as a generator of new species became a topic of much discussion regarding the fauna of Amazonia. Jurgen Haffer, after studying bird distributions in the Amazon basin, came up with a fascinating scenario. During the colder parts of the ice ages, Haffer claimed, parts of the Amazon basin became drier grasslands, resulting in isolated patches of rain forest. Within these isolated patches, Haffer argued, new species of birds had recently developed, helping explain the present distribution of Amazonian forest birds.9 Here, the island-like patches of rain forest had served as refugia in which the isolated forest birds could begin to form new species. This idea became very popular and engendered many concordant views. Trouble is, there was no real evidence that the Amazon basin had ever been dry enough to support extensive grasslands during recent glacial times. Pollen data did show that tree species from higher and cooler altitudes had shifted into the Amazonian lowlands during cool glacial periods. Indeed, the lowland rain forest had changed in species composition, but it had not been replaced by savannas to any extent.10 Also, almost twenty cooler glacial cycles pulsing back and forth over the last two million years allowed precious little time for speciation-by-isolation. Though now discredited, Haffer's theory illustrates the popularity of seeing geographic isolation as a major generator of new species.
Clearly, geographic isolation is a factor in having populations splitting into new and separate species. But wait a minute; remember those 380,000 species of beetles? There simply aren't enough isolated mountains or islands to give us such extraordinary numbers. Also, hundreds of beetle species can live in the same small forest; some are even members of the same genus, living together as separate species. Consider human lice. There are two species; one lives in the hair on our head, and the other species lives in our crotch! That's not very far apart from a geographical point of view. How did this happen?
Worse yet, there is no guarantee that geographic isolation will result in genetic incompatibility. Sycamore trees (in the genus Platanus) from Eurasia and North America can produce perfectly fertile hybrids, despite having been separated geographically by the Atlantic Ocean over tens of millions of years. Hybridizing the eastern Mediterranean Platanus orientalis with the eastern North American P. occidentalis gives us vigorous hybrids used as street trees in many temperate zone cities. Not only are the parental species still inter-fertile, the hybrids are fertile as well, despite millions of years of separation. Such realities discredit any claim that long-term isolation will necessarily produce genetically incompatible new species. In a fine summary of speciation, Menno Schilthuizen concludes that “…scientific data are starting to support the notion that the impact of geographical isolation has been trivial rather than paramount.”11 Surely there must be ways, other than broad-scale geographic isolation, by which populations can form new species.
SPECIATION, PART 2: ECOLOGICAL DIVERGENCE
It was Alfred Russel Wallace, co-discoverer of the theory of natural selection, who first suggested that there might be a local pathway to species formation. Imagine a population living at the outer edge of the range in which most other members of its species are found. Imagine also that the peripheral population is in an environment somewhat different from the environment in which other members of its species are living. It stands to reason that this peripheral population will have to respond to environmental challenges different from those of its conspecifics. As such, the peripheral population will begin to adapt to its new habitat, slowly changing its genetic makeup. Wallace suggested that in such an instance, hybrid individuals—with genes from both the peripheral and the core areas—will be at a disadvantage! They will not be optimally adapted to the core area, nor will they be adapted to the peripheral area. In such circumstances, the hybrids should have decreased success, reducing gene flow between the peripheral population and the larger core species. “It is this inferiority of the hybrid offspring that is the essential point,” wrote Wallace in 1883.12 With one area of the species confronting one set of environmental challenges, while other conspecifics are subjected to a differing set of challenges, it makes sense that gene exchange between the two will be disadvantageous to both. Reduced “fitness” of the hybrids will, in effect, separate the peripheral population from its conspecifics, preventing gene flow between the two. Here is a way by which a species can bud-off small peripheral populations that, with luck, can become new and distinctive species in new and different environments. Maybe that's what happened on the human body, when one set of lice became adapted to a region of our body quite different from the region their fellow lice preferred.13
Early in my work with Costa Rica's flowering plants, I ran into a rather odd conundrum. The black pepper genus (Piper) had over three hundred species names ascribed to little Costa Rica, and hundreds of dried herbarium specimens needing to be identified. Most pipers are shrubs and subshrubs, easy to collect and well represented in museum collections. However, experts had created too many species names, and my job was to get the species numbers to conform with reality on the ground. Working with dried herbarium material easily separated distinctive species, but a crowd of specimens remained that were difficult to separate. The conundrum I encountered with Piper was this: though I had real problems trying to separate some of the dried specimens, in most any Costa Rican woodland even a child could easily distinguish the species. What was going on here? Over time, an answer presented itself: the most closely related—most similar—piper species were not growing together! The two “sister species” might grow on the very same mountainside, but not at the same altitude or in the same type of vegetation. Needless to say, these very similar species, flattened and dried, were the ones that gave me trouble at the museum. The altitudinal distinction in most of these close species pairs was easy to observe in the field, but much more difficult to evaluate in specimens. Later, working with many other Costa Rican species, I found this pattern repeated in other plant families. While not common, such close species pairs sent a clear message: they are the likely products of recent ecological differentiation on environmental gradients. In Costa Rica, species pairs separated on an altitudinal gradient were especially striking because they were all living in wet evergreen forests with similar rainfall. Yet one species would be found above 1,200 meters for example, and it's nearest relative at lower elevations. Most impressive of all, there were no intermediate (hybrid) specimens and no evidence for a gradual change up the mountainside (in what is called a cline). Clearly, it looked as if an ancestral species had given rise to a new species at the edge of its original habitat. This interpretation fits with the fact that these species appear to be each other's closest relatives. In most cases, the lower elevation species were widespread, with the higher elevation montane species restricted to a specific cordillera.14
What might account for the separation of sister species on an altitudinal gradient—especially the fact that each species thrives only in a particular altitudinal range? This puzzle recalled an interesting experience we had at Ethiopia's College of Agriculture in the 1960s. After five years of field trials, our agronomists distributed seeds of wheat to local farmers. This particular wheat variety, called “Kenya Five,” had been tested and found to be particularly well suited to the local climate. Surprisingly, only two months after these seeds were distributed, a number of farmers came back with a serious complaint: their Kenya Five wheat had become infested with rust! How could this have happened after five years of careful trials? The answer was simple! All the testing had been done at two locations: the College campus at 6,500 feet (2,000 m) elevation, and the Bishoftu agricultural station at about 5,000 feet (1,500 m) elevation. Every farmer with rusted Kenya Five had come from elevations above 7,000 feet (2,150 m)! Clearly, Kenya Five was susceptible to rust in the cool and moister environments above 7,000 feet elevation. Surely, similar effects are taking place among wild plants: disease susceptibility is closely coupled with environmental conditions. Because natural vegetation is so complex, we do not see early die-off in a particular species as the highland farmers saw their wheat succumb to rust. Pathogen interactions may be one reason so many species are tightly constrained within a particular ecological or altitudinal zone. This affects humans as well. Malaria is rarely a problem for people living above about 1,000 meters (3,200 ft.) elevation in the tropics since malaria-carrying mosquitoes do not fly above that elevation!
Tropical birds and mammals also display altitudinal stratification, with local highland endemics usually related to more widespread lowland species. In addition, closely related species can be separated by different soil types or regions of contrasting rainfall. Surely Wallace had seen similar patterns during his many explorations. The suggestion that ecological differentiation might be a significant aspect of speciation is now receiving the attention it deserves.15
Recent DNA studies of species of cichlid fishes in Lake Victoria, East Africa, suggest that these hundreds of species arose from only a few introductions less than a million years ago. This should have alerted the scientific community to the possibility of speciation over very short distances. Further south along the Rift Valley, Lake Malawi has more than six hundred cichlid fish species, where a single introduction appears to have produced so many species.16 Lake Tanganyika, located between Lakes Victoria and Malawi, has also had a radiation of cichlids. More important, the three different lakes possess species that have become specialized for similar niches! In other words, the separate lakes bear witness to independent and parallel adaptive speciation.17 In each lake, cichlids gave rise to new species without the presence of significant geographic barriers.18 Avoiding competition by swimming at different depths and pursuing different kinds of prey has created a richer fauna of ever more specialized fish.
The central point regarding ecological differentiation, as envisioned by Wallace, is that it provides an explanation for speciation over small distances: competition and natural selection. If gene flow from distant conspecifics diminishes local survivorship, then any mutation capable of reducing such gene flow will be selected for. In addition, if hybrids or intermediates suffer reduced fitness, they can act as a barrier to gene flow between the core population and its offshoot, just as Wallace suggested. The importance of these general concepts is that they function virtually anywhere on the planet, and continuously! Rising mountain chains, advancing glaciers, or diverging continents are unnecessary. All that is required is adaptive selection for a new and slightly different environment or new and different lifestyle. Continuing speciation, based on environmentally driven divergent selection, I believe, explains much of our world's extraordinary biodiversity.
The notion of ecological differentiation answers another question: Why should a genus have different species? Why not have gene exchange with all one's close relations? The answer would seem to be that in many cases it's better to be a specialist than a generalist. With differing species, a genus can have representatives of its lineage in a variety of different habitats or living different lifestyles. By becoming genetically isolated, these different species can adapt more effectively to their new habitats, free of the effects of nonadaptive genes from other close relatives. Such speciation scenarios may be more common in the tropics, where being an ecological specialist is often successful.
Further evidence for ecological speciation comes from the study of Anolis lizards in the Caribbean. Here, several lineages have produced similarly adapted species on different islands, and they've done this independently! Island isolation has given rise to parallel speciation within the same genus.19 However, and with nature so versatile an enterprise, there are additional ways of forming new species.
SPECIATION, PART 3: OTHER WAYS OF MAKING NEW SPECIES
One of the simplest ways of creating a new species is through sudden chromosomal change. While rare among animals, this has been a significant process in the botanical world. Plants have messed around with chromosomes a lot, especially as regards the process called polyploidy. Recall that sex cells have one set of chromosomes; they are haploid. Most larger organisms are diploid, with each cell having two sets of chromosomes. Sexual union unites haploid gametes to form the diploid egg cell (zygote), which develops into an adult. But plants can do more. Once in a while they produce sex cells that do not divide properly and this, after fertilization, can create individual plants that have three sets of chromosomes (triploid), or four sets (tetraploid), or even more. Polyploids are individuals with more than the standard diploid number of chromosomes. Silly plants, they don't seem to realize something's wrong and often grow just as vigorously as normal members of their species. Better yet, here is a way of getting around “the mule problem.”
Recall the mule: strong and sure-footed, but with chromosomes that just can't line up properly, rendering these animals sterile. What hybrid plants with differing chromosomes can do is to double the entire set of their incompatible chromosomes! Such a doubling may be a very rare event, but rare events in nature can have profound and long-lasting consequences. The individual hybrid offspring with doubled chromosomes experiences no problems during meiosis, because each chromosome now has an appropriate homologue to pair with. Suddenly, with doubled chromosomes, the once-sterile hybrid can produce functional pollen and egg cells. They can become, in effect, a new species. This is exactly what happened in the history of our most important cereal grain: wheat. Some many thousands of years ago, wheat's wild ancestor (einkorn) hybridized with a weedy relative. This may have happened often over time, but hybrid offspring could not persist. Enter the rare event: gametes that did not become reduced in chromosome numbers. When these got together in a new individual, we had a rare doubling of chromosomes. Bingo! Such was the origin of durum wheat, the stuff we use in making pasta; it is a tetraploid, with four sets of chromosomes. And as if all that weren't enough, durum wheat then hybridized with another relative, and BINGO again. This last episode gave us the bread wheats, with six sets of chromosomes (hexaploids). Certainly, ancient farmers tending their wheat fields had no understanding of what was really going on. But they did find and propagate both tetraploid and hexaploid wheat, foods that founded civilizations!
Though rare, chromosome doubling has been an important element in plant diversification. A more recent example involves American cord grass (Spartina alterniflora). Introduced into Britain in the late 1800s, this species hybridized with a native species there. The resulting hybrid was sterile and called Spartina x townsendii. Like its parents, the hybrid had sixty-two chromosomes, and the two pairs of thirty-one were incompatible. Nevertheless, these salt-marsh plants reproduced asexually, and the hybrids maintained themselves. Then, in 1892, a new and fertile cord grass was discovered; a hybrid plant had doubled its chromosomes to 124 and was now able to reproduce sexually. Called Spartina anglica, this new and invasive species has spread around the world! On a wider scale, recent DNA analysis suggests that chromosome doubling events played key roles in the early diversification of flowering plants.20 Numbering over 280,000 living species, flowering plants bear witness to the efficacy of chromosomal change.
Animals, though doing little in the way of chromosome doubling, have many other ways of forming new species. Of these, new behaviors are among the most important. By developing a new and different song, or new and different plumage, populations of animals may be able to diverge and become reproductively isolated. Consider the peacock, with its grand show of fancy feathers. Darwin worried about peacocks; how might natural selection have resulted in so spectacular, but so burdensome, a display? His answer, and the one accepted ever since, was simple: choosy peahens! Apparently, the bigger the show, the more impressed are the ladies, and the more likely they are to mate. All this fuss and bother has an additional payoff: the females can judge the quality of their suitors. Darwin's sexual selection can explain fancy plumage, intricate mating rituals, and even provide new opportunities for speciation. Some of our eastern North American frog species, virtually identical in size, color, and shape, sing different songs at slightly different times. Their springtime songfest in swamps and wetlands is critical to bringing male and female frogs together for reproduction. Their different songs have made clear that these frogs are no longer exchanging genes, isolated by their springtime mating rituals. Perhaps they diverged after one population began singing a song just a little bit different from its neighbors. Along the same line, picture-winged fruit flies of the Hawaiian Islands are famous for having produced hundreds of endemic species; elaborate mating rituals seem to have been a key factor in their explosive diversification.21
WHY AREN'T THERE MORE SPECIES?
Here's a question that I find profoundly naive, a product of our modern, luxurious, famine-free lifestyles. In modern societies, we no longer lose a significant percentage of our children before they reach adulthood. We live in a world that no longer confronts the hazards most other species have to deal with. Before agriculture, we humans were constantly challenged by starvation, predators, parasites, and disease. The “real world” is different. Extinctions, local and global, are the rule, not the exception.
Imagining a world of dynamic equilibria where bad things happened on only rare occasions, many biologists of the 1970s dismissed the notion of “Nature red in tooth and claw!” But bad things happen often, and biological theorists have come back to their senses. Fierce and relentless competition in unpredictable environments not only created the incredibly diverse world we see around us, but continues to shape it. Species-loss and extinction events have occurred throughout the history of life; it is no small wonder that our planet supports as many species as it does.
Surely there must be many ways that population systems can become divided into genetically isolated species or bud off new sister-species. How else might the world have come to support so many species? And why have the great extinction events been followed by a resurgence of species numbers? Once an environment has been devastated, and many of its species sent into oblivion, the forces of renewal begin their work again. Empty environments invite new colonists, and once these are settled in, they seem impelled to further divide the landscape amongst themselves. Intense competition between members of the same species may be the underlying process. In this way, ecological differentiation provides a motive force for building ever greater species numbers. Forget about balanced ecological systems in some imagined equilibrium. Rather, speciation is dynamically driven by incessant competition, happenstance division, pest pressure, and new opportunities. Clearly, speciation has countered extinction effectively, making our world ever richer in species numbers.
Having a better notion of the ways in which new species can be generated, let's turn to the distribution of species numbers around the globe. Species are not peppered randomly over the world's land surfaces; different habitats support many different numbers of species. In chapter 4 we will consider the geography of species richness and how it is distributed around the globe. In chapter 5 we will consider general patterns and unusual anomalies in species distributions. We then examine how local biodiversity maintains itself, in chapter 6, before returning to advancing complexity in further chapters.