By John Palka — Posted July 2, 2017
The natural world is full of diversity that catches our eye. Flowers of many hues. Gigantic trees soaring skywards and tiny mushrooms pushing up through the forest floor. Eagles soaring in the air above and earthworms burrowing in the soil. The varied forms of sea life—whales, octopuses, and barnacles—and the ubiquitous world of organisms too tiny to see with the naked eye.
With this diversity, however, coexists greater or lesser similarity. We recognize birds and butterflies as groups, and no one would confuse them with each other just because they both fly. We recognize wolves, coyotes, and dogs—whether Chihuahuas or St. Bernards—as a natural group manifestly distinct from lions, tigers, and cats. A rose is a rose is a rose, even though the local nursery may have twenty or more cultivated varieties.
All cultures have recognized groups of organisms in nature and have evolved ways of defining these groups. Biological scientists have done likewise, but with a tremendous advantage. Modern techniques allow the determination of evolutionary relationships among organisms, so that the groupings used by systematists—the biologists whose focus is the determination of relationships and the naming of organisms accordingly—reflect not only their similarity today but also their evolutionary history. Let’s explore this perspective on the natural world by looking at a group of plants familiar to those who wander the woods of the Pacific Northwest.
Local Members of the Heath Family (Ericaceae)
Among the abundant and conspicuous trees and shrubs of the Pacific Northwest are madrone, salal, evergreen huckleberry, and native rhododendron. Biologists place all these plants in a single family, the Ericaceae. (Erica is the Latin name for heaths and heathers, which also belong to the family, as do blueberries.) Given the apparent diversity among these plants, what prompts biologists to place them in a single family? Let’s look more closely.
Madrones (Arbutus menzesii) are moderate-sized, evergreen trees that are found predominantly along the shoreline or not too far inland. They are instantly recognizable by their orange-red, peeling bark.
Salal is abundant in many forests. It is a low-growing to mid-sized shrub with shiny, oval, evergreen leaves. Like madrone, it has small white or pinkish flowers.
Evergreen huckleberry is likewise a denizen of the forests, particularly those recovering from long-ago logging. It is evergreen like madrone and salal. The new leaves it forms in the spring are a striking rust color.
Its flowers are much like those of both madrone and salal.
Each spring Northwesterners delight in finding rhododendrons blooming in the native forests. Only three species occur here, whereas there are hundreds in the mountains of southern China, but the local ones are highly appreciated and one of them, the coast rhododendron (Rhododendron macrophyllum), has been selected as the Washington state flower.
Its flowers are as showy as any of the cultivated varieties sold for planting in gardens.
There are number of characteristics that all these plants, and most of the other four thousand or so species in the family Ericaceae, share. We have already noted one of them—they have evergreen leaves. We have also paid attention to the fact that their flowers are similar, except that the flowers of rhododendrons don’t seem to look anything like those of the other species. But flowers turn out to be a prime focus of systematists, so let’s look a little deeper.
Here, for comparison, is the flower of a wild Nootka rose (Rosa nootkensis).
Its color is quite similar to that of the rhody, but look more closely. (For a reminder of how flower parts are arranged and named, look at the post Flower Forms.
- The rhododendron has eight to ten stamens, the rose has many.
- The rhody has a single style for capturing pollen, the rose has many.
- The petals of the rhody are fused edge-to edge. (You can see this clearly in the photo above where one of the flowers has dropped from its stem.) The petals of the rose are free and unfused.
These are just the characteristics that can readily be seen in a surface view; there are many more ways in which these two flowers differ. This suggests that, despite the similar color of the flowers of these two particular species, rhodies and roses are not closely related. Systematists place roses in quite a different family, the Rosaceae.
How about rhodies and the other three species placed in the Ericaceae?
- You can see that the petals of madrone, salal, and evergreen huckleberry flowers are fused, just like those of the rhody.
- You can see a style peeking out of some of their flowers—a single style, as in the rhody.
- You cannot see the stamens—they are hidden within the flowers—but they do in fact number eight to ten like in the rhody.
One of the reasons that systematists place a lot of emphasis on flower characteristics is that these reflect the plant’s pollination mechanisms, and hence its mechanisms of reproduction. One of the properties of species—markedly so among animals though rather less so among plants—is that different species do not cross-fertilize and therefore remain reproductively isolated, usually even when they are found in the same locale. For this reason, characteristics that are related to the process of fertilization, including the characters of flowers, are likely to reflect the evolution of species and thus the history of life on Earth.
To sum up, thus far we have seen that systematists collect in the family Ericaceae shrubs and trees that usually have evergreen leaves and whose flowers usually have a single style, eight to ten stamens, and fused petals. What else is there?
In addition to structural features, there is a nutritional feature that many species in the Ericaceae share. They often grow in poor, acid soil and form rich and structurally distinctive associations with mycorrhizas. As we have seen, mycorrhizal associations are very widespread among plants. However, those found in most Ericaceae are structurally distinctive (they are even called ericoid mycorrhizas) and often are required for the plants to grow, whereas in other cases such associations may be supplementary rather than necessary. So, we can add tolerance of poor, often acidic soils and a dependence on mycorrhizas to our list of the characteristics of the Ericaceae.
The authors of a recent analysis of the family Ericaceae used close to a hundred anatomical characteristics, as well as a series of molecular ones, in their work. It is the combination of all these attributes that underlies the groupings developed by modern systematists.
Classification and Evolution
As we saw at the beginning, the goal of systematists is two-fold. First, they seek to identify order in the fantastically diverse world of nature, order that is comprehensible to us humans, and second, they try to have that order reflect the way that organisms have evolved over the past three billion years or more. This is a daunting task indeed! We will only look briefly at a tiny fragment of this huge field, how plant systematists think about the ordering and evolution of flowering plants.
The field of systematics started in the 18th century, primarily with the Swedish botanist and physician Carolus Linneaus. Linnaeus established the practice of naming plants and animals with two names (in Latin, the language of all European scholarship at that time), the genus followed by the species. In this way close similarities could be acknowledged in the naming process—there could be several species in one genus. Called the binomial (two-name) system, it is still in use today; I have given the Latin binomial designation for all the species we have encountered on Nature’s Depths, for example Arbutus menzesii in this post. In addition, Linnaeus grouped his genera (the plural of genus) into larger assemblages—families, orders, and others—in an attempt to assign them to a single, comprehensive system covering all life as it was then known.
It is notable that this was a system of classification, not one of evolution. Charles Darwin and his contemporary, Alfred Russell Wallace, had not yet formulated the theory of evolution by natural selection, so comprehensive classification was the most that Linnaeus could aspire to. Even this was a heroic task, however—his two-volume study Systema Naturae (The System of Nature), whose tenth edition was published in 1758, covered 4,000 species of animals and 7,700 species of plants. He was the sole author!
Darwin published The Origin of Species almost exactly a century later, in 1859. It was Darwin who explicitly set the task for biologists to develop a system of classification that would reflect evolutionary relationships—a comprehensive tree of life. Over the years many efforts were made, but all that biologists had to rely on in their work were the structure, ecology, and behavior of the organisms they dealt with. The theory of evolution was in place, but the mechanisms of inheritance and variation were as yet unknown. Darwin, and after him the pioneering geneticist Gregor Mendel, simply deduced that they must exist. Only in the 20th century did we finally obtain compelling evidence that genes are made of DNA, that mutations are modifications of the DNA, and that DNA and the genes it encodes are faithfully replicated and transmitted from one generation to the next.
This set the stage for meeting Darwin’s challenge. With the advent of astonishingly cheap DNA sequencing, systematists now can study the DNA of many species and uncover their historical relationships. Kinship can be established in much the same way as human paternity can. This has revolutionized the science of systematics. Let’s look at a simple illustration.
One Family or Two?
Here is a photograph of a one-flowered wintergreen, Pyrola (renamed as Moneses) uniflora.
It is perennial and evergreen. Its flowers have 4 or 5 petals, 8 or 10 stamens, and a single style. This plant grows in forest soil and is heavily dependent on mycorrhizas. Sound familiar? It is not a shrub, and its flower petals are not fused, but otherwise it shares a number of characteristics with members of the Ericaceae. For many years it was placed in a separate family, the Pyrolaceae, but this family was recognized to be closely related to the Ericaceae. New DNA evidence, however, shows compellingly that the Pyrolaceae share a history with the Ericaceae and that, therefore, the two should be recognized as a single group. To indicate this, modern treatments of the Pyrolaceae classify them as a group within the Ericaceae.
Much the same applies to the Indian pipe (Monotropa uniflora) we have encountered before.
This forest dweller has flowers with 4 or 5 petals, 8 to 10 stamens, and one style. It grows from an underground root, a rhizome, and it grows anew each year. In other words, it is not evergreen—indeed it is not green at all, but a ghostly white. Totally lacking in chlorophyll, the Indian pipe is obligately dependent on its mycorrhizal fungi and these are in turn dependent on nearby trees whose roots they also colonize. This is a classic example of mycoheterotrophy.
Like Pyrola, Monotropa was once classified in its own family, the Monotropaceae, closely related to the Ericaceae. Sometimes it was placed in the Pyrolaceae. Nowadays, the Monotropaceae are usually recognized as a group within the Ericaceae, again based primarily on DNA evidence.
So, here we have two examples in which previously suspected familial relationships have been reinforced and clarified by molecular evidence and a modest re-classification has resulted. There are also many examples in which species that were classified in single groups on classical criteria of structure, ecology, and behavior have been separated because molecular evidence shows that they are not closely related by descent. In such cases the observed similarities are interpreted to reflect convergent evolution, the evolution of attributes that fit organisms with different past histories for a particular life style—a specific ecological niche.
The reciprocal situation also occurs. Organisms that are closely related by descent may come to look rather different because they encounter environments that exert diverse selective pressures. This is divergent evolution.Once again, molecular evidence can reveal relationships that visible characteristics mask.
Systematics and the Tree of Life
The modern molecular approach has profoundly enriched our understanding of the diversity of organisms on Earth and of their evolutionary interrelationships. The “System of Nature” (Linnaeus’s term) keeps changing as new evidence comes in. But when we engage in the kinds of observations on which Nature’s Depths is based—when we go out into the woods or stroll on the beach—the old ways of looking at the organisms we encounter still prevail. A good field guide will still prompt us to look at the detailed structure of what we see; it will still give us genus and species; and it will still encourage us to think of the entire ecosystem in which we and our sister organisms find ourselves. Given all this, our experience will be enriched if we also remember that the names that we give to the organisms that surround us, and the names of the families and larger groupings into which we classify them, reflect the path through which they have evolved through our Earth’s deep-time history. Try it!