A Change in the Sources and Forces of Change

My basic understanding of plant evolution has come from courses and books that spoke to the sources of variability in a population, and the forces that impact the survival of certain individuals over others, survival that played out in the production of offspring that would come to define the next generation. Sources relate how new genetic possibilities arise. Mutation is best-known by the public. This happens when wholly new “genes” (really, they are different options, alleles) appear in the genetic material of a population; if a mutation impacts a place where sex cells are made (a pollen sac or an ovule)then new genetic possibilities may become available for future generations. But there are other ways a population can get fresh genetic options, such as through successful sexual encounters with other populations that result in hybrid progeny. We also know that genetic material can be transferred from one plant to another through bacterial and viral activity. Over time, a lot of new options have given rise to new forms in plants.

I can’t recount all the different forces that are normally discussed, pressures on plants from variability in soils and changes in climate, the activity of pollinators and herbivores (which many people, today, call predators.) Pressures mean that certain plants will have greater success in producing viable offspring than others.

Every plant you encounter today has its own complex natural history, and its own, reticulated story of change through eons and countless generations. Most significantly, every living plant today is the current link in an ongoing chain of life that has never been interrupted, not for the billions of years since the first living ancestor(s) of that very plant came into cellular existence. Plant life is not spontaneous; it is serial – life begets life – cells are formed only from nuclei and cytoplasm that “know” how to make cells. Somehow the wondrous differing forms and biologies we call species, each begetting their own kind, reach back through the struggle for survival to common ancestors.

Looking back on what I understood about evolution, I seem to have missed books and teachers giving much attention to the force of disease. We talked about threats to individuals and populations, and I’m certain we mentioned disease, but it seemed casual. And I know that in the last few decades scientists have discussed more about evolution of disease resistance through production of secondary compounds, such as flavonoids. But to my mind, the role of disease in plant evolution is understated.

Plant disease is a big deal. In the last century, disease has proven to be the most immediate source of devastation to plant populations around the world. When E. Lucy Braun studied the Eastern mixed mesophytic forest, she defined the zone based on eight dominant trees. Major populations of two of those dominants, American Chestnut and American Elm, were wiped out in short order through 20th century introduction of diseases from Europe, diseases that had previously been endemic to European relatives. Today, American Beech is threatened by introduction of yet another disease.

Books have appeared in the last two years that portend extinction of cultivated bananas, due to fungal disease that is readily spread. The world’s citrus crop appears in eminent danger of collapse due to spread of what is called greening disease. A native disease of Hevea brasiliensis, the Rubber Tree, doomed commercial production of rubber in Manaus.

None of these diseases is newly evolved. Their potency is no more than ever, it was their dispersal that had held disaster at bay. Human activity, spreading disease and vectors, establishing monocultures that incubate inoculum, is the wind behind the sails of infection. Surely, each disease would made its way further afield without human activity, but not so quickly as today, not so quickly as to defy the steady counter-force of evolutionary selection for resistance.

Seeing how quickly a new disease can annihilate a population reminds us that disease has been a significant player over history. Certainly, disease has not been the force that drove flower shape, or dispersal success, but it has certainly penalized naively successful lineages that came in contact with resistant disease-bearing groups And diseases evolve also, in most cases even more quickly than do plants. So there has been no shortage of the impact of disease on evolution of plant populations.

Interestingly, the ancient formula seems upended. Because humans basically have taken over management of the planet, we have altered, in some cases, neutralized the normal forces of evolution; the historical evolutionary process is dysfunctional. However, the role of disease in plant survival, thus plant evolution, becomes greatly magnified through human activity. Disease has become the great tyrant of modern plant populations. And the Genie has escaped the bottle; there is no going back.

From an agricultural and horticultural perspective, evolution and spread of disease and pestilence appear to threaten all plant life on earth. At the very time humans have elevated the spread and impact of disease, we have also come to fear technology that will prove necessary to conserve plant diversity – especially the diversity of food crops. Genetic modification of plants will, in the end, prove crucial to the survival of some of our most important and wonderful plants. One wonders how society will negotiate the concerns and politics of genetic technology, but the disease-driven trend of plant diversity is creeping extinction.