Populations within species evolve in different ways, adapting to local physical conditions and to a continually changing web of interacting species. Generation after generation, natural selection constantly adjusts the traits of populations. For the most part, however, this relentless evolution is not what most people envision when they think of evolution--it does not lead to the emergence of new species or "directional" change in the traits of a species. These seemingly aimless meanderings are the essential dynamics of evolution, with directional change and speciation as occasional outcomes.
Relentless Evolution by John N. Thompson | Boffins Books
Pathogens and pests that affect people and crops offer some of the most familiar and compelling examples of relentless evolution. Bacteria evolve resistance to antibiotics, insects evolve resistance to pesticides, and plant pathogens evolve to overcome disease-resistant varieties of crops.
In natural systems, biologists have documented hundreds of cases of ongoing evolution in a wide range of species. These examples include changes in morphological and physiological traits, life histories, behaviors, and interactions with other species. Microbial populations can evolve new traits in a matter of weeks, while plants and animals can evolve in detectable ways within a few decades or even a few years.
Relentless Evolution examines the pace and dynamics of evolutionary change and the ecological drivers of ongoing adaptive change in species and populations. It is Thompson's fourth book on evolution, continuing his exploration of the processes that drive evolution and shape the entangled web of life. We did not realize until recently the relentlessness of evolution, because we lacked the tools, and we often looked for it only where we expected we would most likely find it—principally in environments we have greatly changed.
In the old days of research on evolution, just a few decades ago, we hoped at best to catch glimpses of evolution in action. Scientists and nonscientists alike thought that evolutionary processes acted over long periods of time. We thought that any chance of seeing evolution occur would be due to luck or to extremely unusual circumstances.
It was common for biologists to talk about "ecological time" as compared with "evolutionary time. Most of us biologists therefore felt we could ignore rapid evolution as a potential explanation for the changing patterns we often find in populations and biological communities. When asked for examples of evolution occurring over short timescales, we would rely on a few well-studied cases.
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We would point to the increases in dark-winged forms of peppered moths in regions of high industrial pollution, the rapid evolution of resistance to pesticides in some insects, or the continuing evolution of human influenza virus during the past century. There were some other examples from which we could choose, but few had been analyzed in detail. They were collectively viewed as the fortunate exceptions we could study. Those days are over. Well-studied examples of ongoing evolution within our lifetimes are being published in professional journals at such a fast rate that it is hard to keep up with them.
Even those of us who have studied the ongoing evolution of populations have become increasingly impressed by the speed at which some populations are evolving in nature. The examples come from studies in the fields of ecology, epidemiology, medicine, microbiology, agriculture, forestry, wildlife management, marine biology, fisheries biology, population genetics, and molecular biology.
We have now come to expect that insects and weeds will evolve resistance to pesticides, influenza viruses will evolve at speeds that will keep epidemiologists nervous, and new strains of antibiotic resistant bacteria will continue to proliferate and cause concern within the medical community.
We now know that even the simple act of harvesting fish populations has led to marked evolutionary changes in some species. As we have come to realize the sometimes rapid pace of evolution, many biologists and some policy makers and resource managers have increasingly turned to the problem of how to manage it. How do we slow the rate at which insects evolve resistance to pesticides and bacteria evolve resistance to antibiotics?
How do we conserve and restore biological communities amid global change that is driving evolutionary change in some species? How do we control invasive species that are evolving as they spread across new continents and oceans? Amid our growing appreciation of the pervasiveness of evolutionary change, just about every possible view has now been expressed on how human activities may alter the future evolution of species. That discussion, though, only highlights the more long-standing debate in evolutionary biology of what drives ongoing evolutionary change— sometimes quickly, sometimes more slowly, but ongoing nevertheless.
We can point to particular cases and their causes: rising or falling temperatures, changing patterns of rainfall, sexual selection within species, competition, predation and trophic cascades, parasitism, mutualism, the balance between mutation and random loss of genes, and the occasional odd asteroid.
These ad hoc explanations simply underscore the fact that almost all species live in a constantly changing world that demands evolutionary change in populations.
The relentless evolution of pathogenic Escherichia coli.
If we are to interpret how our world is changing through climate change, habitat modification, and the wholesale movement of species among continents, we need to understand much better the background chatter of endless year-to-year evolution and its causes. We need to know the extent to which continual evolutionary change is truly important in shaping and maintaining the web of life at every timescale and across every spatial scale.
This book explores the pace, genetics, and ecological drivers of adaptive evolutionary change. It is about why natural selection is generally stronger and adaptive evolution more dynamic than, until recently, we have thought. The early chapters focus on adaptive evolutionary change in populations over tens, hundreds, and thousands of years rather than millions of years. This is the part of evolutionary change that is most directly and immediately important to the ecological dynamics of biological communities, to the conservation of species, and to human society as species all around us continue to adapt amid environmental change.
Ignoti, sed non occulti.
The later chapters explore the consequences of ongoing evolution for ecological speciation, adaptive radiation, and the continual reformulation of the web of life. The great problem to solve about life on earth has gradually shifted over the past century and a half since Darwin's Origin of Species.
We began with the problem of whether species evolve. The problem has been solved so completely that we are now faced with a problem at the opposite extreme. Why is evolution so relentless, altering populations generation after generation? After all, species generally seem well adapted to the environments in which they live, yet they continue to evolve even in environments that have not undergone major recent changes. Most of these evolutionary changes occur through modification of genes and traits that already have been subject to selection for many thousands of generations.
Superficially, these small changes seem like aimless evolutionary meanderings. Slowly, though, we have come to realize that these continual adjustments in adaptation are often surprisingly important to the persistence of populations. These small changes capture the ecology of evolution. That appreciation has made us realize that we need as deep an understanding of the ecological drivers of evolutionary change as we have tried to develop for the genetic and molecular processes that translate ecological selection into evolving traits.
These chapters explore how our understanding is progressing. We know the component parts of the process of adaptive evolution. It begins with differences among physical environments that impose selection on populations to adapt to local temperatures and the availability of water, light, and nutrients. Without major environmental change, populations become well adapted to their local physical conditions.
Populations and species, though, do not live in a vacuum. They adapt, speciate, and go extinct as parts of continually changing webs of interacting species. Much of the ongoing evolution of each species is about exploiting other species and avoiding exploitation. The result is a process of reciprocal evolutionary change—coevolution—that shapes the web of life in different ways in different environments.
Occasionally those webs are torn apart by huge physical upheavals that lead to mass extinctions, creating new opportunities for diversification. Overall, the physical environments provide the basic templates for adaptation and diversification, but interactions among species multiply and modify, in myriad ways, how selection acts within and among those templates. That much now seems obvious to us after many decades of hard-won paleontological, evolutionary, and ecological data.
We are, though, still struggling with fundamental questions about the ecological structure and dynamics of evolutionary change.