Entering the Sixth Extinction: Evolution, Diversity and the Stewardship of Life

Who does not marvel at the diversity of flowers in an alpine meadow, butterflies in a tropical forest, fish in a coral reef, or even birds at a backyard feeder?

Understanding diversity has historically been, and remains to this day, one of the fundamental quests of biological science. Why is there this number of species in a lake, a grassland, a continent, and not more? Or fewer? Why are there more species in tropical regions than in temperate regions, and in temperate regions than in arctic regions? What is the relationship of biological diversity to the sustainability of ecosystems? And why– as the geneticist J.B.S. Haldane is credited to have mused in the presence of theologians– does God have such an inordinate fondness for beetles? (350,000 described species, to date!)

When encountering the word “evolution”, many individuals typically think of the theory that endeavors to explain the origins of living things. And that would be correct. But evolution has a second component that is rarely discussed, and even less appreciated, outside of scientific circles– and that is the theory that explains the extraordinary diversification of life. It is safe to say that many more biologists spend their careers studying this aspect of evolution than the former.

Scientific questions about biodiversity are explored through a conceptual framework that couples together evolutionary and ecological theory. Indeed, the U.S. National Science Foundation’s marquee program for the study of biodiversity requires grant applicants to integrate evolutionary and ecological approaches. And in many research universities in the U.S., the two disciplinary areas are merged as departments of Ecology and Evolutionary Biology.

How is it that evolution and ecology are so interdependent? In the process of evolution, natural selection “selects” for genetic variants in a population that have the highest fitness (genetic contribution to future generations) in a particular environment. It is these interactions of organisms with their environment that comprise the scientific discipline of ecology. The forces of selection can be physical factors, such as temperature, salinity, light, or nutrient availability. Or, they can be biotic factors, such as herbivory, competition, and predation. Organisms evolve in response to each other, and spin out new species in the process: diversity begets diversity.

Let’s consider a few examples of how evolution and ecology interact to produce new life forms. The first is a recent example, involving the diversification of lake trout (the lyrical Salvelinus namaycush) in the Great Lakes of North America.¹ The seven Great Lakes adopted their current forms during the period 3,000 -10,000 years ago, as meltwater from retreating glaciers filled the basins they left behind. Lake trout were early inhabitants of this lake system which, due to the disappearance of connecting rivers, contained few species of predatory fish. As a consequence, lake trout evolved into scores of varieties (populations), each adapted to a particular ecological niche, and reproductively isolated from other populations. Some lived in the shallows, and some lived at depths hundreds of feet below the surface. Some fed on plankton, some on crustaceans, and some on other fish. Some reproduced in spring, some in summer, and some in fall. Some, at maturity, attained a length of one foot, whereas others approached four feet. Commercial fishermen christened the various forms as bay trout, black trout, moss trout, shoal trout, redfins, yellowfins, and buckskins, among others. Ten thousand years barely registers on an evolutionary timescale; given a longer period, these trout of the Great Lakes would surely have diversified into discrete species. Indeed, this very process of adaptive radiation and speciation is thought to have produced the extraordinary diversity of cichlid fish (~250 species) that evolved from a single ancestral form over 10 million years in Africa’s Lake Tanganyika.²

Our second example of diversification at the interface of evolution and ecology involves plants and the insects that feed on them. Plants and herbivorous insects comprise roughly half of the macroscopic species on earth, nearly 400,000 known species each. Plants rely on an array of defense mechanisms, especially toxic chemicals, to protect themselves from damage by insects. The defenses are generally effective against most, but not all, insects. And most herbivorous insects are specialists, employing adaptations that enable them to breach the defense systems of a small number of plant species. During the Cretaceous period (145 to 66 million years ago), flowering plants and herbivorous insects underwent explosive, simultaneous speciation. The leading scientific theory for this unparalleled diversification invokes an evolutionary arms race of “adapt and radiate”: plants would evolve new chemical defenses, affording protection from insects, and (like the lake trout above) radiate to occupy new ecological niches. Insects would subsequently evolve mechanisms to defeat the plants’ defense, and then experience their own radiation into new food niches. This dynamic feedback loop of ecological interactions adapting, counter-adapting, and counter-counter-adapting over evolutionary time gave rise to much of the rich biological diversity of the world today.

The answer to the question “of what value is biological diversity?” is not only of academic interest; it has enormous ramifications for sustaining the ecological systems that support a healthy planet and the people that inhabit it. Biological diversity contributes to the productivity and resiliency of ecosystems, and thus safeguards the services that ecosystems provide. Those services— such as air and water purification, climate regulation, soil formation, disease prevention, pollination— are valued at $124 trillion/year, 1.7 times the global GDP.³ Many of the resources required for human flourishing, including medicine and food, are products of the world’s rich biological diversity. If you have used any of the 74% of modern medicines derived from plants (e.g., aspirin, taxol, Tamiflu, codeine), you have the evolutionary arms race between plants and insects to thank. If you consume any of the 70% of agricultural crops that benefit by insect pollination, you have a diverse array of pollinators to thank. And, particularly in this era of escalating and widespread environmental degradation, we can thank biological diversity for providing a hedge against future famine and disease.

Despite their relevance, however, those values are largely utilitarian. And, due to their diffuse and long-term nature, they rarely influence the daily decision-making of people. As articulated alike by scientists, ethicists, and environmental theologians, moral arguments may provide a better hope for human valuing of biological diversity. And nowhere should that be truer than in the Christian community. God not only delights in and cares for creation, but chose to share that caretaking responsibility with humans (Genesis 1, Psalms 24 and 104). God also commanded us to care for people living in poverty, the very people who, throughout the world, are affected first and most by the loss of biological diversity.

The earth is now entering its sixth mass extinction event.⁴ Similar in magnitude but different in cause from those that came before, this one has been triggered by humans. The natural rate of extinction is estimated to be 1-5 species per year. The current rate is 1,000 to 10,000 times faster, approaching one species per hour. A recent report by the World Wildlife Fund revealed that worldwide, population declines of mammals, birds, fish, and reptiles have averaged 60% in the last 40 years.⁴ Even if we return to pre-human rates of extinction, recovery of the loss of mammalian diversity would require millions of years of evolution.⁵

Within the secular world, the Christian community is widely recognized for two prevailing attitudes toward the fundamental components of evolutionary theory: a fierce, combative rejection of biological origins, and an utter indifference toward biological diversity. Could it be that our priorities are not fully aligned with God’s priorities?  Would we not think it morally offensive if a group of emergency room doctors spent their energy arguing about how a patient got there, rather than executing their clear Hippocratic duty to care for her?

Two fundamental tenets of the Christian faith are that humans are made in the image of God, and that they have been called to co-labor in the work of redeeming all of creation. It’s who we are. And it’s what we should do.⁶