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How Will the Sixth Extinction Affect Evolution of Species?

Norman Myers and Andrew Knoll


The current extinction crisis, if unchecked, will disrupt evolution to a degree that:

  • earth will see a proliferation of pests and a decline of large mammals
  • the tropics will no longer be powerhouses for the evolution of new species
  • the biodiversity losses will persist for millions of years

May 2001


The Toolache Wallaby (Macropus greyi) went extinct in Australia in 1943. Source: John Gould, F.R.S., Mammals of Australia, Vol. II Plate 19, London, 1863.

Human activities have brought the Earth to the brink of biotic crisis. Many biologists consider that coming decades will see the loss of large numbers of species1-5… these extinctions

  • will alter not only biological diversity
  • but also the evolutionary processes by which diversity is generated
It takes about 5 million years for the world to recover after a major extinction.

A simple consideration of time underscores the magnitude of the challenge to scientists and public alike6. Episodes of mass extinction documented in the geological record were followed by protracted intervals of rediversification and ecological reorganization; five million years can be considered a broadly representative recovery time, although durations varied from one extinction to another7. Suppose, too, that the average number of people on Earth during the recovery period is 2.5 billion (by contrast with the 6 billion today). Under these conditions, the total number of people affected by what we do (or do not do) during the next few decades will be in the order of 500 trillion. We are thus engaged in by far the largest “decision” ever taken by one human community on the unconsulted behalf of future societies.

The Core Concept

One of the first truisms absorbed by biologists is that evolution is not predictable. However, despite our inability to predict the products of evolution… we can make meaningful estimates about evolutionary processes as they will be affected by the depletion of biological diversity. [In other words] we may have little basis for predicting what large mammals might look like two million years from now, but much better reason to suppose that there will be very few of them.

Evolution is being altered by the current extinction.

The evolutionary dimension to the current biotic crisis has been vividly expressed by Michael Soule8: “Death is one thing, an end to birth is something else…” At least as important will be the alteration of evolutionary process, and for a period that is difficult to estimate but must surely measure in millions of years.

Certain biomes, such as coral reefs, may lose all of their inhabitants.

First-Order Effects: There will be several first-order effects stemming from the biotic crisis:

  • a major extinction of species within the foreseeable future, estimated by some to remove between one-third and two-thirds of all species now extant1,2,5,9

  • a mega-mass extinction of populations, proportionately greater than the mass extinction of species, within the foreseeable future10

  • alien invasions and other mixings of biotas11-14

  • progressive depletion and homogenization of biotas, with potential threshold effects on ecosystems15,16

  • biotic impoverishment generally, possibly including a decline of global biomass,16-18

  • gross reduction if not virtual elimination of entire sectors of some biomes, notably tropical forests, coral reefs, and wetlands, all of which have served as centers of diversification in the past19-22

Further Evolutionary Effects: These first-order impacts will likely engender a series of further consequences, including although not limited to:

Gene pools will be so depleted that species may not be able to bounce back.
  • fragmentation of species’ ranges, with disruption of gene flow23-26

  • decline in effective population sizes, depletion of gene reservoirs/pools10,27,28

  • biotic interchanges — introducing species and even biotas into new areas11,14,29

  • these impacts, in turn, might disrupt food chains/webs, symbioses, or other biological associations30,31

Species that have adapted to human environments will dominate.

These consequences could lead to further repercussions such as the following:

  • An outburst of [select] speciation: It is unlikely that speciation will be evenly distributed among surviving lineages; it may be concentrated among particular clades or ecological types that thrive in human-dominated ecosystems [e.g., rodents]32,33
New species may not evolve if tropical forests disappear.
Loss of species means the loss of sub-species.
Large mammals are likely to go extinct.
  • Depletion of “evolutionary powerhouses” in tropics: According to Jablonski20, the tropics have been “the engine of biodiversity” for at least 250 million years. Today, we face the prospect of severe depletion if not virtual elimination of tropical forests, wetlands, estuaries, coral reefs, and other biomes, with their exceptional biodiversity and ecological complexity. Because some of these biomes appear, in some senses at least, to have served in the past as preeminent “powerhouses” of evolution34,35, their decline could entail severe consequences for rediversification as the biosphere emerges from environmental crisis.

  • Decline of biodisparity: Elimination of species is not the only measure of an extinction event. There can be declines, as well, in biodisparity [morphological and physiological variety].36-38

  • An end to speciation of large vertebrates: Even our largest protected areas will prove far too small for further speciation of elephants, rhinoceroses, apes, bears, and big cats, among other large vertebrates28,39,40.

Lessons from the Past?

The geological record is replete with extinction events, their intensity ranging from the small and local to global mass extinctions that shattered Earth’s biological order. Inevitably, extinctions were followed by rediversification, directed in the case of the largest events by ecological reorganization.

The damage caused by the current extinction is probably permanent.

The geologic record contains much evidence of bounce-back processes38,41-46, but … In the present biotic crisis, it is hard to envision a scenario under which the factors that are driving the biosphere toward grand scale biodiversity loss will be mitigated in the wake of such loss. On the contrary, on any time scale we can envisage (and any scenario that does not involve early mass mortality for humankind), the situation becomes bad and then stays bad for some time to come. Thus, on the time scale of the human species, environmental disruption (or at least aspects of it) is permanent. Under these circumstances… the prospects for rediversification are limited.

Conservation Responses

  • Should we be content simply to safeguard as much as we can of the planetary stock of species?
  • Or should we pay equal if not greater attention to safeguarding evolutionary processes at risk?47-49

Is it sufficient for us to maintain, for example, just the two elephant species we already have, or should we try to keep open the evolutionary option of further elephant-like species in the distant future?

There are so few elephants left, it is unlikely that new elephant species will ever emerge.

This is an unusually significant question, with unusually significant implications for conservation strategies. Elephants, along with many other large mammals, are inclined to move around a good deal, a trait that enables them to maintain gene flow across large areas. As a result, their gene pools often tend to be fairly uniform [an elephant in East Africa may not be so different from one 4,000 km away in South Africa50. Regrettably the remaining populations of elephants, substantial and extensive as they are, albeit fragmented and declining fast, are probably already below the minimum numbers to keep open the possibility of speciation51.

Should we focus on protecting only the dying species?
Conclusion: What we do now will set a course for evolution.

In marked contrast to elephants, with their slow breeding rates, many insect species have immense breeding capacities and rapid turnover rates. These latter attributes offer quick adaptability to environmental shifts, whereupon genetic changes are passed along promptly. These attributes not only leave many insect species well suited to survive the environmental upheavals of human activities, but they offer exceptional scope for speciation in comparatively short order. By contrast, elephants, together with other large-bodied species that reproduce slowly and hence possess restricted capacity for genetic adaptation, will be at an extreme evolutionary disadvantage. Does this factor imply that they should therefore receive all of the greater attention from conservationists or that, in a triage situation, they should rank lower in our priorities? Although this is a fundamental question, it has hardly been addressed.

These, then, are some of the issues that we should bear in mind as we begin to impose a fundamental shift on evolution’s course. We are “deciding” on evolution’s future in virtually a scientific vacuum — deciding all too unwittingly, but effectively and increasingly.

Norman Myers is a Fellow at Green College, Oxford University, and has acted as scientific consultant and policy adviser to the White House, U.S. Departments of State and Defense, NASA, the World Bank, seven United Nations agencies, and the European Commission. He is a member of the U.S. National Academy of Sciences, the World Academy of Art and Science, the American Association for the Advancement of Science, and the Royal Society of Arts. Dr. Myers is the originator of the biodiversity “hotspots” strategy, which has generated over $300 million for conservation activities.

Andrew H. Knoll is a professor of biology at the Botanical Museum at Harvard University. His areas of expertise include the evolution of life, the evolution of Earth’s surface environments, and the relationships between the two. Dr. Knoll is a member of the American Philosophical Society and the American Academy of Arts and Sciences. He received an M.A. and a Ph.D. in geology from Harvard University.

How Will the Sixth Extinction Affect Evolution of Species?


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Another article onthe sixth extinction

Niles Eldredge examines the causes of the current mass extinction.

BioScience Article

“Global Conservation of Biodiversity and Ecosystem Services.”
Habitat destruction has driven much of the current biodiversity extinction crisis, and it compromises the essential benefits, or ecosystem services that humans derive from functioning ecosystems. Securing both species and ecosystem services might be accomplished with common solutions. Yet it is unknown whether these two major conservation objectives coincide broadly enough worldwide to enable global strategies for both goals to gain synergy. In this November 2007, BioScience article, Will Turner and his colleagues assess the concordance between these two objectives, explore how the concordance varies across different regions, and examine the global potential for safeguarding biodiversity and ecosystem services simultaneously. Read the abstract, or log in to purchase the full article.

Biodiversity Project

Outreach program to help explain the importance of biodiversity to individuals and media support to interested people. Includes history and resource links.

Do we still need nature?

The author of this article presents a case for the importance of biological diversity.

Human impact on ecosystems

“This article looks at human impact on ecosystems and the consequences for evolution.” (5/01) =0&minscore=50&journalcode=pnas

Test your environmental knowledge

This quiz covers issues that have been discussed in the media. The questions are designed to tell us how much accurate information people are getting from television, newspapers, magazines, and other sources.

Become an environmental activist

The World Wildlife Fund offers hints on the quickest and easiest way to take action for wildlife and the environment. Instructions and samples give you, your group, or community the basics to take environmental action. original lesson

This lesson has been written by a science educator to specifically accompany another article about the 6th extinction on this website. It includes article content and extension questions on the other article (which can be deleted), but the activity handouts for different grade levels are relevant to the above article.

Lesson Title: Extinction: Is It Inevitable?
Levels: high school - undergraduate
Summary: This lesson explores the cycle of extinction and biotic recovery, with special emphasis on the causes of extinction. Students can chart extinction patterns, research La Brea Tar Pit, debunk dinosaur extinction myths, debate endangered species vs. human water needs… and more!

Download/view lesson. (To open the lesson’s PDF file, you need Adobe Acrobat Reader free software.)

Investigate biodiversity

Science students and teachers are transported to remote locations to experience an insider’s view of science in action, while learning first hand tools and techniques that can be applied in their own backyards.

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Suggestions and links on how to help students understand the importance of biodiversity.

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