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Ärende: Evolutionary consequences
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http://www.actionbioscience.org/newfrontiers/myers_knoll.html


How Will the Sixth Extinction 
Affect the Evolution of Species?
By Norman Myers and Andrew H. Knoll
Excerpts from "The biotic crisis and the future of evolution"
articlehighlights 

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
How Will the Sixth Extinction 
Affect the Evolution of Species?  
By Norman Myers and Andrew H. Knoll   
 
 
 
It takes about 5 million years for for the world to recover after a
major extinction.
 
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 

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.   
  
Evolution is being altered by the current extinction.

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. 

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
extant  1,2,5,9
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
biomass16-18
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 

Gene pools will be so depleted that species may not be able to bounce
back.Further Evolutionary Effects: These first-order impacts will likely
engender a series of further consequences, including although not
limited to: 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.

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. 

Loss of species means the loss of sub-species.
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
Large mammals are likely to go extinct.
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.
  
The damage caused by the current extinction is probably
permanent.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 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?

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.  

Conclusion: What we do now will set a course for evolution.

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. 

© 2001, National Academy of Sciences. Excerpts from paper "The biotic
crisis and the future of evolution" published in  Proc. Natl. Acad. Sci.
USA, Vol. 98, Issue 10, 5389-5392, May 8, 2001.
  
About the authors: 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.
http://www.green.ox.ac.uk/fellows.htm#Myers
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.
http://www.eps.harvard.edu/people/faculty/knoll
         

"Tiny green men might have been a better experiment."

Stephen Hawking
(paraphrasing from a "Universe in a Nutshell".
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