Because favourable mutations tend to get swamped in large populations,
modern Darwinism depends heavily on them arising and being fixed in
small populations through inbreeding. For example, Carroll says:
"Eldredge, Gould, and Stanley also suggest that there are aspects of
the speciation process itself that facilitate rapid change. This
argument had previously been made by Mayr (1954, 1963, 1970,
1982). He proposed that rapid change is much more likely to occur in
small populations restricted to limited geographical areas than in
large, widespread species...In small, isolated populations,
inbreeding will lead to homozygosity, so that recessive as well as
dominant traits will commonly be exposed to selection. We can
expect directional selection to operate much more rapidly under these
conditions than it does in widespread species with large population
sizes." (Carroll R.L., "Vertebrate Paleontology and Evolution," W. H.
Freeman & Co: New York, 1988, p577)
But there is a major problem with this scenario as a general
explanation of evolutionary change. Not only are mutations almost always
unfavourable, but inbreeding generally makes matters worse, by bringing
recessive unfavourable mutations to the fore and causing extinction
in those same small populations, as ReMine points out:
"...to get past fitness barriers, evolutionists have proposed the shifting
balance hypothesis. This hypothesis requires various deleterious
mechanisms, of which inbreeding is the most notable. Even if these
mechanisms eventually succeeded, the population would likely lose many
beneficial traits in the process. It is a poor exchange if the population
loses one beneficial trait in getting around a fitness barrier caused by the
beneficial effects of another trait. The cost of the trait would have
already been paid but the population would no longer have anything to
show for it. The trait might then have to be replaced or re-evolved,
adding extra cost to the process." (ReMine W.J., "The Biotic Message,"
1993, p210)
The following article from SCIENCE indicates that inbreeding is
even *worse* at causing extinction that previously thought:
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Inbreeding's Kiss of Death
The hemophilia that plagued Europe's royal families in the 1800s is a
clear example of how mating with first cousins and other close kin
can cripple a gene pool by allowing recessive genes to emerge from
hiding. Less accepted, however, is the notion that inbreeding can
drive a small, isolated population of animals or plants to extinction.
Many biologists have thought that natural events-widespread flooding
from an El Nino, for example, or disease outbreaks-would swamp any
genetic effects.
But a report in this week's issue of Nature suggests that inbreeding
may be a more potent force than previously reckoned. After studying
fragmented populations of a single butterfly species, a team led by
population biologists Ilik Sacheri and Ilkka Hanski of the University
of Helsinki has found a strong correlation between a population's
genetic diversity and whether it went extinct. This link held up after
ecological factors that also influence extinction, such as weather and
population size, were taken into account. "Our study demonstrates
that inbreeding can contribute to extinction in a natural population,"
says Saccheri. The finding, he adds, bolsters the idea that genetic
diversity must be considered when drawing up plans to protect
endangered species.
The issue of whether a meager gene pool can lead to extinction in
already fragmented populations has provoked "a hell of a lot of
controversy," says Richard Frankham of Macquarie University in
Australia. Although some biologists have argued for the power of
inbreeding, a persuasive argument of late, he says, has been that
climatic events and random fluctuations in population size are far
more important in the wild.
But that's not the conclusion suggested by new data from Finland's
Aland islands, home to a large Glanville fritillary butterfly
"metapopulation"-many small, fragmented populations transiently
connected when individuals fly between them. To see whether genetic
diversity plays a role in extinction, the Finnish team in 1996 collected
adult females from 42 populations and analyzed seven of their
enzymes and one genomic DNA section for variants, or
polymorphisms. After watching seven populations wink out in the last
year, the team found that those populations had at least 28% less
genetic variation than the survivors. Using a statistical model they had
developed to predict extinction risk, which incorporates factors such
as a population's size and isolation and habitat size, the researchers
found that inbreeding accounted for as much as 26% of the
differences from population to population in extinction rates.
The study is "as close as you'll get to direct evidence" that inbreeding
figures in extinction, Frankham says. The findings, he and others say,
suggest that wildlife managers should focus scarce resources on those
threatened populations with larger gene pools within a species. Says
Frankham, "This is going to be absolutely critical as we deal with
fragmented populations." -Jocelyn Kaiser
www.sciencemag.org SCIENCE VOL. 280 3 APRIL 1998 35
(Kaiser J., "Inbreeding's Kiss of Death," Science, Vol. 280, 3 April
1998, p35)
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Steve
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Stephen E (Steve) Jones ,--_|\ sejones@ibm.net
3 Hawker Avenue / Oz \ Steve.Jones@health.wa.gov.au
Warwick 6024 ->*_,--\_/ Phone +61 8 9448 7439
Perth, West Australia v "Test everything." (1Thess 5:21)
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