Download "Natural selection drives them all down, while the founder effect

1. How Human Beings Almost Vanished From Earth In 70,000 B.C.
October 22, 201212:33 PM ET
Commentary
ROBERT KRULWICH
Robert Krulwich/NPR
Add all of us up, all 7 billion human beings on earth, and clumped together we weigh roughly 750 billion pounds. That, says
Harvard biologist E.O. Wilson, is more than 100 times the biomass of any large animal that's ever walked the Earth. And we're
still multiplying. Most demographers say we will hit 9 billion before we peak, and what happens then?
Well, we've waxed. So we can wane. Let's just hope we wane gently. Because once in our history, the world-wide population of
human beings skidded so sharply we were down to roughly a thousand reproductive adults. One study says we hit as low as
40.
Forty? Come on, that can't be right. Well, the technical term is 40 "breeding pairs" (children not included). More likely there
was a drastic dip and then 5,000 to 10,000 bedraggled Homo sapiens struggled together in pitiful little clumps hunting and
gathering for thousands of years until, in the late Stone Age, we humans began to recover. But for a time there, says science
writer Sam Kean, "We damn near went extinct."
I'd never heard of this almost-blinking-out. That's because I'd never heard of Toba, the "supervolcano." It's not a myth. While
details may vary, Toba happened.
Toba, The Supervolcano
Once upon a time, says Sam, around 70,000 B.C., a volcano called Toba, on Sumatra, in Indonesia went off, blowing roughly 650
miles of vaporized rock into the air. It is the largest volcanic eruption we know of, dwarfing everything else...
Robert Krulwich/NPR
That eruption dropped roughly six centimeters of ash — the layer can still be seen on land — over all of South Asia, the Indian
Ocean, the Arabian and South China Sea. According to the Volcanic Explosivity Index, the Toba eruption scored an "8", which
translates to "mega-colossal" — that's two orders of magnitude greater than the largest volcanic eruption in historic times at
Mount Tambora in Indonesia, which caused the 1816 "Year Without a Summer" in the northern hemisphere.
With so much ash, dust and vapor in the air, Sam Kean says it's a safe guess that Toba "dimmed the sun for six years, disrupted
seasonal rains, choked off streams and scattered whole cubic miles of hot ash (imagine wading through a giant ashtray) across
acres and acres of plants." Berries, fruits, trees, African game became scarce; early humans, living in East Africa just across the
Indian Ocean from Mount Toba, probably starved, or at least, he says, "It's not hard to imagine the population plummeting."
Then — and this is more a conjectural, based on arguable evidence — an already cool Earth got colder. The world was having
an ice age 70,000 years ago, and all that dust hanging in the atmosphere may have bounced warming sunshine back into space.
Sam Kean writes "There's in fact evidence that the average temperature dropped 20-plus degrees in some spots," after which
the great grassy plains of Africa may have shrunk way back, keeping the small bands of humans small and hungry for
hundreds, if not thousands of more years.
So we almost vanished.
But now we're back.
It didn't happen right away. It took almost 200,000 years to reach our first billion (that was in 1804), but now we're on a
fantastic growth spurt, to 3 billion by 1960, another billion almost every 13 years since then, till by October, 2011, we zipped
past the 7 billion marker, says writer David Quammen, "like it was a "Welcome to Kansas" sign on the highway."
In his new book Spillover, Quamman writes:
We're unique in the history of mammals. We're unique in this history of vertebrates. The fossil record shows that no other
species of large-bodied beast — above the size of an ant, say or an Antarctic krill — has ever achieved anything like such
abundance as the abundance of humans on Earth right now.
But our looming weight makes us vulnerable, vulnerable to viruses that were once isolated deep in forests and mountains, but
are now bumping into humans, vulnerable to climate change, vulnerable to armies fighting over scarce resources. The lesson
of Toba the Supervolcano is that there is nothing inevitable about our domination of the world. With a little bad luck, we can
go too.
We once almost did.
Radiolab regular Sam Kean's new book on genetics, The Violinist's Thumb, tells the story of Toba, the supervolcano, to explore
how human genes record a "bottleneck" or a drastic narrowing of genetic diversity 70,000 years ago. David Quammen's new
book Spillover is about people pushing into forests, swamps and places where viruses have been hiding. Those viruses are now
beginning to cross over into horses, pigs, bats, birds and, inevitably, they threaten to "spillover" into us. For a virus, or bacteria, 7
billion potential hosts look like a fantastic opportunity.
2. European Roots for Native Americans?
An analysis of ancient DNA from a 24,000-year-old Siberian skeleton generates a new model for the original peopling of the
Western Hemisphere.
By
237Bob Grant | October 29, 2013
Models of gene flow from Aisa to the Americas, like this one from a
2007 PLOS ONE paper, may need to be reconsidered.WIKIMEDIA, ERIKA TAMM ET AL.Native Americans may not have
descended from East Asians who crossed the Bering Land Bridge more than 15,000 years ago, according to a new genomic
analysis of a millennia-old Siberian skeleton. A portion of the nuclear DNA recovered from the upper arm bone of a 4-year-old
boy that was buried near the Siberian village of Mal’ta about 24,000 years ago is shared by modern Native Americans and no
other group. But the boy appears to have been descended from people of European or western Asian origin.
Eske Willerslev, a University of Copenhagen ancient DNA expert, announced the findings last week at the Paleoamerican
Odyssey conference in Santa Fe, New Mexico, and the resulting manuscript is in press at Nature. In addition to finding genome
regions shared by modern Native Americans, he and collaborator Kelly Graf of Texas A&M University found that the boy’s Y
chromosome and mitochondrial DNA belonged to haplogroups that are found almost exclusively in Europeans and people
living in Asia west of the Altai Mountains.
Conspicuously absent from the child’s DNA, however, was any connection to modern East Asians, a genetic relationship
present in the genomes of virtually all Native Americans. This means that the population from which the boy came must have
included ancestors of modern Native Americans, upsetting the popular belief that the original inhabitants of North, Central,
and South America descended directly from East Asians.
According to Science, Willerslev and Graf suggest that at some point before 24,000 years ago, the ancestors of modern Native
Americans and those of East Asians split into two genetically distinct groups. The Mal’ta boy belonged to a population that
migrated to Siberia from Eurasia and mixed with the second before the newly formed population moved into the New World,
eventually populating the Americas.
Previously, the presence of European DNA in the modern Native American genome was attributed to interbreeding after
Europeans made contact. This new finding may turn that supposition on its head. “The west Eurasian [genetic] signatures that
we very often find in today's Native Americans don't all come from postcolonial admixture,” Willerslev said during his
presentation. “Some of them are ancient.”
Though reconstructing humanity’s westward expansion into the New World involves some speculation, recovering the intact
DNA of the Mal’ta boy represents a technological feat: his is oldest complete genome of a modern human sequenced to date.
3. Caribbean lizards settle 'founder effect' controversy
February 2, 2012
University of California - Davis
In the first experimental study of the founder effect in a natural setting, researchers found that natural selection does
not overwhelm the founder effect.
This is a founder pair of Caribbean brown anole lizards, one of the actual colonist couples, shortly after release. They were
involved in the first experimental study of the founder effect -- a long-simmering controversy in evolutionary biology -- in a
natural setting. The study by UC Davis researchers and others is published in the Feb. 3 edition of the journal Science Express,
the online publication of the journal Science.
Credit: Manuel Leal/Duke University
When a devastating 2004 hurricane wiped out a Caribbean lizard population, University of California, Davis, researchers had
an unprecedented opportunity to address a long-simmering controversy in evolutionary biology. Their findings -- from the
first experimental study of the so-called "founder effect" in a natural setting -- are published in the Feb. 3 edition
of Science Express, the online publication of the journal Science, which will publish the study in print on Feb. 17.
The founder effect describes the loss of genetic variation that occurs when a new population is established by a very small
number of individuals from a larger population. But the extent to which this effect contributes to evolution has been up for
debate since the early 1940s, when German evolutionary biologist Ernst Mayr first outlined it.
Some scientists argue that the founder effect is pivotally important in how a species evolves. Others argue that it is a bit player
on the evolutionary stage, quickly overwhelmed by the forces of natural selection.
In their new paper, UC Davis scientists, together with researchers from Harvard and Duke, suggest that both sides are right.
Complicating the debate has been the dearth of data from nature: Founder events are rarely observed.
"Founder effects are very hard to study," said Thomas Schoener, a professor of evolution and ecology at UC Davis and a coauthor of the study. "One must be in exactly the right place at the right time to observe the founder event -- and then fortunate
enough to be able to follow a population through time."
That opportunity presented itself in September 2004, when Hurricane Frances submerged several small, low-lying islands
near Great Abaco, Bahamas.
Before the hurricane, these islands supported populations of a Caribbean lizard, the brown anole, Anolis sagrei. After the
hurricane, seven of the islands were thoroughly searched. No lizards were found.
In May 2005, the researchers randomly selected one male and one female brown anole from lizards collected on a nearby
larger island to found new anole populations on seven small islands.
During the next four years, the researchers repeatedly sampled lizards from the source island, from the seven experimental
founder islands, and from 12 nearby islands that served as a control.
The team found that all lizard populations adapted to their environment, yet retained characteristics from their founders.
For instance, lizard limb length correlates with the average diameter of vegetation on an island. Because the founder islands
had smaller vegetation than the source island, the length of lizard limbs decreased, as expected, due to natural selection.
But islands containing lizards with the largest limbs at the beginning of the study still had the lizards with the longest limbs at
the end of the study.
"Natural selection drives them all down, while the founder effect keeps the order the same," said Schoener. "So they're both
right, in a sense."
If natural selection had overpowered the founder effect, lizards' limbs would have converged at the same length, regardless of
how long-legged the founders were. Instead, limb length decreased roughly in parallel, signifying the persistence of the
founder effect.
"Our study is an entirely unique approach to a question of longstanding importance for evolutionary biology regarding the
founder effect: Will it persist in the face of the strong selection that would often exist in the colonized environment?" said
Schoener. "The answer we found is that founder effects can leave a persistent signal as generations replace one another over
time, even as populations adapt to new conditions. Our study of these fundamental evolutionary principles affects our general
understanding of how the biological world works."
Journal Reference:
J. J. Kolbe, M. Leal, T. W. Schoener, D. A. Spiller, J. B. Losos. Founder Effects Persist Despite Adaptive Differentiation: A Field
Experiment with Lizards. Science, 2012; DOI: 10.1126/science.1209566
4. Evolution myths: Mutations can only destroy information
By Michael Le Page, New Scientist, 2008
Biologists are uncovering thousands of examples of how mutations lead to new traits and even new species. This claim not only
flies in the face of the evidence, it is also a logical impossibility
Most people lose the ability to digest milk by their teens. A few thousand years ago, however, after the domestication of cattle,
several groups of people in Europe and Africa independently acquired mutations that allow them to continue digesting milk
into adulthood. Genetic studies show there has been very strong selection for these mutations, so they were clearly very
beneficial.
Most biologists would see this as a gain in information: a change in environment (the availability of cow’s milk as food) is
reflected by a genetic mutation that lets people exploit that change (gaining the ability to digest milk as an adult). Creationists,
however, dismiss this as a malfunction, as the loss of the ability to switch off the production of the milk-digesting enzyme after
childhood.
Rather than get bogged down trying to define what information is, let’s just look at a few other discoveries made by biologists
in recent years. For instance, it has been shown a simple change in gene activity in sea squirts can turn their one-chambered
heart into a working two-chambered one. Surely this counts as increasing information?
TRIMming the genome
Some monkeys have a mutation in a protein called TRIM5 that results in a piece of another, defunct protein being tacked onto
TRIM5. The result is a hybrid protein called TRIM5-CypA, which can protect cells from infection with retroviruses such as HIV.
Here, a single mutation has resulted in a new protein with a new and potentially vital function. New protein, new function, new
information.
Although such an event might seem highly unlikely, it turns out that the TRIM5-CypA protein has evolved independently in
two separate groups of monkeys. In general, though, the evolution of a new gene usually involves far more than one mutation.
The most common way for a new gene to evolve is for an existing gene to be duplicated. Once there are two or more copies,
each can evolve in separate directions.
The duplication of genes or even entire genomes is turning out to be ubiquitous. Without a duplication of the entire genome in
the ancestor of modern-day brewer’s yeast, for instance, there would be no wine or beer. It is becoming clear that every one of
us has extra copies of some genes, a phenomenon called copy number variation.
The evolution of more complex body plans appears to have been at least partly a result of repeated duplications of the Hox
genes that play a fundamental role in embryonic development. Biologists are slowly working out how successive mutations
turned a pair of protoHox genes in the simple ancestors of jellyfish and anemones into the 39 Hox genes of more complex
mammals.
Newly minted
Can mutation really lead to the evolution of new species?
Yes. Several species of abalone shellfish have evolved due to mutations in the protein “key” on the surface of sperm that binds
to a “lock” on the surface of eggs. This might appear impossible, but it turns out that some eggs are prepared to be penetrated
by deviant sperm. The same thing can happen in fruit flies, and likely in many other groups too. In yeasts, the mutations that
led to some new species forming have not only been identified, they have even been reversed.
The list of examples could go on and on, but consider this. Most mutations can be reversed by subsequent mutations – a DNA
base can be turned from an A to a G and then back to an A again, for instance. In fact, reverse mutation or “reversion”
is common. For any mutation that results in a loss of information, logically, the reverse mutation must result in its gain. So the
claim that mutations destroy information but cannot create it not only defies the evidence, it also defies logic.