First the Orcas. Researchers from Durham University in the
UK conducted a genetic study of orca populations around the world. What they
found was that most populations have very low genetic diversity. What does that
mean? It means the members of each group are fairly closely related. This
happens when a species goes through a genetic bottleneck. That is, most of the
species died off and later generations are descended from the few survivors.
Since we became fully human, our species has done this at least once. It might
explain how we separated from our sibling species such as the Neandertals.
Over time, new mutations will appear in a population's
genome creating new diversity. By assuming the existence of a genetic
clock--that is, a steady rate of mutation--it's possible to estimate when a
population passed through a bottleneck. Naturally, it's a lot more complicated
than that, but that's the gist of it. The importance of the current study is
that the genomes of various orca populations around the world show they all
went through a genetic bottleneck about 40,000 years ago. All except one. The
orca population off the coast of South Africa has a high level of genetic
diversity, suggesting they maintained a large population through the event that
cut back all of the other populations.
In naming that event, the Durham researchers point an
accusing finger at the last ice age. That works pretty well. An ice age doesn't
just mean a lot of ice up north and down south; it means the whole global
climate changes. If you've learned anything about the current climate crisis,
that should be it. Global warming doesn't mean it gets a little warmer every
year; it means entire patterns of climate change. For the orcas, what we're
concerned about is ocean currents. Increased cold in the far north and south
doesn't sound like something that should hurt orcas. After all, most of us
think of orcas as cold water animals. But it can hurt them a lot.
Just as with land climate, the ice ages didn't merely chill
the water in the north and south; it changed circulation patterns all over the
globe. To grossly simplify things, there are two patterns of ocean currents:
one on the surface and one in the deeps. In a few places around the world the
two connect. At one end, surface waters get cold and heavy and plunge into the
deep. At the other end, the waters of the deep run into a continent and are
pushed back to the surface. These upwellings bring nutrients to feed the small
organisms that are the base of the entire ocean food chain. There are other
sources, such as river mouths, but the upwellings create enormously fertile
ecosystems.
The Durham researchers point out that most upwellings were
disrupted or completely collapsed during the ice ages. The one that didn't was
the Bengeula upwelling near South Africa. That fact pulls it all together. Less
food at the bottom of the food chain means less food at the top of the food
chain. As apex predators, orcas are very sensitive to disruptions at the bottom
of the food chain. Only the South African orca population had sufficient food
to make it through a bottleneck that affected orcas everywhere else. For future
research, the Durham team suggest looking at other apex predators to see if
they experienced a similar bottleneck.
I'd like to suggest another line of research. Climate and
weather don't move in straight lines, they seesaw. The weather gets cool for a
few years then it gets warm for a few years. This is not a smooth cycle.
Lately, both the colds and warms have been getting warmer. The direction of
these cycles has been getting warmer for the last 150 years and the speed
upwards has been increasing. The seesaw is moving up, rapidly. In a scale of
thousands of years, climate works in bigger seesaws. In the scale of your
lifetime, the weather seesaws up and down. You might not even notice the
pattern that is climate. On a scale of centuries, those seesaws form an even
greater seesaw that helps historians understand the rise and fall of
civilizations. One more step up and it gets interesting.
There wasn't just one ice age. Ice ages come in sets. What
caused the ice age is an incredibly complex question. Roughly put, first the
earth has to be in a state where ice ages are possible--the continents must be
in the right shape, the mountains must be the right height, and the air must
have the right chemical composition. Even then, a series of astronomical cycles
determine whether an actual ice age happens. In the current period, we have had
around nineteen ice ages (called glacial maxima (maximums)). Finally, when all
the conditions are just so, it usually requires a nudge to tip the first
domino. We are in a period in which ice ages are possible, but the seesaws
within seesaws in and out of glacial maxima do not form a regular curve up and
down. From a warm period (called an interglacial), such as the one we are in,
the climate seesaws down to the coldest period, then rather abruptly (in
geological terms) jumps up into the interglacial climate. Right now we should
be seesawing down into the next glacial maximum; instead, our climate is
reaching temperatures not seen in millions of years.
And the orcas? What intrigues me about this study is that
40,000 years ago is not the coldest part of the last glacial period. It's the
second coldest. There was one seesaw colder. The coldest time was 23-17,000
years ago. If the bottleneck for orca diversity happened when they say it did, there
must be another factor involved that they haven't discovered. Finding that
factor could be a major key to understanding how species deal with bottleneck
situations. The Durham team might be onto something with major implications for
managing our current climate crisis.
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