September 2009

In celebration of the Year of Science’s September theme, biodiversity and conservation, this month’s story focuses on diversity‘s nemesis: extinction. The world faces what may be our sixth mass extinction. We are losing species at an unprecedented rate — 100 to 1000 times higher than throughout most of Earth’s history. As conservationists struggle to lower this rate, there is concern about where their efforts should be focused. Less than one quarter of the world’s 8-14 million species have been described, and only 2.5% of those have been evaluated for extinction risk. How can we set our conservation priorities in the face of so much risk and uncertainty? Biologists tackling this problem have discovered that certain traits seem to go along with high extinction risk — for example, being a large vertebrate or living only in a small area. Now, new research suggests another useful factor to consider: evolutionary history.

Where's the evolution?

An organism’s evolutionary history may contain important information about whether it is likely to wind up extinct, say biologists Kaustuv Roy, Gene Hunt, and David Jablonski — and this could help us protect those species. The scientists’ hypothesis, published last month, is based on a study of clams, oysters, and their relatives over the past 200 million years. Since it’s difficult to identify these critters as particular species consistently based on fossilized shells, the team focused on closely related groups, called genera, and the larger families these 1700 genera belonged to. For each family, they counted up how many genera went extinct during a particular time period. They discovered that the extinctions were far from random; rather, they were clustered together in particular families. In other words, extinction risk seems to be a family trait that can be passed on from an ancestor to its descendants.

You can also see this on the evolutionary tree, or phylogeny, of the mollusks. This tree shows the relationships among different mollusk families. The size of the circles represents the extinction rate for each family. The bigger the circle, the more extinction-prone the family is. Notice how the biggest circles are clustered together and not spread out randomly over the tree? Closely related families tend to have similar extinction risks.

Of course, this doesn’t mean that organisms have a specific “extinction gene.” It just means that there’s probably a set of heritable characteristics that influences a species’ risk of extinction. We don’t know exactly what those characteristics are — they could include almost anything, from reproductive strategy to food source — however, we do know that they are passed from ancestor to descendant. Since closely related organisms tend to inherit similar traits from their common ancestors, they also wind up with similar extinction rates.

This is bad news for any species that finds itself on a once blooming, now leafless branch of the tree of life — but it could be useful information for those who want to preserve biodiversity. We have little knowledge of most of the species that inhabit the Earth. Many are on the verge of extinction, but we don’t always know which. If extinction risk does indeed run in families, it could help us set initial conservation priorities until we have more detailed knowledge.

Even more importantly, Roy’s team’s findings point to a worrisome outcome of extinction’s relentless pruning of the tree of life: it’s very likely to lop off entire branches. If extinction were random, we’d expect it to take a leaf here and a twig there, preserving the overall shape and size of the tree. But if some branches have intrinsically higher extinction risks than others, the process is liable to wipe out these whole limbs — and with them, huge swaths of genetic and organismic diversity.

Preserving this biological diversity has inherent value and is important from the standpoint of conservation — but it has practical justifications as well. Distinct branches on the tree of life have often evolved traits and genes that are found in no other organisms. For example, many closely related yew trees contain unique chemicals that effectively treat cancer. If this entire group had been wiped out by extinction, we would have no knowledge of these valuable drugs. Other branches of the tree of life will likely point us towards other useful substances or genes. There is much to learn from these unique segments of evolutionary history. Unfortunately, if extinction does run in families, it is particularly likely to rip out whole chapters from the encyclopedia of life.

This means that if we want to preserve genetic and organismic diversity, we need to focus our efforts on the remaining survivors of lineages with high extinction risks. These “last lineages standing” are irreplaceable — and vulnerable. Roy and his colleagues’ research highlights the importance of taking into account the processes that have shaped these species’ evolutionary past, while trying to save their evolutionary future.