Sweeping gene survey reveals new facets of evolution




Who would have thought that a hand-held genetic test used to find sushi restaurants trading tilapia for tuna could reveal so much about evolution, such as how new species form?

It would have been crazy to look through all five million of these gene pictures, which are called "DNA barcodes" and were collected from 100,000 animal species by researchers from around the world and put in the GenBank database, which is run by the US government.

That would be Mark Stoeckle from The Rockefeller University in New York and David Thaler from the University of Basel in Switzerland. They released results last week that are likely to shake up, if not completely change, many well-established ideas about how evolution works.

It is a basic fact of biology that over time, species with big, spread-out populations, like ants, rats, and people, will become genetically more diverse.

Is that true, though?

"The answer is no," said Stoeckle, who was the study's lead author and whose work was published in Human Evolution.

For the world's 7.6 billion people, 100,000 sandpipers, 500 million house sparrows, and other species, DNA variation "is about the same," he told AFP.

Nine out of ten species on Earth today, including humans, were first seen between 100,000 and 200,000 years ago. This may be the most shocking finding of the study.

"This conclusion is very surprising, and I fought against it as hard as I could," Thaler shared with AFP.

It makes sense to feel that way—how else to explain the fact that 90% of animals are genetically about the same age?

Was there a terrible event 200,000 years ago that pretty much erased everything?

Cheaper and easier

You need to know about DNA barcoding to understand the answer. They have two types of DNA.

Nuclear DNA is the one we are most familiar with. It is passed down from male to female animals and holds the genetic code for each person.

DNA makes up the genome, which is made up of four different types of molecules grouped in pairs. People have three billion of these pairs, which are grouped together into about 20,000 genes.

But all animals also have DNA in their mitochondria. Mitochondria are the very small parts of cells that turn food into energy that cells can use.

There are 37 genes in mitochondria. One of them, called COI, is used to label DNA.

The genes in nuclear DNA can be very different from one species to the next. But all animals have the same set of mitochondrial DNA, which makes it easy to compare them.

It is also much easier and less expensive to separate mitochondrial DNA.

In 2002, Canadian molecular scientist Paul Hebert (who came up with the term "DNA barcode") found a way to tell species apart by looking at the COI gene.

"The mitochondrial sequence has proved perfect for this all-animal approach because it has just the right balance of two conflicting properties," explained Thaler.

"Neutral" changes

On the one hand, the COI gene code is the same in all animals, which makes it simple to find and compare them.

You can tell the difference between each species, though, because these mitochondrial pieces are different.

"It coincides almost perfectly with species designations made by specialist experts in each animal domain," said Thaler.

Researchers looked at the barcodes of over 100,000 species and found a clue that almost all of the animals came into the world around the same time as humans.

They saw a lack of variety in what are known as "neutral" mutations. These are small changes in DNA that don't help or hurt a person's chances of living from one generation to the next.

To put it another way, they had nothing to do with the sexual and environmental forces that pushed development forward.

How close or not these "neutral" changes are to each other is like tree rings: they show how old a species is.

Which brings us back to our original question: why did most of the species that live today appear around the same time?

Darwin was confused.

Jesse Ausubel, who runs the Program for the Human Environment at The Rockefeller University, said that environmental stress is one option.

He told AFP about the study, "Viruses, ice ages, successful new competitors, and loss of prey are all things that can cause times when the population of an animal drops sharply."

"In these periods, it is easier for a genetic innovation to sweep the population and contribute to the emergence of a new species."

But the last real mass extinction happened 65.5 million years ago, when half of all species on Earth and land-dwelling dinosaurs were wiped out by an asteroid. This means that a "bottleneck" in the population is at best only a part of the story.

"The simplest interpretation is that life is always evolving," Stoeckle said.

"It is more likely that—at all times in evolution—the animals alive at that point arose relatively recently."

In this view, a species only lives for a certain amount of time before it either changes into something else or dies out.

The study also found something else that was a surprise: there are very clear DNA borders between species, with not much in between.

"If individuals are stars, then species are galaxies," stated Thaler. "They are compact clusters in the vastness of empty sequence space."

He also said that Darwin was confused by the lack of "in-between" species.