Charles Darwin: A Hole in Theory of Evolution Explained

Scientists explain how flowering plants developed so quickly

Dec. 13, 2012 — -- Charles Darwin must have thought the very foundation of his theory of evolution was in danger when he wrote a note to his friend Joseph Hooker in 1879, two decades after publication of his seminal work, "On the Origin of Species." The great scientist was besieged with questions about a problem that had haunted him for years.

The problem was flowers: So many kinds, flourishing in every corner of the earth, ranging from breathtakingly beautiful to simply weird, expanding their reach so rapidly that they seemed to mock natural selection, the cornerstone of his work.

Evolution, he knew, was a very slow process, so slow that it had taken nearly four billion years for life on earth to evolve from microorganisms to towering trees and intelligent life. Yet the fossil record of the mid 19th century told a story that seemed impossible.

The first flowering plants appeared around 130-140 million years ago. Yet it took only about 45 million years -- an incredibly short period of time, geologically speaking -- for flowering plants to dominate the planet, with possibly as many as 400,000 species, far eclipsing the pines and non-flowering trees and bushes that blanketed the landscape during the time of the dinosaurs.

In his letter to Hooker, Darwin famously described the story of flowers as "an abominable mystery." So it remained for the generations of scientists who followed in Darwin's tracks, and the answer today is still not as definitive as many would like. But ever so slowly, clues are emerging as scholars use new tools to help unravel the mystery.

The fossil record is much richer today than it was in Darwin's time, and it tells a different – though still incomplete – story. Flowering plants, or angiosperms, may have emerged much earlier than had been thought. Some experts believe the ancestors of flowering plants were around more than 100 million years before the first flowers appeared, and possibly even longer. And for the most part it was a gradual process, just as Darwin would have expected, with a few stunning exceptions.

Scientists from a wide range of institutions have been delving into the genetic history of flowering plants in the Ancestral Angiosperm Genome Project, based at Pennsylvania State University, and they have found two "major upheavals" leading up to the first appearance of flowers. Penn State's Claude dePamphilis calls them the "'Big Bangs' for flowering plants."

They are known as "polyploidy events," mutations in which an organism gets a double dose of genetic material. The duplication of genes has been documented in vertebrates as well as plants, and it can be fatal, but sometimes it results in an evolutionary advantage. DePamphilis thinks that in these two cases, at least, the duplication launched a "genomic renaissance," enabling the ancestors of flowering plants to evolve new and better functions.

All flowering plants probably benefited from large-scale duplications of the genome, he has suggested. That genetic diversity may help explain why today we can find flowers in full bloom in areas where common sense would suggest nothing could survive.

Once they were here, they expanded rapidly, aided by the co-evolution of insects that accelerated the spread of genetic material through cross pollination, according to Indiana University botanist David Dilcher. Dilcher has studied the evolution of flowering plants for decades.

Flowering plants also benefited from better plumbing, according to researchers at the University of Tasmania and the University of Tennessee. Tim Brodribb and Taylor Field found that between 140 and 100 million years ago the leaves of flowering plants developed the capacity to hold more fluids, thus doubling their rate of photosynthesis.

Doug and Pam Soltis of the University of Florida, Gainesville, said they believe much of this happened very quickly, possibly within a period of around 5 million years. That's stunning, they said, considering the fact that flowering plants have been around for at least 140 million years.

So Darwin's theory of natural selection still worked. There was just this one case in which things happened remarkably quickly.

But where did it all begin?

Quite possibly, say researchers, on the Pacific islands of New Caledonia. Scientists at the University at Buffalo are leading a multi-institution collaboration that is sequencing the genome of Amborella, described by biologist Victor Albert as a "living fossil."

This small tree, which grows nowhere else on the planet, is believed to be the direct descendent of the common ancestor of all flowering plants.

The researchers hope that by comparing the genetic makeup of Amborella to modern plants they will be better able to figure out how plants cope with threats ranging from drought to global climate change.

"This is work that's related to the human condition in various ways," Albert said in announcing the project last year. "Most of our food comes from flowers. All the fruit crops and grains are flowering plants. Cotton fiber is from fruit, and fruits come from flowers."

So that rose in the front yard is more than just a pretty face. It is a key player in the evolution of the planet from a lifeless sphere to a home full of flowers. And us.