Evolving evidence

NOVEMBER 24th will mark the 150th anniversary of the publication of On the Origin of Species by Means of Natural Selection, in which Charles Darwin spelt out the famous theory. The big discovery, made by Darwin and Alfred Russel Wallace, was not evolution itself, but the mechanism through which it works – natural selection. Some people still challenge the theory, despite it having received massive scientific validation. A recent publication entitled 15 Evolutionary Gemsby H Gee, R Howlett and P Campbell (www.nature.com/nature/newspdf/

evolutiongems.pdf) briefly describes 15 examples published in the journal Natureover the past decade that offer powerful evidence for the theory. I will describe three examples.

1 The origin of feathers

The fossil record is one of the main pillars of evidence on which the theory of evolution stands. But critics claim the fossil record is fatally flawed because of the lack of “transitional forms” that illustrate intermediates in the transition of one major group of animals to another. The critics are wrong, and the first two cases I will cite describe examples of transitional forms.

Evolutionists tell us that modern birds are descended from dinosaurs. A famous fossil that provides evidence for this was discovered in Bavaria in 1861. The fossil is called Archaeopteryx. The creature displays reptilian features, such as teeth and a long, bony tail, but it also has wings and flight feathers like a bird. It is commonly interpreted as a fossil of the earliest known bird. But has the fossil record thrown up any dinosaurs with feathers – unambiguous transitional forms?

Yes. Fossils found in China in the 1980s showed a variety of dinosaurs with feathers and feathery plumage. Many of these feathered dinosaurs could not have flown, which means that feathers evolved for reasons other than flight (heat insulation, for example). Flight was an extra opportunity that was exploited by creatures already carrying feathers.

2 Land-living ancestors of whales

Whales are mammals, like ourselves, but they have lived in the water for millions of years. There is good evidence that mammals originated on land, which means that, originally, the ancestors of whales forsook the land for the water. The fossil record now provides good evidence for this.

There is no shortage of fossils from the first 10 million years of whale evolution of creatures showing whale features (anatomy of the ear, for example) and limbs like those of land-living animals from which they descended. But until 2007, there was no report of a good fossil of the land-living creature from which whales eventually evolved. Work by Hans Thewissen and others described now-extinct creatures called raoellids that looked like small dogs but were more closely related to even-toed ungulates (a group that includes cows, sheep, deer, pigs and hippos). Molecular evidence had already hinted at a deep evolutionary connection between whales and even-toed ungulates.

Thewissen's work shows that one raeollid, Indohyus(oictured), is similar to whales but unlike other even-toed ungulates in various ways (for example, ear and teeth structure) that indicate the creature spent much of its time in the water. The raeollid diet is very unlike the whale diet, suggesting that the impetus to take to the water might have been dietary change. Indohyusis definitely a good candidate for a transitional form.

3 The molecular basis for Darwin’s finches

All living processes and behaviours are underpinned by molecular mechanisms. Evolution works through molecular mechanisms, and these mechanisms are being elucidated by elegant research.

Darwin described several species of finches in the Galapagos Islands that all looked similar except for their beaks. Ground finches had broad, deep beaks, warbler finches had slender, pointed beaks, and so on. Beak size and shape reflected differences in diet. Darwin speculated that all the finches had a common ancestor that originally migrated to the islands, and that natural selection had then evolved a variety of forms from this common ancestor suited to different ecological niches on the island.

Arhat Abzhanov and others have studied the genes that are switched on and off in the developing beaks of finch chicks. They found that differences in beak shape coincided with variations in the expression of the gene for calmodulin, a molecule that regulates the signalling effected by calcium on metabolism and development. For example, calmodulin is expressed stronger in species with long pointed beaks than in species with more robust beaks.

Biologists are advancing from the documentation of evolution at whole animal level to identification of the underlying molecular mechanisms.

William Reville is associate professor of biochemistry and public awareness of science officer at UCC – understandingscience.ucc.ie