Evolution proposes, the embryo disposes
Evolution is not the only driver of change in species: the blueprint of an embryo determines which variations actually occur. This is shown by Leiden biologists in a Nature publication on the evolutionary development of reptile and bird limbs.
Natural selection explains how species remain adapted to their environment. This has been known since the days of Darwin. However, it is becoming increasingly clear that in some cases it is the blueprint of an organism that determines whether, and how, natural selection leads to actual change. ‘The fact that adaptations in certain characteristics would increase a species’ survival chances in a changed environment does not necessarily mean that these adaptations will in fact occur,’ says Leiden Professor of Zoology Mike Richardson. ‘Genetic and physical limitations in how the embryo of a species is able to develop can result in a "desirable" change failing to take place.'
Darwin was convinced that the species that is best able to adapt will have the highest chances of survival. Since the 1980s, however, there has been a vigorous debate in the scientific community on whether the uncontested power of evolution always applies. Stephen Jay Gould of Harvard University was the first to express this doubt. Richardson and his analyst Merijn de Bakker developed a model to investigate this idea.
They chose to study the evolution of digit loss in the transition from leg to wing. Their colleagues thought they were mad. ‘Collecting embryos and studying gene expression took us more than eight years,’ says De Bakker. But as a result he is now the first author of an article about their research, written together with Richardson and their international colleagues, which appears this week in Nature. ‘We looked at the influence the embryo has on the actual transition from leg to wing.’
The team made some surprising discoveries among their unique molecular data. Even if it might be better for the flying abilities of an bird ancestor to lose its little finger, this may nevertheless fail to happen because the genetic code of the little finger is linked to the code of the entire leg. If it were to lose its little finger, the animal would also lose its leg; therefore such a species will not arise. ‘The wing does eventually appear, but in another construction,’ says De Bakker. ‘The animal may lose its thumb, for instance, and it will still be perfectly able to fly.’
Richardson draws a comparison with the principle of supply and demand in the economy. ‘There may be people who want a mahogany wood iPhone which can also be used as a TV remote control,’ he says. ‘Such a device may be practical and stylish, but it is not stocked in the shops, because there are limits to what factories can produce.’ The same applies to evolution. ‘Something that may be a good adaptation for the survival of the species may still fail to develop due to limitations in the development of the embryo.’
(8 July 2013)
'Digit loss in archosaur evolution and the interplay between selection and constraints'
Nature 7 July 2013
Merijn A. G. de Bakker, Donald A. Fowler, Kelly den Oude, Esther M. Dondorp, M. Carmen Garrido Navas, Jaroslaw O. Horbanczuk, Jean-Yves Sire, Danuta Szczerbin´ & Michael K. Richardson
Fundamentals of Science is one of the six themes for research at Leiden University.