Charles Was Right…

Twenty-two year old Charles Darwin embarked on the HMS Beagle in

Plymouth on December 27, 1831, not knowing that the date would become a cornerstone in the history of mankind. Returning home five years later, the observations and discoveries he made during the journey of the Beagle are the foundation for the most explosive book in the history of biology. Destined for the staid life of a provincial church pastor, Darwin had simply been invited for the voyage to keep the captain company. But once on board, he discovered his true vocation as a naturalist.

Along the North Coast of Argentina he gathered a great number of fossils of mammals which had disappeared since the Pleistocene (1.64 million years ago), such as the sloth and the giant armadillo, which he compared with live specimens of the species in South America. His observations on the diversity of the nandus (large flightless birds of the ostrich family), mockingbirds, finches and Galapagos turtles, as well as the marine life of the Pacific atolls, reinforced his conviction that Earth’s life forms were not created by God and left to thrive here and there at God’s pleasure, as the powerful clergy of Darwin’s time would have him believe. Darwin was convinced that the animals we see today are the evolved descendants of their ancestors, better adapted to their environment after millions of years and hundreds of generations of evolution.

While this explanation seemed evident to Darwin, he found it more difficult to put it into writing and his theory was met with scepticism by other naturalists and vehemently opposed by the clergy. As a result “On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life” was only published in 1859. It was neither Darwin’s first nor last publication, but it was the single most talked-about book of its time and earned him equal amounts of admiration and criticism for the theory of natural selection. It was only around 1940 and the fundamental work of Mendeleyev on genetics that his theory was justified and officially integrated into biological theory.

In the West Indies there are many examples of adaptation and diversity. The islands’ fauna and flora largely originate on the South American continent, but although they seem similar, they are by now distinctly different. There is an unbelievably high rate of endemic species, many of which can only be found on certain islands and nowhere else in the world.

There is no need to travel all over the world for signs of evolution, St. Barth and its sister islands St. Martin and Anguilla, which used to form a single island before the sea level rose, now referred to as the Anguilla Bank, provide numerous examples. Six of the twelve reptile species found on St. Barth are endemic to the Anguilla Bank and one is exclusively typical of St. Barth. The other species were either introduced or are found on other islands of the Lesser Antilles.

Among these reptiles, the ground lizard Ameiva plei, locally known as “zanoli d’tèr”, is very common. This lizard has easily adapted itself and its feeding habits to the human presence and is at ease in a variety of West Indian climates. Most likely, the lizard’s ancestors arrived from the Greater Antilles at a time when the sea level was a lot lower than it is now and the islands of the Anguilla Bank still formed one single island.

In this new territory the lizard would have had to adapt his feeding habits and learn to defend itself against new predators. This is where the two fundamental theories of evolution would come into play: the physical adaptation of individuals to their environment and the sexual selection of female representatives of the species. In the eyes of the female lizard, green males with blue spots would appear to be more attractive than males with other colorings. These color spots break up the silhouette of the lizard on the move in the scrubs, making it more difficult to spot for birds of prey and giving it an appreciable advantage in the fight for survival and in fathering future generations.

During the same period, a small group of lizards is isolated on an islet just off what is today Anguilla, and this small group continues to evolve just as the other lizards on the main island. But on the rocky islet, where no trees grow, their green coloring puts the lizards at risk, both versus preying birds and the inclement sun. Here, darker individuals have a better chance of survival, their color allows them to hide more easily in slits in the rock and escape from predators and the sun. Several generations on, only the darker colored individuals would have survived.

On the main island, the species evolved into what we know as Ameiva plei today, and on the smaller island it evolved into Ameiva corax. Thousands of years later, as the water level rises, the island is split into three. On St. Barth and Anguilla, the lizards had no reason to evolve, but on St. Martin they underwent slight changes and ended up forming two subspecies, Ameiva plei plei and Ameiva plei analifera.

Although Darwin proved to be visionary where evolution is concerned, he did err on one point: adaptation does not necessarily take thousands of years, sometimes a few generations suffice. Take the ground lizards of Ile Fourchue, for example. The goat herds have left practically no trees for lizards to hide in, and many of the lizards have already turned brown. It seems probable that in a few generations they will all have turned brown and lost their spots, much like Ameiva corax.

Source: Tropical magazine n°19, season 2009-2010, page 33.

Photos: Karl Questel

Text: Karl Questel

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