Galapagos islands are historically important because they probably inspired Charles Darwin’s first thoughts on evolution when, as a member of the expedition on HMS
Beagle
, he visited them in 1835. They are a collection of volcanic islands in the Pacific Ocean near the equator, about 600 miles west of South America. They are all young (just a few million years old), formed by volcanoes punching up from the bottom of the sea. This means that all the species of animals and plants on the islands must have arrived from elsewhere – presumably the mainland of South America – and recently, by evolutionary standards. Once arrived, species could make the shorter crossings from island to island, sufficiently often to reach all the islands (maybe once or twice every century or so) but sufficiently seldom that they were able to evolve separately – ‘drift apart’ as we have been saying in this chapter – during the intervals between the rare crossings.
Nobody knows when the first iguanas arrived in the Galapagos. They probably rafted across from the mainland just like the ones that arrived in Anguilla in 1995. Nowadays the nearest island to the mainland is San Cristobal (Darwin knew it by the English name of Chatham), but millions of years ago there were other islands too, which have now sunk beneath the sea. The iguanas could have arrived first on one of the now sunken islands, and then crossed to other islands, including those still above water today.
Once there, they had the opportunity to flourish in a new place, just like the ones that arrived in Anguilla in 1995. The first iguanas on Galapagos would have evolved to become different from their cousins on the mainland, partly by just ‘drifting’ (like languages) and partly because natural selection would have favoured new survival skills: a relatively barren volcanic island is a very different place from the South American mainland.
The distances between the different islands are much smaller than the distance from any of them to the mainland. So accidental sea crossings between islands would be relatively common: perhaps once per century rather than once per millennium. And iguanas would have started turning up on most or all of the islands eventually. Island-hoppings would have been rare enough to allow some evolutionary drifting apart on the different islands, between ‘contaminations’ of the genes by subsequent island-hoppings: rare enough to allow the different groups of iguanas to evolve so much that when they eventually met again they could no longer breed together. The result is that there are now three distinct species of land iguana on Galapagos, which are no longer capable of cross-breeding.
Conolophus pallidus
is found only on the island of Santa Fe.
Conolophus subcristatus
lives on several islands including Fernandina, Isabela and Santa Cruz (each island population possibly on its way to becoming a separate species).
Conolophus marthae
is confined to the northernmost of the chain of five volcanoes on the big island of Isabela.
That raises another interesting point, by the way. You remember we said that a lake or an oasis could count as an island, even though neither consists of land surrounded by water? Well, the same goes for each of the five volcanoes on Isabela. Each volcano in the chain is surrounded by a zone of rich vegetation, which is a kind of oasis, separated from the next volcano by a desert. Most of the Galapagos islands have only a single large volcano, but Isabela has five. If the sea level rises (perhaps because of global warming) Isabela could become five islands separated by sea. As it is, you can think of each volcano as a kind of island within an island. That’s how it would seem to an animal like a land iguana (or a giant tortoise), which needs to feed on the vegetation found only on the slopes around the volcanoes.
Any kind of isolation by a geographical barrier which can be crossed sometimes but not too often leads to
Beagle
, he visited them in 1835. They are a collection of volcanic islands in the Pacific Ocean near the equator, about 600 miles west of South America. They are all young (just a few million years old), formed by volcanoes punching up from the bottom of the sea. This means that all the species of animals and plants on the islands must have arrived from elsewhere – presumably the mainland of South America – and recently, by evolutionary standards. Once arrived, species could make the shorter crossings from island to island, sufficiently often to reach all the islands (maybe once or twice every century or so) but sufficiently seldom that they were able to evolve separately – ‘drift apart’ as we have been saying in this chapter – during the intervals between the rare crossings.
Nobody knows when the first iguanas arrived in the Galapagos. They probably rafted across from the mainland just like the ones that arrived in Anguilla in 1995. Nowadays the nearest island to the mainland is San Cristobal (Darwin knew it by the English name of Chatham), but millions of years ago there were other islands too, which have now sunk beneath the sea. The iguanas could have arrived first on one of the now sunken islands, and then crossed to other islands, including those still above water today.
Once there, they had the opportunity to flourish in a new place, just like the ones that arrived in Anguilla in 1995. The first iguanas on Galapagos would have evolved to become different from their cousins on the mainland, partly by just ‘drifting’ (like languages) and partly because natural selection would have favoured new survival skills: a relatively barren volcanic island is a very different place from the South American mainland.
The distances between the different islands are much smaller than the distance from any of them to the mainland. So accidental sea crossings between islands would be relatively common: perhaps once per century rather than once per millennium. And iguanas would have started turning up on most or all of the islands eventually. Island-hoppings would have been rare enough to allow some evolutionary drifting apart on the different islands, between ‘contaminations’ of the genes by subsequent island-hoppings: rare enough to allow the different groups of iguanas to evolve so much that when they eventually met again they could no longer breed together. The result is that there are now three distinct species of land iguana on Galapagos, which are no longer capable of cross-breeding.
Conolophus pallidus
is found only on the island of Santa Fe.
Conolophus subcristatus
lives on several islands including Fernandina, Isabela and Santa Cruz (each island population possibly on its way to becoming a separate species).
Conolophus marthae
is confined to the northernmost of the chain of five volcanoes on the big island of Isabela.
That raises another interesting point, by the way. You remember we said that a lake or an oasis could count as an island, even though neither consists of land surrounded by water? Well, the same goes for each of the five volcanoes on Isabela. Each volcano in the chain is surrounded by a zone of rich vegetation, which is a kind of oasis, separated from the next volcano by a desert. Most of the Galapagos islands have only a single large volcano, but Isabela has five. If the sea level rises (perhaps because of global warming) Isabela could become five islands separated by sea. As it is, you can think of each volcano as a kind of island within an island. That’s how it would seem to an animal like a land iguana (or a giant tortoise), which needs to feed on the vegetation found only on the slopes around the volcanoes.
Any kind of isolation by a geographical barrier which can be crossed sometimes but not too often leads to
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