Why are there no snakes in Ireland? Well, actually many islands have no snakes. It all depends on whether the island is volcanic (was formed by a volcano on the sea floor that grew until it broke the surface) or continental (broke off from a continent).
Volcanic islands, like Hawaii, rise up out of the sea as bare rock. Only plants and animals that can be blown there (small seeds, flying animals) or cross the sea on floating natural rafts of vegetation (coconuts, small reptiles) will colonize the new island. So volcanic islands rarely have snakes, unless they formed very close to another land mass, close enough for snakes to survive on rafts.
Continental islands are another story. When they break off, they carry with them all of the plants and animals that are on them at the time. Continental islands often give us a snapshot of evolution in progress. Madagascar broke off from Africa before monkeys evolved, so there are no monkeys on Madagascar. This allowed the lemurs to survive on the island, whereas on the mainland they were outcompeted by the monkeys and died out. In the same way, Australia broke off from a supercontinent after marsupial mammals had evolved but before placental mammals.
Once the island breaks off, it has its own story. In Ireland, the Ice Age came along and killed off anything that couldn't survive the cold, such as many reptiles. The same thing happened on the continent. However, on the continent, once things warmed up, snakes could migrate back from the warm places further south where they had survived. But they couldn't cross the cold sea to recolonize Ireland.
A better written, longer version of this, and the article that gave me the title of today's post.
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