I’m traveling in the Galapagos currently. Hopefully I’m having fun and seeing great birds and other species! In the meantime here’s a little of what I will hopefully be seeing!
The Galapagos’s Natural and Unnatural History: Founder’s Effect
In the last missive, I laid forth a hopefully straightforward explanation of evolution (by natural selection). Today I’ll explore it’s relevance to the Galapagos. We have a couple species we could explore: the Mockingbirds, the finches, or the tortoises, or a number of plant species. But since we’re here presumably due to a shared passion for birds let’s stick with that class. Did you know, it was actually the mockingbird that inspired Darwin, not the finches named for him?
Now, the initial post, geology, explored the location of the archipelago, 600 miles from the mainland. That is noteworthy. In the third post, human impact, I mentioned that there were discovers before humans and I didn’t mean Doctor Who. When the volcanic islands first emerged from the sea they were very hot barren rock. They cooled, but they were still barren rock. We now know that the invisible air is filled with invisible particles including biotic ones. Dust travels from Africa to become the condensation nuclei in clouds above America; we measured radiation from Japan’s Fukishima reactor in Massachusetts; when the Eyjafjallajökull volcano of Iceland erupted, the aerosol debris grounded flights, stranding people throughout Europe for weeks. I forget now where I read it, but there was fascinating work done looking at bare rock islands in Iceland after an eruption. The focus was measuring how quickly biotic life reached the resurfaced island through the air. It was months. That island was much closer to a biotic source, but it presents a viable mechanism for colonizing the Galapagos. And thus (we presume) it happened. Bacteria and algae made their way to the islands where together they formed a symbiotic union and began breaking down the rock to make soil. They weren’t doing it for any higher purpose; they were only struggling to survive. After all think of all their fellow travelers who missed landing on any island at all and perished. After considerable time, soil formed.
The earliest plants were most likely to reach the island via the airways. The vast majority of plants on the Galapagos have wind-borne seeds. A ha.
Soon (geologically speaking) the animals will arrive. The birds and the bats (of which there are two species) will also arrive by air, but the rest must make their way by some other fashion. The fur seals and the sea lions swam, but the reptiles and the insects were likely swept off the mainland on rafts of plant material aimlessly drifting through the ocean. Mammals, due to their need to eat often, are unlikely to survive the weeks it would take to drift on currents to the islands (but the rice rats did it). Reptiles, being ectothermic (cold-blooded) and requiring less energy consumption can go extended periods without food; insects can either eat the plant material they’re riding on, or could be transported as eggs or in some stage of torpor, eggs being more likely of the last two.
It doesn’t take many. The minimum requirement is a male and a female. These are the founders (think Gilligan’s Island or splinter groups). Imagine a small flock of finches blown off the South American mainland and out to sea. As a group, they resolutely fly forward seeing any shelter in a storm. By great fortune, one might note breaking waves indicating land. They land. Though they do not realize it yet, they are home. There is no going back. They are the first. For the earliest arrivals, storm-drenched and dazed, they are fortunate that there are no predators and no competitors to contend with. A new struggle begins: this time it’s not a battle with the winds, but a battle with the new land. If they cannot cope, they cannot survive. Can they find food, can they alter their behaviors to fit this new land?
If they can find food and begin creating a life for themselves, in essence: if they survive, they can begin reproducing. Recall the importance of mate choice on evolution as outlined in the fourth article. Whoever it is that landed on the island (and survived) will represent all future genes. Their genes are the genes that will rebuild the genetic pool. Because there are so few, the selection of genes is likely to be very skewed. This is the founder’s effect. If only small, light-weight birds were blown away from South America, then the gene pool will predominantly feature genes for smaller birds than on the mainland. Further compounding the Founder’s Effect is that not all arrivals may reproduce. With the diversity of genes present, some individuals may have traits that make them a very poor candidate for survival on the islands, and they perish. Gene pool is further diminished. That is selection. Of all the small lightweight birds that arrived, imagine some were picky eaters and others were less fussy (generalists). The fussier eaters, would be less likely to find food, and thus the future generation would be smaller birds than were more generalist in feeding habits than the population on the mainland.
For now they are the same species. If reunited, they were merge and mix; the differences separating the two groups would be lost within a generation. In this scenario, due to the distance, it is improbable. So each group continues on separate trajectories. The finches on the mainland continue in their lush tropic climate while the new finches in the arid Galapagos begin drifting, and morphing into something new as a result of their new environment. Selection is at work.