someone clever? evolution in regard to island dwarfism?

Question

i am a christian and accept creation but i have also read a lot about evolution and i think it is perfectly logical and sane given sufficient time and the right conditions (which btw i think is a BIG given). many christians try to argue against evolution by saying that organs such as the eye are too complex to have arisen gradually but even that seems perfectly logical to me. I think the only thing that i can't get my head around is island dwarfism, where, when cut off on an island, groups of large animals evolve to be much smaller. i can see the survival advantages, but i cant see how if say a group of elephants got stuck on an island they wouldn't just eat all the food and die out, but fossils of dwarf elephants have been found on many different islands around the world?!?! does anyone have a clear darwinian explanation for this phenomenon? BTW PLEASE DON'T USE THIS FORUM AS A SOAPBOX FOR YOUR RELIGIOUS VIEWS!

Answer 1

If you can buy the idea of time playing it's part in gradually developing things, we can apply that to island dwarfism.

As you stated, islands usually are poor in resources, providing a driving incentive for things to be smaller. A smaller animal has less of a demand for food than a large animal. In an area with little food, it would only make sense that smaller animals would probably survive in situations where a gluttonous big animal would succumb to starvation.

Imagine four guinea pigs somehow making it onto an island. Two guinea pigs, Hugo and Olga, are of a big-boned, beefy stock. Two of the guinea pigs, Lester and Petunia, are still guinea pigs, but of slightly smaller stock. Due to the fact that there is a limit of food, Hugo and Olga eventually succumb to disease brought on by the fact they simply need more energy/food to survive. Lester and Petunia, while pretty thin and sickly at this point, still manage to scrounge up enough to get by and even have a few babies.

Now, let's look at their babies. Ignoring inbreeding depression and other reasons why it would suck to be mating with your brother/sister/close family, their babies would have the same pressures on them as Lester and Petunia faced. Any particularly large babies would go the way of Hugo and Olga. Babies the size of Lester and Petunia would probably end up being hungry and unhappy, but they would probably get by. But, Lester and Petunia also have a few runtish little piglets. These individuals wouldn't need as much food as Lester and Petunia! As such, they'd have more energy for mucking about, mating, being vigilant against predators and fighting disease.

Now let's tighten that waistband a bit. Let's say that a particularly devastating tropical storm hit this porcine paradise. In the chaos of flying coconuts and crashing waves, much of the delicious, low-lying fruit that the pigs depended on for breakfast, lunch and dinner were destroyed. Suddenly, even Lester/Petunia-sized pigs are starving. The runts are hurting, but at least they aren't just wallowing in their misery like the larger set. After a few months, things return to normal concerning the food.

A few months is a long time to live with minimal food. As such, a huge deal of pressure was placed on the Lester/Petunia-sized group of pigs. Many died to starvation/disease/predation. For the runts, many also died to starvation/disease/predation, but less, because their energy demands were lower. You now have a population in which the proportion of big pigs to small pigs has shifted towards the runts. This is to say that there is now a greater percentage of runts in the total population than before.

Now imagine scenarios like this taking place over thousands of years. Every time a food shortage occurred, it would basically reward those with smaller metabolic demands than those with more stringent needs.

One way I like to kind of visualize it is with a calculator. I just type in 1 and multiply it by .99. Multiply the result by .99 again and again and again and again. You will notice the number gets smaller and smaller. This kind of gives you an idea of a situation where a little bit of pressure will eventually bring you to a place quite far from 1. This can be roughly analogous to the guinea pigs on the island. Their average weight/size might be 1 pound, but due to the lack of food, the top 1 percent of pigs don't mate/have babies. The next generation now has an average weight/size of only .99 pounds. The next generation gets trimmed again, going down to .98 pounds. This will go on and on until some sort of equilibrium between the different factors that influence the size of the guinea pigs is reached.

As one final comment, I think your situation with the elephants probably has happened any number of times (although I constantly wonder how an elephant gets on an island in the first place). Many of those groups of elephants simply died. It was only in certain cases where the elephants were able to survive. Perhaps, in these cases, the food pressure wasn't incredible. It was more of a subtle influence that only really cropped up in cases of disaster. In those periods of serious food shortage, then you would have seen the smaller elephants surviving over the bigger ones. This is kind of getting into the idea of punctuated equilibrium, but I simply don't have the time. I have papers to grade. :)

Anyway, I hope that helped a bit. Way to stay curious!

Answer 2

I believe there probably would be a massive die-off of full-sized elephants on an island, but as long as it were a big enough island to barely support a few of the smaller, stunted, malnourished elephants, it could work. Don't forget key deer as well, it's not just extinct examples of stunted populations due to limited range.

Let's say we've got a herd of 5 elephants that can make a round of an island and wipe out nearly every green thing until they come back around to their starting point, everything they had eaten would have -started- to grow back, I would think, and just cuz an animal is hungry doesn't mean that it's natural cycles stop... so this herd of say, five elephants with just enough food on their island to have to make constant rounds... of course new calves wouldn't be able to get the nutrition that a growing elephant needs... so they'd be a bit smaller, and have no choice but to breed with another slightly smaller elephant, and their offspring would be reduced to eating what they could.... it's a delicate balance, but isn't everything?

Answer 3

Normal sized elephants would quickly eat all the available food on the island, but any babies they would have would be smaller than themselves, and their babies would be smaller still. This is because size is not only determined by genetics, but by food supply (or lack there of) as well. If there is not enough food for the baby elephants they will grow, but much slower than the adults did on the mainland where they had plenty of food. They would also stop growing at about the same age as the adults did (due to genetics) and so be smaller. This makes them better able to handle a reduced food supply as they no longer need as much food.

A present day example can be found on the African continent. In Sudan, where human children cannot get the proper nutrition, they will often be much smaller than their parents if they grow to adulthood.

Answer 4

Natural selection kind of explains it - in regions where resources (food, shelter, etc) are scarce, the smaller may have had better chances at surviving due to needing less food to sustain themselves thus "weeding out" the larger members of the gene pool.

You example of elephants is almost there - suppose there was enough to sustain them for a fairly long time but suddenly the elephants outweigh the trees that feed them. Smaller elephants may have found it easier to feed from lower growing foliage. The population may have even hit a bottle-neck during which all of the larger elephants died out leaving only the smaller ones and their offspring to prosper.

That's my take on it anyway.

Answer 5

Similarly, reptiles on islands tend to be larger because they are able to slow metabolism thus negating the lack of food for long periods of time and usually there becomes fewer predators so one dominant predatory reptile has little competition other than among those of its own species therefore the biggest and strongest survive.