Intelligent design proponents make a negative argument for design. According to them, the complexity and diversity of life cannot be accounted for by unguided evolution (henceforth referred to simply as ‘evolution’) or any other mindless natural process. If it can’t be accounted for by evolution, they say, then we must invoke design. (Design, after all, can explain anything. That makes it easy to invoke, but hard to invoke persuasively.)
Because the ID argument is a negative one, it succeeds only if ID proponents can demonstrate that certain instances of biological complexity are beyond the reach of natural processes, including evolution. The problem, as even IDers concede, is that the evidence for evolution is too strong to dismiss out of hand. Their strategy has therefore been to concede that evolution can effect small changes (‘microevolution’), but to deny that those small changes can accumulate to produce complex adaptations (‘macroevolution’).
What mysterious barrier do IDers think prevents microevolutionary change from accumulating until it becomes macroevolution? It’s the deep blue sea, metaphorically speaking. IDers contend that life occupies ‘islands of function’ separated by seas too broad to be bridged by evolution.
In this post (part 2a) I’ll explain the ‘islands of function’ metaphor and invite commenters to point out its strengths and weaknesses. In part 2b I’ll explain why the ID interpretation of the metaphor is wrong, and why evolution is not stuck on ‘islands of function’.
Read on for an explanation of the metaphor.
The ‘islands of function’ metaphor
The ‘islands of function’ metaphor is a variation of another metaphor, the ‘fitness landscape’. If you’re unfamiliar with the concept of fitness landscapes, I encourage you to do some Googling before reading on.
For those who are familiar with fitness landscapes, a brief review. Imagine a three-dimensional landscape, similar to a terrestrial landscape. There are mountains and depressions, ridges and valleys, plains and plateaus. An organism occupies a particular spot on the landscape. Nearby spots represent organisms that are similar, but with slight changes. As you move further away from the spot, in any direction, the organisms represented become less and less like the original organism.
Evolution can be visualized as a journey across such a landscape. Individual organisms don’t move, but their offspring may occupy different nearby spots on the landscape. So too for their offspring’s offspring, and so on. Thus successive generations trace out a path (or paths) on the fitness landscape as changes accumulate.
Clearly, not all paths are possible. Many mutations are deleterious, causing their possessors to die young or to otherwise fail to reproduce. Paths going through such points on the landscape will end abruptly. Other mutations are beneficial, neutral, or only slightly deleterious. Paths going through those points may continue.
Now let’s bring in the third dimension, height. The height of a point on the landscape is an indication of the fitness of the corresponding organism, where fitness equates to the organism’s ability to survive and reproduce. Greater heights correspond to higher fitness, lower heights to reduced fitness. Offspring that move downhill from their parent(s) are less fit, and therefore tend to leave fewer offspring of their own. Offspring that move uphill from their parent(s) are more fit and tend to leave more offspring. Over time, then, a population tends to shift in an uphill direction as the offspring become fitter.
Eventually the population may reach the tip of a peak and get stuck there. From the peak, movement in any direction results in less fitness. Thus the mutants will tend to die off and the population will remain at the tip of the peak.
So far we’ve been imagining a dry landscape. Now suppose that it rains for 40 days and 40 nights. The rain fills up our landscape, forming a vast sea. Only the mountain tops remain above the water as islands – the ‘islands of function’ that IDers are so fond of.
Our populations occupy the islands. Sea level indicates the minimum fitness at which mutants remain viable. Small changes will create viable descendants at different spots on the island, though the population as a whole will gravitate toward the high spots. Larger changes will put the mutants underwater, where they will die out.
The idea, according to ID proponents, is that populations remain stranded on these islands of function. Some amount of microevolutionary change is possible, but only if it leaves you high and dry on the same island. Macroevolution is not possible, because that would require leaping from island to island, and evolution is incapable of such grand leaps. You’ll end up in the water.
There is some truth to the ‘islands of function’ metaphor, but it also has some glaring shortcomings that ID proponents almost always overlook. I will mention some of the strengths and shortcomings in the comments, and I know that my fellow commenters will point out others.
I may add them to the OP as they come up in the comments. If I do this, I will note that I am doing so and I’ll include a link to the place in the comments where each one is discussed.
Have at it!