The Narrow Way

However you may feel about Richard Dawkins’ recent campaign against religion, he is indisputably among the greatest living scholars of natural history. One of the many topics he discusses in his richly informative book The Ancestor’s Tale is the notion that “evolvability” itself may be amenable to natural selection. He suggests that certain watershed developments in life’s history greatly increased the facility of organisms to adapt, and that such developments would themselves have been adaptive. There is tricky footing here; it is important to keep in mind that natural selection never “looks ahead”. But, as Dawkins writes near the end of the book, “we might find with hindsight that the species that fill the world tend to be descended from ancestral species with a talent for evolution.” There are a number of developments that Dawkins cites as having improved life’s “evolvability”: among these are the birth of eukaryotic cells, multicellularism, segmentation, and sex. He also discusses another, less obvious milestone: bottlenecking.

The word “bottleneck” has another, more common meaning in biology, in which it refers to the reduction of a populous species to just a few individuals. This results in a great reduction of genetic diversity, as all the subsequent members of the species are the descendants of the small group of survivors. Such an event befell, for example, the North American bison herds, which were hunted from an estimated population of 60,000,000 before the arrival of Europeans settlers down to a few hundred animals by 1890. Likewise, it is thought that our own species went through a very narrow bottleneck about 70,000 years ago, in which the Toba supervolcano in Indonesia caused catastrophic global climate changes that brought the human population down to as few as as 1000 people.

But this isn’t the kind of bottleneck Dawkins is referring to here. He is talking about the fact that for many eukaryotic species, including all that reproduce sexually, there is a point in the life cycle where the organism is reduced to a single cell: a zygote, or fertilized egg. This is in contrast, say (to use the example Dawkins suggests), to a “straggling water plant” that reproduces by breaking off pieces of itself once it reaches a certain size. Dawkins points out that this sort of bottlenecking has three important consequences, “all of which are certainly good candidates for improvements in evolvability”:

First, evolutionary innovations can be reinvented from the bottom up, rather than as remouldings of existing structures — the equivalent of beating swords into ploughshares. An improvement in, say, a heart, has a better chance of being a clean improvement if genetic changes can alter the whole course of development from a single cell. Imagine the alternative: take the existing heart and modify it by differential tissue growth within its continuously beating fabric. This on-the-trot remodelling would impair the working of the heart and compromise the would-be improvement.

Second, by continually resetting to a consistent starting point in a recurrent life cycle, bottlenecking provides a ‘calendar’ by which embryological events may be timed. Genes may be turned on or off at key points in the growth cycle. Our hypothetical straggling chunk-extruder lacks a recognizable timetable to regulate such switchings on and off.

Third, without bottlenecking, different mutations would accumulate in different parts of the straggling chunk-extruder. The incentive among cells to co-operate would be reduced. In effect, sub-populations of cells would be tempted to behave as cancers, to increase their chance of contributing genes to the extruded chunks. Wiith bottlenecking, since every generation starts out as a single cell, the whole body has a good chance of being made of a uniform genetic population of co-operating cells, all descended from that single cell. Without bottlenecking, the cells of the body might have, from a genetic point of view, “divided loyalties”.

Although I had already been aware, of course, of the marvellous fact that the link between my own 225-pound phenotype and that of my towering teenage son was a single cell — an amazing feat of compression — I had never considered these other implications until Dawkins pointed them out. This outstanding book is densely packed with such fascinating ideas, and I will be taking many of them up in future posts.

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