In response to:
Our Biotech Future from the July 19, 2007 issue
To the Editors:
Freeman Dyson has written his usual insightful essay [“Our Biotech Future,” NYR, July 19], but I differ on one important point regarding how evolution works. He points out that black leaves would be more efficient than green ones at capturing sunlight, and then tries to explain why leaves are not black. In doing so, he implicitly assumes that natural selection, left to itself after enough time, will always zero in on the most efficient means for improving adaptation. However, natural selection proceeds via a narrow point-to-point pathway, not a wide all-encompassing one. In solving any given problem it can make use of only what happens to be available at that particular time.
An anecdote illustrates this better than a
discourse. Many years ago I was in a group of chemists making new antibiotics. One path taken was to synthesize cephalosporins with the ring sulfur atom replaced by oxygen, a chemically profound alteration. These oxy-cephs turned out to be orders of magnitude more potent than the natural ones. Why then did Nature not discover them, given so much time? Because the immediate precursors of cephalosporins on the synthetic pathway Nature had created to make them had a sulfur atom, not an oxygen atom, in the key position. The corresponding oxy precursor either did not then exist or was chemically unsuited to that pathway, and there was no going back a dozen steps to find a missing link that could go on to oxy-cephs, because every step in natural selection must be adaptive in its own right.
The basic premise is that every step in evolutionary change is stochastic and adaptive. No new structure that appears in this way persists unless it is immediately adaptive, even if just one more step might produce something very superior. Thus green leaves dominate because they happen to have come along before black ones, and also because chance uncovered no route from green to black that was adaptive at every new step.
Raymond A. Firestone
Freeman Dyson replies:
Raymond Firestone correctly points out that evolution is constrained by the laws of chemistry and can only move one step at a time. His analogy between antibiotics and leaves is a good one. Evolution has failed to produce black leaves because they must be made of silicon, and the chemical reduction of silicate rock or sand to silicon cannot be done in one step. Although silicon is one of the most abundant elements, it occurs naturally in rock and sand in combination with oxygen, and no form of life has evolved a chemical pathway leading from silicon dioxide to pure silicon.