Natural Selection Image Map

Stephen Gould, continued

Concerning evolution, Gould points out that in the beginning, all species HAD to be simple. Therefore if you drew a graph showing number of species on the y-axis and a scale for complexity (ranging from bacteria to humans, for example) on the x-axis, the curve would not be bell shaped. Especially for the early times of the earth, the graph would show most of the species at the left extreme of the curve. Life cannot begin simpler than simple. As time passes, life can evolve only in one direction – toward more complexity. Draw the graph yourself, so you can see what we mean. 

The other part of his point is that in the early history of earth, the curve for primitive species always had a few species out on the tail of the curve. That is, they – by chance – were more complex than simple bacteria. Some of these more complex species found suitable niches and thus survived. Now the curve becomes fatter, spread out more toward the right, with more complex species. The process repeats. Now the fatter curve still is not bell shaped, but its extreme right-hand tail again has a few more complex species, some of whom will survive. Thus life seems to be purposefully evolving toward more complex forms. But according to Gould’s view, random variation and the laws of probability are driving evolution toward the more complex, given that you can’t drive evolution toward organisms simpler that the simplest life forms.

One controversial idea was his attempt to explain sudden changes in the fossil record. Most scientists believe that the changes were not sudden and that intermediate forms are just poorly preserved or have not yet been found. Critics of evolution say there are no intermediate forms, although intermediate fossil forms have been found for some lines of evolution. Gould argued that in cases where there are no intermediate fossil forms, the evolution was sudden. His idea was that many species exist for long periods without much change, with occasional "bursts" of new species formation. Gould calls this "punctuated equilibrium." His critics sarcastically called the idea "evolution by jerks." Gould quipped in retaliation by calling his critics' theory of slow evolution as "evolution by creeps."  Who says scientists don't have a sense of humor? Actually, most scientists believe that both ideas are probably right, depending on the species.

Modern genetics research lends support to Gould’s idea: one or a few mutations in major regulatory genes generate major changes in body form. In plants, for example, just one inactivated gene changes a bilaterally symmetrical flower into a radial one.


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