Two Hypotheses on Darwin's Gradualism

Abstract

appears that Gould and Eldredge (1983) and I (Penny, 1983) still have different interpretations of what understood by gradualism. I have commented elsewhere (Penny, 1984) that processes can be studied on a time scale from 10-15 to 1016 sec and because of this, words such as or have no meaning until a time scale is defined. For example, a Padiation physicist interested in photon capture and transfer in photosynthesis is studying reactions that occur from 10-15 to 10-9 sec (Penny, 1984). They would consider ATP synthesis a very slow reaction; it takes about 0.1 sec. A physiologist or ecologist would call the same reaction instantaneous. On the other end of the spectrum, a geologist views events in the fossil record with a duration of about 104 years (about 3 x 1011 sec) to be instantaneous. is essential to define a time scale when using words such as gradual. A similar point is made by Gingerich (1984) in distinguishing between continual and continuous processes. also insisted that change did not go on continuously (see Penny, 1983). The two main alternatives for understanding Darwin's views of gradualism are: Hypothesis 1. Gradualism on a biological or time scale. Natural selection can act on the naturally occurring genetic variability within populations to cause small changes between generations. Distinct forms are connected by what called numerous, successive, slight modifications. The long-term rate of change is unpredictable. Hypothesis 2. Gradualism on a geological time scale. Natural selection causes a continual, slow change in morphology which continues over hundreds of thousands, or even millions of years and shows up in the fossil record as continual, slow change. This is phyletic gradualism. Many variations in wording are possible; the time scale is the essential difference. Both hypotheses can claim to be gradual on their appropriate time scale. In their reply, Gould and Eldredge (1983) claimed I argued that, Darwin ... was not a committed gradualist. This is clearly incorrect; the authors failed to notice that I was arguing for gradualism on a time scale. Gradualism is not the point at issue-it is the time scale. My previous article argued that hypothesis 1 is the most accurate description of Darwin's views. There are two main reasons for this conclusion. The first and most direct is simply reading Darwin's works. Several quotations were given about variability in rates of evolution and of morphological changes over a thousand generations (Penny, 1983). Since then a more detailed study by Rhodes (1983) has appeared that reaches a similar conclusion. Indeed, one of Gould and Eldredge's (1983) own quotations, It may be said that natural selection is daily and hourly scrutinising ... , supports the ecological time scale of hypothesis 1. The second reason for suggesting that hypothesis 1 is the correct interpretation