Animal Extinction

Extinction Punctuated Equilibrium Evolutionists for generations after Darwin imagined a slow and steady process of adaptation, in which daily culling and breeding success very gradually adapted a population from one form into another. Such a process would not be dissimilar to unnatural selection, such as that done with domesticated animals, that gradually worked to change a wolf into a little Cairn Terrier or Shih Tzu. However, some have suggested that there might be a more sudden sort of change involved, in which evolution moves suddenly and with great speed.

This theorized form of evolution, called punctuated equilibrium, has been widely debated, but seems to be increasingly accepted by scientists. There appears to be evidence for punctuated equilibrium from laboratory experiments, from field and fossil evidence, from theory and even from Darwin's original work. It is a common misconception that evolution cannot be experimentally studied in laboratories -- actually, a number of studies have been performed which indicate the existence of punctuated equilibrium in laboratory settings.

In 1996 Science magazine, which is a weekly peer-reviewed journal, published an article called "Microbes hint at a mechanism behind punctuated evolution." (Mlot, 1996) The article describes an experiment performed by Lenski and Cooper, in which over 10,000 generations of e. coli were cultivated in a low-sugar solution. Over the generations, the bacteria evolved to survive best in that environment. Later generations of bacteria were larger, more fit, and could produce more offspring within the environment.

Their research clearly show: "Cells tended to persist at one average size for many generations, then suddenly go through growth spurts.. You can very clearly see dynamics that are punctuated, and that it's arising in this extremely simple system... Statistical analysis showed that the data were best represented not with a smooth curve, but with a stepped curve that took several jumps.

That is just the sort of pattern -- stasis followed by rapid change -- that punctuated equilibrium theory suggests." (Mlot, 1996) Though many feel that this research shows that punctuated equilibrium could occur, some argue that if one looks at this at a larger scale of every 500 generations rather than every 100 generations, then the change appears to be gradual. "If you look closely enough at anything, you will eventually see the steps..." (Mlot, 1996) Punctuated equilibrium has also been evidenced in field work and the fossil record.

The year before publishing the results of Lenski and Cooper's research, Science published an article wondering "Did Darwin get it all right?" In this article, the author considers a series of field research programs which have provided evidence for punctuated equilibrium.

One study of bryozoan fossils showed that "Through 15 million years of the geologic record, these species would persist unchanged for 2 to 6 million years, then, in less than 160,000 years, split off a new species that would continue to coexist with its ancestor species." (Kerr, 1995) The definition of species and the accuracy with which science could identify species was challenged by varying morphological characteristics, double checking the reliability of morphology as a distinguisher of species, and finally whether genetics and morphology were actually linked in these cases.

In the end, "Cheetham's fossil species had passed the biological test with flying colors..." (Kerr, 1995) Not only did the research appear particularly strong, but it was duplicated in several further studies with other forms. "Jackson and Cheetham have now extended the earlier work with Metrarabdotos to the genus Stylopoma. And once again, the 19 different fossil species they traced revealed textbook cases of punctuated equilibrium.. [another scientists named] Collins...recently took the same biologically based approach..

Collins and his colleagues also found punctuated equilibrium in the evolution of these snails in California over the past 20 million years." (Kerr, 1995) So it is clear that punctuated equilibrium has good evidence backing it up both in the laboratory and in the field. Theoretically speaking, punctuated equilibrium has a relatively strong case. First, it explains occasional "gaps" in the fossil record.

(It is worth noting that the founders of the theory strongly object to the characterization of this theory as being drawn from "negative proof" of absences in the time line. They say it was actually founded on positive proofs from the fossil record. However, there are segments of the record where nothing seems to change, followed suddenly by massive signs of development. The theory is entirely distinct from saltationism, which is the theory that "New species may arise in single generation through a macromutation.

Saltationism rejects Darwin's model of gradual change through generations." (Broyles, 1997) That theory is generally rejected by reputable scientists owing to its practical difficulties (with whom would the "Adam" or "Eve" of a new species mate?). Punctuated equilibrium still holds to the idea of generational change, but suggests that it happens relatively quickly (geologically speaking). periods of change may last from five thousand years to millions of years -- either is short in terms of the history of the earth.

Between these times of radical change are periods of "stasis," times when no significant evolution occurs. Despite the fact that punctuated equilibrium is frequently advertised, as it were, as being contrary to the ideas of Darwin, this is not actually the case; there is room in Darwin's original theory for punctuated equilibrium. In setting this case straight, Douglas Theobald (2003) explains that there are two forms of gradualism -- one which holds merely that changes.