One Story of Evolution

 Jenny Cai

February 11, 2011

Bio/Eng 223

Dalke & Grobstein

Web Paper # 1

One Story of Evolution

While there are many stories of evolution, one that is commonly shared in the scientific community is that which lies buried. Through geological history, I seek to examine the origin of organisms and their progress over time and in doing so, present one particular story of evolution commonly found in textbooks. My interpretation of progress is the strive towards greater complexity rather than towards advancement and perfection. Through each period in prehistoric time, distinct fossils and geological evidence display great variations. The textbook theory of evolution, composed of prominent evolutionary scientists’ concepts, is one accepted explanation used to explain these geological phenomena.

In this story, the time line of distinct life forms stretches back to when Earth consists only of simple, unicellular organisms, approximately 3.5 billion years ago, to the multi-cellular organisms of today. This progression is not smooth; rather, the time line of evolution is characterized by minor and major peaks and troughs that portray the pattern of biodiversity in our ecosystem. Scientists analyzed scattered fossils and the Earth’s strata over the past 600 million years and speculated that five major mass extinctions occurred worldwide. The most recent of mass extinction happened in the Cretaceous-Tertiary period approximately 65 million years ago that lead to abrupt plankton and marine extinctions. Mass extinctions happen relatively rapidly (a time span of half a million years) that causes 30% of life forms to become extinct.  Using Darwinian concepts, this loss in biodiversity is part of nature’s selection process to eliminate species less equipped to survive in certain environments and increase resources for the surviving species to thrive and speciate. The survivors are then outcompeted by other species who are more equipped to survive and they remain rebuild the ecosystem. Recovery of biodiversity through speciation takes approximately 10 million years. Mass extinction wipes out the normal process of evolution through the occurrence of abrupt unnatural events such as the impact of a 10 km sized meteor prior to the Cretaceous-Tertiary mass extinction. This collision theory is supported by the high presence of iridium in the strata dating back to this period. Possible natural elements that contribute to mass extinction are volcanic eruptions, global climate change, plate tectonic movements and changes in sea level (Benton and Harper 117-181). If depicted on the tree of life, mass extinction would represent a third of the branches that fail to extend or subdivide at the time period coinciding with one of the five major mass extinctions. The fossil record and mass extinction concurs with Gould and Eldridge’s theory of punctuated equilibrium. 

            Fossil records show periods of time where the evolution of life occurs rapidly due to mass extinction and its recovery as well as periods of stasis until the next mass extinction, about a hundred million years later. Based on the fossil record, the theory of punctuated equilibrium claims that evolution is characterized by insignificant net changes for long periods of time and periodic rare, rapid changes. An issue that Darwin juggles with in the Origin of Species is that our current fossil record, though incomplete, does not indicate the existence of the intermediate forms of species. Punctuated equilibrium accounts for this fact because the fossils uncovered most likely represents the species during the stasis period.  According to the theory, speciation occurs so rapidly that evidence for intermediate forms is more difficult to locate, as Darwin similarly claimed. An opposing theory is that of evolution occurring gradually in small increments over long periods of time. Notably, Darwin seems to support this theory through his continual use of “gradual” to describe the process of evolution, yet he also states that species are likely to retain their forms longer than the period of speciation (Jackson, Lidgard and McKinney 196-215; Darwin 269-349). Both punctuated equilibrium and gradualism can complement to a certain degree. Altogether, species are making gradual changes in a scale below the threshold of consciousness and are in the process of punctuated equilibrium where gradual changes and mutations in the genome accumulate over long periods of time to reach a peak where phenotypic changes occur.

             A good and reliable story of evolution is one that is well supported by evidence and captivates a wide audience. One particular story of evolution tells a tale that encompasses the core of Darwin’s concepts of “fitness”, “selection” and “gradualism” as well as the more recent theories that draw upon Earth’s geological history. The hybrid theory of punctuated equilibrium and gradualism builds upon individual stories into achieving one story that may be more commonly accepted by the audience.  Analysis of the available fossil record allowed scientists to add more content to a story of evolution and as advances in technology unearth more history, stories of evolution will continue to expand. Lying beneath us lay many stories untold.

References

Benton, M. J., and D. A. T. Harper. Introduction to Paleobiology and the Fossil Record. Chichester, UK: Wiley-Blackwell, 2009. Print.

Darwin, Charles, and Joseph Carroll. On the Origin of Species by Means of Natural Selection. Peterborough, Ont.: Broadview, 2003. Print.