Sunday, September 28, 2014

The Evolution of Evolutionary Thought

I just found this document on my computer.  It is a short synopsis of the history of evolutionary thought that I wrote for a class in my first year of grad school.  It is not perfect or complete, but I thought it was worth posting.  Please add corrections or comments below.

The theory of evolution by Darwinian natural selection is of critical importance to modern paleontology, and the field of paleontology has had a decisive role in the justification of the theory.  Early ideas on the natural community of life followed Plato’s Principle of Plenitude.  This principle essentially stated that everything that could exist did in fact exist, and that forms could not be created nor could existing forms be destroyed.  This principle was at the heart of the idea of the scala naturae, or the Great Chain of Being, which envisioned the natural world as consisting of a hierarchical ordering of organisms from the simplest to the most complex (read most perfect), from the creeping slimes to the crowing achievement of creation, humans.  Humans were merely a step below cherubim and other various categories of angels. 

Scala Naturae

            Such was the governing ideology when Carolus Linnaeus set about to create an organized hierarchical system of taxonomy using the binomial nomenclature which we use to this day.  Although some fossils had been discovered before Linnaeus’ time, they were not widely considered as evidence of extinct creatures.  Since there was no accepted evidence that species had in fact disappeared or had been created, Linnaeus was a firm believer in the fixity of species.  This ideology is evident in the system of taxonomy adopted by Linnaeus which organized all of life into a hierarchical system of increasingly exclusive categories (Kingdom, Phylum, Class, Order, Family, Genus, Species). Using Linnaean taxonomy, an organism is identified using its unique binomial name, consisting of a generic and specific name (e.g Homo sapiens).

            Discoveries in the field of paleontology discredited the Principle of Plenitude.  From the 1600s onward there was an increase in activity concerned with collecting fossils and studying them.  Over time, it became clear that many of the animals that produced some of these fossils were no longer known to be alive today.  As more and more of the earth began to be explored by European investigators, it became clear that these organisms were not alive anywhere on Earth!  Given the reality of the extinction of species, the Principle of Plenitude became untenable. 

            The acknowledgement that extinction occurred was a far cry from suggesting that new species could arise from existing ones.  This concept, though proposed earlier by workers such as Lamark, was most vigorously propounded by Charles Darwin in his On the Origin of Species published in 1859.  In this massive work, Darwin accumulated a wealth of data suggesting the occurrence of evolution from diverse fields such as embryology, paleontology, comparative anatomy of extant species and others.  Further, he suggested a mechanism, known as natural selection, to explain the process of evolution.  Essentially, Darwin’s theory claims that, in a given population, there are more individuals born than can survive, and these individuals vary in their expression of characteristics.  Since populations normally stay relatively constant in size, a struggle for existence must occur.  In this struggle, those individuals that have some advantage over others will survive, and live to pass their genes on to subsequent generations.  Over time, the differential reproductive success of individuals will result in the change in phenotypic expression of the population.  If this occurs to a great enough degree, speciation can occur. 

Young Darwin

            Darwin’s theory was a masterwork of logical observation, but he still did not have a mechanism for the inheritance of variation from parent to offspring.  This problem was solved in the early 20th century with the rediscovery of Gregor Mendel’s work on particulate inheritance.  Coupled with the work of mathematicians concerned with population biology working in the 1930s, the Neo-Darwinian synthesis was born.  The synthesis conceived of evolution as being driven by several fundamental factors.  Mutation is the random process by which genetic variation is created in an individual.  When these mutations occur in germ cells, this variation can be passed on to offspring.  Genetic recombination occurs when the genetic information on a parent chromosome recombines with sections of the other parent chromosome.  This is another source of genetic variation.   Genetic drift is the change in population gene frequencies resulting from stochastic processes, often involving population isolates.  Gene flow is the change in gene frequencies resulting in the admixture of two or more populations.  Finally, selection is the reduction in genetic variation resulting from the differential survival of phenotypically variable individuals, which results in population evolution. 

            These microevolutionary processes just described work very well to explain small scale changes over short geologic time scales.  Paleontology, however, with its mind-boggling time scales offers challenges to the idea of evolution.  It is difficult to imagine that the sort of massive morphological change occurring over 4 billion years of organic evolution can really be accounted for with just these microevolutionary processes.  A further challenge came in 1972 from eminent paleontologists Eldrege and Gould, who claimed that the fossil record may in fact show long periods of virtual stasis with sudden, rapid bursts of morphological change, which may not preserve well in the fossil record due to their shallow time depth.  This theory of punctuated equilibrium implies that the “gaps” in the fossil record may not be gaps at all, but rather might represent geologically instantaneous speciation events with intervening periods of stasis. These speciation events are cladogenetic events, that is, they represent the splitting of a parent lineage into two daughter lineages.  The implication of punctuated equilibrium is that there are evolutionary processes acting above the level of the individual; such processes are not accounted for under the Neo-Darwinian synthesis.  The alternative to this theory is the theory of phyletic gradualism, which holds that evolutionary change occurs gradually over time through the process of anagenetic speciation, or speciation within a lineage.  In the fossil record, there seems to be evidence consistent with both cladogenesis (stasis punctuated by rapid change with the splitting of lineages) and anagenesis (gradual change over time in a given lineage), so at present it is not possible to resolve this issue. 

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