Example Biology Essay
The Fossil Record and Theories of Evolution.
In general, the term 'evolution' can imply a drastic orgradual change from a very broad perspective. Life on earth, the universe,galaxies, as also the earth in general have evolved through millions of years.In this essay we consider only one aspect of evolution emphasizing on evolutionas a biological tool for change among species and consider fossil record assupportive of both evolution theories and also the other theories contrary toevolution. Evolution is the central unifying concept, a theory thatsuccessfully connects biology, paleontology and other branches of science.Evolution is a gradual descent of organisms accompanied by changes that helpthe organisms to adjust and adapt to the surroundings. 'Descent withmodifications' as Darwin contended implies changes in organisms in successivegenerations (Mayr, 1976). These changes are triggered by the derivation of newspecies and there is a change in the properties of populations of organisms andthese properties tend to transcend the lifetime of any single individual. Newerspecies are modified versions of older species.
Although, individual organisms do not biologically evolve,populations evolve when heritable genetic materials are transmitted from onegeneration to another. Biological evolution can range from very limited changesto drastic transformations on a large scale changing the entire speciesaltogether and bringing in new forms. Evolution can thus be defined asinheritable changes in populations of species that are spread and transmittedover many generations (Zimmer, 2002). It is also more scientifically defined aschanges in the frequency of alleles within a gene pool carried throughdifferent generations as understood in the Darwinian version of the theory(Dawkins, 1989). Evolution studies are supported by detecting changes in genefrequency within a population and the fact that the theory of evolutionemphasizes on a common ancestor, only indicates that two or more species showsuccessive heritable changes in populations since they are separated from eachother as distinct forms (Allen and Briggs, 1989). Most popular definitions ofevolution however highlight not the transmission of heritable traits andchanges but the processes of diversity that has given rise to millions ofspecies from the most primitive organisms. Here however we move on to theevidence for and against evolution theories and the role of fossil record inthis context. Some researchers claim that the theory of evolution has beensupported by four primary sources that serve as evidence (Zimmer, 2002; Allenand Briggs, 1989):
1. The fossil record that tracks changes in early andprimitive forms of life
2. The anatomical and chemical similarities in theconstitutions of different species.
3. The genetic changes observed and recorded in severalliving organisms over several generations
4. The geographical spread and distribution of species thatseems to suggest a definite pattern, and
The Fossil Record
Fossils are buried in rock layers as indentations of dead plantand animal materials. The totality of these artifacts and their impressions onthe rock formations is considered a fossil record. Fossil record as we havebriefly mentioned is the primary source of evidence supporting the theory ofevolution and the gaps in these records ironically also forms the bone ofcontention taken up by anti-evolution theorists. Fossil records are used byscientists to understand the process of evolution in general, and thesubsequent changes in several species at several times of the earth's existence(Donovan and Paul, 1998).
The Fossil Record seems to provide an important clue to thechanges in primitive and even now extinct species and this definitely helps usto frame a conceptual graph on how evolution has taken shape. Fossil and rockrecord forms the primary source of evidence collected by scientists for nearly400 years and the consequent database obtained is mainly observational. Thefossil record among all other evidence gives a large database of documentedchanges in past life on earth. The use of Fossil record to study life forms onearth dates back to pre-Darwinian times and the changes in life forms could bestudied from a sequence of layers of sedimentary rocks and fossils of differentgroups of species were found in each of these successive layers (SA, 1982).Sedimentary rocks are found widely across the earth's surface and are formedwhen small particles of sand, mud or gravel, shell or other materials witheredoff by water or wind accumulates in seabeds and oceans. As these sediments pileup they bury shells, leaves, bones, and parts of living organisms. Layers ofsediments are thus formed for every large period of time and all these layersbecome subsequently cemented to each other to become different layers of sandstone,limestone, shale and so on. Within these layers of sedimentary rocks the plantand animal remains become buried as fossils and are later revealed as fossilrecords (Allen and Briggs, 1989). From these fossil records several specieshave been identified, some of which are extinct and some of which have traitstransitional between different major groups of organisms. Fossils oftransitional forms actually give considerable evidence of species evolutionover time. However there is not enough evidence through fossil records toconclusively prove evolution, as there are still talks of 'missing links' asvery few and according to some, no transitional forms have been actuallydiscovered. The Fossil record data available to us is incomplete and inconclusiveat present.
During the late eighteenth and early nineteenth centuries,William Smith, a British Engineer observed different assemblages of fossilspreserved at different levels and different ages of rocks. These assemblagessucceeded one another in a regular and determinable order (cited in, Wikipedia,2004). This was further bolstered by the fact that rocks collected fromdifferent locations showed similar fossil formations according to the differenttimes they represented. Smith named this correlation of rock fossil data as theprinciple of faunal succession. The occurrence of faunal succession was one ofthe primary arguments of Darwin who used fossil evidence as supporting thetheory of evolution.
Various modern approaches to the theory of evolution havebeen recently developed. Mayr claims that the theory of Punctuation forinstance has two basic points that
1. most or all evolutionary change occurs during speciationevents, and
2. most species usually enter a phase of total stasis afterthe end of the speciation process (which involves formation of new species).
Speciation thus involves transformation of species ingeological time (Erwin and Anstey, 1995). Formation of new species is explainedeither by phyletic gradualism or a gradual steady transformation of species byphyletic evolution highlighting the deficiency of the fossil records, or bysympatric saltational speciation that highlighted punctuational equilibria andbranching of species rather than transformation as lineages as the real explanationfor evolution (Mayr and Provine, 1998). Biologists like Gould and Eldredge havealso supported punctuation theories. Richard Dawkins on the other hand stresseson the principle of gene multiplication where genes as replicators seems to bethe focal point of defining evolution (Sterelny, 2001).
In quite an important paper Volkenstein (1987) suggeststhat there can be no contradiction between punctuated equilibrium and phyleticgradualism if synergetics and theory of information are incorporated within thetheory of evolution. Punctualism can be seen as phase transition maintainingthe directionality of evolution. Volkenstein argues that Punctualism, non-adaptationism and neutralism form the triad ofinternally connected features of evolution.
Problems with Fossil Records
Of course at that point, the absence of a proper theory ofevolution prevented Smith or other researchers from providing an explanation ofthe actual cause of faunal succession. The cause of faunal succession as isknown today is mainly due to evolution of organisms and species that change,transform or become completely extinct, leaving behind their traces on earth asfossils. Age of rocks and the changes in species features are both determinedby fossil record and faunal succession used as tools in biostratigraphy.However fossil data show extremely few records of transitional species,organisms that can conclusively suggest how and when evolution of new anddifferent species occurred (Donovan and Paul, 1998). Darwin himself suggestedthat the geological record itself is imperfect and incomplete and this isfurther strengthened by the fact that transitional species were short lived andhad very narrow geographical range.
Radiometric and Carbon dating have made it possible to identifyfossils more than 3.5 billion years old and have indicated that animal speciesmay have appeared abruptly, a phenomenon which Darwin himself found difficultto accept. Even though one or two forms of organisms which may be considered astransient have been identified, there are no records of transitional plants andthus an evolutionary plant history could not be drawn as of yet. Along withthese issues it has also been seen that most of the fossils found are ofspecies which have existing forms and are either similar to existing species orare completely identical. The intermediate temporary stages as serve to act, aslinks between two related species seems to have been completely downplayed bythe fossil data obtained. Animals seem to have remained more or less unchangedthrough all these years. Despite the collection of a huge number of fossils,nearly all of them being fossils of presently existing animals have createdproblems for the theory of evolution. It is a general belief that based onfossil discoveries already made, there will be little or no evidence thatevolution had actually occurred and continues to occur (Donovan and Paul,1998). If animals die a natural death, they are usually decomposed even beforebeing fossilized. However during sudden catastrophes can bury the animals andembed them deep in the earth. Some rocks and organisms that transformed to showfossils for years and decades were actually deposited within a short period oftime.
Although Darwin based his arguments heavily on fossilrecord, most scientists now believe that fossil record is actually incompatiblewith evolutionary theory as no transitional links or intermediate forms havebeen discovered among this huge collection of fossils in all these years. Thissuggests that there is no real evidential data that the theory of evolution isin fact true. There is no evidence of partially evolved species or intermediateforms either in the past or in the present fossil record and the fossil recordavailable is quite representative of all fossil data that will ever becollected. Evolution seems to point out towards more undefined and partiallyevolved species, fact completely undermined by available fossil record thatshows well-defined organisms rather than gradual gradations. The incompletefossil record is the primary bone of contention in the evolutionary debate andseems to give an edge to non-evolutionists.
Considering all the aspects of the debate and gaps infossil records and weighing this against evolution theories highlighting eithergenerational transformation of lineages or drastic changes and speciation atspecific periods, we can conclude that available gaps in fossil record may bemore indicative and supportive towards speciation and abrupt changes ratherthan gradual evolution through phyletic transformation.