Defining Darwinism And Understanding Evolution Biology Essay
Evolution means descent of organisms with genetic changes over time forming new species. The term 'Darwinism' refers to Charles Darwin theory of natural selection causing "evolutionary change" (Gould, 2002). Darwin observed that though there was an overproduction of offspring, populations remained the same; suggests that variations between the offspring's are selected by nature and only few survive. Offspring with beneficial characteristics, best suited to the environment, survive, reproduce and pass on the beneficial genes to their offspring; resulting in species formation gradually (Darwin, 1884). The purpose of the essay is to discuss the various arguments for and against the use of term 'Darwinism' in relation to evolution.
Homologues structures, such as bird's wings and horse's legs, carry out different functions but are similar in structure, as shown in Figure 1, (Brooker et al., 2007). This suggests that they evolved from a common ancestor, but have adapted to different environments, forming different species; thus supporting 'Darwinism' (Oldroyd, 1980). In contrast, fossil records do not reflect Darwin's theory because if species had evolved over generations, then the fossils should show the intermediate forms of the species as it evolves; but the fossil record is inconsistent (Darwin, 1884). Similarly, Darwin proposed that selection acting on individual, leading to speciation, occurred progressively over many generations. However, during Cambrian explosion a huge diversity of species appeared on earth, over a short period of time - this is supported by fossil records.
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Evolutionary biologists dispute that natural selection, which acts on continuous variation, could not lead to the evolution of a new species. Because, natural selection adapts prevailing characters to the environment causing quantitative changes to occur in an organism, such as increase in organ size or increase in speed of flight, but not qualitative changes; thus selection, due to these slight variations, could not be accountable for appearance of new structures (Kellogg, 1907).
Also, 'Darwinism' is based on the occurrence of variation between species; that causes beneficial changes to the organism which can be transferred from one generation to another. However, non-beneficial modifications being passed on over the generation; for example presence of horny callosities on all horses feet, which serves no purpose, cannot be explained by Darwin's theory, as the modified characters do not help the organisms in their survival. Similarly, sterility is still conserved in the population, even though it is not a beneficial character. Thus, 'Darwinism' describes the method of species adaptation; not species evolution and the term hold back the public understanding of evolution (Oldroyd, 1980).
In addition, selection does not describe the occurrence of 'repeated identical structures' on an organism. For example, carnivores back teeth advantageous for biting into prey are developed over generations from the small skin teeth. This development could have occurred due to selection of a chance variation. But, the development of tooth beside the back tooth in the same manner cannot be explained by selection of a chance variation. Another example is the association of nerve fibres that innervates the muscle cell. Selection, on the basis of accident, could be responsible for the relationship between one muscle cell and the nerve fibres. However, selection due to chance variation every-time could not be responsible for the complex relationship between the organs in the thousands of species (Kellogg, 1907). Therefore, the term 'Darwinism' holds back public understanding of evolution.
Some secondary sexual characters, such as the huge antlers of stag, cannot be explained by selection as they do not aid the organism in their struggle to exist, but can cause harm, since the antlers make them more visible towards the prey. Alternatively, presence of secondary sexual characters can make them more attractive to prospective mates, more likely to breed, produce offspring and pass on the beneficial characteristics; consequently supporting Darwin's theory (Zimmer, 2010).
Darwin compares artificial selection to natural selection, because in both process individuals with beneficial characteristics are selected. However, during artificial selection, new variety of the species is formed not a new species. This suggests that new species could not have evolved just due to selection of advantageous characteristics. Another argument is that when organisms of different variation are artificially crossed they produce fertile offspring, thus conveying Darwin's theory of inheritability of variation; but when two species cross they produce sterile offspring which emphasis "fixity of species" (Stebbins, 1982).
Another difficulty with Darwin's theory is that several studies have revealed that variations are not just due to environmental, but also due to genetic factors; for example recombination during meiosis and chance mutations. Genetic recombination, not included in Darwin's theory, is vital for evolution as new combination of the genetic material enable the individuals in the population to diverge along separate evolutionary lines. However, new combinations are not selected and disappear from the population, if they do not help organisms in their struggle (Margulis & Sagan, 2002).
Additionally, modern evolutionists explain species evolution by combining Darwin's natural selection and Mendel's genetics, as the genetic basis of variation is important for the understanding of evolution; because for the increase of new alleles in the population to occur, due to natural selection, the phenotypic traits must be heritable. Thus, the term 'Darwinism' holds back public understanding of evolution, as 'Darwinism' does not have any genetic basis (Stebbins, 1982).
Finally, Mendel demonstrated that traits of the parents did not combine together to form the offspring's traits, through his pea experiments, as suggested by Darwin. His experiments showed that though the phenotype of the organism might change due to selection; the variations within a population are carried in the allele to the next generation. This occurs because genes do not blend; even if a gene is not expressed in a generation, it can be expressed in the next generation (Stebbins, 1982).
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In conclusion, the term 'Darwinism' is holding back public understanding of evolution to some extent because Darwin's theory, of evolution due to natural selection, is not supported by much strong evidence. According to Darwin, variation in organisms occur due to environment and if the character is beneficial it is passed to offspring; but, modern scientists state that variation in organisms occur due to both genetic and environmental factor combined. Selection aids the survival of existing species, by adapting their structures to the environment; but do not lead to the evolution of a new species by forming new structures. 'Darwinism' does not explain species evolution on its own, as genetic transfer also plays a role in speciation. If 'Darwinism' is combined with the theory of mutation; which causes the variation that is acted on by selection and with Mendelian Genetics, process of new species formation could be clarified fully.
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