The Principle of Heredity states that where no external pressure exists a population of a species, with all its natural variations, will stay the same. If an external change occurs, for example a food source becomes scarce, one variation can prove more successful at dealing with the change and so this variant will become more prevalent (Hide, G. 2011).
When there is increased competition for resources (food, living space etc.) due to the natural tendency of species to overproduce, the variant with the most successful reproductive rate will survive; this is the Principle of Selection. For example, the peppered moth before the British Industrial Revolution was predominantly light coloured to match the light coloured trees common in Britain at the time and camouflage the moth from predators. However, with the advent of the Industrial Revolution and the high levels of pollution that came with it, the trees and buildings around highly industrialized areas were covered in soot and the light coloured moths were now easily picked up by predators against the dark backgrounds. This change in environment meant that moths with the natural variant for darker coloured wings were more likely to survive and thus reproduce and over time the dark coloured moths became the prevalent variant. (Jones, S. 1999)
Using variants of colour in mice as an example, different modes of selection can be shown using graphs (See figure 1.). Stabilising selection removes the extremes from a population. Graph 1 shows that very light coloured mice or very dark coloured mice are more susceptible to predators and so the most successful mice are those that are in between the two extremes. In graph 2, directional selection shows that when an external pressure increases the vulnerability of light coloured mice, the population shifts so that mice become darker over time. Graph 3 shows that when the extremes of a population, for example very dark or very light coloured mice, are the favourable variant a species can diverge into two separate groups and can eventually lead to two separate species forming.
Another mode of selection, balancing selection, is more clearly explained by the sickle cell trait (figure 2). Again, this disease is a random genetic mutation and the balance that it confers to heterozygote humans means that these people are more likely to survive and reproduce as they have the resistance to malaria but not the debilitating effects of sickle cell disease. Sexual selection is also be a factor as organisms are more likely to choose a mate that displays attributes that are conducive to survival and increased reproduction. However, some sexually attractive traits will be ignored if a new external pressure has made this trait detrimental to survival. For example, a study has been made of elephants in Uganda which shows that because of ivory poaching elephants with small or no tusks are becoming more prevalent in the wild. Tusks are an important tool for attracting a mate and also have practical applications but the study shows that the benefits do not outweigh the risk of poaching (Jones, S. 1999).
The evidence for natural selection and evolution is now overwhelming. As Darwin saw in the Galapagos, there is a great deal of physical evidence for natural selection and adaptation that we can see by a cursory study of living creatures, such as the physical resemblance that humans have to orang-utans. The rapid advancement of science since the publication of On the Origin of Species has allowed the theories of Darwin and his contemporaries to be researched and tested thoroughly and with developments in biochemistry and the work on the genetic code, new and irrefutable evidence of the similarities between species on a genetic level has come to light. Comparative anatomy is one of the more easily observed examples of evidence for adaptation and natural selection. Homologous traits, for example the mammalian forelimb, can be seen in organisms that have extremely different phenotypes like the whale and the bat (see figure 3.) which points to a common ancestor with a comparable forelimb that later diverged into separate species. (Hide, G. 2011)
Figure 3.
Similarly comparative embryology is easily observed through microscopy. Many organisms are almost identical at the embryonic stage and diverge slowly as the embryo grows. For example, both fish and human embryos have gill bars, a tail and a two-chambered heart in the early stages of development (Schraer, W. 1992). Also, the notochord, present in all chordate embryos, which later becomes a part of the vertebral column can be seen in fossils of very simple vertebrates from over 500 million years ago. Fossils and geological studies have also provided a wealth of evidence for natural selection. James Hutton and Charles Lyell’s work on uniformitarianism and biogeography, the study of the distribution of animals and plants, heavily influenced Darwin and also gave credence to theories that suggested the Earth was much older than previously thought. Biogeography explains why Australia, isolated earlier in the Earth’s lifetime than other continents, has such unique flora and fauna (Fried, G. 1999) and why Acanthodrilidae, a species of earthworm, can be found in both South America and West Africa (Moriera, M. 2006). By the position of fossils within rock strata, small adaptations in species can be viewed chronologically over a long period of time and more recently we have been able to use radiometric technology to date these changes more precisely. Fossils can also allow the study of long-extinct organisms such as the dinosaurs, for example, fossil records show that birds evolved from a species of small theropod dinosaurs around 150 million years ago. Widely believed to be the first prehistoric bird is the archaeopteryx (see fig.4), the archaeopteryx is reptilian in structure but has feathers and wings, this mix of features has shown conclusive evidence of a link between reptiles and birds (Ostrom, J.H. 1976).
Perhaps the most compelling evidence for natural selection and evolution is at the molecular level. All living organisms are similar at a cellular level, in the way that basic anatomical structures develop and in chemical composition. DNA is the genetic code that determines the structure of organisms and similarities between the DNA of species have been shown by cracking this code. It is to be expected that species that share physical characteristics would share a similarity in DNA, humans and chimpanzees for example, but humans also share 40% of DNA with bananas (Cracraft, J. 2004) suggesting that all living organisms, at some point, shared common ancestors. Junk, or non-coding, DNA is also strong evidence for evolution as these vestigial sequences would not occur if an organism had just simply appeared (Elder, R. 2012).
Darwin’s theories on natural selection, evolution and common ancestry were published at a time when religious creationism was the norm and many people, even scientists, refused to accept the idea that humans were created through sheer chance and descended from animals. Darwin himself was apologetic to the extreme and was wary of challenging the religious status quo (Amigoni, D. 1995) but luckily, for modern science, his ideas gained a following and formed the basis for modern evolutionary theory. The vast amount of evidence that is now available through studies and recent advances in technology mean that the theory of evolution is now accepted, although in some parts of the world creationism seems to be having somewhat of a renaissance, and it will hopefully continue to pave the way for new and exciting discoveries of our past.
Reference List and Bibliography
Amigone, D. and Wallace, J. ed. (1995) Charles Darwin’s The Origin of the Species: New Interdisciplinary Essays. Manchester. Manchester University Press.
Anon. (1993). Oxford Dictionary of Biology. Oxford. Oxford University Press.
Campbell, N. and Reece, J.(2005) Biology. Seventh Edition. San Francisco, U.S.A. Benjamin Cummings. [Figure 3.]
Coyne, J.A., (2009). Why Evolution is True. Oxford: Oxford University Press.
Cracraft, J and Donoghue, M. (2006) Assembling the Tree of Life. Oxford. Oxford University Press.
Darwin, C. [1859] (2008), The Origin of Species: On the Origin of Species by Means of Natural Selection. (6th edition) [Kindle Edition] Boston: Mobile Reference. Available through: Amazon Media EU.
Elder, R. (2012). Molecular Genetics. Lecture, University of Salford, unpublished.
Fried, G. and Hademenos, G.J. (1999) Schaum’s Outline of Theory and Problems in Biology. 2nd Edition. Columbus, OH. McGraw-Hill Professional.
Hide, G. (2011). Darwinian Evolution. Lecture, University of Salford, unpublished.
Jones, S. (1999). Almost Like A Whale. London. Transworld Publishers Ltd.
Moriera, M.S. Siqueira, J.O. and Brussaard, L. (2006) Soil Biodiversity in Amazonian and other Brazilian Ecosystems. Oxfordshire. CABI Publishing Series.
Ostrom, J.H. (1976) “Archaeopteryx and the origin of birds.” Biological Journal of the Linnean Society. 8: 91-182.
Pinker, S. (1994). The Big Bang. In: M, Ridley ed. 1997. Evolution. Oxford. Oxford University Press. 58.360-361
Sadava, D. et al., (2011). Life: The Science of Biology. 9th Edition. Sunderland, MA. Sinauer Associates Inc.
Schraer, W. and Stoltze, H.J. (1992). Biology: the study of life. Essex. Prentice Hall.
The Virtual Fossil Museum. 2008. Archaeopteryx. [Photograph] Available at: http://www.fossilmuseum.net/fossilpictures-wpd/Archaeopteryx/Archaeopteryx.htm. [20/04/2011] [Figure 4.]
University of Michigan (2010). Evolution and Natural Selection. [web lecture] (10/10/2010) Available at: http://www.globalchange.umich.edu/globalchange1/current/lectures/selection/selection.html [Accessed: 10/02/2012]
