Q1: Darwin's Origin consisted of two parts. The first described observations in support of common descent, the second a mechanism to explain how it was the case. Provide examples of the observations Darwin used (1 page).

‘On the origin of species by means of natural selection : or, The preservation of favoured races in the struggle for life’ by Charles Darwin (1860) was the first book to put forward the scientific theory that populations evolve over a period of generations by the process of natural selection. The first part of the book described observations in support of common descent.

One such observation is that of the homologous structures possessed by a wide variety of animals. Darwin used the fact that lizards, bats, whales, birds, frogs cats and even humans all contain a pent dactyl limb to support his theory that all life is descended from one common ancestor. As he says himself , “What can be more curious than that the hand of a man, formed for grasping, that of a mole for digging, the leg of the horse, the paddle of the porpoise, and the wing of the bat, should all be constructed on the same pattern, and should include the same bones, in the same relative positions?”(Darwin 1860)

Another observation of common descent from the first part of Darwin’s book is that of convergence of form. This is the presence of similar biological traits in members of different species. Darwin illustrated this using the example of the finches. He observed a variety of forms encountered in related species that converge upon forms in unrelated species adapted to similar environments( Dr John Finarelli 2012).

The similarity in embryology between members of different species is another example of an observation used by Darwin to support the theory of common descent. Detours seen in embryonic development - e.g. gill clefts in tetrapod (including human) embryos – are consequences of shared evolutionary history ( De Beer, G. 1958). Darwin argued that the remarkable differences which occur during development are evidence of evolutionary history.

The presence of rudimentary or vestigial organs in the body of certain organisms was proposed by Darwin as another example of evidence of common descent. Rudimentary organs can often be detected in the embryo, but are lost later during development (e.g., teeth in the upper jaws of embryos of whales and ruminants) (David H.A. Fitch 1997). Vestigial structures point to an ancestor that possessed a functional version of the structure, such as the appendix of a human or eye-spots on a blind cray-fish.

Many organisms possess imperfect bodily structures and in the first part of ‘The Origin Of Species’, Darwin used this observation as yet another example of his theory of common descent. He argued that the fact that these structures are imperfect in the modern day anatomy of the organism suggests evolution from a similar organism to which the structure was more suited. “What you have is evidence that a system evolved from a starting point, optimizing what it had to begin with.”( Dr John Finarelli 2012)

Bibliography:

• de Beer, G. (1958), DARWIN'S VIEWS ON THE RELATIONS BETWEEN EMBRYOLOGY AND EVOLUTION. Journal of the Linnean Society of London, Botany, 56: 15–23. doi: 10.1111/j.1095-8339.1958.tb01704.

• Dr John Finarelli 2012, lecture at UCD on 4/11/2012 at 14.00

• Darwin, C. (1860), On the origin of species by means of natural selection : or, The preservation of favoured races in the struggle for life. London:Murray.

• David H.A. Fitch, 1997. Darwins Evidence: Rudimentary Organs. Available at; http://www.nyu.edu/projects/fitch/courses/evolution/html/rudimentary_organs.html (accessed 11/11/2012)

Q2: Provide a description of Darwin's mechanism (1 page). Provide as much detail and give examples as needed.

The theory of natural selection is the mechanism used by Darwin to describe evolutionary change. Natural selection is the process “by which each slight variation, if useful, is preserved.” (Darwin, 1860). There are four preconditions to the theory of natural selection; individual members of a species are different from one another, variation is inheritable, there is excess reproduction and finally there is a struggle for existence( Scott and Maume, 2007).

The fact that individual members of a species are different from one another is known as variation. This is critical to the theory of natural selection as it allows for some organisms to be better suited to their environment than others of the same species, thus allowing for ‘survival of the fittest’. “These individual differences are highly important for us, as they afford materials for Natural Selection to accumulate”.( Darwin, 1860)

Darwin’s proposal that variation is inheritable was a major break-through at the time. He proposed that traits, and in particular favourable traits, are passed down from generation to generation, leading to evolution.

One of the core ideas of natural selection is that there is excess reproduction, i.e. that there are more organisms than there is resources to support them. This aspect of Darwin’s mechanism was heavily influenced by Thomas Malthus, who proposed that organisms reproduce at a geometric rate, whereas food supply grows linearly and so ecosystems cannot support all potential offspring. “population, when unchecked, increases in a geometrical progression of such a nature as to double itself every twenty-five years.” (Malthus ,1798).

Darwin’s observation that there is a struggle for existence is closely linked to the precondition that there is excess reproduction in nature. In any population, more offspring are born than can survive to reproduce. This struggle for existence is due to limited resources in the environment .With this comes competition which leads to Individuals varying in form and behaviour to benefit as much as they can from the limited resource. Much of this variation is heritable.( Dr John Finarelli, 2012). The link between competition and excess reproduction is best explained by Darwin himself : “Owing to this struggle for life, any variation, however slight and from whatever cause proceeding, if be in any degree profitable to an individual of any species, in its infinitely complex relations to other organic beings and to external nature, will tend to the preservation of that individual, and will generally be inherited by its offspring”.

Bibliography:

• Darwin, C. (1860), On the origin of species by means of natural selection : or, The preservation of favoured races in the struggle for life. London:Murray

• Maume, K. and Scott S. (2007). New Senior Biology. Dublin: Folens

• Malthus, T.(1798) An Essay on the Principle of Population, as it Affects the Future improvement of Society with Remarks on the Speculation of Mr.Goodwin, M. Condorcet, and Other Writers. London: Printed for J. Johnson, in S t. Paul's Church-Yard.