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Monday, August 6, 2007

Darwin's theroy of evolution

Darwin's Theory of Evolution
Charles Darwin is renowned in many countries across the globe as the man who developed the Theory of Evolution. His theory sets about explaining the progressive changes that occur within species down the generations, as well as the formation of new species, when environmental pressures have differential effects on the reproductive success of all individuals.
Darwin's theory has two aspects to it, namely Natural Selection and Adaptation, that work together to shape the inheritance of alleles (forms of a gene) within a given population. Darwin made the following five fundamental observations, from which three inferences can be drawn.
Darwin's observations
All species have such great potential fertility that their population size would increase exponentially if all individuals that are born go on to reproduce successfully.
Populations tend to remain stable in size, except for seasonal fluctuations.
Environmental resources for things such as food and shelter are limited.
Individuals of a population vary extensively in their characteristics (to the extent that no two individuals are exactly alike) which impacts upon their own ability to survive and reproduce.
Much of this variation is genetic and is therefore heritable.
Inferences drawn from observations
Due to the limited resources, there is a struggle for existence among individuals - often with only a fraction of offspring surviving through each generation to reproduce successfully.
It is not a random process that determines which individuals will reproduce and which will not, as it depends in part on the genetic / hereditary constitution of those surviving individuals. Those individuals whose inherited characteristics best suit them to that environment in which they live are likely to have more offspring than those that are not so well adapted to the environment. This, by definition, is natural selection.
The unequal ability between individuals to survive and reproduce will lead to gradual evolution of the population, with favourable characteristics accumulating over the generations through natural selection.
Natural selection shapes adaptations and differentiates between the reproductive success of individuals. Adaptations are anatomical structures, physiological processes, or patterns of behaviour that contribute to ancestral survival through the unique suitability of those traits / characteristics (Crawford, 1998). There are three key conclusions to be drawn from the theory of evolution:
Natural selection is differential success in reproduction.
Natural selection occurs through an interaction between the environment and the variability inherent among the individual organisms making up a population.
The product of natural selection in the adaptation of populations of organisms to their environments.
Things to remember about evolutionThis information is taken from the lecture notes for 175.202 students at Massey University written by Professor Andy Lock. Click here if you want to be taken to the site where a fuller account is given.
Evolution is not progressive - Evolution is not designed to produce the best quality products, it only seeks to design adaptations 'that will do the job' most efficiently and economically. Therefore, evolution used the 'mammal template' throughout the development of all mammal species - eg. pentadactyl limbs, mammary glands, spinal curvature, pelvis structure - and made the necessary modifications to suit the niche that species lived in.
Evolution is not an argument for the status quo - Evolution does not dictate why things are the way they are. It must be remembered that some of an animals features may be trade-offs or by-products of the evolution of an unrelated adaptation.
Evolution provides constraints - What has gone before sets physical limits on what we can do now. This is seen in the fact that we learn some things much more readily than others. In this sense evolution has also constrained what things we are able to perceive and attend to.
Evolution provides complexity from simplicity - Species that depend on each other for food (predator - prey relationships) often enter into 'arms races', as they try to outdo each other.

1 comment:

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