3.2.1 Variation

Darwin’s theory of evolution by natural selection rested on the observation that there is a great variety of life out there. By the time Darwin published his book ‘On the Origin of Species’ in 1859 evolution was not a new theory. Indeed, scientists of one type or another had asked the question for centuries – ‘why is there such variety of life?

What Darwin observed was that there was variation between species (interspecific variaition), but also within species (intraspecific variation). A good way to think of this is that cats are a different species from dogs (interspecific) and there are different breeds of dog (intraspecific).

Dog breedsThese differences are what results in differential survival rates between and within a species. Some individuals have features which mean they are better suited to the environment they live in. They are more likely to survive and if they are more likely to survive then they stand a better chance of passing on their genes.

Of course, those that aren’t suited to the environment are not likely to survive to reproductive age. They die, and that’s natural selection.

Where does variation come from?

Well, some of it is coded for in our genes and some variation is a product of the environment an organism lives in. In many cases there is an interplay between genes and environment which results in a broad spectrum of features within a species.

In the lesson we measured some intraspecific variation – we looked at arm length and we plotted a graph (actually a histogram) of our results. It looked something like the one below.

normal distributionThis is called a normal distribution and is the shape of histogram that all features that vary continuously produce. Continuous variation is produced when a characteristic is controlled by a large number of genes. Arm length, height, weight are all examples of such characteristics.

Whenever you sample a population of living things you need to make sure that your sample is large enough so that random ‘freak’ individuals, anomalies, do not skew your mean and thus affect your normal distribution.

In the lesson we calculated the standard deviation. Remember – this is a measure of how much our data deviated from the mean value. You can see 3 standard deviations marked on the graph above. The second set of blue lines tells you that 95% of all your data lies within 2 standard deviations of the mean (the balck line on the graph).

Here’s a video explaining the importance of standard deviation.



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