Imagine that we have a population of bugs, what factors would we need to see within this population in order for natural selection to occur? Firstly we need a variation in reproductive success, some bugs have no offspring, some have 1, 2, 3 and so on. There also has to be a variation in traits, say for example some of the bugs are coloured brown, and some are coloured green. Thirdly there has to be a no-zero correlation between reproductive success and the trait, in this instance say the brown bugs get eaten by birds more often than the green bugs because they are less camouflaged, and therefore the green bugs are more likely to have more reproductive success because they aren’t being eaten before they have a chance to breed, whereas the brown bugs are. Finally there has to be an inheritability of the trait, the offspring inherit the colour of their parents. If these four factors are in place then we have selection. In this case brown colouration is being selected against because there is a negative correlation between the trait and reproductive success, and the green colouration is being selected for because there is a positive correlation between the trait and reproductive success.
So the four major requirements for natural selection are:
- Variation in reproductive success
- Variation in a trait
- A non-zero correlation between reproductive success and the trait
- Inheritability of the trait
If all of these are in place except for the 3rd criteria then we have what is known as random ‘drift’. These two kinds of evolution, selection and drift produce different results. Natural selection produces adaptations, things such as eyes, ears, noses, brains, hands and so forth. Drift connects variation with phylogenetics and allows us to analyse variation in DNA sequences to infer the history of life – which relates microevolution to macroevolution.
Strong selection can produce some amazing results, any of the ingenious defence mechanisms, ferocious predatory tricks and strategies, camouflage, and glorious display colouring and patterns, and much more are the result of selection. It produces exquisite adaptations that are found all over nature.
Most traits in an organism will have been, at some point, shaped and designed by selection. The history left by microevolutionary processes is known as macroevolution – the 3.9 billion year history of life on this planet. I shall examine it in my next post.