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What is the difference between natural selection and genetic drift?

Ok, I know the basic definition of the terms, but I'm having trouble separating the two clearly in my mind. It's still somewhat unclear; what is an example where a widespread change in a population over time would be considered genetic drift, and another where it would be considered natural selection? Real or made-up examples. And what makes genetic drift random, and natural selection non-random?

Public Comments

1. The core difference between Natural Selection (NS) and Genetic (or allelic) Drift (GD) is cause. Both are methods of genetic change in a population however one happens randomly (GD) while the other is a direct response to and environmental challenge (NS). Now for the examples; Natural Selection as you probably know, is the mechanism by which a species (or genetic makeup of a breeding group) changes in response to an environmental challenge. For example, imagine a population of brown rabbits in a field. They are happy, breeding and being eaten occasionally by foxes until over time, the environment changes so that the field is covered in snow. Now, the brown rabbits stick out and the foxes have an easy time of hunting them. Consequently, the number of brown rabbits decreases dramatically and they are threatened with extinction. The genetic mutations always present in the population (and which increase with in-breeding in small populations) throw up random variations as always but now, some are more beneficial. For instance white rabbits which used to be caught and eaten quickly before the snow came, are now much better adapted. As such, they are more likely to survive, breed and pass on their white genetic make up and hence more white rabbits are born - they are naturally selected by the snow and the foxes; their environment. GD is also a change in genetic make up of a population however it is not stimulated by the environment. Imagine our population of rabbits again. 50% of them have blue eyes and 50% have green. The eye colour makes little difference to their survival chances and is just a natural variation. A new born rabbit will statistically have a 50% chance of blue eyes and 50% chance of green eyes. In a big population, the proportion of blue to green is likely to stay at or around 50%. However that is not the case in a small population. Imagine there are now only 20 rabbits: 10 with blue eyes, 10 with green. Purely by chance, some of these rabbits will not breed, or some breed more often. Let's say - by chance - one green-eyed rabbit gets run over and doesn't breed. There are 10 blues and 9 greens. That means that there are now 53% blues and 47% greens. These proportions will now have a greater impact on the consequent generation since there are more blues, there will be a greater chance of blues appearing in the next generation and less chance of greens. Populations like this constantly vary due to any number of random events. In small populations, those random events become more important since they represent a greater proportion of the total population. In large populations, this drift is small and frequently reversed however in small populations it is almost inevitable that one of the two (or more) traits will eventually be lost from the population. So with NS, a specific trait increases in the population because it is better adapted to the changed environment. With GD, a specific trait increases in the population simply because a random event caused there to be slightly more of one and less of another leading to the more populous being more likely to breed.