What is natural selection and genetic drift?

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.

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GENETIC DRIFT:-When the frequency of some genes gets changed in a small population due to some accidental reasons, such phenomenon is known as Genetic Drift. Genetic drift can give rise to different changes in sub-populations. It is random and rarely produces adaptations to the environment.

NATURAL SELECTION:- It may be defined as a process that results in the increased

survival and reproductive success of individuals, who are well adjusted to the

environment. Theory of natural selection was given by Charles Robert Darwin.

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Genetic Drift

Allele frequencies in small populations do not generally reflect those of larger populations since too small of a set of individuals cannot represent all of the alleles for the entire population. Genetic drift occurs when the population size is limited and therefore by chance, certain alleles increase or decrease in frequency. This can result in a shift away from Hardy-Weinberg equilibrium (HWE). Unlike natural selection, genetic drift is random and rarely produces adaptations to the environment.

Natural Selection

Although population genetics by itself is important, one of the objectives of this field is to assess how changes in allele frequencies affect the evolution of a population. Evolution in its modern form was first explored by Charles Darwin in 1859. In his book On the Origin of Species, Darwin outlined what he called “descent with modification” and what we now refer to as evolution. He speculated that all species evolved from a common ancestor. Over time, faced with new environments and habitats, populations of species acquired modifications, which allowed them to better adapt to their environment.11

Darwin termed these changes within populations, natural selection, and he proposed the idea of “survival of the fittest.” Individual variations which proved beneficial would be preserved within a population, whereas variations that were lethal to the organism would be destroyed. Under natural selection, some individuals in a population have modifications that allow them to more successfully survive and reproduce, making their adaptations more common as a whole due to their increased reproductive success. Over a long period of time, this change in the characteristics of a population can lead to the production of a new species