Natural Selection

I should probably confess that at the time of writing this post, I did not have any clue of what natural selection is (or for that matter, evolution). Looking back, I just see how silly I was. Six years of experience in biological and medical institutes has taught me to understand and appreciate science in its broad and pristine perspective. Let me explain the idea behind natural selection.

Natural selection is one of the mechanisms through which evolution can happen. Evolution can happen if any of these five key factors are present in a population system:

1. Mutation

2. Natural selection

3. Non-random mating

4. Genetic drift &

5. Gene flow

If none of these are present, then at least for diploids, the allelic and genotypic frequencies will remain constant (the famous Hardy-Weinberg equilibrium) through generations and no evolution will take place. At the end of the day, evolution is just a gradual change in the allele frequencies across generations and it is not synonymous with natural selection. Although selection need not be the only factor in evolution, it is a key mechanism when the population is sufficiently large. Selection requires at least three necessary and sufficient conditions in order to maintain evolution:

1. Variation of traits within a population
2. Heritability of traits
3. The traits should confer fitness (reproductive) or survival advantage.

First, selection requires variation in phenotypes which are amply provided by mutations or polymorphisms. Technically, mutations provide variations in genotypes that account for phenotypes through the principles of molecular biology. They provide the variety (or phenotypic traits) so that nature can pick and retain the traits that are beneficial for survival/reproduction. 

The second key factor for selection to take place is the heritability of traits. Selection can act only on phenotypes and not on genotypes. Therefore, for a trait to evolve in a population, it should be heritable in the first place. If a trait is not heritable, it will not be passed into future generations and the trait will go into an evolutionary dead-end.

Third, the traits should benefit survival or reproduction, otherwise known as fitness advantage. The traits that are not beneficial are weeded out through environmental or genetic machineries.

Natural selection is simple yet a profound concept. Selection works based on a subset of population rather than individual(s). It is a statistical property, like color for instance. Individual atoms/molecules may not have a specific color, but color can emerge due to aggregation. Similarly, when a group of individuals in a population develop a certain trait that is more favorable to survival/reproduction, assuming the trait is heritable, there is a good chance of passing it on to the next generation by sheer probability.