Natural selection

There are different kinds of selection, but there are only few that cause evolution. We shall examine them:

1) Directional selection

2) Selection against recessive alleles.

3) Sexual selection

Directional selection:

Directional selection works by selecting against one end of a range of phenotypes, or by selecting for one end of range of phenotypes. Some useful alleles will become more visible in the gene pool and harmful ones will be removed.

This means that an undesirable range of genes is removed or a useful set of traits is passedon to the offspring. When this happens to 95% of the population, the trait will be present in only a few and in the next generation, it will disappear. The table shows this clearly.

       average = 0       Distribution changed  average = 1.5

      |   .:.   |               |         |:  

      | .:::::. |Select  --->   |       .:|:: 

      |:::::::::|For            |     .:::|:::

    .:|:::::::::|:.             |   .:::::|::::

  .:::|:::::::::|:::.         ..|:::::::::|:::::..

  +--+--+--+--+--+--+       +--+--+--+--+--+--+--+

 -3 -2 -1  0  1  2  3      -3 -2 -1  0  1  2  3  4 

(positive numbers indicate useful traits and negative nos. indicate harmful characters.)

5 populations have been taken, and the individuals have been put into the categories - unfit, average, and useful according to the phenotypes (traits) they have. In the next table, the effect of natural selection has been shown. You will notice that the offspring of the respective populations have less of unwanted phenotypes. The number of people with useful characters has increased. The unwanted characters increase in respect with the number because chances of mutations are higher.

Selection against recessive alleles:

In the first example, we talked about a dangerous recessive allele and how natural selection played its role in removing it. This is called as selection against recessive alleles. This is the most common form of selection.When a certain allele 'a' is recessive and harmful, it is removed from the gene pool. The table shows the effect of natural selection on the offspring.Selection also works against others like dominant alleles, heterozygotes etc. But they are not very widely noticed.

Sexual selection:

Males of many species have many characteristics called secondary sexual characteristics. But are actually dangerous in terms of survival. The brightly coloured tail of a peacock, vocal sounds of a frog are a few examples.

It might not be very wise for a frog to yell its heart out when a snake lurks nearby. But why has natural selection not eliminated these traits by now? It is because it helps them to find a mate. To reproduce, most animals need a partner

Attractiveness is one of the few qualities that help an animal to find a mate. Sexual attractiveness has more weightage compared to survival as it helps them to mate and produce young.

Traits, which help a male, find mates are therefore passed on to the offspring sacrificing survival needs. This is known as sexual selection.

Is natural selection "The survival of the fittest"?

NO! This is a major mistake and Charles Darwin never said anything like this. It was Herbert Spencer who coined this phrase. Now that you have read so much about natural selection, you know that it favours higher reproductive ability and also survival.

BUT this phrase actually says that natural selection is concerned mainly with survival of an animal, which is not the case. In fact this phrase is circular - the fittest means who are fit to survive.

So the phrase becomes "survival of those are fit to survive" and this is a circular argument. So when you talk about natural selection, make it a point to talk about reproductive capacity and not "the survival of the fittest."

This has been a very long topic. You have learnt about mutation, natural selection and random genetic drift. Now you know why small-scale changes are caused. Proceed further to learn about large-scale changes like rise of new species and extinctions.