Natural selection and its three types

Selection changes the gene pool

In the last lesson, natural selection was one of the five factors that move a population away from equilibrium. Here is how it works. Individuals with some genotypes leave more young than others. The alleles of these successful individuals become more common in the next generation, and the alleles of the others become less common.

NCERT gives the facts on which selection rests. Natural resources are limited. The sizes of populations stay stable, apart from seasonal changes. The members of a population differ from one another in their features. Some of the differences are inherited.

Reading a bell curve

Many characters, such as height, body mass or beak size, vary in a population in a bell-shaped way. Most individuals have values near the mean. Few have values at the two ends, called the peripheral values. The height of the curve at any value is the number of individuals with that value.

NCERT describes three ways in which natural selection can change this curve.

• Stabilising selection: more individuals acquire the mean value of the character. The extremes are removed. The curve becomes narrower and taller. The mean stays at the same place.
• Directional selection: more individuals acquire a value other than the mean, on one side. The whole curve moves.
• Disruptive selection: more individuals acquire peripheral values at both ends of the curve, and fewer have the mean value. The curve gets two peaks.

NCERT gives no examples for these three. The common examples in other books are these.

• Stabilising: the mass of human babies at birth. Babies that are very small or very large have a higher death rate than babies of the average mass.
• Directional: the dark form of the peppered moth, and resistance to a drug or a pesticide. The mean moves to a value that survives the drug.
• Disruptive: the bill size of the African black-bellied seedcracker finch, whose two kinds of seed are best handled by a small bill or a large bill.

You should see that stabilising selection narrows the red curve and does not move it, directional selection moves it without changing its shape, and disruptive selection makes it wider with two peaks.

What happens to one allele

This part is not in the NEET syllabus, but A-Level and IB ask it. NEET asks only the idea that selection changes allele frequencies.

Let the relative fitness of $AA$ and $Aa$ be 1, and of $aa$ be $1 - s$. The number $s$ is the selection coefficient. If $s = 1$, no $aa$ individual survives to breed. Then the breeders of the next generation are $AA$ and $Aa$ only, and the frequency of $a$ falls to

This is the case of a lethal recessive. Start with $q = \tfrac12$. The next values are $\tfrac13$, $\tfrac14$ and $\tfrac15$. After $n$ generations, $q = \tfrac{1}{n + 2}$. The fall is fast at first and then very slow, because a rare $a$ is hidden in carriers, and selection cannot act on a carrier who looks healthy.

If $aa$ is only partly selected against, the fall is slower. For any $s$ the new frequency is

Selection can also keep an allele in a population. In regions with malaria, a person who is heterozygous for the sickle-cell allele does better than either homozygote. This heterozygote advantage keeps both alleles in the population. This is not asked in NEET.

Try it in Python

The program follows an allele that is lethal when homozygous, starting from $q = \tfrac12$. It uses exact fractions.

Practise with new numbers

Common mistakes

• Thinking that stabilising selection means no selection. It is selection against both extremes.
• Saying that the mean changes in disruptive selection. The mean can stay the same. It is the extremes that become common.
• Saying that selection acts on genes directly. It acts on the whole individual, and the result is a change in allele frequencies.
• Thinking that selection removes a harmful recessive allele quickly. When the allele is rare, most copies are in healthy carriers, and the fall is slow.
• Mixing up selection and mutation. Mutation makes new alleles. Selection changes how common they are.

Remember these

• Stabilising: the mean value of the character becomes more common. Directional: a value on one side of the mean. Disruptive: values at both ends.
• Selection changes allele frequencies by differences in fitness.
• A lethal recessive falls as $q' = q \div (1 + q)$, fast at first and then slowly.
• Carriers hide the recessive allele from selection.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. In a field, plants that are very tall fall in the wind, and very short plants are shaded. Which type of selection is this?

2. In a dry year, only large seeds are left on an island, and finches with larger beaks survive. What happens to the mean beak size?

3. What does the curve look like after disruptive selection?

4. A lethal recessive allele has $q = \tfrac13$. What is $q$ in the next generation?

5. Why does selection against a rare recessive allele become so slow?

References

• NCERT (2023). Biology, Textbook for Class XII, chapter 6.
• Clark M. A., Douglas M. and Choi J. (2018). Biology 2e, OpenStax, chapter 19.3, Adaptive Evolution.