Question

In a species, the weight of a newborn ranges from 2 to 5 kg. 97% of the newborn with an average weight between 3 to 3.3 kg survive whereas 99% of the infants born with weights from 2 to 2.5 or 4.5 to 5 kg die. Which type of selection process is taking place?

• A. Cyclical selection
• B. Directional selection
• C. Stabilising selection
• D. Disruptive selection

Answer 1

The correct option is C Stabilising selection

Natural selection is defined as an evolutionary force that causes organisms which are better adapted to their environment to survive and produce more offspring.

The three different types of natural selection that can occur are:

Stabilising selection: When natural selection operates in such a way that individuals with mean (average) value of the trait are favoured, it is called stabilising selection. Hence, stabilising selection acts to eliminate extreme phenotypes and favour individuals with intermediate phenotypes. This leads to the multiplication of intermediate phenotypes. This mode of selection reduces variation and tends to maintain a particular phenotypic character.

Considering the data given in the question, 97% of the new born with an average weight between 3 to 3.3 kg survive whereas 99% of the infants born with weights from 2 to 2.5 or 4.5 to 5 kg die. The probable reason for this could be that if the birth weight of a new born is too low, the baby will be very weak and will have health problems. On the other hand, if it is too high it could lead to complications. An average birth weight is the most favoured trait. Hence, due to the selection pressure, the babies with medium weight are selected over underweight and overweight babies. Hence, this is an example of stabilising selection.

Directional selection: When natural selection operates in such a way that individuals with extreme value of the trait are favoured thereby shifting the curve for the phenotypic character in that direction, it is called directional selection. Directional selection operates in response to gradual changes in the environmental conditions.

Disruptive selection/Diversifying selection: When natural selection operates in such a way that individuals at both the extreme values of the trait are favoured and individuals with mean value of the trait are eliminated, it is called disruptive selection. Consequently, the original population is disrupted into 2 or more separate groups that later evolve into new species.