Question
Directional, Stabilizing and Disruptive selection.
| Directional selection | Stabilizing selection | Disruptive selection |
| 1. Natural selection operating in a linear direction is called directional selection. | 1. Natural selection operating to balance or stabilize the population is called stabilizing selection. | 1. Natural selection which disrupts the mean characteristics of a population is called disruptive selection. |
| 2. In directional selection, more individuals acquire characters other than the mean character value. | 2. In stabilizing selection, more individuals of a population acquire a mean character value. | 2. In disruptive selection, more number of individuals acquire extreme or peripheral character value. |
| 3. Directional selection eliminates one of the extremes of the phenotypic range and favour the other. | 3. Stabilizing selection tends to favour the intermediate forms and eliminate both the phenotypic extremes. | 3. Disruptive selection favours extreme phenotypes and eliminate intermediate. |
| 4. It streamlines variations. | 4. It reduces variations. | 4. It increases variations. |
| 5. This kind of selection is the most common. | 5. This kind of selection is common. | 5. This kind of selection is rare. |
| 6. Directional selection operates for many generations, it results in an evolutionary trend within a population and shifting a peak in one direction. E.g. Industrial melanism, DDT resistance in mosquito, etc. | 6. This selection leads to evolutionary change but tend to maintain phenotypic stability within population. E.g. All the populations which have adapted to their environment. | 6. It ensures the effect on the entire gene pool of a population, considering all mating types or systems. E.g. African seed cracker finches with different sized beaks |
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| Name of polluting gas | Source | Effect |
| —————— | ————– | Combining with haemoglobin and causing respiratory problems |
| $NO_2$ | Chemical industries | —————– |
| $CO_2$ | ————— | —————— |
| ——————- | Biogas plants | Greenhouse effect |