Biology · Rung 2 of 4

Natural selection

Natural selection occurs when heritable differences influence reproductive success. Variants associated with a greater contribution to future generations can increase in frequency. Advantage depends on the environment. Populations evolve across generations; individuals do not choose adaptations.

Where this sits

Comes after: Heritable variation
Opens up: Population change, The origin of life

Exercises from this rung

Generated by the app, from easiest to hardest

The fastest lizards escape the cat best. What kind of selection is that?
- Directional: it pushes towards one extreme
- Artificial: a person steers it
- Stabilising: it favours the middle
- Disruptive: it favours both extremes
Why Directional: it pushes towards one extreme. One end of the variation leaves more offspring than the other, and generation after generation the population's average moves towards it. It is what you see when the environment changes in one direction: more heat, harder seeds, a new predator.
easier
For natural selection, which outcome matters more than being the strongest individual?
- Always having the largest body
- Living alone for as long as possible
- Leaving more offspring that can also reproduce
Why Biological fitness concerns contribution to future generations in a particular environment. Strength, lifespan or size may help, but are not the criterion on their own. A trait supports selection when it contributes to reproductive success and is heritable.
in between
In a wood there are 300 light moths and 400 dark ones. On the blackened bark the birds spot the light ones more easily: 40% of the light moths survive and 90% of the dark ones. What percentage of the survivors are dark?
Answer 75
Why Light: 300 × 40% = 120. Dark: 400 × 90% = 360. Of 480 survivors, 360 are dark: 75%, up from 57% before. Selection did not invent the dark moth: it was already there, and selection only made it more common. Repeated every generation, this is England's peppered moth.
harder