Lesson 2 of 5 / Variation, selection and species
Explain why an environment favours a variant
How can predation or an antibiotic change reproductive success?
In this lesson: Use environmental examples to explain differential survival and reproduction.
About 5 min
The key ideaAn environmental factor is a selection pressure when inherited variants differ in their success under it. Survivors must reproduce for the change to reach future generations.
Explore the idea
Change the background, then compare survival
Choose the environment before applying predation. Changing the background changes the advantage; it does not deliberately change each beetle into the needed colour.
Original illustrative counts, not measured predation rates. Colour is assumed heritable and other influences are held constant.
Explanation
A selection pressure is an environmental influence that makes variants differ in survival or reproductive success. Predation, food availability, drought and exposure to an antibiotic can all play this role, depending on the organism and inherited characteristic.
Consider a beetle population containing inherited green and brown colour variants. On brown ground, birds may detect green beetles more easily. If brown beetles consequently survive and reproduce more successfully, alleles contributing to brown colour can become more common in later generations.
In a bacterial population exposed to an antibiotic, resistant variants may survive while susceptible bacteria are killed or have reproduction inhibited. Resistant survivors reproduce, increasing their representation. The antibiotic does not need to create the resistance allele in order to select for it.
An advantage depends on context. A colour that conceals a beetle on one background can make it conspicuous on another. Avoid saying the strongest organism always wins: biological fitness concerns contribution to later generations in the actual environment, not physical strength alone.
Step by step
- 1
Name the variation
State the inherited difference already present or newly arising without direction.
- 2
Name the pressure and advantage
Explain how the environment changes survival or reproduction for that difference.
- 3
Pass it on
Successful individuals leave descendants carrying the relevant alleles.
Worked example
A change of background
An original model starts with equal numbers of green and brown beetles. Birds remove more visible beetles. Predict what changing the ground from brown to green could do.
One way to explain it
It could reverse which colour has a camouflage advantage. Green beetles may then survive and reproduce more successfully, provided colour is heritable and other factors do not override the effect.
Why this answer works
- The environmental background changes detection.
- A selective advantage is conditional, not a permanent property of one colour.
Is this true? "An antibiotic teaches each bacterium how to become resistant."
Selection favours resistant variants among a population. Mutations are not instruction-driven responses to the required survival outcome.