Lesson 10 of 10 / Inheritance
Natural selection and evolution
How can an environment change a population without individuals choosing to change?
In this lesson: Explain differential survival and reproduction, environmental selection and inherited population change over generations.
About 6 min
The key ideaWhen heritable variants differ in survival and reproductive success, their frequencies can change over generations. Natural selection can therefore contribute to evolution.
Both light and dark heritable forms exist before selection. Changing the background does not instantly change their genes.
Light/dark shapes: inherited colour forms. Gold rings: better-camouflaged individuals in the advantage view. The later 6:2 display is an illustrative possible composition, not a predicted rate or a claim that survivors changed colour.
Competition for limited resources also affects reproductive success. Natural selection requires inheritance and unequal reproduction; being difficult to see is useful here only because it can affect those outcomes.
Explanation
Individuals in a population vary. Some differences are heritable, arising from genetic variation. More offspring may be produced than can all survive, and organisms can compete for limited resources such as food, light, water or space.
An environmental factor can make one heritable variant more successful under the current conditions. Predators may detect some prey more easily; drought may favour plants with traits that conserve water; an antibiotic can select resistant bacteria. The advantage depends on the environment.
Individuals with an advantageous heritable variant are more likely to survive and reproduce. Their offspring can inherit the relevant alleles. Over generations, those alleles and the associated characteristics can become more common in the population. Differential reproduction matters: survival alone does not transmit the variant.
Evolution is change in the inherited characteristics of a population over generations. Natural selection is one mechanism that can produce this change. An individual becoming stronger through exercise is not itself evolution, because this does not demonstrate an inherited population change.
Selection acts on existing variation; it does not cause organisms to choose mutations that match a need. A variant advantageous in one environment may be less advantageous after conditions change. Best fitted means greater reproductive success in the relevant environment, not simply the largest or strongest organism.
Step by step
- 1
Name the heritable variation
Identify the different inherited forms present before selection.
- 2
Explain the advantage
Connect a particular environmental factor to survival and reproduction.
- 3
Follow the next generations
Offspring inherit the advantageous variants, which can become more common over time.
Worked example
Camouflage changes after an environmental change
A population contains inherited light and dark colour forms. On dark backgrounds, birds detect light individuals more easily. Explain a possible change over generations.
One way to explain it
Dark individuals are less easily detected, so they may survive and reproduce more successfully. Their offspring can inherit dark-colour alleles. Over generations, the dark form may become more common through natural selection.
Why this answer works
- Colour variation exists before the change in selection.
- Connect visibility to unequal survival and reproduction.
- Finish with inherited population change, not an individual changing colour by choice.
Is this true? "The environment gives every individual the same useful mutation."
Selection acts on variation already present. Unequal survival and reproduction can change the frequency of heritable variants; it does not direct every individual to mutate in the same way.