7.1 Introduction to Natural Selection
- Syllabus
- 2025
- Topic
- 7.1
- Level
- —
Learning objectives
7.1A—Describe the causes of natural selectionDescribe the causes of natural selection.• Natural selection is a major mechanism of evolution.• According to Darwin’s theory of natural selection, competition for limited resources results in differential survival. Individuals with more favorable phenotypes are more likely to survive and produce more offspring, thus passing on those favorable traits to subsequent generations.7.1B—Explain how natural selection affects populationsExplain how natural selection affects populations.• Evolutionary fitness is measured by reproductive success.• Biotic and abiotic environments can fluctuate, affecting the rate and direction of evolution. Different genetic variations can be selected in each generation.
What Causes Natural Selection?
Natural selection is a major mechanism of evolution. It occurs when limited resources create competition and individuals with different heritable phenotypes do not survive and reproduce equally.
Limited resources → competition → some phenotypes perform better in that environment → their carriers are more likely to survive and produce offspring → those favorable traits are passed to more members of the next generation.
Survival matters because it can create an opportunity to reproduce, but reproduction is the step that transmits a favorable trait. Repeated differences in reproductive contribution allow a trait to become more common over generations.
Individuals do not evolve because they compete or try to adapt. Natural selection sorts existing heritable variation through unequal survival and reproduction; the population changes across generations.
How Natural Selection Changes Populations
Evolutionary fitness is measured by reproductive success: the contribution of an individual's heritable traits to later generations. Natural selection affects a population when individuals with some genetic variations leave more offspring than others.
Environmental condition → unequal reproductive success among variants → different contributions to the next generation → change in trait or allele frequencies in the population.
| Environmental change | Selection consequence |
|---|---|
| A biotic factor changes, such as competitors or pathogens | A different phenotype may gain higher reproductive success |
| An abiotic factor changes, such as temperature or water availability | The strength or direction of selection may change |
| Conditions fluctuate between generations | Different genetic variations may be favored at different times |
Fitness does not mean strongest, longest-lived, or healthiest in every setting. It is environment-dependent reproductive success, and the evolutionary result is measured at the population level across generations.