Meaning and mechanism
Among the heritable variations, some contribute to reproduction in the environment, and these may be passed on to the next generation at different ratios compared to other types. This is the core of natural selection.
Genetic drift arises by chance, even when there is no difference in fitness. In small populations, the relative variation of a sample becomes large, making it easier for alternative alleles to disappear or become fixed.
Relationships and conditions
The simulation first calculates the expected allele frequency after selection, then samples 2N gene copies. Distinguish the expected value from one generation’s realized result.
Real evolution also involves mutation, migration, linkage, and complex fitness structures. Repeated trials under the same conditions do not guarantee results in the same direction.
What to observe in the simulation
If neutral, the expected frequency remains the same, but individual trajectories are shaken. In large groups, the relative size of the drift is reduced and trends in selection become easier to read.
This Wright–Fisher model assumes fixed population size, random mating, and no immigration or emigration. Each generation samples 2N alleles with replacement. Fitness is a simulated relative measure combining survival and reproductive contribution, with genotype fitness AA=wA², Aa=wAwa, and aa=wa². Rabbit colors illustrate incomplete dominance: AA light, Aa intermediate, aa dark; they do not predict real fur-color inheritance. Mutation has equal probability in both directions; linkage, age structure, and real evolutionary rates are omitted. The picture summarizes the first replicate with up to 100 rabbits.
Key relationships and symbols
p selection=pwA/[pwA+(1−p)wa] → mutation → Binomial(2N,p)/(2N)
| Symbol | Meaning | Unit |
|---|---|---|
| p | Allele frequency | 0~1 |
| w | Relative fitness | Dimensionless |
| N | Number of diploid individuals | individuals |
Worked example
How do natural selection and genetic drift differ?
Selection is directional due to differences in contribution to survival and reproduction, and drift is a contingency of a finite sample that occurs even with the same probability. The two can work simultaneously.
Common misconceptions
- Natural selection is not a process where organisms create the specific mutations they need or desire.