this image shows the relative frequency of banded and non-banded snails in a population. if the frequency of banded snails decreased to 60% and non-banded snails increased to 40%, would you consider the population to be evolving?
Yes. A shift from 80% banded / 20% non-banded to 60% banded / 40% non-banded means the population would be considered to be evolving.
Why this counts as evolution
In biology, evolution at the population level is defined as a change in the genetic composition (often measured as allele or trait frequencies) of a population over generations.
Shell banding is a heritable trait in these snails, so when the proportion of banded vs. non-banded individuals changes from one generation to the next, it indicates that the underlying allele frequencies have changed.
So:
- Initial state: 80% banded, 20% non-banded.
- New state: 60% banded, 40% non-banded.
That change in trait (and thus allele) frequencies over time is exactly what biologists mean by microevolution in a population.
What might be causing the change?
Several processes could be driving this shift:
- Natural selection
- Perhaps non-banded snails are now better camouflaged in a changed environment, so they survive and reproduce more successfully.
- Genetic drift
- In a small population, random events (like storms, disease, or accidents) could, by chance, remove more banded snails than non-banded ones, altering frequencies without any advantage.
- Migration (gene flow)
- Non-banded snails could have migrated into the population from another area, increasing their frequency.
- Mutation
- New mutations affecting banding are less likely to explain such a large, rapid change alone, but they can contribute.
In many textbook and quiz contexts (including problems about banded snails very similar to this one), the “expected” answer is explicitly that yes, the population is evolving because the trait frequencies have changed over time.
One-sentence TL;DR
Because evolution in this context is defined as a change in heritable trait (allele) frequencies in a population over generations, the shift from 80/20 to 60/40 means the population is evolving.
Information gathered from public forums or data available on the internet and portrayed here.
