Selection Pressures Shape Guppy Populations
- In natural environments, traits evolve under the influence of selection pressures, which determine survival and reproduction.
- John Endler’s experiments with guppies provide a vivid demonstration of how natural and sexual selection shape populations.
Natural and Sexual Selection: Key Differences
- Natural selection favors traits that enhance survival.
- Sexual selection favors traits that improve mating success, even at the cost of survival.
Think of natural selection as a filter that removes harmful traits, while sexual selection acts as a spotlight, favoring traits that attract mates.
Endler’s Experimental Setup
- Endler divided guppies into three groups, each exposed to different levels of predation:
- Pond Q: No predators.
- Pond R: Killifish (Rivulus hartii), a weak predator.
- Pond S: Pike-cichlid (Crenicichla alta), a strong predator.
- The key trait observed was the number of spots on male guppies:
- Spots are heritable, influencing both mating success (favored by females) and predation risk (favored by predators).
Results: How Selection Pressures Influence Traits
Pond Q (No Predators): Sexual Selection Dominates
- What happened: The average number of spots increased over time.
- Why: Without predators, sexual selection was the primary force. Females preferred males with more spots, leading to an increase in this trait.
Pond R (Weak Predators): A Balance Between Selection Pressures
- What happened: The number of spots remained stable.
- Why: Killifish posed a moderate threat, balancing natural selection (fewer spots for camouflage) and sexual selection (more spots for mating).
Pond S (Strong Predators): Natural Selection Dominates
- What happened: The number of spots decreased significantly.
- Why: High predation pressure from pike-cichlids strongly favored males with fewer spots, as they were less visible to predators.
Evolution can occur rapidly when selection pressures are strong, as shown in Endler’s experiments.
Broader Implications of Endler’s Work
- Conservation Biology: Understanding how traits evolve under different pressures helps develop strategies to protect endangered species.
- Human Impact on Evolution: Human activities, such as introducing predators or altering habitats, can disrupt natural selection, leading to unintended consequences for biodiversity.
- How might human activities, such as pollution or habitat destruction, create new selection pressures in natural populations?
- Can these pressures lead to unintended evolutionary consequences?
