Natural Selection on Quantitative Traits

Overview

Natural selection acts on the phenotypic variation within a population. When environmental conditions favor certain trait values over others, the trait distribution shifts across generations. It's one thing to read about this in a textbook; it's another thing entirely to watch it happen.

Darwin's finches are the classic case. Peter and Rosemary Grant documented natural selection in real time on Daphne Major in the Galápagos: during the 1977 drought, large-beaked birds that could crack the remaining tough seeds survived at higher rates, and average beak depth measurably increased in the next generation. Similar stories play out with peppered moths during industrial pollution, guppies under predation pressure, and countless other systems.

What You'll Do

Start with a population showing natural variation in a quantitative trait (beak depth, body coloration, running speed—depending on the system you choose). Introduce a selection event: a drought, a new predator, or an environmental shift. Measure the trait distribution before and after selection acts on the population.

Calculate the selection differential (the difference between the mean of survivors and the mean of the original population) and apply the breeder's equation, R = h²S, to predict the evolutionary response. Then run the simulation forward and see how well your prediction matches the actual change in the next generation. Repeat across multiple generations and compare directional, stabilizing, and disruptive selection regimes.

Learning Objectives

1. Measure and graph phenotypic trait distributions in a population
2. Calculate the selection differential and apply the breeder's equation (R = h²S)
3. Distinguish between directional, stabilizing, and disruptive selection based on trait distribution changes
4. Predict evolutionary response to selection given heritability estimates

Background

Quantitative genetics provides the mathematical framework linking selection to evolutionary response. The breeder's equation, R = h²S, was developed in the context of agricultural breeding programs but applies equally well to natural populations. R is the response to selection (how much the trait mean shifts between generations), h² is narrow-sense heritability (the proportion of phenotypic variance attributable to additive genetic effects), and S is the selection differential.

The Grants' work on Geospiza fortis (the medium ground finch) on Daphne Major remains the most famous empirical demonstration. Beak depth is highly heritable (h² ≈ 0.65), and the 1977 drought imposed strong directional selection favoring larger beaks. The predicted and observed responses matched closely, providing textbook-quality evidence that natural selection can produce measurable evolutionary change within a single generation.

The simulation also includes peppered moths (Biston betularia), where industrial pollution shifted selection on wing coloration, and Trinidadian guppies (Poecilia reticulata), where the presence or absence of predatory fish drives rapid evolution of male coloration and life history traits.