Natural Selection & Fitness
- Explain the four conditions Darwin identified that produce natural selection
- Define fitness as reproductive success and separate it from survival alone
- Distinguish directional, stabilizing, disruptive, and sexual selection
Darwin’s logic in four steps
Natural selection is not a force that pushes organisms toward perfection — it is an outcome that follows automatically whenever four conditions are met. Variation: individuals in a population differ in their traits. Heritability: at least some of that variation is passed from parent to offspring through genes. Overproduction: populations produce far more offspring than the environment can support, so there is a struggle to survive and reproduce. Differential reproduction: individuals whose traits happen to suit the current environment leave more offspring than those whose traits do not. Put these together and the helpful variants become more common each generation — with no goal, no plan, just arithmetic.
Fitness means reproduction, not brute strength
In everyday speech "fitness" suggests being strong or fast. In evolution it means one thing only: relative reproductive success — how many surviving, fertile offspring an individual contributes to the next generation compared with others in the population. A peacock dragging a heavy tail is not "fit" in the athletic sense, yet if that tail wins more mates it has high fitness. Survival matters only insofar as it lets an organism reproduce; an animal that lives long but never breeds has a fitness of zero. Fitness is also relative to a specific environment — a trait that is advantageous in a drought may be a liability in a wet year.
Four patterns of selection
When we plot a trait’s distribution across a population, selection can reshape that bell curve in distinct ways. Directional selection favors one extreme, shifting the whole curve toward it (e.g. larger beaks during a drought when only big seeds remain). Stabilizing selection favors the intermediate and trims both extremes, narrowing the curve (e.g. human birth weight — very small and very large babies both fare worse). Disruptive selection favors both extremes over the middle, splitting one hump into two (e.g. birds with small or large beaks thrive but medium beaks match no available seed). Sexual selection is a special case in which traits spread because they improve mating success — through competition between one sex (large antlers) or mate choice by the other (bright plumage) — even when those traits reduce survival.
A drought strikes an island of finches. Only large, tough seeds remain, and birds with deeper, stronger beaks crack them while small-beaked birds starve. Over several generations the average beak depth in the population increases. Identify the type of selection and walk through why the shift occurs.
- 1.Check the four conditions: beak depth varies among birds, beak depth is heritable, more chicks hatch than the drought can feed (overproduction), and deep-beaked birds survive and breed more (differential reproduction).
- 2.Ask which part of the trait distribution is favored: one extreme (deep beaks) is favored while the other extreme (shallow beaks) is selected against.
- 3.Favoring a single extreme shifts the entire distribution toward that extreme over generations — the signature of directional selection.
A large, aggressive male sea lion holds a territory and mates with many females, while smaller males rarely mate at all. Even though the large body size costs enormous energy and shortens lifespan, the trait becomes more common. Which mechanism best explains this?
Natural selection acts on phenotypes within a population — it does not craft a "better" individual on demand, and individuals do not evolve during their lifetime. Variation must already exist; selection only edits the frequencies of variants that are already present. Avoid saying an organism "tried to" or "wanted to" adapt.
Human birth weight clusters tightly around an intermediate value: both very low and very high birth weights are associated with higher infant mortality. Which type of selection maintains this pattern?
On free-response questions, do not just name the type of selection — justify it by pointing to which part of the trait distribution is favored (one extreme = directional, the middle = stabilizing, both extremes = disruptive) and tie the advantage to reproductive success in that specific environment.
Answer the 2 checkpoints as you read.
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