L22. Natural Selection: Who Survives and Reproduces?

Natural Selection and Evolution

R-report

L22. Natural Selection: Who Survives and Reproduces?

If a new bird starts hunting in your backyard, which insects will still be there next year—and why?

A backyard mystery: beetles on two paths

Imagine a school garden with two walking paths. One path is fresh, pale gravel; the other is dark soil where bugs hide. At the start of spring the garden has both light-colored and dark-colored ground beetles in similar numbers. After a new flock of small birds begins foraging on the pale gravel, students notice fewer dark beetles on that path a month later and more light beetles. The beetle mix on the dark soil path hardly changed. This change is our phenomenon to explain. It’s specific: the same place, the same beetle species, but different outcomes on two adjacent surfaces. We will use this scene to learn how natural selection sorts which organisms survive and leave offspring.

What natural selection needs (a quick checklist)

Natural selection is not a force that decides what an individual “wants.” It is a process that changes populations because some traits help organisms survive and reproduce better in certain conditions. You already studied mutations and genetic variation; that lesson explains where differences come from. Here we focus on how the environment filters those differences.

• Variation: Individuals differ (light vs dark beetles). Heritability: These color differences are passed to offspring (parents produce similar-colored young). Differential survival/reproduction: On pale gravel, light beetles are harder for birds to spot, so they survive and have more young; dark beetles get eaten more often.

How the beetle scene becomes evolution

When the birds prefer beetles they can see easily, the survivors will be the better-hidden beetles. If light beetles survive more and those survivors reproduce, the proportion of light beetles in the garden will grow over several beetle generations. That shift in the population’s traits over time is evolution by natural selection. Important details for middle school learners: natural selection acts on traits that affect survival or reproduction in a particular environment. If the garden surface changes (for example, the pale gravel is replaced by dark mulch), the advantage can switch. Selection works on whatever already exists in the population; it cannot produce perfect organisms—only ones that are “good enough” where conditions favor them.

Predicting and testing selection in the garden

Students can make testable predictions from this idea. For example: if birds keep foraging mostly on pale gravel, then light beetles will rise in frequency on that path over the next few months. To test this, you could count beetles on both paths every week and record which color lays eggs or appears as larvae. Good tests track survival and reproduction separately: counting adults tells you about survival, watching young tells you about reproduction. Watch for alternative explanations (maybe light beetles moved from nearby areas, or the gravel warms faster so beetles prefer it). Designing simple controls—like sampling a nearby garden without new birds—helps show whether bird predation is the likely cause.

Big idea

Natural selection happens when heritable differences mean some individuals survive or reproduce better in a specific environment. In the school garden example, bird predation on a pale gravel surface favored beetles that matched that surface, so light-colored beetles increased.

Evolution by natural selection shapes populations over generations. It depends on existing variation and on the current environment; change the environment, and the direction of selection can change too. Careful observation and simple tests—counting adults and young, and comparing places with and without the predator—help show when natural selection is at work.