HS-LS4-3: The Statistics of Natural Selection
Also called NGSS HS-LS4-3 · Topic: Natural Selection and Evolution · Life Science
The Official Wording
Apply concepts of statistics and probability to support explanations that organisms with an advantageous heritable trait tend to increase in proportion to organisms lacking this trait.
Clarification statement: Emphasis is on analyzing shifts in numerical distribution of traits and using these shifts as evidence to support explanations.
Assessment boundary: Assessment is limited to basic statistical and graphical analysis. Assessment does not include allele frequency calculations.
NGSS Lead States, 2013 · Read it on nextgenscience.org · checked October 15, 2026
What HS-LS4-3 Means in Plain English
Students use probability, percentages and graphs to show that an inherited trait which improves survival becomes a larger share of a population over generations. They track how the proportion of organisms with the trait shifts and use that shift as evidence for natural selection.
The math stays basic, with tallies, percentages and bar or line graphs, and allele frequency calculations are not required. It builds on HS-LS3-3 and HS-LS4-2 and leads to HS-LS4-4, where selection adapts whole populations.
HS-LS4-3 I Can Statements
- I can calculate the percent of a population with a trait.
- I can graph how a trait's share changes across generations.
- I can use a shift in proportions as evidence of selection.
HS-LS4-3 Worked Example
A dice simulation based on rats that inherited resistance to a common rat poison.
If rats that resist poison survive more often, how quickly does the resistant share of the population grow?
Set It Up: A cup of 30 counters (27 white for normal rats, 3 red for resistant rats), dice and a data table.
- Set the rules: each year of poison, roll one die per rat. A normal rat survives only on a 6, and a resistant rat survives on 2 through 6.Start: 3 of 30 resistant (10%)
- Roll for every rat, remove those that die, and give each survivor two young of its own color.Survivors: 5 normal, 3 resistant. Next: 10 normal, 6 resistant (38%)
- Repeat the rolls for two more generations.Generation 2: 71% resistant. Generation 3: 89%
- Combine results from all groups and graph percent resistant by generation.Every group's line climbs steeply
Resistant rats rose from 10% to about 90% of the population in three generations, and every group saw the same upward trend even though single rolls varied.
Why it works: Probability favors resistant rats in every round of poisoning, so they leave more young each generation and their share keeps compounding.
Common Mistakes with HS-LS4-3
Comparing raw counts
“There are more resistant rats now” while the total keeps changing
Try this: Convert counts to percentages of the whole population each generation, because the total size rises and falls while selection acts.
Thinking the poison causes resistance
“The poison made the rats resistant”
Try this: The resistant trait was already present in a few rats. The poison only changed which rats survived to reproduce.
Trusting one small trial
One group's odd result treated as the answer
Try this: Pool trials from several groups, since chance makes small samples jumpy, and look for the overall trend across generations.
Where HS-LS4-3 Fits
Practice HS-LS4-3
Quick Check
- A population of 200 beetles has 50 green ones. What percent are green?
- Resistant rats stay at 40 while the total drops from 400 to 100. Did the trait's share change?
- Why does a helpful trait spread faster when the survival difference is larger?