Students who demonstrate understanding can: Use mathematical representations to support explanations of how natural selection may lead to increases and decreases of specific traits in populations over time.
Clarification statement: Emphasis is on using mathematical models, probability statements, and proportional reasoning to support explanations of trends in changes to populations over time.
Assessment boundary: Assessment does not include Hardy Weinberg calculations.
Official wording from the Next Generation Science Standards (NGSS Lead States, 2013). NGSS is a registered trademark of WestEd. Neither WestEd nor the lead states and partners that developed the NGSS were involved in the production of this page, and they do not endorse it. View on nextgenscience.org
Natural selection becomes easier to see when you count. Imagine a population of 1,000 moths where 10 percent are dark and 90 percent are light. If pollution darkens tree bark and birds now catch light moths far more often, the dark moths will make up a larger share of each new generation. Within a few generations the population might be 30 percent dark, then 55 percent, then mostly dark. The traits of individuals did not change; the proportions in the population did.
Students use mathematical representations, such as tables, bar graphs and percentages, to support explanations of these trends. They calculate the fraction of a population that has a trait in each generation, graph the change over time, and connect the pattern to which individuals survived and reproduced. A trait can decrease as well as increase, for example when the environment changes back.
Calculations stay at the level of percentages and proportions; formal population-genetics formulas such as Hardy-Weinberg are not part of this standard.
A single moth does not change color. Natural selection changes the proportion of a trait in the population across generations.
Trait frequencies follow the environment. If conditions reverse, a trait that was increasing can decline.
If the total population changes, counts can mislead. Comparing percentages shows whether a trait is truly becoming more common.
A moth population has 1,000 moths with 100 dark ones. In the next generation there are 800 moths with 240 dark. In the following generation there are 900 moths with 450 dark. Calculate the percentage of dark moths each time and explain the trend.
Answer: Dark moths made up 10, 30 and then 50 percent of the population, showing natural selection increasing the frequency of the dark trait.
Run a simulation over several rounds, recording the number of each trait after every round in a class table, then convert to percentages and graph. Ask students to annotate the graph with the 'selection event' that explains each change.
Assessment excludes Hardy-Weinberg calculations; it expects students to use proportions and graphs as evidence in a written explanation.
Original questions written for this standard. Choose an option or type your answer, then press Check. Every question has a worked explanation.
Answer: 25 (also accepted: 25%, 25 percent)
50 / 200 = 0.25, which is 25 percent.
Answer: A) Resistant bacteria survived and reproduced more
The antibiotic killed non-resistant bacteria, so resistant ones made up more of each new generation.
Answer: B) Percentages show the share of the trait regardless of population size
Percentages show whether the trait is becoming more common relative to the whole population.
Answer: 10 (also accepted: 10 percentage points, 10%)
90/300 is 30 percent and 120/300 is 40 percent, an increase of 10 percentage points.
Answer: D) Natural selection depends on the current environment
When the environment changed back, light moths again had better survival odds.
Answer: A) The proportion of individuals with a trait in the population
Natural selection shifts trait frequencies in the population over time.
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.
Construct an explanation based on evidence that the process of evolution primarily results from four factors: (1) the potential for a species to increase in number, (2) the heritable genetic variation of individuals in a species due to mutation and sexual reproduction, (3) competition for limited resources, and (4) the proliferation of those organisms that are better able to survive and reproduce in the environment.
A full lesson with slides, activities and an exit ticket on natural selection by the numbers, pitched to grades 6-8 and editable in PowerPoint or Google Slides.
Make a lesson βA printable, differentiated worksheet on MS-LS4-6 with an answer key, ready in about a minute.
Make a worksheet βTurn natural selection by the numbers into a quiz students answer online that marks itself, with a class summary for you.
Build a test βMathematical models, probability statements and proportional reasoning, such as percentages of a population with a trait across generations.
No. The assessment boundary specifically excludes Hardy-Weinberg calculations.