MS-LS4-4: Construct an explanation based on evidence that describes how genetic variations of traits in a population increase some individuals’ probability of surviving and reproducing in a specific environment. [Clarification Statement: Emphasis is on using simple probability statements and proportional reasoning to construct explanations.

By the end of grade 8. Genetic variations among individuals in a population give some individuals an advantage in surviving and reproducing in their environment. This is known as natural selection. It leads to the predominance of certain traits in a population and the suppression of others. In artificial selection, humans have the capacity to influence certain characteristics of organisms by selective breeding. One can choose desired parental traits determined by genes, which are then passed on to offspring.

MS-LS4-6: 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.

By the end of grade 8. Adaptation by natural selection acting over generations is one important process by which species change over time in response to changes in environmental conditions. Traits that support successful survival and reproduction in the new environment become more common; those that do not become less common. Thus, the distribution of traits in a population changes. In separated populations with different conditions, the changes can be large enough that the populations, provided they remain separated (a process called reproductive isolation), evolve to become separate species.

Evidence Statements

MS-LS4-4

1. Articulating the expression for phenomena

   1. Students articulate a statement that relates the given phenomenon to scientific ideas about the cause-and-effect relationship between the inheritance of traits increasing the chances of successful reproduction and natural selection.

   2. Students use evidence and reasoning to construct an explanation for the given phenomenon.

2. Evidence

   1. Students identify and describe given evidence (e.g., from students’ own investigations, observations, reading materials, archived data) necessary for constructing the explanation, including:

      1. Individuals in a species have genetic variation that can be passed on to their offspring.

      2. The probability of a specific organism surviving and reproducing in a specific environment.

      3. The traits (i.e., specific variations of a characteristic) and the cause-and-effect relationships between those traits and the probability of survival and reproduction of a given organism in a specific environment.

      4. The particular genetic variations (associated with those traits) that are carried by that organism.

3. Reasoning

   1. Students use reasoning to connect the evidence and support an explanation that describes the relationship between genetic variation and the success of organisms in a specific environment. Students describe a chain of reasoning that includes:

      1. Any population in a given environment contains a variety of available, inheritable genetic traits.

      2. For a specific environment (e.g., different environments may have limited food availability, predators, nesting site availability, light availability), some traits confer advantages that make it more probable that an organism will be able to survive and reproduce there.

      3. In a population, there is a cause-and-effect relationship between the variation of traits and the probability that specific organisms will be able to survive and reproduce.

      4. Variation of traits is a result of genetic variations occurring in the population.

      5. The proportion of individual organisms that have genetic variations and traits that are advantageous in a particular environment will increase from generation to generation due to natural selection because the probability that those individuals will survive and reproduce is greater.

      6. Similarly, the proportion of individual organisms that have genetic variations and traits that are disadvantageous in a particular environment will be less likely to survive, and the disadvantageous traits will decrease from generation to generation due to natural selection.

MS-LS4-6

1. Representation

   1. Students identify the explanations for phenomena that they will support, which include:

      1. Characteristics of a species change over time (i.e., over generations) through adaptation by natural selection in response to changes in environmental conditions.

      2. Traits that better support survival and reproduction in a new environment become more common within a population within that environment.

      3. Traits that do not support survival and reproduction as well become less common within a population in that environment.

      4. When environmental shifts are too extreme, populations do not have time to adapt and may become extinct.

   2. From given mathematical and/or computational representations of phenomena, students identify the relevant components, including:

      1. Population changes (e.g., trends, averages, histograms, graphs, spreadsheets) gathered from historical data or simulations.

      2. The distribution of specific traits over time from data and/or simulations.

      3. Environmental conditions (e.g., climate, resource availability) over time from data and/or simulations.

2. Mathematical modeling

   1. Students use the given mathematical and/or computational representations (e.g., trends, averages, histograms, graphs, spreadsheets) of the phenomenon to identify relationships in the data and/or simulations, including:

      1. Changes and trends over time in the distribution of traits within a population.

      2. Multiple cause-and-effect relationships between environmental conditions and natural selection in a population.

      3. The increases or decreases of some traits within a population can have more than one environmental cause.

3. Analysis

   1. Students analyze the mathematical and/or computational representations to provide and describe evidence that distributions of traits in populations change over time in response to changes in environmental conditions. Students synthesize their analysis together with scientific information about natural selection to describe that species adapt through natural selection. This results in changes in the distribution of traits within a population and in the probability that any given organism will carry a particular trait.

   2. Students use the analysis of the mathematical and/or computational representations (including proportional reasoning) as evidence to support the explanations that:

      1. Through natural selection, traits that better support survival and reproduction are more common in a population than those traits that are less effective.

      2. Populations are not always able to adapt and survive because adaptation by natural selection occurs over generations.

   3. Based on their analysis, students describe that because there are multiple cause-and-effect relationships contributing to the phenomenon, for each different cause, it is not possible to predict with 100% certainty what will happen.