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Overview: This 14-minute film describes the connection between the infectious parasitic disease malaria and the genetic disease sickle cell anemia - one of the best-understood examples of natural selection in humans. Author/Source: Howard Hughes Medical Institute Grade level: 13-16 Time: 15-30 minutes Teaching tips: HHMI provides a variety of teacher resources for use with this video: an in-depth film guide, student quiz, two worksheets and three student lessons. Concepts: Correspondence to the Next Generation Science Standards is indicated in parentheses after each relevant concept. See our conceptual framework for details. There is a fit between organisms and their environments, though not always a perfect fit. An organism's features reflect its evolutionary history. Some traits of organisms are not adaptive. There is a fit between the form of a trait and its function, though not always a perfect fit. Evolution is often defined as a change in allele frequencies within a population. Evolution results from natural selection acting upon genetic variation within a population. Evolution results from mutations. Natural selection acts on phenotype as an expression of genotype. Phenotype is a product of both genotype and the organism’s interactions with the environment. New heritable traits can result from mutations. Mutation is a random process. Organisms cannot intentionally produce adaptive mutations in response to environmental influences. Inherited characteristics affect the likelihood of an organism's survival and reproduction. Over time, the proportion of individuals with advantageous characteristics may increase (and the proportion with disadvantageous characteristics may decrease) due to their likelihood of surviving and reproducing. Traits that confer an advantage may persist in the population and are called adaptations. Depending on environmental conditions, inherited characteristics may be advantageous, neutral, or detrimental. Natural selection can act on the variation in a population in different ways. Natural selection sometimes favors heterozygotes over homozygotes at a locus. Heterozygote advantage preserves genetic variation at that locus (i.e., within the population, it maintains multiple alleles at that locus).

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