--- title: "Relative Fitness | General Biology I" description: "Relative fitness in General Biology I compares an organism's reproductive success to others in a population, showing how natural selection changes trait frequencies." canonical: "https://fiveable.me/college-bio/key-terms/relative-fitness" type: "key-term" subject: "General Biology I" unit: "Unit 19" --- # Relative Fitness | General Biology I ## Definition Relative fitness is a measure of how well a genotype reproduces compared with other genotypes in the same population. In General Biology I, it is used to describe natural selection and adaptive evolution. ## What It Is Relative fitness is the reproductive success of one genotype or phenotype compared with others in the same population. In General Biology I, you use it to ask not just whether an organism survives, but whether it leaves more offspring than its competitors. The idea is relative, so the number only makes sense when you compare it to something else. A genotype with the highest offspring output in a given environment has the highest relative fitness, even if all organisms in the population produce fewer offspring during a harsh year. Biologists often treat the best-performing genotype as a reference point and give it a fitness value of 1.0, then express other genotypes as fractions of that value. For example, if one genotype produces twice as many surviving offspring as another, the second genotype has lower relative fitness, not because it is "unfit" in an absolute sense, but because it contributes fewer genes to the next generation. This comparison matters because natural selection does not favor traits just for being good in general. It favors traits that work better under the current conditions. A trait that improves camouflage in one habitat may raise relative fitness there, while the same trait may do little or even hurt reproduction in a different habitat. Relative fitness can change when the environment changes, when predators or pathogens shift, or when the population itself changes. That is why adaptive evolution is not a one-time event. The genotype that is favored today can lose its advantage later if the selective pressure changes. A common mistake is to confuse relative fitness with strength, speed, or health. Those traits might affect fitness, but relative fitness is only about the number of viable, fertile offspring compared with others in the population. In biology, survival matters mainly because it changes who gets to reproduce. ## Why It Matters Relative fitness is the number that connects variation in a population to natural selection. Once you know which genotype leaves more offspring, you can explain why one trait becomes more common over generations and another becomes less common. This term also keeps you from using vague language like "the best trait wins." In General Biology I, fitness is always tied to a specific environment. A plant genotype that does well in dry soil may have high relative fitness in a drought year, but not in a wet one. It is also the bridge between genotype and population change. You start with inherited variation, compare reproductive success, and then track how allele frequencies shift. That sequence is the core logic of adaptive evolution. When you see graphs, data tables, or short scenarios, relative fitness is the tool that tells you which phenotype is being favored and why. It turns observations about offspring number, survival rate, or mating success into an explanation of evolution by natural selection. ## Connections ### Natural Selection Relative fitness is the measurement natural selection acts on. If one phenotype leaves more offspring than another, natural selection can increase the alleles behind that trait in the population. Without a fitness comparison, you cannot explain why a trait becomes more common over time. ### Adaptive Evolution Adaptive evolution is the long-term result you get when traits with higher relative fitness become more common. Relative fitness gives you the short-term, generation-by-generation reason the population shifts. It connects a single environmental pressure to evolutionary change across many generations. ### Genotype Relative fitness is usually compared among genotypes or among phenotypes that come from different genotypes. In problems, you may be asked which genotype has the higher fitness based on offspring counts, survival, or mating success. The genotype with the highest contribution to the next generation has the highest relative fitness. ### [Environmental Constraints](/college-bio/key-terms/environmental-constraints) Environmental constraints can change which traits have the highest relative fitness. A trait that works well in one setting may be less useful when temperature, food supply, predators, or disease changes. That is why fitness is not fixed and why adaptation is always tied to context. ## On the AP Exam A quiz question may give you two or three genotypes, then ask which one has the highest relative fitness based on the number of surviving offspring. Your job is to compare reproductive success, not just count how many organisms are alive at one moment. In a data table or graph, look for the trait that leads to more viable, fertile offspring in that environment. In a short-answer or lab question, you might explain why a trait increased in frequency after a change in conditions. Use relative fitness to connect the environment, the trait, and the change in allele frequencies. If a genotype survives well but produces fewer offspring than another genotype, it does not have the higher relative fitness. ## relative fitness vs absolute fitness Absolute fitness is the raw reproductive success of an organism or genotype, such as the number of offspring it produces. Relative fitness compares that success to other genotypes in the same population. Biology questions often use relative fitness when they want you to explain which trait is favored by natural selection. ## Key Takeaways - Relative fitness compares reproductive success between genotypes or phenotypes in the same population. - The genotype with the most surviving, fertile offspring has the highest relative fitness in that environment. - Natural selection changes populations by favoring traits that raise relative fitness, not traits that are simply good in general. - Relative fitness can shift when the environment changes, so a trait's advantage is never permanent. - Do not confuse relative fitness with absolute fitness, which is just raw reproductive output. ## FAQs ### What is relative fitness in General Biology I? Relative fitness is how much reproductive success one genotype has compared with others in the same population. It is usually based on the number of surviving, fertile offspring. In evolution questions, it tells you which trait is being favored by natural selection. ### How do you calculate relative fitness? A common way is to divide the reproductive success of one genotype by the reproductive success of the most successful genotype. If the best genotype is assigned 1.0, the others are expressed as fractions of that value. That makes it easier to compare genotypes in a population. ### How is relative fitness different from absolute fitness? Absolute fitness is the actual number of offspring or surviving offspring an organism produces. Relative fitness compares that number to others in the population. A genotype can have decent absolute fitness but still have lower relative fitness if another genotype reproduces even more. ### What does relative fitness show in natural selection? It shows which traits increase the chance of passing genes to the next generation. If a trait raises relative fitness, the alleles behind it tend to become more common over time. That is the population-level pattern of adaptive evolution. ## Related Study Guides - [19.3 Adaptive Evolution](/college-bio/unit-19/3-adaptive-evolution/study-guide/5lMBZYnmdRaxqUhd) ## About This Document Canonical Fiveable pages are available as Markdown at the same path plus `.md`. - [llms.txt](https://fiveable.me/llms.txt): index of Fiveable's sections and URL patterns - [llms-full.txt](https://fiveable.me/llms-full.txt): complete subject and unit listing - [MCP server](https://fiveable.me/mcp): call Fiveable as tools instead of fetching pages (`https://fiveable.me/api/mcp`) - [MCP server for AP teachers](https://fiveable.me/mcp/teachers): a teacher's classes, assignments and AP-rubric grading (`https://fiveable.me/api/mcp/teacher`) ## Structured Data ```json {"@context":"https://schema.org","@graph":[{"@type":"LearningResource","@id":"https://fiveable.me/college-bio/key-terms/relative-fitness#resource","name":"Relative Fitness | General Biology I","url":"https://fiveable.me/college-bio/key-terms/relative-fitness","learningResourceType":"Concept explainer","educationalLevel":"AP® / High School","about":{"@id":"https://fiveable.me/college-bio/key-terms/relative-fitness#term"},"audience":{"@type":"EducationalAudience","educationalRole":"student"},"dateModified":"2026-07-03T02:21:11.424Z","isPartOf":{"@type":"Collection","name":"General Biology I Key Terms","url":"https://fiveable.me/college-bio/key-terms"},"author":{"@type":"Organization","@id":"https://fiveable.me/editorial#content-team","name":"Fiveable Content Team","url":"https://fiveable.me/editorial","parentOrganization":{"@id":"https://fiveable.me/#organization"},"member":[{"@type":"Person","name":"Amanda DoAmaral","sameAs":["https://www.linkedin.com/in/adoamaral"]},{"@type":"Person","name":"Becky Bahr","sameAs":["https://www.linkedin.com/in/becky-bahr-b52743132"]}]},"publisher":{"@type":"Organization","name":"Fiveable","url":"https://fiveable.me"}},{"@type":"DefinedTerm","@id":"https://fiveable.me/college-bio/key-terms/relative-fitness#term","name":"relative fitness","description":"Relative fitness is a measure of how well a genotype reproduces compared with other genotypes in the same population. 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