Exam 2: The Evolution of Behavior

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Narrow-sense heritability (h2) refers to the proportion of phenotypic variation in a trait that is due to additive genetic effects, or the genetic variation that can be passed on from parent to offspring. It is often estimated using methods such as parent-offspring regression and the twin study method.

One method for measuring narrow-sense heritability is through parent-offspring regression, which involves comparing the phenotypic values of parents and their offspring. By plotting the average phenotypic value of parents against the average phenotypic value of their offspring, we can estimate the slope of the regression line, which provides an estimate of narrow-sense heritability.

Another method for estimating narrow-sense heritability is through twin studies, particularly the classical twin study design. By comparing the similarity of monozygotic (identical) twins, who share 100% of their genetic material, to that of dizygotic (fraternal) twins, who share, on average, 50% of their genetic material, we can estimate the contribution of genetic factors to phenotypic variation.

Narrow-sense heritability is fundamentally different from broad-sense heritability in that it specifically measures the additive genetic effects on a trait, while broad-sense heritability encompasses all genetic effects, including additive, dominance, and epistatic effects. In other words, narrow-sense heritability only considers the genetic variation that can be passed on from parent to offspring, while broad-sense heritability includes all genetic variation that contributes to phenotypic differences among individuals.

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1. Variation in traits within a population: In order for natural selection to operate, there must be genetic variation within a population. This means that individuals within the population have different traits or characteristics, such as different colors, sizes, or behaviors.

2. Inheritance of traits: The traits that contribute to the variation within a population must be heritable, meaning they can be passed on from one generation to the next. This allows for the potential for advantageous traits to become more common in a population over time.

3. Differential reproductive success: Natural selection operates through the process of differential reproductive success, where individuals with certain advantageous traits are more likely to survive and reproduce, passing on those traits to future generations. This differential success leads to a change in the frequency of certain traits within the population over time.