Problem 20 - Animal Behavior: An Evolutionary Approach

Question

Imagine a population of 1000 fruit fly larvae in which there are two hereditarily distinct foraging phenotypes, rover and sitter. Imagine that there are 195 rovers and 805 sitters. Let's say that both types survive to adulthood equally well and both types have 1.2 surviving offspring on average. What were the frequencies of the two behavioral types in the parental generation? What will the frequencies be in the generation composed of their offspring? What would happen if rovers had 1.1 surviving offspring on average, whereas sitters had \(0.9 ?\) What's the point of this question?

Step-by-Step Solution

Answer

Initial frequencies: Rovers 0.195, Sitters 0.805. Offspring frequencies (equal fertility): Rovers 0.195, Sitters 0.805. With differential fertility: Rovers 0.228, Sitters 0.772. This demonstrates natural selection's effect on phenotype frequencies.

Step 1: Calculate Initial Frequencies in Parental Generation

First, calculate the frequency of each phenotype in the initial population. The total population is 1000 larvae. - Frequency of rovers = \(\frac{195}{1000} = 0.195\)- Frequency of sitters = \(\frac{805}{1000} = 0.805\)

Step 2: Calculate Frequencies in Offspring Generation with Equal Fertility

In this scenario, both rovers and sitters have 1.2 surviving offspring on average. The frequencies of rovers and sitters in the offspring generation remain the same because their reproductive success is equal. Thus, the frequencies for rovers and sitters remain: - Rovers: 0.195 - Sitters: 0.805

Step 3: Calculate Frequencies with Differential Fertility

Now, recalculate the offspring frequencies with rovers having 1.1 offspring and sitters having 0.9 offspring. Number of rover offspring = 195 rovers \( \times 1.1 = 214.5\)Number of sitter offspring = 805 sitters \(\times 0.9 = 724.5\)Total offspring = 214.5 + 724.5 = 939- Frequency of rovers in the offspring = \(\frac{214.5}{939} \approx 0.228\)- Frequency of sitters in the offspring = \(\frac{724.5}{939} \approx 0.772\)

Step 4: Interpret the Results

The change in frequencies when rovers have higher reproductive success implies natural selection favoring rovers. Over time, this shift would lead to more rovers in the population if the trend continues.

Step 5: Conclusion Step: What's the Point?

This exercise illustrates the effect of differential reproduction on phenotype frequencies, demonstrating principles of natural selection. It shows how even small differences in reproductive success can shift population dynamics.

Key Concepts

Phenotype Frequency

Phenotype frequency is a fundamental concept in evolutionary biology. It refers to how common a particular phenotype is within a population. In this exercise, we had 1000 fruit fly larvae with two distinct behavioral phenotypes: rover and sitter. To find the frequency of each phenotype, we divide the number of individuals with a given phenotype by the total population.
For example, the initial frequency of rovers is calculated as follows:

• Rovers: 195 individuals over 1000 total, resulting in a frequency of 0.195.
• Sitters: 805 individuals over 1000 total, resulting in a frequency of 0.805.

Understanding phenotype frequency is crucial because it helps us understand the genetic makeup of populations and how these distributions might change over time.

Differential Reproduction

Differential reproduction means that organisms with certain phenotypes produce more offspring than others. This difference can lead to changes in phenotype frequencies in subsequent generations.
In the given exercise, if rovers and sitters both had 1.2 offspring on average, then their relative numbers in the next generation would remain unchanged. However, if each rover has 1.1 surviving offspring and each sitter has only 0.9, the proportion of rovers increases in the next generation:

• 214.5 rovers versus 724.5 sitters, with frequencies of 0.228 and 0.772 respectively.

Differential reproduction is a driving force of natural selection, as organisms that are better adapted to their environment tend to leave more offspring, shifting the phenotype distribution in the population.

Population Dynamics

Population dynamics examines how populations change over time and space, focusing on factors like birth rates, death rates, and migration. In our fruit fly example, we observe dynamic changes based on reproduction rates.
With both phenotypes initially reproducing equally, the dynamics remain static—the phenotype distribution does not change. However, introducing differential reproduction by altering offspring numbers per phenotype causes a shift in population dynamics.

• If rovers reproduce more successfully, their frequency increases over generations.
• Sitters become less represented if their reproductive success decreases.

Population dynamics help scientists predict how populations will evolve with changing environmental pressures, offering insights into long-term survival and adaptation.

Evolutionary Biology

Evolutionary biology is the study of organisms' origin and changes over time through natural selection and genetic variation. It explains how differential reproduction and resulting changes in phenotype frequencies drive evolution.
In the fruit fly scenario, even a small mismatch in reproduction rates between rovers and sitters illustrates a key principle in evolutionary biology: Natural selection favors phenotypes that improve reproductive success.

• Over time, a consistently higher reproduction rate among rovers decreases the relative frequency of sitters.
• This shift demonstrates evolution in action, with advantageous traits becoming more common.

Understanding evolutionary biology allows us to explore these intricate processes, helping to explain biodiversity and the adaptation of species to their environments.