Question

A hypothetical population of frogs consists of 50 individuals in each of two ponds. In one pond, all of the individuals are green; in the other pond, half are green and half are brown. During a drought, the first pond dries up, and all the frogs in it die. In the population as a whole, the frequency of the brown phenotype has gone from 25 percent to 50 percent. Has evolution occurred? Has there been natural selection for the brown color morph?

Short answer

Yes, evolution has occurred due to the change in phenotype frequencies, but there was no natural selection for the brown color.

Step-by-step solution

Initial Population Overview

Initially, there are two ponds each containing 50 frogs. In the first pond, all frogs are green, while in the second pond, there are 25 green frogs and 25 brown frogs. This means the total population of frogs is 100.

Calculate Initial Phenotype Frequencies

Calculate the initial frequency of the brown phenotype among all frogs. There are 25 brown frogs out of 100 total frogs. Therefore, the initial frequency of the brown phenotype is $\frac{25}{100}=0.25$ or 25%.

Changes Due to Drought

A drought causes the first pond to dry up, resulting in the death of all 50 green frogs in that pond. Now, only frogs from the second pond remain, which include 25 green and 25 brown frogs.

Calculate New Phenotype Frequencies

With the new total frog population being the 50 frogs from the second pond, the 25 brown frogs now make up $\frac{25}{50}=0.5$ or 50% of the remaining population.

Assess Evolution and Natural Selection

Evolution is defined as a change in allele frequencies in a population over time. Here, the frequency of the brown phenotype has increased from 25% to 50%, indicating that evolution has occurred. However, since the change in phenotype frequency was due to the external environmental factor (the drought) and not differential survival based on phenotype, natural selection for the brown morph has not occurred.

Key concepts

Phenotype Frequency

In population genetics, phenotype frequency refers to how often a particular phenotype appears in a population. This can be crucial for understanding evolution and biodiversity within ecosystems. In the case of our hypothetical frog population, initially, we had two distinct phenotype frequencies across two ponds. The first pond hosted only green frogs, contributing 100% green phenotypes to its local population. The second had a 50-50 split, with equal numbers of green and brown frogs. By calculating these frequencies, we can track changes over time.
For instance, the initial brown phenotype frequency across the entire population of 100 frogs was 25%, as there were 25 brown frogs. As environmental changes occurred, these numbers shifted, increasing the brown phenotype frequency to 50% after the drought eliminated all green frogs in the first pond.
By observing and calculating phenotype frequencies, scientists can gain insights into evolutionary trends, which reveal the adaptability and survival strategies of species over time.

Natural Selection

Natural selection is a key mechanism of evolution described by Charles Darwin. It occurs when certain traits enhance an organism's survival and reproductive success in a specific environment, leading to those traits being passed on more frequently to future generations. Traits that offer a survival advantage tend to become more common over time, while those that do not may gradually disappear.
In the exercise concerning the frog population, while the brown phenotype increased in frequency, this change was not the result of natural selection. The drought acted as an external environmental force that removed the pond of green frogs. This wasn't due to brown frogs having better survival traits related to the drought, but rather was a consequence of geographical location. Hence, without differential survival and reproduction favoring brown frogs over green ones, natural selection wasn’t at play in increasing the brown phenotype.

Environmental Factors in Evolution

Environmental factors greatly influence evolutionary processes by affecting the survival of organisms. Such factors can introduce selective pressures that determine which traits are advantageous, neutral, or disadvantageous. Events like climate changes, droughts, food scarcity, or habitat destruction can dramatically alter the composition of species within an ecosystem.
In our example, a drought led to all frogs in one pond perishing. This event shifted the population's genetic makeup significantly as it removed part of the gene pool entirely. As a result, the frequency of phenotypes, in this case, shifted notably across the population. Although natural selection wasn't the driving force behind this change, environmental factors still exerted a profound impact, demonstrating how external conditions can shape evolutionary trajectories by influencing population dynamics.
Understanding the role of environmental factors helps researchers predict how species might cope with changes such as climate variations, giving insights into potential evolutionary paths and biodiversity conservation strategies.