The figure illustrates the effect of an ethanol-rich and an ethanol-free environment on the frequency of the Drosophila Adh^F allele in four populations in a 50-generation laboratory experiment. Population 1 and population 2 were reared for 50 generations in a high-ethanol environment, while control 1 and control 2 populations were reared for 50 generations in a zero-ethanol environment. Describe the effect of each environment on the populations, and state any conclusions you can reach about the role of any of the evolutionary processes in producing these effects.

Verified step by step guidance

1. Step 1: Observe the initial frequency of the Adh^F allele in all four populations at generation 0, noting that they start at similar frequencies around 0.35 to 0.4.
2. Step 2: Compare the changes in allele frequency over 50 generations between populations reared in the high-ethanol environment (Population 1 and Population 2) and those in the zero-ethanol environment (Control 1 and Control 2). Notice that in the high-ethanol environment, the frequency of Adh^F increases significantly, approaching fixation in Population 1 and rising substantially in Population 2.
3. Step 3: Examine the zero-ethanol environment controls, where the frequency of Adh^F either decreases or fluctuates without a clear directional trend, indicating no strong selection for the allele in this environment.
4. Step 4: Conclude that the high-ethanol environment exerts positive selection pressure favoring the Adh^F allele, likely because this allele confers a fitness advantage in metabolizing ethanol, while in the zero-ethanol environment, there is no such advantage, and genetic drift or other evolutionary forces may influence allele frequency.
5. Step 5: Recognize that natural selection is the primary evolutionary process driving the increase in Adh^F frequency in the ethanol-rich environment, whereas in the ethanol-free environment, genetic drift or neutral evolution may be responsible for the observed allele frequency changes.

Key Concepts

Natural Selection

Natural selection is the process where individuals with advantageous traits have higher survival and reproduction rates, leading to an increase in those traits in the population. In this experiment, the increase in AdhF allele frequency in high-ethanol environments suggests that this allele confers a selective advantage under ethanol stress.

Allele Frequency and Genetic Variation

Allele frequency refers to how common a particular allele is in a population. Changes in allele frequency over generations indicate evolutionary processes at work. The graph shows how the AdhF allele frequency changes differently in ethanol-rich versus ethanol-free environments, highlighting the role of environmental factors in shaping genetic variation.

Role of Environmental Pressure in Evolution

Environmental pressures, such as the presence or absence of ethanol, can influence which alleles are favored by natural selection. The experiment demonstrates that ethanol acts as a selective pressure, increasing the frequency of the AdhF allele in populations exposed to it, while in ethanol-free environments, allele frequencies fluctuate without a clear directional trend.