Suppose that you are starting a long-term study of a population of annual, flowering plants isolated on a small island. Reading some recent papers has convinced you that global warming will probably cause long-term changes in the amount of rain the island receives. Briefly outline the observations and experiments you would need to do in order to document whether natural selection occurs in your study population (what requirements must be met to establish that adaptation is happening?). What traits would you measure, and why?
Directional natural selection can be understood as a process by which fitter traits (or genes) increase in proportion within populations over the course of many generations. It must be understood that the relative fitness of different traits depends on the current environment. Thus, traits that are fit now may become unfit later if the environment changes. Conversely, traits that have now become fit may have been present long before the current environment arose, without having conferred any advantage under previous conditions. Finally, it must be noted that fitness refers to reproductive success relative to alternatives here and now—natural selection cannot increase the proportion of traits solely because they may someday become advantageous.
To document whether natural selection occurs in your study population and if adaptation is happening, you would need to perform a combination of observations and experiments. Here is an outline of the steps you could take:
1. Data collection on the study population: Start by gathering baseline data on the population of annual, flowering plants on the small island. This should involve documenting the population size, distribution, and genetic diversity.
2. Environmental changes: Monitor and record the long-term changes in the amount of rain the island receives. Gather historical climate data, and establish a monitoring system to track rainfall patterns over time.
3. Trait measurement: Identify specific traits or characteristics that may be influenced by the changes in rainfall. Look for traits that are known to vary among individuals within the population and can potentially confer selective advantages or disadvantages under different rainfall conditions. Examples could include seed size, flowering time, leaf size, root length, or water-use efficiency.
4. Data analysis: Monitor the changes in the selected traits over time, and analyze the data to observe any trends or patterns. Look for correlations between trait variations and changes in rainfall patterns. Statistical analysis methods, such as regression analysis or correlation analysis, can be employed to determine the strength and significance of these relationships.
5. Experimental studies: In addition to observations, consider conducting controlled experiments to directly test the impact of rainfall variations on selective pressures. This could involve manipulating rainfall levels in specific plots or monitoring naturally occurring variations in rainfall across different parts of the island.
6. Replication and duration: To establish that adaptation is occurring, it is important to replicate your study over multiple generations and for an extended duration. Natural selection and adaptation are long-term processes, so monitoring the population for several years or longer will provide more robust evidence.
By performing these observations and experiments, you can establish whether natural selection is occurring in the study population. You will determine if certain traits are being selected for or against in response to changes in rainfall patterns, which in turn would indicate whether adaptation is happening.