Question

A forest has two groups of plants that appear to be very similar both phenotypically and genotypically. The major difference is that one plant group releases its pollen in February, while the other group releases its pollen in April. Design a plan for collecting the data needed to determine whether the plants are members of one population with great diversity or members of two separate species.

Answer 1

In order to know whether the plants are members of one population with great diversity or actually members of the same species we can attempt to find out whether they reproduce or not. That's one of the main aspects of two organisms of the same species - fertility.

We could start by setting up some kind of artificial environment where both plants would pollenise at the same time, or we could also collect the pollens from both plants and store it to use them in a different time of the year than that of their production. 

After having the pollens available from both plants we could fertilise a group of both plants with each other's pollens (the actual object of study), and also fertilise a group of both plants with their own pollens (so we can have a control for the quality of the pollens and the plants - in this group it is expected to have offspring, if there isn't we cannot take into account any other results).

After the fertilisation, we should now count the offspring. If there is offspring resulting from the crossed plants, they are probably of the same species. We could also compare these plants with the offspring of the normal crossing to check whether there were major differences (such as health issues, or offspring number) that would lead to conclude that still there wasn't compatibility. 

By creating a hybrid between this two groups, even if they are from the same species, we may have to take also into account that they may have different required conditions than their parent plants.

The study should be repeated a few times or the number of plants involved should be large enough to be statistically relevant.