The following questions refer to the evolutionary tree in Figure 22.2.
The tree's horizontal axis is a timeline that extends from 100,000 years ago to the present; the vertical axis represents nothing in particular. The labeled branch points on the tree (V-Z) represent various common ancestors. Let's say that only since 50,000 years ago has there been enough variation between the lineages depicted here to separate them into distinct species, and only the tips of the lineages on this tree represent distinct species.

-Some members of a photosynthetic plant species are genetically resistant to an herbicide, while other members of the same species are not resistant to the herbicide. Which combination of events should cause the most effective replacement of the non-herbicide-resistant strain of plants by the resistant strain?
1) The presence of the herbicide in the environment
2) The absence of the herbicide from the environment
3) The maintenance of the proper conditions for one generation
4) The maintenance of the proper conditions for many generations
A)1 and 3
B)1 and 4
C)2 and 3
D)2 and 4

Question

The following questions refer to the evolutionary tree in Figure 22.2.
The tree's horizontal axis is a timeline that extends from 100,000 years ago to the present; the vertical axis represents nothing in particular. The labeled branch points on the tree (V-Z) represent various common ancestors. Let's say that only since 50,000 years ago has there been enough variation between the lineages depicted here to separate them into distinct species, and only the tips of the lineages on this tree represent distinct species.

-Some members of a photosynthetic plant species are genetically resistant to an herbicide, while other members of the same species are not resistant to the herbicide. Which combination of events should cause the most effective replacement of the non-herbicide-resistant strain of plants by the resistant strain?
1) The presence of the herbicide in the environment
2) The absence of the herbicide from the environment
3) The maintenance of the proper conditions for one generation
4) The maintenance of the proper conditions for many generations
A)1 and 3 
B)1 and 4 
C)2 and 3 
D)2 and 4

Quizplus · Accepted Answer

The presence of the herbicide in the environment (event 1) will select for the herbicide-resistant strain, leading to its increased frequency in the population. However, for the resistant strain to completely replace the non-resistant strain, this selection pressure must be maintained for many generations (event 4) to allow for the resistant allele to become fixed in the population. Maintaining proper conditions for one generation (event 3) may not be sufficient for complete replacement, as it would only result in selection for the herbicide-resistant individuals in that generation. The absence of the herbicide (event 2) would not provide any selection pressure for or against either strain, so it will not affect the replacement of the non-resistant strain.

The Following Questions Refer to the Evolutionary Tree in Figure