Why is it that a plant species that reproduces only by self pollination can adapt better than a plant species that reproduces asexually?

The answer to your question is that in asexual reproduction
there is no exchange of genetic information to create offspring that are different from the
parents. In the case of plants, the new plants created are clones of the parent plant. It is
important for genetic information to be shared so that the new plants created have genes which
are different from the parent plant. If the new genetic make-up is better than that of the
parents, the plant is able to flourish better and the species
evolves.

Now, self pollination is a process where the same plant
provides the pollen and the egg for the new plant that is created. Though this is not as
efficient a way of improving the genes of the plant as cross pollination, it is in a way better
than asexual reproduction.

This is the reason, plants only self
pollinate when they are unable to cross pollinate. This could be due to a small number of other
plants of the same species in the surrounding environment or no medium to carry the pollen from
one plant to another.