Although evolution occurs through modification of individual members of a species, the
population ecologists argue that it is more important to understand evolutionary dynamics at
the level of the population. When the environment changes or when a new species makes an
inroad on the resource niche traditionally held by another, ultimately the change is reflected
in population structure. Because members of a species tend to share similar strengths and weaknesses, it is the whole species that tends to survive or fail. Although some individual
members may be fitter than others, they are often not as fit as the incoming species and thus
tend to share the fate of their population in the long run.