One of the questions in last year's Level 3 Bio exam asked students to consider the impact of human predation on fish evolution.
Most fish stocks, in New Zealand & around the world, are intensively harvested by fishermen. The mesh sizes used in their nets mean that the fish they catch are mostly the larger and older indivduals. This is a form of selection.
Explain what is likely to happen to the fish populations if this pattern of selection continues to operate.
Well, I'd predict that over time the average size of fish in the target species would get smaller. Why? Because the fishing pressure described in the question is a form of directional selection. It's removing a disproportionate number of the older, larger fish. So the breeding pool is likely to become dominated by those fish that reach sexual maturity earlier & at a smaller size. If these characteristics have a heritable component, then over time you'd expect to see a shift in the average size of the fish in question.
And I've found a couple of articles that support that answer. One, published in Science two years ago (Stokstad 2007), documented how today's adult Atlantic cod are around 20% smaller than they were 30 years ago, with females reaching sexual maturity a year earlier than they used to. Well, attributing that to human fishing pressure sounds plausible - but this would become even more convincing if it could be shown that any change in size of non-harvested species occurs more slowly.
This is provided by an analysis (Darimont et al. 2009) of research papers detailing changes in the breeding patterns and & body size/size of body parts in 40 'exploited prey' species. Next, these data were compared with data on evolutionary change in Galapagos finches, and with another database compiled on species that aren't hunted but are still affected by human influences.
The results: on average, species that were hunted by humans changed three times faster than the Galapagos finches, and 50% faster than species affected by other human influences. And the changes were downward - hunted species bred earlier, or got smaller - or both. It might be that not all these changes are genetic, but even then, they indicate how profoundly humans can impact on other species and, by extension, their ecosystems.
C.T. Darimont, S.M. Carlson, M.T. Kinnison, P.C. Paquet, T.E. Reimchen, & C.C. Wilmers (2009) Human predators outpace other agents of trait change in the wild. Proceedings of the National Academy of Sciences 106(3): 952-954 doi 10.1073/pnas.0809235106
E. Stokstad (2007) The incredible shrinking cod. ScienceNOW Daily News, 2007 131 (3)