What is Selection Pressure?
Selection pressure is the result of effective mortality, that is, the mortality rate in terms of seeds or disseminating agents present at the end of the growth period, not immediately after herbicidal treatment. If resistance genes are already found in a population; the greater the effective mortality, the greater the probability of selecting resistant individuals. It is considered that genes coding for herbicide resistance are commonly present in weed populations even before the introduction of a herbicide and not due to the introduction of resistant individuals from other areas or created by the herbicide per se (LeBaron, 1984). It is just the selection pressure imposed by the herbicide what determines the enrichment of the population with resistant individuals.
Selection pressure is an abstract force that shapes organisms as they evolve due to mutation, natural selection, and genetic drift. For instance, a selection pressure on the ancestors of giraffes for longer necks led to modern giraffes. Selection pressures on small mammals living in open plains led to muscular legs and adeptness at running. Selection pressure operates over durations as short as a few generations, and as long as millions of generations. A selection pressure can derive from practically anything, as long as it acts in a relatively consistent fashion over long timeframes, and actually impacts the reproductive or survival rates of a species. Potential sources of selection pressure may include availability of prey, presence of predators, environmental stresses, competition with other species (including humans), and intra-species competition. In the eyes of evolution, differential reproductive capacity is all that matters — if a certain predator only consumes old animals tha
