In population genetics, directional selection is a mode of natural selection in which an extreme phenotype is favored over other phenotypes, causing the allele frequency to shift over time in the direction of that phenotype. Under directional selection, the advantageous allele increases as a consequence of differences in survival and reproduction ...
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http://en.wikipedia.org/wiki/Directional_selection
(from the article `evolution`) The distribution of phenotypes in a population sometimes changes systematically in a particular direction. ( the centre column of the figure.) The ...
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http://www.britannica.com/eb/a-z/d/54
Fitness increases/decreases with trait magnitude. E.g. selection on bill size in Darwin`s medium ground finch â€` Geospiza fortis. Population crashes in 1977, 1980 and 1982, due to droughts, selected for increased bill depth due to abundance of hard seeds of Tribulus cistoides. Deep beaks are needed to crack these seeds and therefore large bills...
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http://www.encyclo.co.uk/local/20206
The adjustment, through natural selection, of a pest population to a change in for instance host resistance or of a host population to a change in the parasitic ability of its pests.
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Preferential change in a population, favoring the increase in frequency of one allele over another
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https://www.encyclo.co.uk/local/20125
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