Improvement in the utility of one aspect of phenotype in one context that results also in a decline in utility in a different context.

Antagonistic pleiotropy typically is measured in terms of Darwinian fitness, with improvements in fitness in one situation corresponding to declines in fitness in another. These situations can be especially different environments or instead different .

One thus can view this …

1. As occurring within a single organism simultaneously,
2. As occurring within the same organism but with a gap between benefits and costs, or
3. As occurring within the same or different organisms where benefits associated with carrying a given allele are found in one environment while costs associated with the same allele are found in another.

We might view the first of these concepts as equivalent simply to tradeoffs. The more common use of the concept of antagonistic pleiotropy, however, is associated with the second concept, and particularly is used towards gaining an evolutionary understanding of () as a description of how alleles that may be beneficial early in life may be costly later in life.

Substitute "life" with "life stages" and this idea of a temporal distinction may be generalized to include tradeoffs in parasites between, for example, within-host and between-host ability.

"Differences in fitness depending on environment" is so common a theme in evolutionary biology that it would seem to describe it as an example of antagonistic pleiotropy, though, strictly, it is. That is, for all organisms there is for specific niches (some broader, some narrower) and without question there exist adaptations which increase within one niche that can serve to reduce competitiveness outside of that niche (i.e., antagonistic pleiotropy, though in the third of the presented senses of that term).

With microorganisms this concept of different environments often is limited to the idea of different hosts within , e.g., adaptation of a parasite to one host with the result of a reduction in fitness, associated with a given allele or genotype, given or of a host with different properties.

The following video is of a wide ranging but nevertheless somewhat fascinating conversation: