When a crop host is taken to another part of the world, some of its parasites may be left behind in the area of origin, as happened with tropical rust, when maize was taken from the New World to Africa. If the parasite arrives in the new area at a later date, it is described as a re-encounter parasite. A re-encounter parasite is usually very damaging because the crop host tends to lose horizontal resistance during the absence of that parasite. See also: Old encounter, New encounter. Relative measurements horizontal resistance can be measured only in terms of the level of parasitism. Because this level is affected by so many other factors, it is impossible to devise an absolute scale of measurement of horizontal resistance. Consequently, we can measure horizontal resistance only in terms of its relation to the level in other cultivar of known field performance. That is, we can say that cultivar 'A' has more resistance to a given parasite than cultivar 'B'. But we are unable to develop a scale of resistance similar to the Celsius scale of temperature. Renfrew hypothesis
This is the hypothesis that the proto-Indo-European language (PIE) spread across Europe, and across the Middle East to India, with the cultivation of wheat. It postulates that wheat farmers had population densities about fifty times greater than those of hunter-gatherers, and they gradually spread into the hunter-gatherer territories and swamped both their languages and their genes with their superior numbers. See also: Austronesian family of languages, Coconut. Reproduction
There are several kinds of biological reproduction. First perhaps is the distinction between r-strategists (quantity breeders) and K-strategist (quality breeders). Second, there is the distinction between sexual and asexual reproduction. The most rapid reproduction occurs with asexual r-strategists that may also be crop parasites. It is the population explosions of such parasites that can be so damaging and so difficult to control. The vertical subsystem (i.e., gene-for-gene relationship) evolved to stabilise such population explosions by operating as a system of biochemical locks and keys. Reproductive advantage and disadvantage
In ecological terms, individuals within a population which have a reproductive advantage (e.g., more resistance to parasites) tend to proliferate, while those with a reproductive disadvantage (e.g., less resistance to parasites) tend to disappear. See also: Micro-evolution.
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