Tests of the Theories

1. Medawar's hypothesis

At the foundation of the evolutionary theories of senescence are the theoretical predictions of Medawar (1952). He suggested that in a natural environment a population would experience a high level of random, age-independent, mortality due to harsh environmental conditions. As a result, there is an ever-decreasing proportion of individuals that survive to reproduce at older ages. Harmful genes whose time of expression is beyond the natural range of life span for a particular species, can accumulate in a population with little or no check because the effect of mortality from external causes will be to reduce the force of selection in each successive portion of the life span. As a result, natural selection is ineffective at preventing an accumulation of deleterious genes that affect late-ages because they have a minimal impact on the population. Medawar concluded that aging is the result of the cumulative expression of deleterious genes in individuals that live longer than the average life span of the species in its natural environment. He expanded this further to suggest that in populations removed from their natural environment, a larger percentage of individuals will live to relatively late ages, and it is only under these circumstances that the expression of late deleterious genes would be evident and senescence could be observed. A direct test of Medawar's hypothesis would be to compare the demography of a single species under natural conditions and under artificial conditions in which life span was artificially extended. The previously unpublished study described here is the first direct test of this hypothesis.

The objective of this experiment was to evaluate the mortality of a single species in its natural environment and under idealized conditions. The experiment was done with Rumex hastatulus, a dioecious, wind-pollinated, weedy colonizer of disturbed sites, which is known to show substantial variation in individual life span from annual, to biennial, to short-lived perennial (Radford et al, 1968). For the field experiment, the objective was to grow the plants under the same local conditions under which selection had been acting and had shaped the life history. The observed mortality pattern would then be an accurate assessment of natural field mortality. For this experiment, 646 three-week old seedlings were planted into the field, 20 cm apart, and except for an initial mowing, immediately prior to planting, the natural vegetation was undisturbed. For the experiment in the greenhouse, the objective was to grow plants under idealized conditions and to increase the proportion of individuals in the population living to late ages. To do this, there were two treatments. In the first, undisturbed census treatment, 637 plants were grown in individual pots, and watered and fertilized on a regular basis. In the second greenhouse treatment, 50 plants were inhibited from flowering by removing any flower buds from the base of the plant as soon as they appeared. This latter treatment was used to assess the effects of reproduction on mortality under these idealized conditions. The field and greenhouse populations were all censused monthly until every individual had died, and the mortality patterns of the different populations were contrasted.

The results of these experiments show that survivorship in the field and in the greenhouse was very different (Fig. 23-1). The mean life span in the field was 146 ± 80 days. Only 4 of 646 individuals survived until the second growing season and only one of these individuals flowered. Mortality was high, constant, and age-independent, in this experimental field population. This pattern is consistent with Medawar's conjecture about species in their natural populations. A similar high rate of mortality was observed for a population of natural seedlings of this species, which were marked in the field following emergence (Roach, unpublished).

Survivorship in the greenhouse was very high: 84% of the population survived to the day of first flowering during the second growing season (day 350). Yet, post-flowering survival


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