44
RICHARD S. MILLER
occur, there is often a strong interference component that limits the
number of eggs laid, or a sequence of rapid sera1 changes in the oviposition site that restrict egg laying to a relatively short period. For
example, several burying beetles (Necrophorus) may collaborate in the
burial of a carcass, but aggressive interference ensures that only one
pair of adults will finally remain in possession of it (Wynne-Edwards,
1962). Such interactions are not confined to intraspecific encounters,
as I have witnessed the same thing among different species of Necrophorus. In this example, competitive interference limits the size of the
F, generation and consequently reduces the probability of heavy exploitation among the larvae. The ecological advantages of this arrangement are fairly obvious. Laboratory studies of competition have
demonstrated that direct exploitation in crowded populations may lead
to extremely high mortality of larvae and severe reductions in the size
of the F, generation. If climatic or other environmental controls do not
prevent the occurrence of potentially harmful rates of exploitation,
there will be a selective advantage in the development of effective
interference mechanisms within and between species populations.
It was noted earlier that the traditional emphasis on selective
elimination in laboratory studies of competition has tended to obscure
the more interesting fact of extended coexistence that is frequently
observed in these populations, even when the conditions of the experiment are designed to achieve maximum competition. The experiments
with D. melanogaster and D. simulans adults and larvae (Miller, 1964c)
established the fact that interactions between sibling species in a uniform environment can have equal value, but this series of experiments
involved events in only one generation. Conventional competition theory
would argue with some justification that a competitive advantage,
however slight, is bound to accrue to one species or the other in a
stochastic projection of a mixed-species system through several generations. If we grant this objection to the models developed earlier, we
must then ask "What degree of species difference or environmental
fluctuation would permit indefinite coexistence of the two species?"
Moore (1952) found that D. melanogaster eliminates D. simuluns when
they compete in population cages kept a t a temperature of 25"C, but
the competitive advantage is reversed at 15°C. However, the progress
of competition and its outcome depended partly on the age and quality
of the food, which was a function of the rate at which the food medium
was renewed. Conceivably coexistence could have been maintained indefinitely either by keeping the populations at some intermediate
temperature between 15" and 25"C, by periodic fluctuations in temperature, or by selecting an appropriate program of food renewal. Even
with the experimental design that was followed, D. melanogaster was
RICHARD S. MILLER
occur, there is often a strong interference component that limits the
number of eggs laid, or a sequence of rapid sera1 changes in the oviposition site that restrict egg laying to a relatively short period. For
example, several burying beetles (Necrophorus) may collaborate in the
burial of a carcass, but aggressive interference ensures that only one
pair of adults will finally remain in possession of it (Wynne-Edwards,
1962). Such interactions are not confined to intraspecific encounters,
as I have witnessed the same thing among different species of Necrophorus. In this example, competitive interference limits the size of the
F, generation and consequently reduces the probability of heavy exploitation among the larvae. The ecological advantages of this arrangement are fairly obvious. Laboratory studies of competition have
demonstrated that direct exploitation in crowded populations may lead
to extremely high mortality of larvae and severe reductions in the size
of the F, generation. If climatic or other environmental controls do not
prevent the occurrence of potentially harmful rates of exploitation,
there will be a selective advantage in the development of effective
interference mechanisms within and between species populations.
It was noted earlier that the traditional emphasis on selective
elimination in laboratory studies of competition has tended to obscure
the more interesting fact of extended coexistence that is frequently
observed in these populations, even when the conditions of the experiment are designed to achieve maximum competition. The experiments
with D. melanogaster and D. simulans adults and larvae (Miller, 1964c)
established the fact that interactions between sibling species in a uniform environment can have equal value, but this series of experiments
involved events in only one generation. Conventional competition theory
would argue with some justification that a competitive advantage,
however slight, is bound to accrue to one species or the other in a
stochastic projection of a mixed-species system through several generations. If we grant this objection to the models developed earlier, we
must then ask "What degree of species difference or environmental
fluctuation would permit indefinite coexistence of the two species?"
Moore (1952) found that D. melanogaster eliminates D. simuluns when
they compete in population cages kept a t a temperature of 25"C, but
the competitive advantage is reversed at 15°C. However, the progress
of competition and its outcome depended partly on the age and quality
of the food, which was a function of the rate at which the food medium
was renewed. Conceivably coexistence could have been maintained indefinitely either by keeping the populations at some intermediate
temperature between 15" and 25"C, by periodic fluctuations in temperature, or by selecting an appropriate program of food renewal. Even
with the experimental design that was followed, D. melanogaster was