The evolution of rates of successful and unsuccessful predation

Summary The question, how will evolutionary change in a predator or in its prey change the ratio of the rate of successful captures to the rate of unsuccessful capture attempts is addressed. I argue that this ratio is not a good index of the predator's adaptation to prey capture, because decreased costs of capture attempts or increased assimilation efficiency (both favored by natural selection in the predator) will usually reduce the ratio. In addition, the evolution of increased ability to capture prey may lead to a reduction in the success/failure ratio. Analysis of several simple models suggests that this result is robust. The presence of unsuccessful predation does have an important influence on the evolution of predator traits that increase its rate of encounter with the prey; the presence of unsuccessful predation may cause the predator to increase its adaptations for prey detection in response to an increase in the prey's ability to avoid detection.     

THE INCIDENCE OF SPLENIC METASTASIS OF CARCINOMA

The incidence of metastases to the spleen observed at autopsy in a series of 1,000 consecutive cases of malignant neoplasms of epithelial origin was 9 per cent; in a group of 104 cases of malignant neoplasms of non-epithelial origin, the spleen was involved in 45 per cent. These data do not support the contention that metastasis to the spleen is rare, or that the spleen possesses anti-neoplastic properties.     

Limiting similarity and the form of the competition coefficient

The question of whether there is a limit to the similarity of competing species has previously been investigated by a number of authors. These studies have all used the Lotka-Volterra model of competition, and have assumed that the competition coefficient α_ij may be calculated using the expression, α_ij = ∝ U_i(R) U_j(R) dR/∝ (U_i(R))² dR. In this paper, the generality of this formula is questioned and two alternative expressions for α_ij are proposed. When these expressions are used in an analysis of limiting similarity, qualitatively different conclusions emerge regarding the existence and nature of this limit, using either deterministic or stochastic models. The relevance of these findings to theories of character convergence and similarity barriers is discussed. The available field evidence does not strongly support the validity of the formula for α_ij used in previous studies. Since a given method of calculating α_ij must be derived from a higher level model, it is suggested that the Lotka-Volterra model is not sufficient in an investigation of limiting similarity.