Understanding evolutionary forces that maintain sexual reproduction has been a central question in evolutionary biology, because the well‐known “two‐fold” cost of sex demands an explanation, especially since asexual reproduction (vegetative reproduction and parthenogenesis) often arises by mutation and hybridization. Geographic parthenogenesis, where asexual reproduction by close relatives of sexual lineages tends to be found in “marginal” habitats, has been considered a key pattern for studying the maintenance of sex. Recent studies of sexually reproducing populations have revealed that mating attempts by males can be harmful to females due to sexual conflict. Where that occurs and where there is a mixture of both sexual and asexual females, if male mating harassment is more harmful to asexual females than to sexual females, this may prevent the invasion of rare asexuals. Here, we use a simple ecological model with density‐dependent male mating harassment to show how asexual reproduction can become prevalent in habitats when environmental conditions are poor (i.e., low carrying capacities). The maintenance of sex is possible in a bistable system where abundant males are so harmful that they prevent the invasion of rare asexuals. However, male density is not large enough to suppress asexuals and sex is lost when carrying capacities are low or when sex ratio is highly male‐biased. While previous theoretical studies employed a frequency‐dependent model to study male mating harassment, we demonstrate that density‐dependence may be important to explain the dynamics in low population densities where there is geographical parthenogenesis.
In accordance with the diversification and efficiency promotion of visual communication, various new services have been desired recently. One such service is facsimile which can transmit both black‐and‐white pictures and photographic pictures. However, it is rather difficult to process these two kinds of pictures with a single algorithm. To fulfill the demand for high transmission efficiency and high quality of the reproduced pictures, it is desirable to segment the documents into two categories and process them separately. For this purpose, the extraction of the photographic area included in documents becomes a very important theme.
In this paper an extraction algorithm of the photographic area from the document images using the characteristics of edge elements and background gray levels is proposed. The features of this algorithm are: 1( automatic setting of parameters adaptable for each picture is possible; 2) area segmentation corresponding to the various gray levels of backgrounds is possible; and 3 segmentation accuracy is rather high. In the experiment presented in this paper, 125 document images which consist of 512 × 512 pixels with 7‐bit gray levels have been used. The background gray levels of these images are 27, 36, 42, 57 and 77, and into these images photographic images, which occupy 50, 60, 70, 80 and 90 percent of the total area have been inserted. Experimental results show that in every image the photographic areas have been segmented successfully in the sense of practical use. For 93.8 percent of the images, the differences between the inserted areas and extracted areas are only 0 – 7 blocks (0 – 27 pixels), and the usefulness of this algorithm has been proved categorically.
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