Potato (Solanum tuberosum subsp. tuberosum) production has increased six‐fold (per unit area) in the USA since the 1920s. Direct comparison of potato cultivars released during the past century can help us understand how potato breeding has contributed to these production improvements and to other important traits associated with marketing and utilization. Our objective was to study trends in potato genetic improvement during four subjective breeding periods (BP) (pre‐1900 = BP I; 1930 to 1949 = BP II; 1950 to 1969 = BP III; 1970 to present = BP IV), and also to compare performance between and within the three major cultivar types (round‐white, long, and red‐skinned). In field trials conducted from 1990 to 1992, under best management practices (with scheduled irrigation) in Michigan, the greatest total yield potential was observed in several cultivars released during BP I and II. These cultivars also had late vine maturities. On average, BP II had the greatest marketable yield. Cultivars released in BP III had the lowest total yield, earliest vine maturity, highest scab resistance and most favorable tuber appearance. General trends over periods were for earlier maturity and improved tuber appearance. Round‐white cultivars improved for chip‐processing ability and dry matter content over breeding periods, while long types increased in percent marketable yield only in BP IV. No trends were observed for scab resistance. When cultivars were grouped according to tuber type, there were no differences in total yield; however, the long types had the lowest marketable yield and the red‐skinned types had lowest dry matter content.

Mice and rats vary the size of their nests in response to experimentally induced ambient temperature changes (Thorne 1958' Kinder, 1927Koller, 1955) and hdrmonai changes (Richter, 1941;Lehrman, 1961). Behavioral responses in nest building apparently compensate for extrinsic temperature changes and intrinsic hormonal inbalance. Nest size, therefore, may provide a suitable quantitative measure of the behavior associated with heat conservation. If genetic variations are associated with environmental temperature gradients from north to south, the size of the nest constructed by animals from cold regions should be larger than those constructed by more southernly distributed forms under constant temperature conditions. Within a species or species group, a behavioral cline might be expected. Among species, similar responses to the same environmental factors can be expected. The purpose of this study was to test the behavioral cline hypothesis within the species group Peromyscus maniculatus and the adaptive similarity of sympatric species between the species P. polionotus and P. floridanus.The distributions of the forms tested are illustrated in fig. 1. Two subspecies of Peromyscus maniculatus and the closely related P. polionotus were selected because of their genetic similarity over this wide range. Peromyscus floridanus is sympatric with P. polionotus in the southern parts of its range and is the only species of a different subgenus, Podomys. Both the effects of phylogenetic origin and the environment

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