The Evolution of Life-History Traits in Mosquitofish Since Their Introduction to Hawaii in 1905: Rates of Evolution, Heritabilities, and Developmental Plasticity

Abstract

In 1905, mosquitofish were introduced to sugar plantation reservoirs in Hawaii. Collections of at least 250 fish from each of 24 reservoirs, 4 stable and 20 fluctuating in water level, demonstrated that there were small but significant differences in the life history traits of fish from stable and fluctuating classes of reservoirs, and large and significant differences among stocks from individual reservoirs. Fish from 2 stable and 4 fluctuating reservoirs were then raised in individual containers with controlled food and temperature. Age and length at maturity, growth rates, and size of offspring all differed significantly among stocks. Broad-sense heritabilities were significantly greater than zero for female age at maturity for fish from one of two stable reservoirs, and for male maturation traits for fish from two of four fluctuating reservoirs. Rates of evolution, calculated from the maximum difference between the means of lab-raised stocks and assuming 140 generations since 1905 and continuous change, ranged from 0.1% to 0.5% of the average value of the trait per generation. The traits that changed more rapidly were also more phenotypically plastic, thus suggesting that phenotypic plasticity cannot account for stasis in the fossil record. The concept of plastic trajectories is introduced and exemplified, and predictions are made about how age and length at maturity should alter under stress for organisms with different demographic histories.