Yukio Yasui

Yukio Yasui (安井 行雄, born 1964) is a Japanese evolutionary biologist and professor at Kagawa University. He is known for theoretical work on the evolution of polyandry, gametic sexual reproduction, and evolutionary bet-hedging. Yasui is recognized for articulating complex evolutionary concepts in intuitive, non-mathematical language, nicknamed nonmathematical theoretician.

Yukio Yasui
安井 行雄
Born	1964 (age 60–61)
Nationality	Japanese
Alma mater	Kyoto Prefectural University Hokkaido University
Known for	Good sperm model, evolution of gametic sex, evolutionary bet-hedging
Notable work	Digital Encyclopedia Birdwing Butterflies
Awards	Japan Ethological Society Prize (2018)
Scientific career
Fields	Evolutionary biology
Institutions	Kagawa University

Career

Yasui earned his B.Agr. and M.Agr. degrees from Kyoto Prefectural University in 1987 and 1989, and completed his Ph.D. at Hokkaido University in 1993. He served as a postdoctoral fellow at the Japan Society for the Promotion of Science (JSPS: 1993-1996), the Japan Science and Technology Agency (JST: 1999-2000), and the National Institute for Environmental Studies (NIES: 1999-2000). He was also a COE Research Fellow at the Center for Ecological Research, Kyoto University (1997-1998), and a visiting research fellow at the University of Western Australia (2010).

From 2000 to 2023, he was Associate Professor in the Faculty of Agriculture at Kagawa University, and became a full Professor in 2024.

Research

Yasui's research focuses on the evolutionary mechanisms of female multiple mating (polyandry), the origin of gametic sex and anisogamy, and adaptive strategies under environmental uncertainty.

In 1997, he proposed the Good Sperm Model in The American Naturalist, which suggests that females may increase offspring quality by mating with multiple males whose sperm compete for fertilization.^[1] The model is often discussed in relation to Sexy son hypothesis#Sperm models and Sperm competition#Mate choice.

He further re-evaluated the genetic benefits of polyandry in 1998 in Trends in Ecology & Evolution. He pointed out that while it is true that polyandry increases genetic diversity within a clutch produced by females, polyandry requires a genotype-environment interaction (genotype α is suited to environment A, and genotype β is suited to environment B) for it to be selected. If there is a genotype γ that is suited to both A and B (= good genes) exists, it will become fixed, rendering genetic diversity—and thus polyandry—unnecessary.^[2]

In 2016, he revived the theory that polyandry is a bet-hedging strategy to avoid total reproductive failure of a female.^[3]

In 2022, he co-authored a hypothesis on the evolutionary origins of gametic sexual reproduction and anisogamy in Journal of Ethology.^[4] This work has been linked to the seesaw effect hypothesis and the “Inflated isogamy” hypothesis.

That same year, he published a reinterpretation of evolutionary bet-hedging in Ecological Research, proposing a new framework for understanding the mean–variance trade-off of fitness in unpredictable environments.^[5] His contribution has been featured in the section Evolutionary Bet-Hedging Reconsidered of the Wikipedia article on bet hedging.

In recent years, he has empirically tested bet-hedging theory through studies of multiple mating in crickets.^[6]

As an experimental researcher, he was the first to study sperm competition in mites from the perspective of male reproductive strategy, clarifying its relationship with pre-copulatory mate-guarding behavior. In the species Macrocheles muscaedomesticae^[7], the first male to mate with a female monopolizes fertilization. Males guard females before they molt to secure virgin females. On the other hand, in the species Parasitus fimetorum^[8], mating order is unrelated to sperm precedence, so males do not engage in guarding behavior. Males with high sperm competition ability succeed in fertilization and pass this ability on to their offspring. This led to the conception of the good-sperm hypothesis.

Through field surveys conducted on foot over several tens of days, he revealed that intraspecific variation in the growth and reproduction of the fairy shrimp species Branchinella kugenumaensis^[9] occurs in response to anthropogenic environmental changes on an extremely small geographical scale (a group of terraced rice fields located in a valley about 1.5 km length in Japan), where paddies with different owners had different water management practices (e.g., irrigation and drainage). This resulted in significant differences in water retention duration and the period during which the shrimp could grow and reproduce. Specifically, in paddies where water persists for extended periods, fairy shrimps continue to grow after reaching maturity, achieving maximum size and producing large quantities of eggs. In contrast, shrimps in paddies where water soon disappears mature early, cease growing, and lay a limited number of eggs at once. This pioneering research on the life history evolution of branchiopod crustaceans has significantly contributed to the development of his bet-hedging theory.

Yasui is also the author and editor of the 2025 open-access digital resource Digital Encyclopedia Birdwing Butterflies (Takashi Ohya Collection), which compiles high-resolution taxonomy, distribution, and morphology of Ornithoptera and related genera. Ohya, T., & Yasui, Y. (2025-02-16). Digital Encyclopedia Birdwing Butterflies (Takashi Ohya Collection). Kagawa University Academic Repository. https://www.ag.kagawa-u.ac.jp/birdwingbutterflies/en/#gsc.tab=0

Selected publications

Yasui, Yukio (1997). "A "good-sperm" model can explain the evolution of costly multiple mating by females". The American Naturalist. 149 (3): 573–584. doi:10.1086/286006.

Yasui, Yukio (1998). "The genetic benefits of female multiple mating reconsidered". Trends in Ecology & Evolution. 13 (6): 246–250. doi:10.1016/S0169-5347(98)01383-4.

Yasui, Yukio; Garcia-Gonzalez, Francisco (2016). "Bet-hedging as a mechanism for the evolution of polyandry, revisited". Evolution. 70 (2): 385–397. doi:10.1111/evo.12847.

Yasui, Yukio; Hasegawa, Eisuke (2022). "The origination events of gametic sexual reproduction and anisogamy". Journal of Ethology. 40 (3): 273–284. doi:10.1007/s10164-022-00760-3.

Yasui, Yukio (2022). "Evolutionary bet-hedging reconsidered: What is the mean–variance trade-off of fitness?". Ecological Research. 37: 311–325. doi:10.1111/1440-1703.12303.

Awards

Japan Ethological Society Prize (2018)

Journal of Ethology Editor’s Choice Awards (2021–2024)

Ecological Research Top-Cited Article Award (2024)

Springer-Nature Research Highlights 2022 - Evolutionary Biology

Miyadi Award, Ecological Society of Japan (2000)

Editorial and professional service

Yasui served as Associate Editor (2002-2004) and later Chief Editor of the Journal of Ethology (2011–2016), and remains on the journal’s editorial board. He has reviewed for national grant agencies in Japan and served on the selection committee of the Japan Prize Foundation (2021–2022).