My study interest is in evolution of diversity. I am studying mechanisms underlying (1) speciation, (2) adaptive radiation, and (3) evolution of trait polymorphism using mathematical and simulation models.
Plant-pollinator mutualistic systems show huge diverity in both the number of species and forms of interactions. I am studying how compex biological interactions in pollinations systems drive evolutionary diversification.

(1) Roles of predators in evolutionary diversification of plant-pollinator systems
Predators of pollinators such as crab spiders are very common. How these antagonists of plant-pollinator mutualisms affect evolutionary diversification in plant-pollinator mutualisms? Our computer simulation suggested that predators drives active co-evolutionary dynamics in plants-pollinator-predator ecosystems. In the simulation, such co-evolutionary dynamics dramatically increased the possibility of evolutionary diversification in plant-pollinator mutualisms. This result suggest that antagonists of mutualisms could be a driver of diversification in plant-pollinator mutualisms.

(2) Evolutionary mechanisms underlying discrete flower color polymorphisms in food-deceptive plants
Food-deceptive plants produce rewardless flowers which bear appealing traits to attract pollinators but do not provide them with any reward (e.g. nectar). Some food-deceptive plant species show flower color polymorphism (e.g. coexistence of yellow and red flowers within single populations). Previous studies have suggested that learning behavior of pollinators to avoid rewardless flowers creates a fitness advantage of rare flower colors, facilitating evolution and maintenance of floral polymorphism. To test the theoretical plausibility of this hypothesis, we simulated flower color evolution in food-deceptive plants by using an individual-based model. The model incorporates learning and color discrimination by pollinators. Simulation results confirmed the plausibility of the hypothesis; pollinators' learning promoted the evolution of floral polymorphism. Interestingly, we found that the number of flower color variations is determined by the accuracy of color discrimination by pollinators. In simulations with accurate color discrimination, food-deceptive plants evolved to show continuous flower color variation. In contrast, only two or three discrete flower colors evolved with inaccurate color discrimination by pollinators. In such cases, intermediate flower colors in between two existing flower colors could not arise because pollinators who have learnt to avoid the two existing colors most strongly avoided the intermediate colors. Animal behaviors based on inaccurate recognition may be a general evolutionary agent creating discrete trait variations in nature.

Understanding the mechanisms of rapid adaptive radiation has been a central problem of evolutionary ecology. Recent molecular evidence suggests that interspecific hybridization creating novel phenotypes could have promoted several adaptive radiations. However, since hybridization can also cause merge and collapse of existing taxa, the plausibility of the idea that hybridization can promote adaptive radiation remains unclear. To clarify whether and under what conditions does hybridization promote adaptive radiation, we simulated genomic evolution following hybridization between two allopatrically evolved lineages. The simulation demonstrated that hybridization can facilitate adaptive radiation into novel vacant ecological niches. Especially, hybridization could be a necessary cause for adaptive radiation when novel niches are highly different each other and phenotypic effects of mutations were too small to jump over fitness valleys between adaptive peaks. Additionally, simulation results suggested that the effect of hybridization to promote adaptive radiation will be most strong when the degree of genetic differentiation between parental lineages is moderate. Our results provide theoretical basis for roles of hybridization in causing adaptive radiation.

Research articles

Kagawa, K., Takimoto, G. 2014. Predation on pollinators promotes co-evolutionary divergence in plant-pollinator mutualisms. American Naturalist 183: 229-242.

Takahashi, Y., Kagawa, K., Svensson, E. I., Kawata, M. 2014. Evolution of increased phenotypic diversity enhances population performance by reducing sexual harassment in damselflies. Nature Communications 5: 4468.

Kagawa, K., Takimoto, G. 2016. Inaccurate color discrimination by pollinators promotes evolution of discrete color polymorphism in food-deceptive flowers. American Naturalist 187: 194-204.

Kagawa, K., Takimoto, G. 2017. Hybridization can promote adaptive radiation by means of transgressive segregation. Ecology Letters accepted.

Symposium
Kagawa, K., Takimoto, G. 2012. How are novel traits evolve? New insights from theoretical studies (in Japanese). The 28th Annual Meeting of Society of Population Ecology, S4, Funabashi, Chiba, Japan.

Suzuki, M., Kagawa, K. 2014. Understanding plant evolution from animal cognitive behavior: toward coevolution of plant ecology and behavioral ecology. (in Japanese) The 61th Annual Meeting of Ecological Society of Japan, W03, Hiroshima, Japan.

Presentations at scientific meetings

Kagawa, K. 2012. Why are rewardless flowers divers? A simulation study. The 97th Annual Meeting of Ecological Society of America, PS102-168, Portland, Oregon, US.

Kagawa, K. , Takimoto, G.2017. Inaccurate Color Discrimination by Pollinators Promotes Evolution of Discrete Color Polymorphism in Food-Deceptive Flowers. XIX International Botanical Congress, T2-07-06, Shenzhen, China.

Presentations without review

Kagawa, K., Takimoto, G. 2010. Is asymmetric structured food web stable for evolutionary changes? (in Japanese). The 57th Annual Meeting of Ecological Society of Japan, P2-077, Tokyo, Japan.

Kagawa, K., Takimoto, G. 2011. A possibility of sympatric speciation via pollination interaction (in Japanese). The 58th Annual Meeting of Ecological Society of Japan, P2-291, Hokkaido, Japan.

Kotaro, K. , Takimoto, G. 2012. Why rewardless flowers are diverse? The 5th East Asian Federation of Ecological Societies, P2-140A, Shiga, Japan.

Kagawa, K. 2012. Why and how color polymorphism in rewardless flowers evolved? ~A simulation study~ (in Japanese). The 28th Annual Meeting of Society of Population Ecology, S4-3, Funabashi, Chiba, Japan.

Kagawa, K. , Takimoto, G.2014. What kind of lineages will undergo adaptive radiation? ~Evolutionary roles of hybridization and sexual selection~ (in Japanese). The 61th Annual Meeting of Ecological Society of Japan, PA2-010, Hiroshima, Japan.

Kagawa, K. , Takimoto, G.2015. Roles of hybridization and magic traits in adaptive radiations. The 62th Annual Meeting of Ecological Society of Japan, I1-25, Kagoshima, Japan.

Kagawa, K. 2016. Under what conditions does hybridization promote adaptive radiation? ANU-UC-ISM Joint Symposium on Environmental Statistics 2016, Canberra, Australia.

Figures from simulations

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Paintings

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Photographs

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