Abstract
Batesian mimicry, a phenomenon in which harmless organisms resemble harmful or unpalatable species, has been extensively studied in evolutionary biology. Model species may differ from population to population of a single mimetic species, so different predation pressures might have driven micro-evolution towards better mimicry among regions. However, there is scant direct evidence of micro-evolutionary change over time in mimicry traits. Papilio polytes shows female-limited Batesian mimicry. On Okinawa, one mimicry model is Pachliopta aristolochiae, which was not present on the island until 1993. In P. polytes, the size of the hind-wing white spot, a mimetic trait, is maternally heritable. Among specimens collected between 1961 and 2016, the average white spot size was unchanged before the model’s arrival but has rapidly increased since then. However, white spot size showed greater variance after the model’s establishment than before. This suggests that before 1993, white spot size in this population was not selectively neutral but was an adaptive trait for mimicking an unpalatable native, Byasa alcinous, which looks like P. aristolochiae apart from the latter’s hind-wing white spot. Thus, some females switched their model to the new one after its arrival.
Highlights
Recent studies in evolutionary biology have demonstrated that micro-evolutionary change is directly observable in the field when natural selection is strong[1], with some remarkable examples of the rapid evolution of morphological traits[2, 3]
The swallowtail butterfly, Papilio polytes, is a common species throughout South-East Asia. This species has a female-limited polymorphism in Batesian mimicry, in which females exhibit four morphs, whereas males are always monomorphic within a population[6, 10]
There was no significant genetic correlation between the white and red spot sizes. These results indicate that the white spot size in the mimetic females can change in response to natural selection
Summary
Recent studies in evolutionary biology have demonstrated that micro-evolutionary change is directly observable in the field when natural selection is strong[1], with some remarkable examples of the rapid evolution of morphological traits[2, 3]. Theseus (Fig. 1I) is limited to the true tropics, where they share their range with ‘red plain models’ such as Pachliopta aristolochiae (black form) and Pachliopta hector[6] Their wing morphology is characterized by many small red spots on the black hind wings. By a conspicuous white spot in the centre of the hind wing surrounded by several small red spots (Fig. 1D–F) These various wing colour patterns suggest that regionally different predation pressures related to differences in model morphology drove micro-evolution towards better mimicry in each region. Two years after Uesugi’s report, in 1993, P. aristolochiae established a population on Okinawa[15] (the distribution record of Clarke and Sheppard[3] has a mistake: P. aristolochiae was not distributed on Okinawa in 1972) This provides us with another fortunate opportunity to directly observe the presumable micro-evolutionary change in the mimetic wing colour patterns. Using specimens in museum and private collections and some captured by us, we analysed quantitative changes in the size of the white spot over more than five decades in the field populations of Okinawa and other islands (the Miyako and Yaeyama groups) of the Ryukyus
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