Monarchs across the Pacific: the Columbus hypothesis revisited

Abstract

The 'Columbus hypothesis' suggests that the annual north-south return migration of Danaus plexippus in North America is a very recently evolved behaviour, less than 200 years old. This hypothesis rests, in part, on an analysis of the 19th century spread of the monarch across the Pacific that assumes a continuous east to west movement and is based predominantly on one publication. We review all the contemporary literature and present new analysis of the data. The movement of the monarch across the Pacific in the second half of the 19th century is best explained by a model which involves no more than three spot introductions, directly or indirectly aided by human movement, followed by natural spread of the monarch across island groups. Contemporary records refer to 'boom' and 'bust' population cycles on newly settled islands, which may have led to high rates of monarch movement. We see no evidence in the records to suggest an east to west sweep by monarch populations as suggested by the Columbus hypothesis.

Serial founder effects and genetic differentiation during worldwide range expansion of monarch butterflies

“…Additionally, colonization was probably affected by between-location distances and wind patterns [27]. The distances between locations inhabited by monarchs in the Iberian Peninsula and northern Africa are much shorter (and often connected by land) than the distances between oceanic islands in the Pacific.…”

“…We focus on the monarch butterfly, Danaus plexippus, which has expanded its ancestral range to colonize locations around the globe [25]. Monarchs are famous for their annual migration from the eastern parts of Canada and the United States to overwintering sites in central Mexico [26]; however, monarchs also occur in locations around the world, ranging from the New World tropics to the Pacific Islands [25,27] and southern Europe [28]. Most monarch populations outside of North America are, in fact, non-migratory [29,30], or locally travel only short distances in the form of modest range shifts [31].…”

“…Monarchs likely colonized locations around the world through serial founder effects (stepwise dispersal) or multiple independent colonization events [27,33], and were aided in reaching new locations through human transportation of both milkweeds and monarchs [34], and extreme weather events [35]. For example, monarchs were recorded in Australia in 1870 and were most probably carried there on cyclonic winds from a source population in New Caledonia [35].…”

Host plant adaptation during contemporary range expansion in the monarch butterfly

“…Although best known from its ancestral range in North America, the monarch can now be found in locations around the world, where it forms geographically isolated, generally nonmigratory, year‐round breeding populations. In most cases, establishment dates for these populations are well documented, with a wave of out‐of‐North America expansion taking place over the last ∼180 years (Vane‐Wright ; Zalucki and Clarke ). The monarch's introduction dates and population genetics are consistent with a natural wave of expansion (Pierce et al.…”

The genetics of monarch butterfly migration and warning coloration

“…More recently, monarchs dispersed across the Pacific, throughout Oceania and Australia, and they also dispersed across the Atlantic to Europe and Africa 17 . It is unknown whether the Pacific and Atlantic dispersal events were independent, but both are thought to derive from North American migratory monarchs 17,23 . There is also an enigmatic non-migratory population in south Florida 15 , which may result from overwintering migratory butterflies that fail to return north.…”