Holly is the Ilex genus of 400 tropical and temperate species. Ilex are most diversified in East Asia and South America, but are found in Europe, North and Central America, and the temperate Middle East. Only one species is native to tropical Africa, and only one species is found in tropical Australia. A few occur on the Pacific Islands, including Hawaii and Tahiti. Genetic studies show that different lineages of hollies hybridized as they evolved, giving them a complex history.
Holly is not an abundant pollen producer and its pollen is locally distributed by bees or other insects, rather than widely broadcasted on wind. Thus, fossil holly pollen is a reliable record of an ancient site’s local climate. The oldest fossil holly pollen is Australian and dates to the Cretaceous, 90 million years ago. Five Cretaceous pollen sites occur in Africa, southeastern Australia, northwestern Borneo, California and Russia. Wide distribution suggests holly evolved by the early Cretaceous, when continents were still joined.
By the beginning of the Tertiary, 64 million years ago, dinosaurs were disappearing, but holly thrived from Europe to China. When the colder Quaternary began, fewer than 3 million years ago, northern Eurasia varied from polar desert to semi-desert steppe (prairie) or tundra, and holly disappeared from those treeless landscapes. Even tropical Asia was dry and cold by comparison to today’s climate, and holly was driven into refuge sites where conditions favored their survival. Different species developed at different refugia. When the climate again favored holly expansion, these species hybridized.
The Quaternary was a time of global cooling that included the Pleistocene glaciations. Today’s four existing closely related species of European holly are the progeny of one hardy ancestral “Ice Age” survivor. They are Ilex canariensis, from Portugal and the Spanish Canary Islands off the coast of Morocco; Ilex colchica from Russia; Ilex perado Aiton of the Canary Islands, Portugal, the Portuguese Madiera Islands off the coast of Morocco; and Ilex aquifolium, “English Holly,” which is widely distributed in England, southern Europe, northern Africa and some temperate areas of the Middle East.
Holly is widely distributed in the Tertiary pollen of North America, where Pleistocene glaciation drove holly to refugia in the current southeastern states. In South America, fossil pollen as old as 60 million years is found in the southern areas and 40 million years in the northern areas. The Pleistocene brought colder, drier climate and treeless prairie vegetation. Holly survived among refuge conifer forests and later diversified. Rainforest emerged in southeastern Brazil at the mouth of the Amazon 15,000 years ago.
New Zealand Ilex Extinction
Absence of holly from the Pacific island of New Zealand is an example of Ilex extinction during global cooling where no retreat to a refuge niche was possible. During the Pliocene cooling, which preceded the Pleistocene glaciation, holly was present in New Zealand, as were temperate trees such as eucalyptus, acacia, bombax, and the conifer-like casuarina “she-oak.” This diversity was lost during climate change.