Today we have a Darwin-themed text-and-photo contribution by Athayde Tonhasca Júnior, and on his favorite topic: pollination (and my favorite topic, speciation). Athayde’s IDs and narrative are indented, and you can enlarge his photos by clicking on them.
Parting ways
As superlatives go, it would be difficult to beat the South African Platland Baobab [Adansonia digitata]. Its 10.6-m diameter trunk was large enough to accommodate a bar inside its hollow trunk. The massive tree, now deceased, was also old – it had been on this Earth for about a millennia.
There aren’t many places where you can order a pint inside a tree like the Platland or Sunland Baobab © South African Tourism, Wikimedia Commons:
Leaving aside its connection to thirsty pilgrims, the Platland Baobab was not exceptional: other specimens belonging to the same African baobab (Adansonia digitata) species are similarly big and old. The African baobab’s size, age and the somewhat bizarre shape (the ‘upside-down tree’) inspired many legends and superstitions. Beyond the mythical, baobabs have practical uses to some rural communities in parts of Africa: fruits and leaves are rich in vitamin C, the bark can be used for making rope, and tree hollows serve as water reservoirs. Wildlife also feed on baobab’s parts, sometimes in excess: elephants eat baobab bark during the dry season, resulting in significant tree mortality when elephant numbers are high.
One African titan squaring up to another © Ferdinand Reus, Wikimedia Commons:
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Like the vast majority of flowering plants, the African baobab is a hermaphrodite: its flowers have male and female reproductive organs. And like most hermaphrodite plants, baobab flowers are self-incompatible; they can’t fertilise themselves. Therefore, pollinators have to come to their reproductive aid. That’s particularly important for African baobabs, which often grow in isolation, with an average of 2 trees/ha.
When researchers started investigating baobab reproduction in West and East Africa in the 1930s and 40s, bats were soon singled out as their likely pollinating agents. It made sense: the white, large (up to 200 mm in diameter) pendulous flowers open at night and release a musty smell, all signs of chiropterophily, or pollination by bats. But things are a bit more complex. Flowers in west and east Africa are mostly visited by the straw-coloured fruit bat Eidolon helvum (Eidolon helvum) and the smaller Egyptian fruit bat (Rousettus aegyptiacus), respectively. However in southern Africa, baobab flowers have no appeal to bats, but do attract hawk-moths. These regional differences are linked to floral features such as shape, scent and nectar volume. In west Africa, flowers are larger, have longer peduncles, longer styles and more nectar than flowers in east and southern Africa. East African flowers are smaller and sturdier, with less nectar but enough to encourage visits by the Egyptian fruit bat. Flowers in southern Africa are smaller still and produce nectar in volumes just enough for moths (Venter et al., 2025). And while baobabs flowers from the three regions release bat-attracting sulphur compounds, southern African flowers also produce β-caryophyllene, a chemical known to lure moths (Karimi et al., 2021).
Below: A) A straw-coloured fruit bat in west Africa feeding on a baobab flower while a hawk-moth thieves, that is, it takes nectar but does not pollinate. B): an Egyptian fruit bat in east Africa landing briefly to lick nectar. C:) a long-tongued and a short-tongued hawk-moths feeding in southern Africa © Venter et al., 2025:
The African baobab is by no means unique; many other species comprise populations of diversified floral traits that suit particular pollinators and local environmental conditions. Ecologists refer to each of these populations as pollination ecotypes, species complexes, geographical races or ecological races. Pollination ecotypes have one possible outcome of exceptional importance: given enough time, they may drift further apart in their morphological and physiological traits to the point of becoming reproductively incompatible with each other.
Examples of pollination ecotypes. Long-spurred Platanthera bifolia pollinated by the hawk-moth Sphinx ligustri (a) and a shorter-spurred form pollinated by the hawk-moth Hyloicus pinastri (b); short-tubed Gladiolus longicollis pollinated by hawk-moths with short probosces (c) and a long-tubed form pollinated by hawk-moths with long probosces (d). © Johnson, 2025:
It’s worth emphasising the meaning of such an outcome. Different forms – or morphs – in each ecotype associated with their own pollinators will eventually become different species, a process that has become widely acknowledged (Johnson, 2025). Speciation via ecotypes supports Darwin’s view that species and infraspecies taxa (varieties, subspecies, forms, morphs, etc.) represent a continuum: In short, we shall have to treat species in the same manner as those naturalists treat genera, who admit that genera are merely artificial combinations made for convenience. This may not be a cheering prospect; but we shall at least be freed from the vain search for the undiscovered and undiscoverable essence of the term species (Darwin, 1859). Such a continuum implies that speciation is much more common and frequent than one may expect (Mallet, 2008).
The roles of insect pollinators as safeguards of biodiversity, crop production and human health are well known and celebrated. But the tale of African baobab pollination ecotypes reminds us of another fundamental aspect: pollinators greatly contribute to the radiation and diversification of angiosperms, the largest and most diverse group in the plant kingdom and largely responsible for the functioning of all terrestrial ecosystems. It’s a hefty responsibility upon tiny shoulders.
Accumulated diversification of insect families through time. Dotted lines indicate the Permian–Triassic (P–T), Triassic–Jurassic (T–J), and the Cretaceous–Paleogene (K–Pg) mass extinctions © Peris & Condamine, 2024:
References
Darwin, C.R. 1859. On the Origin of Species by Means of Natural Selection. John Murray.
Johnson, S.D. 2025. Pollination ecotypes and the origin of plant species. Proceedings of the Royal Society B 292: 20242787.
Karimi, N. et al. 2021. Evidence for hawkmoth pollination in the chiropterophilous African baobab (Adansonia digitata). Biotropica 54: 10.1111/btp.13033.
Mallet, J. 2008. Hybridization, ecological races and the nature of species: Empirical evidence for the ease of speciation. Philosophical Transactions of the Royal Society of London B 363: 2971-2986.
Peris, D. & Condamine, F.L. 2024. The angiosperm radiation played a dual role in the diversification of insects and insect pollinators. Nature Communications 15: 552.
Venter, S.M. et al. 2025. Regional flower visitor assemblages and divergence of floral traits of the baobab Adansonia digitata (Malvaceae) across Africa. Botanical Journal of the Linnean Society boaf085.