How Did Bushbabies Become Different Species?

Africa’s bushbabies (family Galagidae) are among the continent’s most distinctive primates, with their large, prominent eyes and bushy tails. Bushbabies are endemic to sub-Saharan Africa, where there are over 20 recognised species. Yet despite decades of research, scientists still debate a fundamental question: how did the many species within the family Galagidae evolve?

A new study, published in the Biological Journal of the Linnean Society, suggests that bushbaby diversification is more complex than a simple evolutionary story would imply.

Speciation is the evolutionary process through which one species splits into two or more distinct species, usually after populations become genetically different and reproductively isolated. There are many well-documented mechanisms that can generate diversification and speciation.

The first is allopatric speciation, in which a population may be separated by a geographic barrier, such as a river, mountain range, or inhospitable desert. Over time, isolated populations accumulate genetic differences and become distinct species, even in similar environments known as niches. This process is often associated with ecological niche conservatism, whereby species retain ancestral niche traits.

Speciation can also occur parapatrically or sympatrically, whereby populations diverge while sharing the same geographical range or with some overlap in range due to intrinsic changes. In this scenario, ecological adaptation can play a larger role, and ecological niches may diverge as populations respond to different environmental pressures through ecological niche divergence. These adaptations gradually lead to reproductive isolation and the formation of new species.

This study investigated whether diversification within Galagidae was more consistent with ecological niche conservatism or ecological niche divergence, and how these patterns relate to different modes of speciation.

The researchers examined several pairs of closely related sister species across Africa. They collected thousands of occurrence records and combined them with climate data on rainfall, temperature, and seasonal variation.

Using ecological niche modelling, they estimated the environmental conditions most suitable for each species and compared how similar or different the sister species’ niches were.

If closely related species occupied very different niches, that would support ecological divergence. If they occupied similar niches despite being separate species, that would point to geographic isolation and niche conservatism.

The results did not reveal a single evolutionary pattern across Galagidae; the divergence analyses suggest that ecological niche evolution, measured by niche divergence and conservatism, plays a limited but mixed role in interspecific speciation.

Some studied species appeared to occupy very similar ecological niches, while others showed significant differences. Overall, the results suggested that ecological adaptation has played only a limited role in generating Galagidae diversity.

In other words, most Galagidae species do not appear to have evolved mainly in response to different climates or environments. Rather, geographic isolation may have been more important.

The clearest example of niche conservatism in this study came from sister species in the genus Paragalago.

These species occupied remarkably similar niches despite being distinct species. Their environmental needs were more similar than would be expected by chance, supporting previous studies proposing that they evolved through geographic separation during climatic fluctuations over the last few million years.

Climatic records suggest that during cooler, drier periods, African forests repeatedly expanded and contracted. These changes may have isolated ancestral populations in separate forest refuges, allowing them to diverge genetically while remaining adapted to broadly similar environments.

The findings provide one of the strongest examples of niche conservatism in the study.

However, researchers identified one particularly intriguing case that may reflect ecological divergence.

Two species in the genus Euoticus exhibited non-equivalent ecological niches. This result was notable because the species occur in close geographic proximity and are separated mainly by the Sanaga River in Cameroon.

The researchers argue that this may suggest that adaptation to different environments contributed to divergence within this lineage, although additional research is needed to determine the extent to which ecological adaptation influenced speciation.

The finding is particularly interesting because the Gulf of Guinea forests are already known for extraordinary biodiversity and high endemism. The study raises the possibility that ecological adaptation has played a larger role in generating species diversity in this region than previously recognised.

Among all the groups examined, Euoticus provided the strongest evidence that ecological adaptation may have contributed to diversification, although the authors stress that the evidence remains indirect.

The models also revealed which environmental factors appear most important for Galagidae.

Rainfall emerged as a major predictor of habitat suitability. Species generally preferred areas with higher, more reliable precipitation, especially during the driest parts of the year.

This makes ecological sense. Rainfall influences plant growth, insect abundance, and the availability of food resources throughout tropical ecosystems.

Many species showed negative responses to large seasonal temperature swings and extreme heat. The authors suggest that smaller-bodied galagos may be especially sensitive to temperature extremes, as they lose heat more rapidly than larger mammals.

Together, these results suggest that many Galagidae thrive in relatively predictable environments.

One of the study’s most important conclusions is that present-day geographic distributions do not necessarily reveal how species evolved. The key takeaway is that location alone cannot explain the origins of Galagidae.

Some species with overlapping ranges occupied quite different niches, while others separated by large distances occupied surprisingly similar niches. There was no consistent relationship between geographic overlap and ecological similarity. This means scientists cannot simply look at where species live today and accurately infer how they originated.

Instead, the evolutionary history of Galagidae appears to involve a mixture of processes, including geographic isolation, climatic change, ecological adaptation, and potentially other mechanisms such as competition, mate choice, vocal differences, and sensory drive.

The study ultimately argues against a simple explanation for the diversification of Galagidae species.

Rather than finding strong evidence that adaptation to different environments drove speciation across the family, the researchers found a patchwork of evolutionary histories. Geographic isolation appears to have been important in many cases, but ecological divergence may have contributed in some lineages, particularly within Euoticus, where the strongest evidence for niche differentiation was observed.

The result is a more nuanced picture of evolution: one in which different species can arise through distinct evolutionary pathways, even within the same animal family.

For these primates, the story of speciation is the product of multiple evolutionary forces operating across Africa’s changing landscapes over millions of years, and truly understanding it will require far more research to paint this intricate picture.