Stephen J. Gould dedicated his monumental 2002 scientific testament, The Structure of Evolutionary Theory, to the two colleagues he felt closest to him, calling them his fellow “musketeers”: paleontologists Niles Eldredge and Elisabeth Vrba. With them he had shared many scientific battles for an extension of neo-Darwinian evolutionary theory beyond gene reductionism, strict gradualism, and adaptationism, somehow recovering Charles Darwin’s own original pluralism.

On February 5, Elisabeth Vrba passed away in New Haven at the age of 82 from the aftermath of a fall. In 2003 she had been a guest speaker at the Genoa Science Festival specifically to remember Gould and to celebrate the Italian edition of that book, the first not in English.

Born in Hamburg in 1942, fatherless at age two, Elisabeth had moved with her mother to Namibia. She took her doctorate in zoology and paleontology at the University of Cape Town, where she conducted fundamental studies on the evolution of African mammals by scouring the limestone caves of the Transvaal (including Sterkfontein, inhabited by australopithecines). In 1986 she moved to Yale, where she remained for the rest of her brilliant career.

To Vrba we owe some of the most relevant formulations of macroevolutionary theory, namely the “turnover pulses” hypothesis, an extension of punctuated equilibrium theory according to which at certain crucial episodes in natural history – because of large-scale climatic and environmental changes – species undergo a rapid turnover process due to multiple coordinated extinctions and speciation. Creativity and destruction together.

In particular, Vrba understood that in the early Pleistocene, habitat fragmentation caused by climate change produced adaptive radiation in many African mammal groups, including our own, the hominins. Rapid environmental change around 2.5 million years ago dwarfed the previous adaptations of many mammals, forcing them to migrate to more congenial habitats, to extinction or to survival through readaptation and diversification. The fragmentation of environmental niches multiplied speciation and colonization of new habitats, triggering rapid processes of turnover among species. Perhaps out of that climatic disruption had emerged, among the australopithecines, the first forms of the genus Homo. Something similar had happened 5 million years ago. The idea, still debated, had anyway the merit of highlighting the role of global ecological changes in the evolution of African mammals. The existence of a dramatic bottleneck in human populations that began 900 thousand years ago, caused by climate cooling, recently confirmed one of her predictions.

Elisabeth Vrba also proposed that long-term evolutionary trends (e.g., the growth of the brain in the genus Homo) were due to a phenomenon of “species sorting”, i.e. the differential survival not only among individuals, but also among entire species, due to the traits of the populations within the species themselves. Here “sorting” is a screening less strong than “selection”, because it does not imply the emergence of species traits, heritable, that establish their greater or lesser fitness at the species level, but rather the propagation to the species of properties of organisms and populations.

In her view, in fact, evolution occurs at multiple hierarchical levels, from genes to species and ecosystems, with effects that can propagate from one level to another, either top down or bottom up (she wrote about this in 1984 with Niles Eldredge). Selection of adaptations among organisms (e.g., being generalists or specialists) can generate concomitant indirect and incidental effects, at a higher level, on speciation rates (“effect hypothesis”). Generalists tend to speciate less and remain more stable. Specialists to fragment into more species, which last less.

Studying species of antelope and other Pliocene families in southern Africa, Vrba in 1988 then formulated the “disturbance resistance” hypothesis, according to which extinction corresponds to the crossing of a critical threshold: biota are conservative and resist change until environmental disturbance becomes unsustainable and leads to extinction and species replacement in a global turnover pulse. In 1983 in Science she explained that macroevolution (evolutionary change at and above the species level, triggered by strong ecological perturbations) is an autonomous dimension that cannot be fully extrapolated from the accumulation of small changes in gene frequencies.

These examples alone illuminate sufficiently the absolute originality of her anti-reductionist and pluralist contribution to evolutionary theory, but Elisabeth Vrba’s name is associated above all with a neologism, coined by her in 1982 and soon adopted by her colleague and friend Stephen J. Gould: exaptation, or functional co-option. This is now defined as all those traits that in the course of evolution became established in connection with some adaptive function (or without any function at all – the spandrels – for example, as side effects of other changes, as structural constraints, or as products of neutral processes such as genetic drift) and then were engaged in different adaptive tasks. In other words, the current function of a trait does not necessarily coincide with its historical origin. The frequency of these opportunistic functional conversions (now observed at all levels: from gene re-functionalizations to the reuse of neural areas) teaches us that evolution is not a process of optimal engineering refinement, but a bricolage that starts from the existing (with its constraints and prior history) and reworks it as environmental circumstances change. Evolution is transformation of the possible.

Independent, free-spirited, intolerant of anything that diverted her from research, she did not write bestsellers, but her papers are still widely read. Elisabeth Vrba was a specialist – and as such perhaps the world’s foremost authority on evolution, taxonomy, paleobiogeography, phylogeny and paleoecology of bovids – but also a refined theorist. She spoke fast and wrote with a great clarity that reflected her lucidity of reasoning and criticism. She was always short of time, and she spent it well. She was fond of repeating that her interest was in pushing the boundaries of science, not sailing in calm seas already explored by others. Without spotlight or hype, she succeeded.

Today all those involved in macroevolution are like dwarfs on the shoulders of this great musketeer.

Photo: Elisabeth Vrba, 2009. This picture was taken at Senckenberg Museum, Frankfurt am Main, Germany. Credits: Gerbil, CC BY-SA 3.0, via Wikimedia Commons