The evolution of mammals has been marked by significant events, such as the mass extinction that wiped out non-avian dinosaurs, which is believed to have facilitated the rise of mammals.
In a study published in
Science, combining information on current and extinct species, scientists found that mammals began to diversify before this mass extinction, and that the extinction primarily affected species with low speciation rates, meaning those that generate few species.
One of the great questions in evolutionary biology is why biodiversity varies so dramatically from one group of organisms to another. In mammals, for example, the group of marsupials and placentals includes over 6,500 recognized living species, while their sister group, the monotremes (which include the platypus and the echidna), has only 5 currently recognized living species, an asymmetry that seems to have persisted since their origin.
To understand why diversity varies in this way, we need to examine changes in diversification, which is the net result of the generation of new species (speciation) and their loss (extinction). Throughout the over 200 million years of mammalian evolutionary history, several events have certainly influenced their diversification, including the acquisition of evolutionary innovations such as the placenta, the emergence of flowering plants, their interaction with dinosaurs, and the mass extinction that killed all dinosaurs except birds (K-Pg mass extinction), among others.
The scientists studied the evolutionary radiation of mammals by combining information on thousands of phylogenetic and fossil species with a new flexible model that provides fine resolution on how diversification has changed over time and across lineages. Integrating existing and extinct species at this large scale has proven essential for better reconstructing the dynamics of diversity, speciation, and extinction in mammals over deep time.
The scientists found that mammals, particularly those with a placenta, had already accelerated their diversification well before the dinosaur extinction, contrasting with the idea that dinosaurs limited the richness of early mammals. During the mass extinction that followed, around 66 million years ago (at the K-Pg boundary), which killed most dinosaurs, mammalian diversity was also severely impacted, with only about one-third of species surviving.
Complete radiation of mammals, incorporating the tree of existing species and lineages that have gone extinct or have not been sampled to date (a representative complete tree sample); warmer colors represent higher speciation rates. The surrounding colored radial arcs identify the diversity of the 14 mammalian clades, with species silhouettes and Roman numerals for identification. The dotted gray lines specify the time scale every 40 million years into the past, and the solid gray lines mark, in order, the K-Pg mass extinction event.
© Ignacio Quintero Interestingly, the mammalian lineages that survived the K-Pg mass extinction were not those with lower background extinction rates, but rather those with higher background speciation rates—that is, those producing more species at the time of the event. The extinction of slow-speciation lineages resulted in increased average speciation rates at the beginning of the Cenozoic, giving an evolutionary advantage to some lineages over others, particularly placentals relative to marsupials.
However, even within these groups, speciation speeds vary considerably between lineages, suggesting that periods of rapid species generation depend on an unpredictable and transient combination of different factors. The study reveals a contingent view of macroevolution, where the accumulation of biodiversity is not explained by a few remarkable innovations or environmental events but depends on lineages experiencing temporally optimal speciation conditions.
References:
Imbalanced speciation pulses sustain the radiation of mammals
Quintero, I., Lartillot, N., Morlon, H.
Science, May 30, 2024, DOI:
10.1126/science.adj2793