When any species goes extinct, there is a loss of information from the Earth's evolutionary history. However, for extinctions caused by anthropogenic (human) impacts on the environment, there is some concern among scientists that more evolutionary history may be lost than in past extinction events.

This is a challenging comparison to make because past extinction events are tracked using the fossil record, whereas current extinction events are tracked using what are called "molecular phylogenies," which are based on DNA sequencing. In a study published recently by PNAS, researchers showed that these two data types can be compared effectively, suggesting that we can analyze present extinctions in light of our planet's past extinction events.

In paleontology, a species is considered to be the same species for as long as its physical traits remain the same. This is a consequence of the fossil record; scientists must classify species based on whatever information about their physical traits can be obtained from their fossils. This analysis can allow new species to branch off even while a “parent” species remains intact, a phenomenon known as “budding cladogenesis.”

In comparison, modern molecular phylogenies are based in DNA sequence changes among species. This type of classification results in a “parent” species splitting into two “daughter” species as DNA differences accumulate, a process known as “bifurcating cladogenesis.” Therefore, there are discrepancies between the representations of past and present speciation events, which makes comparisons difficult.

The goal of the new analysis was to examine how the loss of evolutionary history is different for budding phylogenies versus bifurcating phylogenies—are there differences between paleontological phylogeny and molecular phylogeny? To explore this question, the researchers ran a set of simulations that probed whether excessive loss of species in one phylogeny type, either molecular or paleontological, would be detectable in data obtained from the other type.

These simulations assumed that all speciation events followed the "budding cladogenesis model" as referred to above and thus maximized the difference between ages of species. Evolutionary trees were simulated for low and high extinction rates, and each tree was subjected to different forms of species age bias in extinction risk—that is to say, for each phylogenetic tree subjected to a simulation, there were several simulations that each had a preference for killing off either older or younger species in the tree. For each simulation, the researchers calculated the loss of evolutionary history as measured by molecular phylogeny and paleontological phylogeny.

The two types of phylogenies are quite different, since molecular data and fossils are assigned ages in very different ways. Nevertheless, the simulations revealed that estimated evolutionary history losses during past and present extinctions were similar. This finding indicated that the two types of phylogeny were comparable in terms of how they tracked evolutionary history loss.

The researchers' simulations also revealed that extinction events typically remove evolutionary history more rapidly in the paleontological paradigm than in the current molecular one, where the loss of this information is mitigated by the shared history of many species. They found that the modern tracking of species via DNA (molecular phylogeny) protected the loss of evolutionary history somewhat, as compared to the tracking of species using just the fossil record.

Most notably, the paper's analysis indicates that there's no naturally occurring equivalent for the loss or endangerment of older species, which we're currently seeing in human-caused extinctions (although the authors don't precisely define their use of the term "older species".) This finding indicates that humans are affecting the loss of evolutionary history in a distinctive and marked way.

The researchers note that they think a focus on the loss of evolutionary history alone is likely to underestimate the loss of distinct biological features that can result from extinctions of younger species. Loss of younger species could also suggest that human impacts have the potential to suppress the origin of species in certain parts of the phylogenetic tree. This might be another novel way in which humans are affecting evolution, one that has not previously been seen in the fossil record.

In their conclusions, the researchers write that the literature shows that anthropogenic impacts endanger some long-lived species and lineages, which may in turn lead to their extinction and a consequent loss of evolutionary history. However, it is still unclear whether these effects on extinction will be a unique evolutionary legacy for humans. There is a large archive of past extinctions preserved in the fossil record that may be the key to answering this question.

The research presented in this article shows that the differences between molecular and fossil data do not prevent comparison of past and present extinctions. The team behind it thinks that these comparisons could provide important insights into the nature of the extinction process and could assist in the development of more informed conservation priorities.

PNAS, 2015. DOI: 10.1073/pnas.1409886112 (About DOIs).

Listing image by Image courtesy of the National Park Service Paleontology Program