Philosophical Society of Washington

Minutes of the 2317th Meeting

President John Ingersoll called the 2,317th meeting to order at 8:15pm April 26, 2013 in the Powell Auditorium of the Cosmos Club. Mr. Ingersoll announced the order of business and introduced two new members of the Society, including the speaker of the evening.

The minutes of the 2,316th meeting were read by Corresponding Secretary Robin Taylor and approved.

Mr. Ingersoll then introduced the speaker, Mr. R. Alexander Pyron of the George Washington University. Mr. Pyron spoke on "The Origins of Amphibian Diversity."

Mr. Pyron began by noting that the number of observed species is extraordinarily massive, but that this biodiversity exhibits important spatial and temporal variations. There is a significant latitudinal gradient in diversity, with the highest density of mammal and amphibian species in tropical South America, Africa, and south-east Asia and with the lowest density found near the poles, he said. He explained that this gradient also applies to communities of species, traits exhibited by species, and diversity of function, but species and communities are the simplest to observe at the broad scale. Mr. Pyron stated that the goal of his research is to identify simple mechanisms responsible for the observed diversity patterns.

Mr. Pyron then discussed three mechanisms that can change the number of species that exist and can be found in a specific geographic area. He explained that speciation is the accumulation of new species as they divide over time, extinction is the loss of a species, and dispersal is the historical process of a species moving to new geographic locations. Considering the interaction of these core mechanisms through time enables a generalized model that accommodates the only independent factors that could lead to the spatial and temporal variations observed.

Mr. Pyron explained that a phylogeny diagram shows the evolutionary relationship between species using molecular genetic data provided by DNA sequencing, allowing researchers to trace speciation and estimate extinctions. Another existing framework for describing the molecular evolutionary timescale of species is a timetree diagram, which provides a temporal scale as well as relatedness information. Generally speaking, branch length within the diagram is associated with speciation rate, but branch length can also be related to traits or preferred climate.

Mr. Pyron has used these tools to study amphibians, a diverse group of terrestrial vertebrates that includes frogs, salamanders, and caecilians. He noted that the geographical distribution of the roughly 7000 known amphibian species is astounding, with both latitudinal and longitudinal gradients and strong ecological correlations. He has constructed a very large amphibian phylogeny based on available sequenced DNA, then combined it with existing range map data for those species to associate their ecological preferences. His goal is to determine where amphibians originated geographically, which regions they have occupied the longest, and whether mechanism rates vary by climate, region, or diversity.

Mr. Pyron used tree-based analysis to determine whether climate or range area cause rates of speciation or extinction to vary. He found a latitudinal gradient in speciation rate much larger than could be explained by random range shuffling, with more phylogeny branches formed in tropical ecological niches as speciation rate rises and extinction rate drops. Additionally, he found the rates of dispersal from tropical to temperate areas to be low, but the rates to be high for the reverse direction.

Mr. Pyron further analyzed 66 families of ecologically distinct species called clades to determine whether the same patterns appear in all groups. He found that clades became more species-rich at the equator, the speciation rate was higher at the equator, and the extinction rate was higher at the poles. Additionally, he estimated how many species from a phylogeny could coexist and found that temperate communities have lower carrying capacities, so the associated clades are more saturated. He explained that, as species accumulate and occupy niches, there is less available space for new species to diversify into. He believes species in tropical environments finely subdivide the available ecological space to allow more species to coexist.

Mr. Pyron then used area-based analysis to determine whether the number of species in geographical regions was related to the time of region occupation. He found that occupied time had no effect on species richness, but that the diversification rate is strongly related to species richness and diversification rate is strongly influenced by the energy of each area. Areas with higher throughput of water evapotranspiration, which appears to be related to increased speciation rates, showed an increase in species richness, he said. Further, temperate regions have been occupied much longer, so most fossils and ancient amphibians alive today are found in those regions. Tropical South America is the second most recently colonized area, but the associated speciation rates are very high and the region accounts for nearly a third of known species. Mr. Pyron believes tropical climates provide more open niches, which can be occupied by more species that can then diversify at a faster rate without reaching saturation as quickly. Conversely, temperate regions appear to have a smaller range of available niches occupied by fewer species.

With that, he closed his talk and Mr. Ingersoll invited questions.

Someone wondered how mass extinction events might affect biodiversity. Mr. Pyron explained that the Cretaceous-Paleogene boundary is the only mass extinction event that amphibians would have experienced and that researchers have suggested an increase in the rate of diversification after that event since many competitors became extinct.

Another question concerned the possibility of global climate change increasing speciation as average temperatures rise. Mr. Pyron noted that amphibians tend to be sensitive to large temperature changes and many species might become extinct, but agreed that extant species could undergo faster speciation.

After the question and answer period, Mr. Ingersoll thanked the speaker, made the usual housekeeping announcements, and invited guests to apply for membership. At 10:00pm, President John Ingersoll adjourned the 2,317th meeting to the social hour.

Attendance: 65
The weather: Mostly Cloudy
The temperature: 15°C

Respectfully submitted,

Justin Stimatze,
Recording secretary

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