Strotz and Allen (1) examined our recently published phylogeny of Cenozoic macroperforate planktonic foraminifera (2) to assess the relative frequency of anagenesis (evolution within a single evolving lineage) and cladogenesis (lineage branching) in the production of new morphospecies. They conclude that anagenesis is much less prevalent than indicated in our phylogeny.
We disagree with the authors’ criteria for distinguishing anagenesis from cladogenesis in the fossil record, which is based on the assumption that “morphospecies that coexist temporally are reproductively isolated, and therefore independently evolving.” This forces Strotz and Allen to interpret as separate, cladogenetically produced entities (i) all extant morphospecies and (ii) all fossil morphospecies with a substantial overlap in their stratigraphic ranges. Here we show why neither interpretation is safe.
Historically, planktonic foraminifera have been widely used for biostratigraphic research; as a result, morphologically intergrading populations have been subject to considerable taxonomic splitting to better subdivide geological time (2). The range overlap of many morphospecies is an artifact of the typological approach used for defining species in the geological record and does not necessarily imply cladogenesis.
Consider one of the most abundant extant species, Globigerinoides sacculifer sensu lato. This species has four commonly recognized synonyms: quadrilobatus, trilobus, sacculifer, and immaturus. We conservatively recognize only trilobus and sacculifer in our morphospecies phylogeny and unite these morphs into a single lineage in our derived evolutionary lineage phylogeny. Under their methodology, Strotz and Allen (1) must infer cladogenesis for this biostratigraphic split due to the substantial range overlap of these morphospecies (∼12 My). However, experiments have shown that these morphs are ecotypic variants of a single lineage that are related to food supply and other environmental variables (3), a hypothesis that is also supported by genetic analysis (4).
Strotz and Allen (1) allow fossil morphospecies to belong to the same lineage as long as they do not coexist for more than 0.6 My in the Neogene or 2.4 My for the Paleogene. These figures are entirely arbitrary, seemingly drawn from just two studies of evolutionary rates within lineages that are not representative of the group as a whole.
We support the integration of fossil and molecular data as the most robust method for determining the phylogenetic history of a group, but we deliberately avoided genetic evidence in constructing our fossil phylogeny so that it could be considered independent. We acknowledged that many more speciation events might have occurred than are resolved in our phylogeny. Strotz and Allen (1) claim to have integrated molecular data with the fossil data we presented (2); however, there is no description in their methods section of how this was done.
In summary, Strotz and Allen (1) conclude what they assume: that the names given by biostratigraphers to morphospecies generally refer to genetically and evolutionarily separate entities, despite the statements of biostratigraphers to the contrary both in the primary literature (5) and in our own description of our methods (2).
Footnotes
The authors declare no conflict of interest.
References
- 1.Strotz LC, Allen AP. Assessing the role of cladogenesis in macroevolution by integrating fossil and molecular evidence. Proc Natl Acad Sci USA. 2013;110(8):2904–2909. doi: 10.1073/pnas.1208302110. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 2.Aze T, et al. A phylogeny of Cenozoic macroperforate planktonic foraminifera from fossil data. Biol Rev Camb Philos Soc. 2011;86(4):900–927. doi: 10.1111/j.1469-185X.2011.00178.x. [DOI] [PubMed] [Google Scholar]
- 3.Bijma JC, Hemleben C, Oberhänsli H, Spindler M. The effects of increased water fertility on tropical spinose planktonic foraminifers in laboratory cultures. J Foraminiferal Res. 1992;22(3):242–256. [Google Scholar]
- 4.André A, et al. The cryptic and the apparent reversed: Lack of genetic differentiation within the morphologically diverse plexus of the planktonic foraminifer Globigerinoides sacculifer. Paleobiol. 2012;39(1):21–39. [Google Scholar]
- 5.Pearson PN, Olsson RK, Huber BT, Hemleben C, Berggren WA, editors. Atlas of Eocene Planktonic Foraminifera. Vol. 41. Fredericksburg, VA: Cushman Foundation for Foraminiferal Research; 2006. [Google Scholar]