“To see the world in a grain of sand…”, this is the first line of William Blake´s poem “Auguries of Innocence” which describe a series of paradoxes about innocence, evil and corruption. But in a biological sense, this line can also describe how “a grain of sand” could gives a glimpse of how evolution works using the remains of planktonic foraminifera which resemble grains of sand to the naked eye and date back hundreds of millions of years.
Foraminifera are an important group of single celled protozoa with shells of different composition and granuloreticulose pseudopodia. The first record of the group is from the Early Cambrian and extend to the present day. Their size range is from about 100 micrometers to almost 20 centimeters long.
Planktonic foraminifera are ideal subjects for testing how species evolve over time. They are a diverse extant clade that have an exceptional fossil record, due to extremely large population sizes and widespread species distributions. They also can record the climate and environmental conditions on their calcium carbonate shells.
It seems that gradual morphological trends do not strictly reflect the rate of speciation or its mode within the clade. In a paper published in 1998, Kucera & Malmgren, showed that gradual change in the Cretaceous planktonic foraminifera Contusotruncana fornicata probably resulted in a shift in the relative proportion of high conical to low conical forms through time, yet isotopic data indicated a rapid separation of the population.
Using stratigraphic, phylogenetic and ecological data from the exceptional fossil record of Cenozoic macroperforate planktonic foraminifera, Dr Thomas Ezard from the University of Southampton, explains how the fossil record contains signals of biological processes that drive genetic evolution. He used a complete phylogeny of those Cenozoic foraminifera to provide palaeontologically calibrated ages for every divergence within the clade that are independent of molecular data. Their hypothesis is that speciation provokes a burst of rapid genetic change, giving molecular evolution a punctuational component. This rapid burst helps isolate the new species from its ancestor.
The study shows how the fossil record contains signals of biological processes that drive genetic evolution and promotes the importance of using fossil records in conjunction with the molecular models.
Ezard, T. H. G., Thomas, G. H., Purvis, A. (2013), Inclusion of a near-complete fossil record reveals speciation-related molecular evolution. Methods in Ecology and Evolution, 4: 745–753. doi: 10.1111/2041-210X.12089