The species problem

Biological biases in the fossil record manifest themselves in different ways.  Firstly, we should consider species concepts.  Traditionally, a species was defined by its morphology.  That is, a species is a collection of individual organisms with similar morphology (or other traits) which differ from other such groups.  Most of us can quite confidently tell that the animal in figure 1a is not in the same species as 1b.  Other than being striped, furry and legged, they are completely different morphological entities. 

Figure 1            a) tiger                                                                  b) bumblebee

The biological species concept was borne from modern day biology, when species were studied in their environments and is defined by their breeding habits.  A species being groups of [potentially and/or actually] interbreeding populations which are reproductively isolated from other such groups.  In practice, this concept is not used in the fossil record because we lack information on breeding habits.  The modern-day phylogenetic approach is used in extant species using DNA sequencing.  The phylogenetic species is an irreducible cluster of individuals which can be diagnosed as completely different from other such groups (this can be traits or genes).  Recently this approach has uncovered the cryptic species concept, a group of organisms which are morphologically very similar, but genetically very different to related species.  

Figure 2         Male (columns 1 and 3) and females (columns 2 and 4) of four Perichares species.  Left is ventral and right is dorsal view (from http://www.pnas.org/content/105/17/6350/F3.expansion.html)

Defining extant species seems to be quite difficult.  Defining extinct species from fossils is even more of a challenge because we are left with ancient remains, no or little information about breeding, and usually no DNA sample.  This leaves the morphological concept.  Unfortunately, the morphology of some organisms can reduce their fossilisation potential.  Species without identifiable hard parts are may only be fossilised in specific conditions (e.g. jellyfish are best fossilised by the hardening of their surrounding substrate, which then leaves a natural mold).  This creates a bias in the paleorecord towards those organisms which have a more ‘fossilisable’ anatomy, or indeed those fossils which are easy to identify and find (e.g. bivalve molluscs). 

Due to the complexities in identifying distinct extinct species, past diversity is measured using higher taxonomic classifications (such as genus or family).  This automatically fills in the gaps because a genus is still alive until the last individual species goes extinct.  This also has its drawbacks because some genera are not uniformly species rich, meaning a genus of 2 species can be more susceptible to extinction than a genus with 200 species (figure 3).  Therefore genera with only few species (who may also be rare) are likely to be under-represented by the fossil record and potentially, more likely to go extinct without either being fossilised, or being discovered as a fossil.  Furthermore, higher taxonomic units are not comparable between groups because there is no consistent genus/family concept (i.e. it is not temporally standardised). 



 Figure 3   Uneven distribution of species among: a, eutherian orders, with rodents being the dominant group; b, rodent families, with murids being dominant; and c, murid genera (from Purvis and Hector, 2000)

Given the issues highlighted, it is unsurprising that it is difficult to pin down the vital statistics of past global biodiversity.  Paleobiologists have been aware of some of them for some time, and thus potential solutions have been suggested.  But given the uncertainty of our past, how can we compare the extant with the extinct when we have two completely datasets?  I will highlight some of the issues in comparing past with present, and some of the ways the aforementioned biases have been addressed. 

References

Barnosky, A., et al. (2011), Has the Earth’s sixth mass extinction already arrived?  Nature, 471, 51-57.

Purvis, A., and Hector, A., (2000), Getting the measure of biodiversity.  Nature, 405, 212-219

Purvis, A., Jones, K., Mace, G. (2000), Extinction.  Bioessays, 22, 1123-1133