Looking at diversity: phylogenetic approach

Phylogenies are used to assess the relatedness of species, genera or family to their respective ancestors and relatives.  They are particularly useful because they allow us to separate some of the different aspects of biodiversity.  For example:  there are around 1.36 million extant animals known and there are 36 animal phyla (taxonomic classification of groups in the kingdom Animalia).  If the mean number of species is around 38000 species per phylum, what do you think the MEDIAN number of species per phylum is? 

1. 3
2. 33
3. 330
4. 3300
5. 33000

Well given that over one million species are within the phylum Arthropoda, and two phyla contain just one species, the median is 330.  Species richness is not evenly distributed, meaning some taxa dominate over others.  Why might this be?  Well taxa at the same rank may be older and so have had more time to diversify.  Or maybe it’s just chance?  Phylogenies can help to explain the correlates of biodiversity, and thus expose traits which might predispose species to diversification (in this blog) and extinction (in the next blog).  For example, it’s well known that big animals are less diverse than small animals (see figure 1 for Mammalia).  We cannot simply test our hypothesis that rodents are more diverse than elephants because they are small.  The problems in testing are due to confounding variables and non-independence. 

Figure 1  Phylogeny of the mammals

One lineage might have evolved into grazers, and to be a good grazer being big is key.  This lineage might produce 5 families of big grazers (figure 2: green dots).  Another lineage from the same ancestor may have evolved strong female mate choice, which in turn speeds diversification (figure 2:  red squares).  In this lineage there’s high speciation rate and so has greater species richness. 

Figure 2  Schematic of two lineages diverging with different traits described in the text

At face value, the large grazer families are less species rich than the smaller families, but not because of body size!  Phylogenies remove this non-independence problem by comparing sister clades because they are automatically the same age.  Nested comparisons (red in figure 3a) can be made between  matched pairs. 

Figure 3a Two pairs (sister clades) with number of species S in each lineage (yellow) and total species in the sister clade (red).  The trait X is body size (yellow, red is mean body size).  3b  Correlation of species richness and trait. 

Here are two sister clades, within each matched pair, does having more species (S) correspond to having a larger or small mean trait (X)?  If there is no correlation between the two, then the null hypothesis (X is larger 50% of the time) cannot be rejected.  A regression (figure 3b) can be used to look at the:

                                                

relative rate difference (for ΔS)=

                      

These kinds of analyses are particularly useful when testing hypotheses about co-radiation (i.e. have two completely different lineages such as flowering plants and beetles driven each other’s diversification), types of selection for traits (i.e. dichromatism in birds shows strong sexual selection), and other evolutionary phenomena. 

This has been a brief introduction into phylogenies, and they can be used to predict correlates of extinction in a very similar way that they predict correlates of diversification.   Using the fossil record, we look at traits which are predisposed to extinction and extrapolate those traits into contemporary diversity. 

Reference:  All personal communication Prof A Purvis.