Human Evolutionary Studies Program and Department of Archaeology, Simon Fraser University
Saturday All day, Plaza Level
Different species are often represented by different numbers of specimens in fossil hominin phylogenetic studies. The potential impact of this variability on the reliability of the resulting phylogenies has been highlighted recently. Using simulated data, Hawks (2004) found that only 12% of the true trees were the shortest when sample sizes varied among taxa. He argued that character states were erroneously assigned to taxa with small sample sizes and that this resulted in many autapomorphies. Given the importance of a reliable fossil hominin phylogeny for palaeoanthropology, there is a need to investigate this issue further.
This study evaluated the impact of heterogeneous sample sizes on phylogenetic reconstruction using craniodental data from extant hominoids and two outgroup taxa. Taxa were randomly assigned sample sizes ranging from the maximum per taxon to three individuals. The size-corrected data were then converted to discrete character states using divergence coding. Next, maximum parsimony and bootstrap analyses were conducted in PAUP*. Lastly, to assess the impact of sample size heterogeneity, trees yielded by the heterogeneous samples were compared with the trees obtained when homogeneous sample sizes were used.
The main effect of sample size heterogeneity was an increase in the number of unresolved clades. This was due to taxa with smaller samples often being found to be statistically indistinguishable from taxa with larger samples in the character coding phase, and therefore being assigned the same character state. Thus, heterogeneous sample sizes likely negatively impact fossil hominin phylogenies but not in a manner suggested by Hawks (2004).
Research funded by the Canada Research Chairs Program, the Canada Foundation for Innovation, the British Columbia Knowledge Development Fund, and Simon Fraser University.