Twelve different species of the mygalomorphae spider genus myrmekiaphila, are described using a phylogenetic tree of morphological data based on genitalia. While the tree was accurate in that it grouped the species based on matching genitals, it was questionable as to weather or not it would agree with the genetic data of the spiders and the tree it would produce. (Bond and Platnick 2007)
Using the genetic data, the morphological tree, and the computer program MEGA the resulting information supported the idea that the morphological data was not enough to properly classify the species of spider. This conclusion was brought about by inputting the genetic data of the species in the morphological tree into a program called MEGA in order to analyze the different species and compile them into a more accurate genetic tree.
Upon investigation of the genetic tree discrepancies were found when compared to the morphological tree. Since the morphological tree did not account for prezygotic barriers it was unable to give the correct phylogeny, Thus the phylogenetic trees for morphological and genetic data did not agree when classifying the male myrmekiaphila spiders by their genitalia.
The southeastern area of the United States is home to a particularly clever genus of megalymorphae arachnid known as myrmekiaphila. These spiders are for the most part homogeneous aside from the distinct difference in the male genitalia amongst the different species. Using the different shapes of genitalia a morphological tree was made to distinguish the spiders, and this is shown in figure 1.
For the purpose of the experiment each species was labeled as either branched; M_neilyoungi, M_howelli, M_millerae, M_fluviatillis, M_torreya, and M_jenskini, or unbranched; M_coreyi, M_comstocki, M_tigris, and M_foliata, Promyrmekiapila and Aptostichus; Aptostichus being the outgroup of the tree. In order to determine the accuracy of the morphological tree a genetic tree was made for comparison to see if they agreed when classifying spiders by genitalia.
It was hypothesized that the two trees would not agree and that prezygotic barriers such as geographical reproductive isolation would take place making the true phylogeny of the spiders different from the morphological tree. This hypothesis was derived from the idea that each of these species while existing in the same relative area were at the same time separated and inhabited different location throughout the southeast, This is shown by figure 3.
The process of analyzing DNA sequences in order to properly create a genetic tree for a particular genus can take quite some time, fortunately certain technologies make it much easier. Through the use of MEGA, a computer program used to format and manipulate data sequences creating a phylogenetic tree became a much easier task. Using this program the data for the DNA sequences of the Myrmekiaphila genus was analyzed and arranged into maximum likelihood of reiterations using the Bootstrap method. These iteration were then organized into a genetic tree based on the Jukes-Cantor model. These parameters are what organized the The following tree in figure 2 is the resulted genetic tree of the data analyzed in MEGA.
Using the tree in Figure 2, it is possible to figure out weather or not it will agree with the morphological tree on the classification. Upon comparison of the morphological and genetic trees the first noticeable difference is the placement of the species along the tree’s. Where in the morphological tree the spiders were grouped according to the appearance of their genitalia, the genetic tree grouped unbranched spiders with the branches ones distinguishing each species not by their physical features, but by their location in the southeastern reaches of the US. For instance M. torreya is a branched species that overlaps with location with M. Coreyi which is an unbranched species, and while they have different reproductive organs they are more closely related to each other then to other branched or unbranched species. This new tree provides support for the hypothesis that the species’ were subject to reproductive isolation and that the two trees do not agree when they classifying the spiders by their genitalia.
Figure 3: (Bond and Platnick 2007)
Discussion and analysis,
In summary, whether or not the morphological tree for spider genitalia would agree with the genetic tree, depended on the genetic data for the spiders DNA which was formatted into a phylogenetic tree to find the proper ancestry, and if the spider genitalia was passed on from a common ancestor then the morphological tree would agree, but if it evolved through convergent evolution amongst the different population on their own then the trees would not agree.
Based on figure 3 the resulted genetic tree in figure 2, indicated that the species had evolved separately and that the difference in genitalia did not mean there was common ancestry amongst the spiders with alike genitals. This is visible by the placement of the species along the tree, and between which other species they share the closest ancestry with. The idea that species’ can evolve similar traits while not sharing a common ancestor comes from the idea of convergent evolution which states that under the same circumstances a species occupying the same niche as another can evolve the same way. (D.Stern 2013)
Geographical reproductive isolation is the most likely cause of separation in ancestry. Since the spider population is widely spread throughout the southeast and some species were more prevalent in some locations than others they would not have come into contact with the other species and even if they did they would need to have they same genitalia in order to mate, because of this it is likely that the reason the spiders share morphology and not ancestry is convergent evolution. (S. Prakash 1972)
Since the genetic tree did not group the spiders based on their genitalia and instead grouped them by their location the means by which the species evolved was not due to a simple common ancestor between branched and unbranched genitals. Due to this difference in the two trees they will not agree on the classification of spiders based on genitalia and instead be based on the area of which they are from. (Creative Commons Bio 2e 2018)