Springer distinct separation between members of the Paenungulates from

Springer et al. wrote an article about the
placental mammal tree. This article is about the reconstruction of the
placental mammal tree. They explained in the beginning that until recently
studies involving molecular and morphological traits have only been able to
limitedly explain the relationships between placental mammals. Their goal in
writing this article was to present the changes that have been made to this
phylogenetic tree with that data that was used to support these changes. They
also explain what problems are still occurring with the tree and what studies
may be conducted to help further resolve these problems. The information
presented by the authors of this article show the importance of the
reconstruction of the mammalian phylogeny. The main reason why it is important
to understand the placental mammal tree is because it can be used as a model to
research other organisms.

            Some of the changes that were made
to the mammalian phylogeny were made after studies had been done using
molecular science. In the 1980s, comparative sequencing was performed on
certain proteins. The information found from sequencing these proteins showed a
distinct separation between members of the Paenungulates from the rest of the
ungulates. They were then able to be grouped with aardvarks in a new
classification called Afrotheria.

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            Sequencing of mitochondrial genomes
dominated studies on interordinal relationships. The hypotheses involving this
have been contradicted by morphological and molecular evidence. The PCR was
used to make sequencing of nuclear genes possible. The analyses of the nuclear
genes were unstable and created misleading topologies. When the nuclear genes
were analyzed individually, they supported a few of the clades that were
created such as Afrotheria as mentioned above. The division of all placentals
into the four major clades was supported from studies involving DNA sequences
from multiple nuclear genes. These clades were further supported by analyses on
sequences of mitochondrial tRNA and rRNA.

            The four clades have further been
resolved by molecular data. Some examples of this include: Paenungulata being
within Afrotheria; a sister-group relationship between elephant shrews and
golden moles/tenrecs (supported by fetal membrane structures); the split
between Glires and Euarchonta within the Euarchontoglires. These data have
resolved many relationships within orders. Many unexpected results have arisen
as well.

            Other changes to the mammalian
phylogeny were made involving studies on morphology. One way that was used to
resolve the phylogeny was to determine what traits were homologous between two
species or groups and what were analogous. Homologous traits lead back to a
common ancestor, while analogous traits are similar but evolved from different
lines. It is very difficult to determine the difference when it comes to anatomical
features. An example of a homologous pairing is the relationship between Glires
and Paenungulata. An example of an analogous trait is the similar features in
bats and flying lemurs. These traits evolved independently from different
ancestors between the two separate orders these mammals are placed in.

            While all of these studies have made
the mammalian phylogeny more accurate, there are still many problems that need
to be assessed. The major problem with the phylogeny involves resolving the
placental root. One hypothesis that has to do with biogeography is the
separation of the Xenarthrans of South America and the Afrotheses of Africa.
They became separate about 100 million years ago, which is about when the plate
tectonics separated South America and Africa. There are also miniscule problems
with relationships within the major clades.

            There are a few approaches to
resolving the rest of the problems with the mammalian phylogeny. One involves
fixing the misleading information that comes from mitochondrial DNA. A way to
resolve this problem is by increasing the sampling of taxon and removing the
data that contradict the model. There are also problems with the grouping of extinct
species that can be resolved by re-examining them and comparing them to
currently living species. This challenge can lead to new methods in analyzing
phylogenies by reassessing morphological traits. 


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