Using Phylogeny to Establish Evolutionary Relationships

`

A phylogeny is the evolutionary history of a species or group of species.

Example-legless lizards look like snakes yet they are more closely related to legged lizards than they are to snakes. The phylogeny of legless lizards demonstrates this.

Fused Mobile Short tail Legs eyelids jaw post. anus

Legged Lizards no no no yes

Snakes yes yes yes no

Legless Lizards no no no no

Systematics is the process used to classify organisms and establish evolutionary relationships.

Systematics uses DNA, biochemistry, fossils, embryology to establish phylogenies. The classification of organisms is based on evolutionary relationships. Organisms are given two names-Based on Latin. The first is the genus name, the second is the species name. Related species are grouped together in the same genera, related genera are grouped together in the same family and so on. There are three Domains based on ribosomal sequencing- Eukarya, Archae, and Bacteria

Classification-Linnaean Classification is based on a hierarchy. Each species has two names, based on Latin. The first is the genus classification and the second is the species name. The first genus is capitalized and the species is lower case. Both names are italicized. Similar genera are in the same family. Similar families are in the same order. Similar orders are in the same class. Similar orders are in the same phyla. Similar phyla are in the same Kingdom. Kingdoms are placed in domains.

evolutionary history. Often it mirrors hierarchal classification of an organism. Node #2 represents the most recent common ancestor of the weasel and dog family, whereas node # 1 represents the most recent common ancestor of the canine genus.

A taxon is a taxonomic unit at any of these levels. Higher units of taxons are usually defined by morpho-logical differences rather than bio-chemical differences. Phylogenic tree- is a branching diagram that represents

`

Phylogenetic trees cannot 1.  Does not indicate absolute age of a species. It

only indicates the most recent ancestor. 2.  It does not assume that one taxon on a

phylogenetic tree evolved from the taxon next to it or on the same level.

There are five recognized super groups in the domain Eukarya

1.  Excavata 2.  Chromalveolata 3.  Rhizaria 4.  Archaeplastida (plant kingdom

found in this group) 5. Unikonta (Animal and Fungi

Kingdom found in this group)

All the tan boxes represent groups that used to be in the Protista Kingdom.

,

Homologies can be extended to DNA sequences. The more closely related two species are, the closely their DNA sequences will be alike.

Analogous structures result from two different species in two different environments with similar selection pressures. The two plants below are not closely related but are very similar due to similar selection pressures.

Cladistics is a systematic approach in classifying organisms. Organisms are placed in groups called clades which includes common ancestor with a particular unique trait. This unique trait is passed on to all of its descendents. Therefore a clade contains the ancestor and all of its descendents.

Clades, like taxonomic ranks are nested within larger clades. A clade is only considered a taxon IF it contains an ancestral all of its descendents. Taxon 1 is considered to be a clade whereas taxon 2 and taxon 3 are not considered to be clades.

Monophyletic relationship in grouping is a true clade whereas paraphyletic grouping (b) will leave out one or more descendant groups in this case J. Polyphyletic grouping (c) has two or groups without a common ancestor. B is the common ancestor but it is left out.

Cladistics is based on SHARED ANCESTRAL CHARACTERS and SHARED DERIVED CHARACTERS. Ex-Mammals have backbones but that is not unique to mammals. All vertebrates have backbones and therefore the backbone is a shared ancestral character that originated in the ancestor of the taxon. Hair is shared by all mammals but not the ancestors of mammals thus hair is considered to be a shared derived character that separates them from all the other taxa.

Example hair is considered to be a shared derived characteristic of all mammals but not turtles and retractable claws is a shared derived characteristic of members of the family Felidae (cat family) but not found in wolves, horses or turtles.

Clades, like taxonomic ranks are nested within larger clades. This shows how a cladogram is reflective of classification and it hierarchal nature.

Below is a chart containing SHARED DERIVED CHARACTERS for a variety of vertebrates and a cladogram based on the chart. Lancets are considered to be an out-group in that they diverged from the group before vertebrates evolved and do not have a vertebral column. Lancets are closely related to vertebrates but are not in the in-group.

Cladograms and phylogenetic trees are a ways to diagram evolutionary history. They are very similar and quite often used interchangeably. A phylogenetic tree measures the degree of similarity and its evolutionary history whereas a cladogram measures the degree of shared derived characters from some common ancestor.

Both can be based on either morphological characteristics or molecular characteristics.

For example, if the ancestral state has an A at some nucleotide position and some of your samples have a G at this position, it is assumed that those sharing the G have inherited this change from the same source and so are related to each other by descent.

Of course you need to look at many characters to determine what is related to what. In phenetics, you do not need to determine what is ancestral and what is a shared derived character. You only care about who appears most similar to whom at one point in time. Cladistics is generally considered to be more reliable for the determination of evolutionary relatedness.

The first classification system created by Linnaeus considered only two kingdoms, Plant and Animals. Prokaryotes were considered plants because of their stiff cell wall, and fungi were considered plants because the grew in the ground like plants. Later it was recognized that prokaryotes, protists, and fungi were so different from plants, that there should be five kingdoms.

Monera-prokaryotes Protista-single-celled eukaryotes Plantae-multicellular, eukaryotes that are autotrophs Fungi- multicellular, eukaryotes that are absorptive heterotrophs. Animalia- multicellular, eukaryotes that are ingestive heterotrophs

One challenge has been evidence that there are two distinct lineages of prokaryotes. It is based on the sequencing of the rRNA found in the small subunit. One lineage of prokaryotes is more closely related to eukaryotes than it is to the other lineage of prokaryotes. Most taxonomists now recognize the domain as the largest taxon. The three taxa at the domain level are the Bacteria, Archae, and Eukarya

Within those domains are super groups. The kingdoms are found within the super groups. Of the five kingdoms only three have survived-Animalia, Plantae, and Fungi. The Moneran and Protista kingdoms are no longer recognized.