Phylogenetic tree

Most of the modern classification systems that exist are based on the evolutionary relationships that exist between organisms, that is, on the phylogeny of organisms. Classification systems that are based on phylogeny organize species or other groups to reflect our understanding of how these groups evolved from their common ancestors .

What does a phylogenetic tree represent?

A phylogenetic tree is in charge of representing the evolutionary relationships between various species or between other types of entities that are believed to have a descent in common . Each line that we can observe in the phylogenetic tree has a different representation.

The lines that reach the top are those that represent the species that exist today while the lines that do not reach the top are those that represent species that are already extinct . Every time we find a bifurcation or when the branch of the tree divides, it represents that two different species have been produced but that have the same ancestor , which is represented by the original branch before the bifurcation .

What is it for

The phylogenetic tree is a structure that serves to show the different relationships that exist between species that had a common offspring using information obtained from fossils and from different comparisons of the structure and molecules that organisms have.

Types

The two types of phylogenetic trees that exist are the following:

• Rooted phylogenetic tree:  This is a type of direct tree, which has a single node that corresponds to the most recent common ancestor that all the entities of the leaves of the tree have. The most common method for rooting trees is through the use of an external group uncontested and close to allow the inference of the data sequence or features, but the same time must be sufficiently distant so that it can be an external group evident.
• Phylogenetic tree without root:  this type of tree is responsible for illustrating the relationship between the nodules of the leaves without making assumptions about ancestry . They can always be generated from rooted trees when the root is omitted, since a root can not be inferred from a tree without roots without the mechanisms adequate to identify the descent .

Parties

The parts of a phylogenetic tree include the following points:

• The “types” that have the branches of the trees and that represent the taxonomic group that is in the study.
• The taxa that may be located at any taxonomic level : Orders, species, populations, etc. These taxa can also be called OTU or operational taxonomic units.
• The lines that are placed inside the tree are called the ” branches “.
• The points to which the branches connect, or the tips of the branches, are called nodes . The internal nodes are responsible for connecting the branches and the external nodes are the types that represent the taxa .
• Some phylogenetic trees will have a basal node, known as the ” root “.
• A grouping of an ancestor and all his descendants is known as a clade .

How to make a phylogenetic tree

The first step we must take is to find the set of organisms in order to start organizing our tree. We must also look for and choose the characteristics that the organisms we have chosen have, both physical , behavioral and biochemical .

The next step is to determine which of the characteristics we have is ancestral and which of them is derived . When we speak of ancestral character, we must refer to one that was present in the ancestor of the species we are studying. The derived character is the one that arises at a certain moment as part of the lineage that descends from the original ancestor.

Grouping the organisms depending on the characteristics is how we begin to build the tree. It usually starts with the character that has the greatest number of organisms and so on. Then we must look for the character that is shared by most of the organisms.

How to interpret a phylogenetic tree

It is important to know that each sample having the tree belongs to a fragment in the sequence of DNA . In this place you must specify what type of gene or genes were used for the construction of the tree.

Each of these sequences is separated by branches , connected by knots . The closer the knots are between one or another sequence, it means that the species from which they were extracted were also. close If the sequences are separated by several knots , it means that they are much more distant phylogenically .

The length of the branch certain indicates changes in pairs of sequences of DNA that have been analyzed, compared with a sequence sister or what is called ‘outgroup’ . At the end, the sequences are grouped into clades , in which the closest sequences are shown phylogenetically, which are grouped in a longer branch.

Examples

• Phylogenetic tree of life:  living things are divided into three kingdoms: Archea, Bacteria, and Eukarya . The first two are prokaryotic, and the third corresponds to eukaryotic organisms. The phylogenetic tree shows us that living things have common ancestors . In the early stages of the development of life on our planet, there was more exchange of genetic material between the first organisms that emerged, this being the basis of future species .
• Phylogenetic tree of plants:  the first plant that existed was called ancestral alga.  From it the bryophytes , pteridophytes and spermatophytes were derived . Bryophytes gave rise to mosses , liver plants, and anthocera . Spermatophytes formed gymnosperms and  angiosperms .