12 julio, 2024

Cladogram: what is it, what is it for, examples

What is a cladogram?

A cladogram It is a diagram or branching scheme of the characteristics shared by a group of organisms, representing the most probable evolutionary history of the lineage. The reconstruction is carried out following the methodology proposed by the biologist Willi Hennig.

The cladograms are characterized because they group the taxa based on their synapomorphies or derived characters that are of a shared character.

What are cladograms used for?

Cladograms allow you to visualize the phylogenetic relationships between a group or groups of organisms of interest.

In evolutionary biology, these diagrams make it possible to draw up phylogenetic trees and, therefore, reconstruct the evolutionary history of a group, helping to define its classification and taxonomic ranks.

In addition, it helps to elucidate evolutionary mechanisms by examining the way in which organisms change over time, the direction of this change, and the frequency with which it changes.

As elaborated?

One of the main goals of evolutionary biologists is to find the position of species on the «tree of life.» To achieve this, they analyze different characteristics in organisms, whether they are morphological, ecological, ethological, physiological or molecular.

The morphological characteristics of the individuals have been widely used to establish their classification; however, there comes a point where they are not sufficient to discriminate on specific branches of the tree. In this case, molecular tools help discern these relationships.

Once the character is chosen, the hypotheses of the kinship relationships between the species of interest are constructed and represented schematically.

In this diagram the branches represent hypothetical ancestors where an event of cladogenesis or separation of evolutionary lineages occurred. At the end of each branch are located each of the taxa that were included in the initial analysis, whether they are species, genera, among others.

homologies

In order to establish relationships between a group of organisms, homologous characters must be used; that is, two characteristics that share a common ancestor. A character is considered homologous if they acquired their current state by direct inheritance.

For example, the upper limbs of humans, dogs, birds, and whales are all homologous to each other. Although they serve different functions and look very different to the naked eye, the structural pattern of the bones is the same across the groups: they all have a humerus, followed by the radius and ulna.

In contrast, the wings of bats and birds (this time based on the structure for flight) are not homologous because they did not acquire these structures by direct inheritance. The common ancestor of these flying vertebrates was wingless and both groups acquired it convergently.

If we want to deduce phylogenetic relationships, these characters are not useful because, although they are similar, they do not adequately indicate the common ancestry of the organisms.

Shared primitive and derived characters

Now, a homologous character of all mammals is the vertebral column. However, this structure does not serve to differentiate mammals from other taxa, because other groups—such as fish and reptiles—have backbones. In cladistic language this type of character is called shared primitive character or symplesiomorphy.

If we want to establish the phylogenetic relationships between mammals using the vertebral column as a criterion, we could not reach any reliable conclusion.

In the case of hair, it is a character shared by all mammals that does not exist in other groups of vertebrates. Therefore, it is a shared derived character –synapomorphy– and is considered an evolutionary novelty of a specific clade.

To elaborate a cladogram, phylogenetic systematics proposes the formation of taxonomic groups using shared derived characters.

Classification schools: cladism

To establish the classification and phylogenetic relationships between organisms, it is necessary to resort to objective norms that use a rigorous method to elucidate said patterns.

In order to avoid subjective criteria, schools of classification arise: traditional evolutionary taxonomy and cladism.

Cladism (from the Greek cladeswhich means «branch») or phylogenetic systematics was developed in 1950 by the German entomologist Willi Hennig, and is widely accepted for its methodological rigor.

Cladists construct cladograms that represent genealogical relationships between species and other terminal taxa. Similarly, they look for ordered sets of shared derived characters, or synapomorphies.

This school does not use shared ancestral characters or symplesiomorphies and only grants validity to monophyletic groups; that is, groupings that include the most recent common ancestor and all descendants.

Paraphyletic groups (groups of organisms that include the most recent common ancestor, excluding some of its descendants) or polyphetic (groups of organisms from different ancestors) are not valid for cladists.

parsimony principle

Building a cladogram may yield several graphical representations that show different evolutionary histories of the same group of organisms. In this case, the most “parsimonious” cladogram, which contains the fewest number of transformations, is chosen.

In light of parsimony, the best solution to a problem is the one that requires the least number of assumptions. In the field of biology this is interpreted as fewer evolutionary changes.

Differences between cladograms and phylogenetic trees

Taxonomists generally establish technical differences between a cladogram and a phylogenetic tree. It is necessary to clarify that a cladogram is not strictly equivalent to a phylogenetic tree.

The branches of a cladogram are a formal way of indicating a nested hierarchy of clades, while in a phylogenetic tree the branches are representations of lineages that have occurred in the past. In other words, the cladogram does not imply an evolutionary history.

To obtain a phylogenetic tree, it is necessary to add extra information: additional interpretations related to ancestors, the duration of the lineages in time and the amount of evolutionary changes that have occurred between the studied lineages.

Therefore, cladograms are the first approximations for the final creation of a phylogenetic tree, indicating the possible pattern of branching.

Examples of cladograms

amniotes

The amniote cladogram represents three groups of tetrapod vertebrates: reptiles, birds, and mammals. All of these are characterized by the presence of four layers (chorion, allantois, amnion, and yolk sac) in the embryo.

Note that the concept of «reptile» is paraphyletic, since it excludes birds; for this reason it is rejected by cladists.

apes

The ape cladogram includes the genera: Hylobates, Pongo, Gorilla, Bread and Homo. Popularly, the concept of ape is paraphyletic, because it excludes the genus Homo (We humans).

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