Why is an Outgroup needed for Phylogenetic Analysis? 

Outgroups are significant in studying cladistics or phylogenetics (that describe the evolutionary relationship between different organisms). Further, it is also important to understand the differences and similarities between different organisms. 

" Difference between outgroup and ingroup"


Cladistics is the most common method that incorporates the information into phylogenetic gene trees. It is defined as a classification method that helps categorize taxa hierarchically into the discrete form of sets and subsets. This categorization is broadly used in biological systematics.  Will Hennig, the German Entomologist, made the methods of cladistics that were called by the name of Phylogenetic systematics in the later period. These methods are useful in determining the relationship between different taxa. The closely related taxa are called sister groups. They are hypothesized through the character analysis, which could be physiological, ecological, molecular, and behavioral. The difference between the terms apomorphic and plesiomorphic present in the cladograms’ base. Apomorphic represents a character state that is derived or specialized, whereas plesiomorphy represents an ancestral trait. Varying groups are used for defining the relationship that exists between different organisms. 

Importance of Phylogeny 

A phylogeny is a group of evolutionary evidence of genetically based similar organisms of the ancestor species and pre-living or existing species. Phylogeny provides a sequence and order of the ancestors and their prehistorical evidence. This phylogenetics helps in describing the relationship between the organisms and their ancestors. 

Note: The use of tree structures normally represents this phylogenetic information. 

Phylogenetic tree 

" Phylogenetic tree "

The Phylogenetic tree contains various groups of branches that indicate the different subgroups and their recent common ancestors, and it is a common point at the bottom of the tree. The large and old phylogeny has multiple branches, and the short and young phylogeny have few branches. Thus, it helps to determine the experimental ancestors of both younger and older individuals. 

The branch of the cladogram does not represent the genetic changes or other lengths of time. Instead, the branch that is present in the phylogenetic tree represents a length change in its structure.  


The ingroup is defined as a taxa group that is specifically considered in identifying evolutionary relationships. The taxa that are specified in an ingroup are related closely. Thus, they are recognized as sister groups, and they tend to share a common ancestor. So, taxa in an ingroup are descendants that are characterized to divide from the same node in the cladogram. The cladogram is a diagram used in cladistics to represent the relationships of the organisms. Cladograms are diagrams that are used to show the relationship between the group of species known as clades. The representation of these relations of cladograms has reconstructed the phylogeny of the taxon. Cladograms are normally constructed by the grouped organisms based on their shared characteristics. 

Note: Taxon in ingroup are descendants that split the same node in the cladogram. 

Note: The clade is a grouping of common ancestors and descendants. 

”Example of cladogram”


The outgroup is considered a part of the cladistics that provides the relationship between the distantly related organisms. This information specifically will act as the reference group for identifying the evolutionary relationship between the other group of organisms called ingroups. The term outgroup was coined in the early period of the 1970s. It stems from the tree’s base as it represents the distantly related taxon to the group of interests.  In short, outgroups do not belong to the category of groups that comes under the evolutionary relationships’ investigation; rather, it acts as a comparison point for the category of ingroup when phylogeny is being rooted. 

Significance of outgroups 

The ingroup and outgroup are important concepts in the subject of biology. The determinations of these groups provide a fundamental structure and meaning of the social information and also helps in providing fundamental guidelines, experiment condition rules, and regulations for social behavior. 

Outgroup choice 

The outgroup that is particularly chosen is hypothesized to be less closely related to the category of the ingroup than the relation of the ingroup to itself. The conclusion derived in terms of evolution from these types of relationships is that the species of outgroup has a common ancestor with the ingroup that is basically considered to be older than the ingroup’s common ancestor. Choice of outgroups has the ability to modify the phylogeny’s topology. Thus, in the analysis of cladistics, phylogeneticists tend to use multiple outgroups. Using multiple outgroups is useful in providing a more robust form of phylogeny, which is capable of buffering against poor candidates of outgroup and testing the hypothesized monopoly of the ingroup. 

In order to qualify as an outgroup, a specific taxon sampling must be able to satisfy the following two characteristics: 

  • It must not represent the ingroup by its group membership. 
  • It must be related to the category of ingroup, closely enough for thoughtful comparisons to the ingroup. 


  1. Placental mammals, Great Apes, Chordates, and  Angiosperms are examples of the ingroup. 
  2. Gibbons, Echinoderms, Marsupials, and Gymnosperms are some examples of the outgroup. 

In each example provided above, a phylogeny of organisms in the ingroup may be rooted by scoring the same character states for one or more outgroup members. 

Context and Applications 

This topic is significant in the professional exams for both undergraduate and graduate courses and research side, especially for: 

  • Bachelors in zoology 
  • Bachelors in Genetics 
  • Bachelors in General Biology 
  • Masters in Genetics  
  • Masters in General Biology 
  • Masters in zoology 
  • Phylogenetic tree 
  • General Biology 
  • Evolution 

Practice Problem 

Question: What are the features of a cladogram? 

Solution: The cladogram is made up of the nodes, roots, and some other groups. This root shows the ancestors of organisms in the cladogram. In cladogram, the nodes are two types; these are internal nodes and terminal nodes. Internal nodes in the cladogram represent the point of divergence; the terminal node is the ending part of the cladogram. Outgroups may also involve in cladogram to compare the other similar group of organisms. The cladogram also contains a sister group in structure. These are closely related to the sister species. 

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