What Is A Cladogram?
A Cladogram is a type of diagram that shows hypothetical relationship between groups of organisms, including their common ancestors. The term “cladogram” comes from the Greek words clados, which means “branch,” and gramma, which means “character.” The diagram resembles the branches of a tree that extend outward from a trunk. However, the shape of the cladogram isn’t necessarily vertical. The diagram can branch from the side, top, bottom, or center. Cladograms can be very simple, comparing only a few groups of organisms, or highly complex, potentially classifying all forms of life. However, cladograms are more often used to classify animals than other forms of life.
A Cladogram consists of the organisms being studied, lines, and nodes where those lines cross. The lines represent evolutionary time, or a series of organisms that lead to the population it connects to. Nodes represent common ancestors between species.
There are many shapes of cladograms but they all have lines that branch off from other lines. The lines can be traced back to where they branch off. These branching off points represent a hypothetical ancestor (not an actual entity) which can be inferred to exhibit the traits shared among the terminal taxa above it.
Cladograms indicate common ancestry, but they do not indicate the amount of evolutionary time between an ancestor and a descendant group. While the lines of a cladogram may be different lengths, these lengths have no meaning. In contrast, the branch lengths of a phylogram are proportional with respect to evolutionary time. So, a long branch indicates a longer time than a shorter branch.
How to Construct a Cladogram
Cladograms are based on comparing similarities and differences between groups of organisms. So, a cladogram could be constructed to describe relationships between different types of animals, but not between individuals. Follow these simple steps to construct a cladogram:
- Identify separate groups. For example, the groups could be cats, dogs, birds, reptiles, and fish.
- Make a list or table of characteristics. Only list characteristics that can be inherited and not those that are influenced by environmental or other factors. Examples include vertebrae, hair/fur, feathers, egg shells, four limbs. Continue listing traits until you have one trait common to all groups and enough differences between other groups to make a diagram.
- It’s helpful to group organisms before drawing the cladogram. A Venn diagram is useful because it shows sets, but you can simply list groups. For example; Cats and dogs both are vertebrates with fur, four limbs, and amniotic eggs. Birds and reptiles are vertebrates that lay shelled eggs and have four limbs. Fish are vertebrates that have eggs, but lack four limbs.
- Draw the cladogram. The shared common trait is the root. All of the animals in the example are vertebrates. The first node leads to the branch of organisms with the least in common with the other groups (fish). The next node off the trunk leads to another node that branches off to reptiles and birds. The final node off the trunk branches to cats and dogs. You may be wondering how to decide whether the second node leads to reptiles/birds or to cats/dogs. The reason reptiles/birds follow fish is that they lay eggs. The cladogram hypothesizes the transition from shelled eggs to amniotic eggs occurred during evolution. Sometimes a hypothesis may be incorrect, which is why modern cladograms are based on genetics rather than morphology.
Facts About Cladogram
- A Cladogram is a type of diagram that shows hypothetical relationship between groups of organisms, including their common ancestors.
- Cladograms isolate certain aspects by ignoring properties shared in common and highlighting differences. Also, some types of Cladogram may help discover selection bias in the data.
- There are many shapes of Cladograms but they all have lines that branch off from other lines. The lines can be traced back to where they branch off. These branching off points represent a hypothetical ancestor (not an actual entity) which can be inferred to exhibit the traits shared among the terminal taxa above it.
- A Cladogram can be simple, comparing only two or three groups of organisms or it can be enormously complex and contain all the known forms of life.
- Some Cladograms chose to show extinct species, while others omit them. Any particular Cladogram is formulated specifically for the use it is needed.
What Is A Phylogenetic Tree?
A phylogenetic tree (also phylogeny or evolutionary tree) is a branching diagram or a tree showing the evolutionary relationships among various biological species or other entities based upon similarities and differences in their physical or genetic characteristics. All life on Earth is part of a single phylogenetic tree, indicating common ancestry. Phylogenetic relationships provide information on shared ancestry but not necessarily on how organisms are similar or different.
A phylogenetic tree can be read like a map of evolutionary history. Many phylogenetic trees have a single lineage at the base representing a common ancestor. Scientists call such trees rooted, which means there is a single ancestral lineage (typically drawn from the bottom or left) to which all organisms represented in the diagram relate.
In this tree, the vertical branches represent a lineage, which is a taxon, shown at the tip, and all its ancestors. The nodes are where lineages diverge, representing a speciation event from a common ancestor. The trunk at the base of the tree is actually called the root, and the root node represents the most recent common ancestor of all of the taxa represented on the tree. Time in this particular style of tree is represented vertically, proceeding from the oldest pictured here at the bottom to the most recent at the top.
Construction Of A Phylogenetic Tree
There are currently two main categories of tree-building methods, each having advantages and limitations. The first category is based on discrete characters, which are molecular sequences from individual taxa. The basic assumption is that characters at corresponding positions in a multiple sequence alignment are homologous among the sequences involved. Therefore, the character states of the common ancestor can be traced from this dataset. Another assumption is that each character evolves independently and is therefore treated as an individual evolutionary unit. The second category of phylogenetic methods is based on distance, which is the amount of dissimilarity between pairs of sequences, computed on the basis of sequence alignment. The distance-based methods assume that all sequences involved are homologous and that tree branches are additive, meaning that the distance between two taxa equals the sum of all branch lengths connecting them.
Facts About Phylogenetic Tree
- A Phylogenetic tree also referred to as phylogeny, is a diagram that depicts the lines of evolutionary relationship of different species, organisms or genes from a common ancestor.
- Phylogenetic relationships provide information on shared ancestry but not necessarily on how organisms are similar or different.
- Phylogenetic analysis is important for gathering information on biological diversity, genetic classifications, as well as learning developmental events that occur during evolution.
- In a Phylogenetic tree, the species or groups of interest are found at the tips of lines referred to as branches. The tips of the tree represent groups of descendants’ species and the nodes on the tree represent the common ancestors of those descendants.
- Phylogenetic trees compare organisms over evolutionary time and the amount of change that has occurred over time to figure out the relationships.
Difference Between Cladogram And Phylogenetic Tree In Tabular Form
|ELEMENTS OF COMPARISON
|A Cladogram is a diagram used in cladiastics, it shows hypothetical relationships between groups of organisms.
|A Phylogenetic tree is a diagram used to depict evolutionary relationships among organisms or group of organisms.
|A Cladogram consists of the organisms being studied, lines and nodes where those lines cross.
|In a Phylogenetic tree, the species or groups of interest are found at the tips of lines referred to as branches.
|Cladograms are concerned with the way organisms are related to common ancestors through shared characteristics.
|Phylogenetic trees compare organisms over evolutionary time and the amount of change that has occurred over time to figure out the relationships.
|Cladogram is based on the morphological characters of the organisms.
|Phylogenetic trees are based on morphological characters and genetic relationship of the organism.
|Cladogram is drawn with equal-length. The length of the branch does not represent an evolutionary distance.
|Branch length of a Phylogenetic tree indicates the evolutionary distance.
|Cladogram represents a hypothetical evolutionary history of organisms.
|To some extend Phylogenetic tree indicates the true evolutionary history of organisms.
|The shape of the Cladogram shows the relatedness among a group of organisms.
|The distance of the branch depends on the amount of inferred evolutionary change.
|Originally, Cladograms were organized based on morphological features but modern cladograms are more often based on genetic and molecular data.
|Phylogenetic trees can be drawn in various equivalent styles. Rotating a tree about its branch points does not change the information it carries.
|Relation Among Species
|Cladograms have lines that branch off other lines. These branching off points represent a hypothetical ancestor (not an actual entity) which can be inferred to exhibit the traits shared among the terminal taxa above it.
|In Phylogenetic trees, two species are more related if they have a more recent ancestor and less related if they have a less recent common ancestor.