Why Do Evolutionary Trees Typically Show Ancestor Branching Into Two New Species?

In the realm of evolutionary biology, one of the most common depictions used to visualize the relationships between species is the phylogenetic tree. These trees are simplified representations that illustrate the evolutionary history and relationships of different species. An important question often arises: why do evolutionary trees typically show an ancestor branching into two new species, rather than one or three?

Phylogenetic Trees and Simplification

Phylogenetic trees are designed to capture the change over time and the complex relationships between species. These trees are not always strictly binary, where each branch splits into exactly two daughter branches. However, the common depiction of branching as a Y-shape primarily represents a simplified explanation of speciation events. Typically, a species exists, and then a population evolves away from the norm, leading to the formation of a new species. Later, another population of the original species may branch off, creating the Y-shape.

Binary Branching vs. Polytopies

It is important to note that not all phylogenies are strictly binary. In some cases, internal nodes with more than two child nodes are called polytomies. This is not only due to the inherent complexity of the evolutionary process but also due to limitations in the data and methods used to construct these trees. For example, highly conserved genetic data, such as histone amino acid sequences, may result in polytopies. Alternatively, rapid diversification events can make binary branching impossible to resolve accurately.

Speciation and Population Isolation

A species can indeed diverge into more than two daughter sets, and this has been observed in nature. However, it is less common for simultaneous speciation events to occur from the same ancestral population in the same location. If a species becomes genetically isolated into three separate populations, the emergence of three different species is perfectly plausible. Nevertheless, the odds of this happening simultaneously are extremely low. Even if two new species and the old species simultaneously branched off, it would be represented as two separate two-branch divisions rather than one into three.

Understanding Polytopies

Many evolutionary trees use the shorthand of polytopies to indicate uncertainty in the branching order of certain lineages. This is a widely accepted practice because resolving the exact branching order can be challenging. In such cases, multiple lineages are brought together in a shared point to reflect the current state of knowledge.

Conclusion

In summary, while binary branching is a common depiction in phylogenetic trees, it is not always accurate. The complexity of evolutionary processes and the limitations in data and methods can lead to polytomies. The typical Y-shape in phylogenetic trees is a simplified representation to illustrate speciation events. Understanding these complexities is crucial for accurate interpretation and construction of phylogenetic trees.

Keywords: evolutionary trees, phylogenetic trees, speciation events