Greetings Professors, scientists and lecturers,
In phylogenetic tree analysis/evolutionary trees, most of them have only two branches. Why is that?
Though there are subdivision of small branches from the ancestors, everything divides into two but not three or four or five or six? Why?
Please clarify my query.
Thank you in advance
In trees, two species are more related if they have a more recent common ancestor and less related if they have a less recent common ancestor.
all the trees have had nice, clean branching patterns, with just two lineages (lines of descent) emerging from each branch point. However, you may see trees with a polytomy meaning a branch point that has three or more different species coming off of itt. In general, a polytomy shows where we don't have enough information to determine branching order.
TL;DR: true polytomies are very rare, and true polytomies cannot be methodologically identified.
Graphically, polytomies have become a convention for saying “relationship unknown at this node”. But that isn’t really answering your question, for which I have two major points.
A polytomy (more than two branches originating from a node) leading to three branches would be saying “at this point in time, one lineage gave rise to three simultaneously”. Can such a polytomy be “true”? If we think about speciation (in this case defined as a lineage undergoing genetic isolation), that process is going to take a very short amount of time relative to the length of the internode branch. For example, cats and dogs derive from a common ancestor living around 55 million years ago. But the duration of speciation might have only been thousands or tens of thousands of years; 0.01 - 0.1% of the branch length. For, say, weasels (who share a common ancestor with dogs and cats) to also speciate during that time-period is very unlikely (but not impossible). So, given these small time-spans, it is extremely unlikely that two speciation events would overlap.
But, this is probably more important to understand: even if there was a true polytomy, how would we ever identify it? Let’s say five lineages truly did originate simultaneously. A million years later, we do a phylogenetic analysis. That analysis will always be looking at the differences, by definition, between the lineages, but there is no evidence that could logically support a true polytomy - the method can only draw five terminal branches and four internal nodes (for a rooted phylogeny). What part of the algorithm would be saying “if you see this characteristic in the data, the answer is a polytomy”? It cannot. The purpose of all phylogenetic inference is to identify binary splits: telling the method to allow non-binary splits violates the logic of phylogenetic inference itself.
Given these points, it is always more rational to draw phylogenetic trees as bifurcating (having two branches from a node) for two reasons: true polytomies are extremely rare, and true polytomies cannot be identified through any data or method.
it explain taxonomy from ancestor to species level
it has five branches
Just wondering why polyotomies can't be a real phenomenon. For example a virus could give rise to multiple descendant types, each differing by one mutation from the parent type. These could then evolve into different subspecies. Why does evolution in trees have to be portrayed as binary?
Debra de Assis Rosa , you are right, polytomies are a real phenomenon. Equally, evolutionarily-isolated branches re-joining are a real phenomenon. But both are rarer than binary splits. As such, the bifurcating phylogenetic tree is a useful simplification of true evolutionary dynamics: it gives us a good general overview, but cannot be truly "correct". It is merely an inference - the best hypothesis, given the data and the model.
If you can find unambiguous signal in data which supports a true polytomy (and rejects the hypothesis "there is not enough signal to resolve node order"), and can invent a method to pick this out, then you will revolutionise the field of phylogenetics :)
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