Here's something to try - on a piece of paper, draw a nested tree of lines branching down from a single vertical line at the top. This represents a crude evolutionary tree with time running top to bottom (although the orientation doesn't really matter). Now, count every line segment that ends with a split, and every line segment that just terminates without dividing. This will give you a rough count of common ancestors (line segments that end with a split, i.e. speciation), and non-ancestral species (line segments that terminate with no split, i.e. extinction without speciation).Based upon all those missing common ancestors that cant be found?
From this, you should get a rough idea of the proportion of fossil species we find that are likely to be common ancestors. It's probably higher than you might expect. The difficulty is in definitively identifying which are which when you only have a few random samples from the tree.
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