Lineage-splitting and extinction
Lineage-splitting and extinction have taken place often throughout the history of life they are not rare, one-time only events. That means that if scientists have a lot of information about a clade, they can calculate its rate of extinction or the rate of lineage-splitting (known as the rate of diversification).
The balance between a clade's rates of diversification and extinction determines whether or not the entire clade will go extinct. If, for example, your school's yodeling club is losing members faster than it can bring them in, it won't be around for long.
That means that looking back in the fossil record, we see many different trilobite and ammonite lineages, but few that lasted for long periods of geologic time. And of course, for both of these clades, in the long run, extinction happened more frequently than lineage-splitting, with the result that both are now entirely extinct.
Understanding rates of extinction and diversification also gives us a new way to look at diversity. For example, a clade might be large because it is generating new members (high rate of diversification), or it might be large because it rarely loses members (low rate of extinction).
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