1-3-1-7-1 The Pace
1-3-1-7-2 Diversity in clades
Imagine that you've traveled back in time to around 350 million years ago, give or take 50 million years. Your goal is to check out the cool insects living at this point in time. You see a lot of little insects that look like modern silverfish - no big deal.
But something interesting and significant is happening that you can't see - a lineage has split into two. One of these newly isolated lineages will eventually give rise to about 400 extant species that look a lot like the ancient insects you see. But the other lineage will give rise to millions of extant insect species, the bulk of animal life on Earth today. Why is there such a big difference in diversity between these two lineages? After all, they were indistinguishable 350 million years ago...
Why would one lineage lead to millions of species
and the other to only 400?
Being in the right place at the right time is a reason that one clade might be more diverse than another.
Download this graphic from the Image library.
a. Explosion: About 530 million years ago, a huge variety of marine animals suddenly burst onto the evolutionary scene. (Of course, "suddenly," in geological terms, means in perhaps 10 million years). These animals had a variety of new body forms that evolution has been using to produce "spin-offs" ever since, such as these representatives from the Burgess Shale.
b. Extinction: About 225 million years ago, over 90% of the species alive at the time went extinct in fewer than 10 million years. Some groups that were dominant before the extinction never recovered. The cause of this extinction is the subject of much debate, but of equal significance is that it set the stage for a massive diversification of taxa that filled the empty niches.