Diversity in clades
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?
- Opportunity knocks: One possibility is that the now-diverse lineage happened to be in the right place at the right time. The environment presented opportunities, and the lineage was able to take advantage of them. What sorts of factors in the environment might encourage diversification?
All of these factors might be at work in some situations. Consider a plant-eating insect that colonizes a tropical island. On its mainland home, the insect's population size and range of resources is constrained by other species competing for the same resources. But the lack of similar species on the island means open niches and reduced competition from other species. Further, the island offers new kinds of food in the form of plants that the insect has never seen before. Selection might allow some insects to specialize on these new plants. Hanging around each kind of plant might mean that the insects get to mate with insects on a different plant less frequently, encouraging reproductive isolation. All of these factors can drive diversification but only if the population has the genetic variation to take advantage of the opportunities presented by the environment.
- The environment may have offered opportunities for specialization.
- A fragmented environment might make reproductive isolation likely.
- The environment may have provided a release from competition with other insects.
Being in the right place at the right time is a reason that one clade might be more diverse than another.
- Adaptive Radiation: If all of this diversification happens in a short amount of time, it is often referred to as an adaptive radiation. Although biologists have different standards for defining an adaptive radiation, it generally means an event in which a lineage rapidly diversifies, with the newly formed lineages evolving different adaptations. The rapid diversification of mammals shown below may constitute an adaptive radiation.
- Historical changes in diversity: Many events have left their marks on the diversity of life on Earth, pruning or growing the tree of life, but a few stand out as unusually important:
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.