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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?
- 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?
- 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.
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.
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 lifeóbut 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.
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