Patterns in macroevolution
You can think of patterns as "what happened when." All of the changes,
diversifications, and extinctions that happened over the course of life's
history are the patterns of macroevolution.
However, beyond the details of individual past events such as, when
the beetle radiation began or what the first flowers looked like biologists
are interested in general patterns that recur across the tree of life:
- Stasis: Many lineages on the tree of life exhibit stasis, which just means that
they don't change much for a long time, as shown in the figure to the right.
In fact, some lineages have changed so little for such a long time that they are
often called living fossils. Coelacanths comprise a fish lineage that branched off
of the tree near the base of the vertebrate
clade. Until 1938,
scientists thought that coelacanths went extinct 80 million years ago. But in 1938,
scientists discovered a living coelacanth from a population in the Indian Ocean that
looked very similar to its fossil ancestors. Hence, the coelacanth lineage exhibits
about 80 million years' worth of morphological stasis.
A coelacanth swimming near Sulawesi, Indonesia
Character change: Lineages can change quickly or slowly.
Character change can happen in a single direction, such as evolving additional segments,
or it can reverse itself by gaining and then losing segments. Changes can occur within a
single lineage or across several lineages. In the figure to the right, lineage A changes
rapidly but in no particular direction. Lineage B shows slower, directional change.
Trilobites, animals in the same clade as modern insects and crustaceans,
lived over 300 million years ago. As shown below, their fossil record
clearly suggests that several lineages underwent similar increases
in segment number over the course of millions of years.
- Lineage-splitting (or speciation): Patterns
of lineage-splitting can be identified by constructing and examining
a phylogeny. The phylogeny might reveal that a particular lineage
has undergone unusually frequent lineage-splitting, generating a "bushy"
tuft of branches on the tree (Clade A, below). It might reveal that a lineage has
an unusually low rate of lineage-splitting, represented by a long branch with very
few twigs coming off (Clade B, below). Or it might reveal that several lineages
experienced a burst of lineage-splitting at the same time (Clade C, below).
Extinction: Extinction is extremely important in the history of life. It can
be a frequent or rare event within a lineage, or it can occur simultaneously across many
lineages (mass extinction). Every lineage has some chance of becoming extinct, and
overwhelmingly, species have ended up in the losing slots on this roulette wheel: over
99% of the species that have ever lived on Earth have gone extinct. In this diagram, a
mass extinction cuts short the lifetimes of many species, and only three survive.