Encyclopedia of Evolution.pdf - Online Reading Center

Chicxulub
Pritchard, J., and Dolph Schluter. “Declining competition during
character displacement: Summoning the ghost of competition
past.” Evolutionary Ecology Research 3 (2000): 209–220.
Rundle, H. D., S. M. Vamosi, and Dolph Schluter. “Experimental test
of predation’s effect on divergent selection during character displacement
in sticklebacks.” Proceedings of the National Academy
of Sciences (USA) 100 (2003): 14,943–14,948.
———. “Experimental evidence that competition promotes divergence
in adaptive radiation.” Science 266 (1994): 798–801.
———. “Ecological character displacement in adaptive radiation.”
American Naturalist, supplement, 156 (2000): S4–S16.
———. “Frequency dependent natural selection during character displacement
in sticklebacks.” Evolution 57 (2003): 1,142–1,150.
Vamosi, S. M., and Dolph Schluter. “Character shifts in defensive
armor of sympatric sticklebacks.” Evolution 58 (2004): 376–385.
Chicxulub See asteroids and comets.
chordates See invertebrates, evolution of.
cladistics Cladistics is a technique that classifies organisms
on the basis of shared derived characters, which are similarities
that they have inherited from a common ancestor. This
method assumes that two species with a greater number of
shared derived characters are more closely related than two
species that have fewer shared derived characters. The common
ancestor of two species represents a branch point (Greek
clados means branch) in the evolutionary histories of these species.
Cladistic analysis can be applied to any set of evolutionary
lineages, not just to species. Cladistic analysis is also called
phylogenetic analysis. This method was developed in 1950 by
German entomologist Willi Hennig. A similar method, which
was developed by American botanist Warren Wagner, contributed
methodologies that are now part of cladistic analysis.
Cladistic analysis generates clusters of branches entirely
on the basis of the characters of species. The diagram that
represents the branching pattern is called a cladogram. The
older systems of classification, in contrast, attempt to reconstruct
the evolutionary history of the organisms in question.
By making no a priori evolutionary assumptions, cladistic
analysis generates a set of possible evolutionary relationships
and allows the investigator to choose the one that most
closely matches the data of modern organisms or, if known,
the data from the fossil record.
Grouping the species objectively on the basis of similarities
has two advantages over older systems of classification.
• All of the species are placed at the tips of the branches,
rather than at branch points. Modern green algae and
modern flowering plants are descendants of a common
ancestor, but the lineage leading to green algae has undergone
less evolutionary modification than the lineage leading
to flowering plants. Green algae and flowering plants
are at separate branch tips of the cladogram. The ancestral
population of organisms from which both green algae and
flowering plants are descended no longer exists.
• Cladistics allows a more objective approach to classification
than the traditional systems. The cladogram can be
generated by mathematical rules. This is often done by a
computer. The investigator’s preferences do not influence
the results.
As a result of this approach, cladists will not say that one
modern group evolved from another modern group. Consider
the example of green algae and flowering plants. If one could
actually have looked at their common ancestor, it would have
looked like green algae. Cladists will not generally say that
flowering plants “evolved from” green algae. They will say
that green algae and flowering plants evolved from a common
ancestor.
Choosing the traits
The investigator must choose which traits are to be used in
the analysis. Some traits are ancestral (plesiomorphies) and
some are derived (apomorphies). There are four categories of
traits:
1. Shared ancestral traits (symplesiomorphy). Ancestral
traits are shared by almost all of the species in the analysis;
ancestral traits cannot be used to distinguish among the species.
In a cladistic analysis of flowering plants, for example,
chlorophyll would not be a useful trait, since almost all
flowering plants have chlorophyll in their leaves, a trait they
inherited from their common ancestor (see angiosperms,
evolution of). In human evolution, most hominin characteristics
are symplesiomorphies.
2. Unique traits (autapomorphy). Characteristics that are
totally unique to one species also cannot be used to distinguish
among the species. Consider a cladistic analysis of flowering
plants in which one of the species is a parasitic plant,
lacking chlorophyll in its leaves. The lack of chlorophyll is
not a useful trait for this analysis, since only one of the species
has this trait. An example from human evolution would
be the unique skull characteristics, such as the projecting face
and strong bite, of the Neandertals.
3. Convergences (homoplasy). In many cases, a trait
may evolve more than once (see convergence). This could
happen in either of two ways. First, the trait may have
evolved into a more advanced form, then reverted back to
the primitive form, during the course of evolution. This
would cause a species to resemble a distantly related species
that had retained the ancestral form all along. Second, two
species with separate origins may have evolved the same
trait. Evolutionary biologist Caro-Beth Stewart calls homoplasy
the “ultimate trickster” of cladistics. Consider examples
of homoplasy from plants, from nonhuman animals,
and from humans:
• A cladistic analysis of flowering plants may include two
parasitic plant species that lack chlorophyll, one of them a
parasitic relative of heath plants (such as genus Pterospora),
the other a parasitic relative of morning glories (genus
Cuscuta). The common ancestor of these two plants had
chlorophyll. The chlorophyll was lost in two separate evolutionary
events. It would be incorrect to classify Pterospora
and Cuscuta together into one group, with the other
heath and morning glory plants into another group.