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Matt Johnson
Lecture Notes
ORNITHOLOGY
(Humboldt State Univ. WILDLIFE 365)
LECTURE 9 – SPECIES & SYSTEMATICS
I.
II.
Species
A. Species number varies over time. For birds it probably has been as
high as 50,000, but today it is a bit under 10,000 (close to 9,800).
B. Species changes by three processes:
1. Phyletic evolution - the gradual change of a single lineage (does
not alter species number but changes species identity) - probably
rare.
2. Speciation - the splitting of a single phyletic line into two or more
lineages (adds species)
3. Extinction - the elimination of species.
C. What is a species? There are two conflicting views:
1. Biological Species Concept (BSC) - "groups of interbreeding
natural populations that are reproductively isolated from other
such groups" (Earnst Mayr - BSC champion and ornithologist
from Harvard). Reproductive isolation (either real or inferred
from geography) is the criterion used to distinguish closely related
groups. E.g., Western vs. Florida Scrub Jay are different species
due to morphological and range differences, but various forms of
Fox sparrows are not because they are close enough to frequently
interbreed. OVERHEAD
2. Phylogentic Species Concept (PSC) - "smallest diagnosable
cluster of individuals within which there is a parental pattern of
ancestry and descent" (Cracraft 1983). Here, species are judged
based on behavioral, morphological, and molecular characters
subjected to a "cladistic" analysis.
Cladistics.
A. Terms:
1. Homologous - characters shared in two or more taxa that are
derived from a common ancestor. E.g., gull wing and penguin
flipper. Homologous traits result from a common ancestry.
2. Analogous - characters shared in to two or taxa that are not
derived from a common ancestor e.g., gull wing and butterfly
wing. Analogous traits result from "convergence", or a common
environment.
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B. The mantra: "Shared derived characteristics reveal evolutionary
relationships." This approach is called cladistic analysis. Figure 3-4
in Gill OVERHEAD
1. That is, homologous characteristics are the most useful in
reconstructing evolutionary relationships. We are related to apes
not because we have four limbs...turtles and frogs have four limbs
too. We're related to apes because we have upright posture,
opposable thumbs, large brain cases, binocular vision, etc. Its the
derived (advanced) characteristics that we share that unites us.
2. To reconstruct evolutionary relationships, we adopt the
assumption of parsimony, that is, the most likely evolutionary
scenario is the one that provides the simplest progression from
one taxa to another (fewest origins of new character states).
3. Convergence complicates our ability to reconstruct phylogenies.
Convergence makes characters appear to be shared and derived
that in fact are only analogous.
4. We assume that homology is more common than analogy in
biology. (the reason parsimony works is because homology is
more common than analogy; convergence is the exception and not
the rule)
5. So, if we look at ENOUGH characters, and find lots of shared
derived characters, we can assume they are shared due to a
common ancestry, and not due to convergence.
6. But how many characters in enough?
Gray wolf
Long doggy snout
Howls
Sharp canine teeth
Placental
Coyote
Long doggy snout
Howls
Sharp canine teeth
Placental
Tasmanian wolf
Long doggy snout
Howls
Sharp canine teeth
Marsupial
Kangaroo
Blunt snout
No howls
Chewing teeth
Marsupial
Now we know, the Tasmanian wolf is more closely related to a
kangaroo than a gray wolf, despite multiple analogous
similarities. If we looked at many more characters, the true
homologous similarities between Tasmanian wolves and
kangaroos would begin to outnumber (outweigh) the fewer
analogous traits between gray and Tasmanian wolves. (also, we
would recognize that some characters are better for reconstructing
phylogenies than others, e.g., howls vs. no howls is not
evolutionarily meaningful….but placental vs. marsupial is)
7. This is problem in bird evolution. Early birds have much in
common with therapods, there's no doubt of that. But is that due
to ancestry or convergence?
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8. Similar questions are scattered about the relationships of extant
birds as well. Vulture resemble raptors. But in some characters
they also resemble storks. Which similarities are due to ancestry,
and which are convergence? What about Flamingoes? Are they
storks with duck-like bills, are they ducks with stork-like legs?
9. To try to solve questions like these, systematists employ an
arsenal of information types to classify birds:
C. Systematic information:
1. Morphology.
a. Oldest approach. Best when use is restricted to conservative
characters, that is, characters that change slowly such as
skeletal features rather than characters that change quickly,
such as coloration.
b. Trouble is that the environment can change an animal’s
morphology, even some "conservative" characters. James
(1983) transplanted red-wing blackbirds from Colorado to
Minnesota and found that transplanted birds grew tarsi that
resembled the Minnesota red-wings more than their Colorado
parents. OVERHEAD
2. Behavior. Behavior is a character much like morphology. And
it’s not as flexible as you might think. Examples of behavioral
characters used to classify birds include nest building behaviors
(related birds build similar nests), song types (e.g., flycatchers),
scratching techniques.
3. Parasites. Feather parasites have co-evolved with their hosts.
Their branching patterns should thus be parallel.
4. Molecular techniques. These techniques are becoming
increasingly popular as new technologies make new approaches
possible. Explosion of molecular analyses in the literature in the
last 20 years. But remember that these are just more characters,
which can be homologous and analogous just like others.
a. Electrophoresis. Comparison of the composition of certain
proteins (especially egg whites and special allozymes) have
revealed some relationships. In these analyses, species with
shared protein structures are suggested to share a common
ancestry.
b. This approach now widely replaced by DNA-DNA
hybridization. OVERHEAD
i.
DNA of two species are isolated.
ii.
The paired strands are separated, then single strands
from each species are brought together to "hybridize."
iii.
The more closely the two species are related, the more
complementary their DNA stands will be, so the more
tightly they will hybridize.
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iv.
The hybridized strands are heated until they
disassociate. The higher the temp needed to
disassociate them, the more tightly they were bound
together, so the more closely related the species are to
one another. OVERHEAD
v.
Recent findings published by Sibley and Alquist
(1990) based on 25,000 DNA hybridizations of nearly
all bird families of the world suggest some major reclassifications. These are still questioned. Only time
will tell.
D. What we’re looking for…..
1. Monophyletic groups. A monophyletic group is one that contains
all the descendent taxa of a single common ancestor.
Monophyletic groups are the only ones that reveal much about
evolutionary relationships (history). Important to remember that
monophyly is a relative term. Passeriformes are monophyletic
(we’re pretty sure), and so are Corvids, and so are Acids, etc. If
you go back far enough in time, big groups of taxa can be
monophyletic. Indeed, life itself is probably monophyletic.
2. Polyphyletic groups are those that do not share a common
ancestor.
3. Paraphyletic groups are those that share a common ancestor, but
not all descendents are included in the group.