Evolutionary processes (anagenesis
and cladogenesis) produce a pattern
phylogeny: the history of
organismal evolution
[cf.
genealogy: the history of a
single family]
Diagrams of phylogeny
resemble trees
living
species are the terminal twigs
extinct
species are the interior twigs
genera,
families,
orders are successively older & more
inclusive branches & limbs
Systematics:
the science of organizing the history of organismal evolution
science of ordering
Identification: recognizing the
place of an organisms in an existing classification
E.g.,
use of dichotomous keys to identify organisms
Taxonomy
(Nomenclature): assigning scientific
names according to legal rules
E.g.,
use of ICZN Green Book
in Zoology: Priority & Stability
Classification: determining the evolutionary
relationships of organisms
A "Natural Classification"
will accurately reflect phylogeny
Classification
should
be a hypothesis of evolutionary relationships
Alternative classifications are possible (and widely used):
An arbitrary classification cannot help us understand
evolution
Ex: If all 'marine mammals' combined in single
Order,
implication
that aquatic adaptations evolved only once
If
seals (Pinnipedia), toothed (Odontoceti),
& baleen whales (Mysticeti)
evolved separately,
understand differences in their physiology.
Traditional taxonomy
emphasized analysis of similarity
Modern taxonomy (Phylogenetic Analysis)
emphasizes cladistic ancestry
Analysis of distribution of shared character
states:
Evolutionary Genetics provides
protein & DNA sequence allelic variants
Homologous characters 'similar'
because of descent from common ancestor
Therefore useful for classification
bat wing vs. kangaroo arm: from Therapsid forelimb
ostrich 'wing' vs. penguin 'wing': from Archeopteryx-like
wing
bat
vs. bird forelimb
from reptile forelimb (cf. analogous wing structures)
INSIGHT: Homologous
characters evolve over
time =>
***Homologous
characters
need
not look / function alike**
Characters unchanged from ancestors: 'ancestral'
or plesiomorphic
Characters changed in descendants: 'derived'
or apomorphic
[Avoid 'primitive'
and 'advanced': false
connotations]
Homologous
characters
of
two types:
Shared ancestral characters:
similar to each other and ancestor
'ordinary homologies': symplesiomorphic
characters
Usual sense of 'homology" taught in introductory
courses
Ex.: scales in lizards & crocodiles evolved
from Diapsida
Shared derived characters:
similar to each other & different from ancestor
'special homologies': synapomorphic characters
Ex.: carnassial pair (P4/M1) a synapomorphy
of Canidae &
Felidae
derived
from molariform teeth in Creodonta
Characters unique to particular taxa: autapomorphic
Ex.: wings in Chiroptera
unique among mammals
HOMEWORK: Are there
such things as auto-plesiomorphic characters? Give an
example.
The nature of homology
depending on the taxa under analysis
Ex.: "Hair"
Among turtle, lizard, bird, & cat:
unique
character of Mammalia
Among turtle, lizard, cat, & kangaroo:
shared
derived character of therian Synapsida
Among kangaroo, bat, cat, &
whale: shared ancestral
character of terrestrial non-cetaceans
Also:
wings an autapomorphy of the order Chiroptera [they evolved once]
wings also a synapomorphy of suborders Mega-
& Microchiroptera [they are related]
Use of analogous characters
lead to polyphyletic
groups:
loosely: groups that do not have a common ancestor
[but
everything
has a common ancestor]
Accurately: groups that do not include
their common ancestor
Ex.: Pinnipedia
(marine carnivores) once thought polyphyletic
walruses & "eared" sea lions related to bears,
earless ("true") seals related to weasels
[This turns out not
so]
Polyphyletic groups often defined by "absence"
characters
Amphibia: scaleless
tetrapods
Earliest
terrestrial Tetrapoda (Devonian Amphibia) had scales
Modern Lissamphibia [salamanders
(Caudata), frogs (Anura), & caecilians (Gymnophiona)
secondarily scaleless [adaptation for dermal respiration]
& possibly independent lineages
Analogous characters & polyphyletic groups
rejected as basis of
taxonomy
Use of homologous characters
results in monophyletic
groups:
loosely, groups descended from single common ancestor
accurately, groups that include common ancestor of
group
Use
of
shared ancestral characters results in paraphyletic groups:
a
monophyletic group that includes ancestor and
some
but
not all of its descendants.
Grade: group
defined by combination of shared ancestral & derived
characters
describes level of biological organization
Ex.: among traditional
taxonomic Classes of Vertebrata
Agnatha: jawless
descendants of first vertebrates
Comprises hagfish (Myxiniformes)
& lampreys (Petromyzontiformes)
[jawlessness may be secondary adaptation for parasitism]
gnathostomous
(jawed) relatives of Craniata (Chondrichthyes, "fish")
not included
Osteicthyes: fish
with bony skeletons
amniotic relatives of
Sarcopterygia (lungfish)
not included
Reptilia: scaly tetrapod descendants of first
amniotes
feathery diapsid &
furry synapsid relatives not included
Paraphyletic groups
in modern Phylogenetic Taxonomy ("Cladistics")
Classification should reflect relationship
only, not similarity
Relationship determined objectively, e.g., by
molecular methods
Organismal similarity & differences to be explained
Grades subjective
& perpetuate
evolutionary myths
Which characters
important?
'Scaly' reptiles leftover after removal of 'feathery'
birds & 'hairy' mammals
Why
not
take out "finny" Icthyosauria (marine
reptiles)?
"Reptiles & lungfish aren't variable. Their body plans limited evolutionary
possibilities."
"Dinosaurs more like reptiles than birds. Dinosaurs
are reptiles."
"Teeth in modern mammals evolved from toothless edentate or
insectivorous ancestors."
*** Grades units for evolutionary analysis ***
Ex.: Evolution of pagophilic (ice-breeding) behavior in
phocid seals (Perry et al. 1995; Carr & Perry
1997)
Phoca vitulina (harbor seals) breed on land,
other
seals
(e.g., Phoca groenlandica & Halichoerus grypus)
breed & nurse young
on ice
Traditional taxonomy
suggests ice-breeding has evolved several times:
Separate
explanations
for each pagophilic species required:
e.g., ice-breeding a polar bear avoidance behavior
Phylogenetic
taxonomy (supported by molecular analysis)
suggests ice-breeding is ancestral:
Phoca groenlandica a separate genus Pagophilus
groenlandicus,
&
more
closely related to ice-breeding seals like Cystophora
=> Phoca shows recent
evolutionary shift to terrestrial breeding,
special
explanations
for ancestral pagophilic behavior not required
Exclusive use of shared
derived characters results in holophyletic
groups:
monophyletic
group that includes the ancestor and all of its
descendants
Clade: group defined by shared
derived character (s)
describes complete ancestor-descendant lineage
Ex.: Among traditional Classes of Vertebrata
* Placodermi: gnathostomes with hinged craniovertebral joint
in skull [*extinct]
Chondricthyes: gnathostomes
with hyostylic jaw
suspension
Mammalia: cynodont therapsids
with dentary-squamosal jaw
suspension & hair
Aves: Archosauria with
feathers
Saurischia: amniotes
with diapsid skull includes both Mammalia & Aves
Holophyletic
groups (called Monophyletic) are accepted by cladistic
taxonomy,
Contrast traditional versus cladistic
taxonomies of Primates
Hominidae (Homo)
separated from Pongidae (Pan,
Gorilla, Pongo) (Great Apes)
perceived similarity of Apes &
assumed distinctiveness of Homo
emphasized,
relationship of Pan & Homo obscured
Versus Homininae
groups Homo, Pan, & Gorilla
Homo & Pan grouped as Homini (or Panini)
relationship emphasized
Does this obscure ape similarities?
Text material © 2022 by Steven M. Carr