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]
[[[ 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
taxonomy
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 cladistic
taxonomy
of
Primates
Homo, Pan, & Gorilla grouped as Homininae
Homo & Pan grouped as Homini (or Panini)
relationship emphasized
Does this obscure ape similarities?
Text material © 2020 by Steven M. Carr