Download Evolution Week 11a File

Major Evolutionary Patterns
Aims
• Discuss major evolutionary patterns
• The common occurrence of certain
features or sets of features
• Evolutionary implications
• Investigation with cladistics
Forms of evolution
• There are a number of types of
evolutionary patterns and trends that
repeatedly occur and can be seen in both
extant taxa and the fossil record.
• These do not generally have ‘strict’
definitions and are given titles as a way of
labeling them for convenience.
• Like many things in biology, these form
more of a continuum than discreet states.
Forms of evolution
• Divergence: evolution away from a
common form.
• Convergence: evolution of unrelated taxa
towards a common form.
• Parallelism: evolution of similar forms in
similar ways by related taxa.
• Iterative evolution: repeated evolution of a
structure of form (i.e. over time, multiple
convergences).
Divergence
What is convergence?
• Convergent evolution is where two (or more)
species (or clades) who do not have a
recent shared common ancestry
independently evolve very similar structures
to ‘solve’ a similar problem.
• It is generally ecologically or functionally
driven.
• Before cladistics, convergent structures
were often thought to be homologous and
thus led to erroneous conclusions about
systematic relationships.
Detecting Convergence
• In a cladistic context convergence can be
easy to detect. Two taxa with superficially
similar structures that are not closely
related can be assumed to have gained
the feature convergently.
• Outside of cladistics, convergence may be
detected by how the structure has been
formed.
• For example to elongate the snout
phytosaurs elongated the premaxilla, and
crocodilians the maxilla.
Crocs & Phytosaurs
• Although both are
archosaurs and
therefore relatively
closely related,
they do not share a
recent common
ancestor with
elongate jaws.
• The feature of
elongate jaws has
evolved
convergently.
Derived theropods
Spinosaurs
Basal theropods
Pterosaurs
Crocodilians
Aetosaurs
Phytosaurs
Famous examples of convergence
Gliding
Fast bipedal herbivores
Leg loss in aquatic and fossorial herptiles
Multiple effects
• Convergence can lead to a whole suite of
characters changing in unison, even across
the entire anatomy of the animal.
• In the case of the formicivores, digging
requires adaptations to the claws, hand
shape, ulna and humerus, ribs, and
vertebrae, while the elongate snout is
usually accompanied by a reduction in the
teeth (number, size and complexity) and
the development of a long tongue.
Formicivores
Marsupial Convergence
• Marsupials (both extant and extinct)
provide an interesting case of multiple
convergences.
• With their isolation in Australasia, multiple
lineages of marsupials have evolved to
mimic eutherian mammals in other parts of
the world to fill certain ecological niches.
• Some cases are unique (kangaroos as
grazers) and others are only loosely similar
(e.g. the numbat) while others show
striking similarities (e.g. marsupial moles).
Eutherians vs Marsupials
Parallelism
• This is the evolution of parallel forms from
homologous structures in animals with a
shared ancestry.
• The structures do not get much more similar
(convergence) or more dissimilar (divergence)
but evolve in similar trajectories.
• These traits have evolved independently, but
their origin and development is facilitated by
their underlying similar developmental
pathways making them relatively easy to
acquire, and to evolve multiple times.
Spines in mammals
Parallelism in Archosaurs
Iterative evolution
• This is the repeated evolution of a
structure or form by a lineage in response
to a recurring evolutionary pressure.
• It essentially represents multiple lineages
over time acquiring similar (or even
identical) characteristics.
Quadrupedality
• While the earliest dinosaurs
were bipeds, at least 4
separate lineages (3 of the
ornithischians) adopted
facultative (blue) or obligate
(red) quadrupedaility as an
adaptation towards larger size.
Quadrupedal Ornithischians
Sabers
Changes
• Obviously, these terms are dependent on
how the changes are perceived or the
evolutionary relationships between the
taxa in question.
• These terms are not mutually exclusive –
one could consider the archosaurian
filaments to be iterative (repeated
modification of scales into filaments at
different times) or parallelism (related taxa
evolving filaments).
Differing phylogenies / information
• If pterosaurs are
derived archosaurs
then their body
covering would likely
be considered
parallelism with those
of dinosaurs. If they
are more basal this
would probably be
termed convergence.
Dinosaurs
Dinosauromorphs
Pterosaurs ?
Crocodiles
Euparkeria
Pterosaurs ?
• If basal theropods,
sauropodomorphs or
dinosauromoprhs
are ever discovered
with body fibers then
these features might
be shown to be truly
homologous and not
convergent, or a
result of parallelism.
Derived theropods
Basal theropods
Sauropodomorphs
Ornithischians
Dinosauromoprhs
Pterosaurs
Vestigiality
• Vestigiality is the massive reduction and even
loss of non-functioning systems.
• Since these are not being used they are under
neutral selection, or if they are a hindrance may
be actively selected against.
• Determining if systems (be it biochemical,
anatomcial, behavioural etc.) are truly nonfunctional can be very hard.
• Classic examples of vestigiality (such as the
legs of whales) have later been found to have a
function.
Alverezsaurs
Vestigial digits
• Derived alvarezsauroids
have a single hypertropied
manual digit (II) and two
much reduced fingers.
• However the recently
discovered Linhenykus
suggests that these other
digits are vestigial and under
neutral selection.
Atavism
• Atavisims are
reversions to ‘ancestral’
conditions. The
reemergence of
characteristics through
developmental
anomalies but which
are coded for in the
genome of the species.
In conclusion
• These labels are primarily one of
convenience – there is no strict definition
or rule as to what is considered parallelism
or iterative evolution.
• However, they are important and can be
used to describe complex patterns simply.
• Of course they also demonstrate the
complexity of evolution and the way
systems and organisms evolve in
response to ecological challenges.
Further reading
• http://www.zo.utexas.edu/courses/THOC/
Convergence.html
• http://www.nature.com/scitable/topicpage/
atavism-embryology-development-andevolution-843