BI111 Lecture Notes - Lecture 16: Paraphyly, Convergent Evolution, Marchantiophyta
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BI111 – Week 5 – Phylogeny and Plants
Systematics
Twin goals of systematics – reconstruction of evolutionary history and classification of species
Phylogenetic trees come about through successive events of speciation (branching), in which
species gives rise to two (or more)
Reconstructing these trees requires comparison of traits (with shared genetic ancestry) in
multiple species
o Homologies are good, analogies are not (where similarities are not due to being related)
Use Characters to Categorize
Binary nomenclature – Genus and species (reflect the nested hierarchy of life)
Taxon designation has 3 components
o Name – what group does it belong to?
o Rank – where does it fit into the phylogeny
o Content – relevant to species concept
All phylogenetic trees are “guesses” about how species are all related
What do Similarities Reflect?
Can be similar because they share a common ancestor – passed on to descendants (possibly
modified)
o Example – whales, dolphins, sharks
o Similar – similar bodies, fins, torpedo shaped body
o Dolphins and whales - Due to homologies – due to the common ancestor they share
o Sharks and whales – look similar due to similar selective pressures to fill the niche
(homoplasties)
No common ancestor
Want to base phylogenetic trees on homologies not homoplasties
Descent with Modification
Homologous structures reflect
underlying genetic similarities
o E.g. bones that support wings of
bats, birds and pterosaur all look
like modification of pentadactyl
limb
o Develop from comparable
embryonic structures
o Same core bone structure –
developed differently for each
species
Another example is looking at the bone
structures across many species
o Birds have really high ankles, but the basic structure is similar to other species
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Often development of an individual recreates its evolutionary history – “ontogeny recapitulates
phylogeny”
o When looking at certain species at different points of development (such as the
embryo), you can see traits that were part of the ancestors (such as the “old” jawbone
in the opossum embryo)
o Eventually they develop to the present-day traits
o Another example is the appearance of baby blue herons – look like dinosaur birds
(except for the beak)
When no homologies exist, traits are analogous
o Wings of insects are distinct from those of birds or bats
Why do Analogous Structures Evolve
Adaptation to similar selection pressures
E.g. insect pit fall trap “pitcher” structures in plants developed in response to low soil nitrogen
Similar niches – similar forms
o E.g. Australians marsupials vs placental mammals
Convergent evolution – come to the same superficial traits with no ancestor
Parallel evolution – diverged to different species (related) with similar traits
Ancestral and Derived Structures
Individuals show evidence of Mosaic evolution
Combination of ancestral and derived characters in evolutionary lineage
o Both are based on perception and the point in time being examined – e.g. your ancestral
traits may have been your ancestors derived characters
Derived characters determined from fossils and embryos
o E.g. the opossum – the jaw of the embryo was similar to the ancestral jaw bone, and
later develops into the derived characteristic
Derived characters appear later in development
o E.g. baby chimps and humans look similar, but they have many more different traits as
adults
Outgoing comparison for derived characters
Cladistic Phylogenies based on
Principle of Monophly
o All groups in a clade includes a common ancestor and all of its descendants
o Idea of nesting – more specialized groups within more general groups
Nested hierarchy of life
o To be completely representative of branches in the tree
Principle of Parsinomy
o Simplest explanation and most likely to be correct unless other data suggests more
complicated explanation
o E.g. hair only evolved once and was then passed along
o Evolutionary change is rare
Evolution of character twice is extremely unlikely
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