Chapter 25: Phylogeny and Systematics. phylogeny – evolutionary history of a species or group of...
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Transcript of Chapter 25: Phylogeny and Systematics. phylogeny – evolutionary history of a species or group of...
Chapter 25:Phylogeny and Systematics
phylogeny – evolutionary history of a species or group of species
systematics – analytical approach to understanding the diversity and relationships of present and past
organisms
Phylogenies based on common ancestors using fossil, morphological
and molecular evidence
the fossil record – based mostly on the sequence in which fossils have
accumulate in sedimentary rock strata (layers)
Sedimentary rock forms when silt builds up on bottom of waterway. More deposited on top, compress
older sediments into rock.
morphological and molecular homologies
• organisms with similar morphology or similar DNA closely related
analogy is not homology; analogy is a similarity due to convergent evolution
(organisms adapting to similar environment/niche but no common
ancestor)
• distinguishing analogy from homology critical to constructing phylogenies
homoplasies – analogous structures that have evolved independently
Marsupial mole(Australia)
eutherian mole(North America)
No recent common ancestor
molecular homologies
• compare nucleic acids of two species; if very similar, organisms closely related
• a hard to tell how long ago they shared a common ancestor; must also look at fossil record
• mathematical tools can distinguish between distant homologies from coincidental matches
Systematics connects classification with evolutionary history
taxonomy – ordered division of organisms
into categories
bionomial nomenclature – scientific name
• binomial – 2- part scientific name developed by Linnaeus “Linnean system”
• genus – first part of name• specific epithet – second part of name
Homo sapiens = humans, means “wise man”
Heirarchical classification
• Domain• Kingdom• Phylum• Class• Order• Family• Genus• Species
mnemonic to help you remember:
“Dreadful King Phillip Came Over From Great Spain”
Linking classification with phylogeny
phylogenetic trees – branching diagrams that depict hypotheses about evolutionary relationships.
• uses groups nested within more inclusive groups• constructed from series of dichotomies (2-way branch points)• each branch point represents a divergence of two
species from a common ancestor
cladogram – shows patterns of shared characteristics
clade – group of species that includes an ancestral species and all of its
descendants
cladistics – analysis of how species grouped into clades
• clades can be nested inside larger clades
– ex. cat family within a larger clade that includes dog family
monophyletic group – ancestral species and all of its descendants
paraphyletic group– when we lack information about some members of
the clade
polyphyletic group– several species that lack a common ancestor (need
more work to uncover species that tie them together into a monophyletic
clade)
Shared Characteristics – types of homologous similarities
Chordate characteristics
Shared primitive character – shared beyond the taxon.
Shared derived character – evolutionary novelty unique to that
clade.
Ex. hair only found in mammals
Why morphology alone does not show evolutionary relationship:
• Closely related organisms not always similar in appearance (rapid environmental change leads to rapid evolution; also, small changes in genes can lead to large morphological differences)
• Organisms that appear similar not always closely related (convergent evolution)
• Just because 2 groups share primitive characters does not mean they are closely related
outgroups – species or group of species closely related to the ingroup
• less closely related than members of the ingroup
• have a shared primitive character that predates both ingroup and outgroup members
phylogenetic trees – show estimated time since divergence
• chronology of a phylogenetic tree is relative; not absolute
phylograms – length of a branch reflects number of changes in a DNA sequence
ultrametric trees – length of branch reflects amounts
of time
maximum parsimony “Occam’s Razor”– first investigate the simplest explanation
that is consistent with the facts• Aim is to find the shortest tree that has the
smallest number of changes
The top tree has the most parsimony
maximum likelihood – given
certain rules of how DNA changes over time, a tree can be found that reflects the most likely sequence of
evolutionary events
Often the most parsimonious tree is also the most likely
phylogenic trees are hypotheses of how the organisms are related to each
other
• Best hypothesis is one that fits all the available data
• May be modified when new evidence introduced
• Sometimes there is compelling evidence that the best hypothesis is not the most parsimonious
Organisms’ genomes document their evolutionary history
• importance of studying rRNA and mitochondrial DNA (mtDNA)
rRNA changes very slowly; used to study divergences that happened a
very long time ago
mtDNA changes very rapidly; used to study divergences that happened
recently• – useful for studying relationships between
groups of humans
ex. how Native Americans
descend from Asian population that crossed the
Bering Land Bridge 13,000 years ago
• can lead to further evolutionary changes
Gene duplication one of most important types of mutation in
evolution because it increases # of genes in genome.
gene families – groups of related genes in an organism’s genome
• result of repeated duplications• have a common ancestor
orthologous genes – homologous genes passed in a straight line from
one generation to the next.• can diverge only after speciation• can be found in separate gene pools due to
speciation
paralogous genes – result of gene duplication. Found in more than one
copy of the same genome• can diverge in the same gene pool
Genome evolution
• Orthologous genes are widespread and can extend over huge evolutionary distances
• 99% of genes in humans and mice are orthologous; 50% of genes in humans and yeast are orthologous
– demonstrates that all living organisms share many biochemical and developmental pathways
Molecular clocks – way of measuring absolute time of evolutionary change
• based on some genes seem to evolve at a constant rate
• # of nucleotide substitutions in orthologous genes is proportional to time elapsed since the species branched from common ancestor
neutral theory – for genes that change regularly enough to use as a molecular clock, these changes are
probably a result of genetic drift and are mostly neutral (neither adaptive
nor detrimental)
• conclusion: much evolutionary change has no effect on fitness; therefore, not influenced by selection.
• most new mutations are harmful and therefore removed quickly
The universal tree of life
• Genetic code universal to all forms of life so all life must share a common ancestor
• Researchers trying to link all organisms in a “tree of life”
• use rRNA genes for this; as they evolve most slowly
The Tree of Life has three domains: bacteria, archae and eukarya
The early history of these domains is not yet clear
• due to horizontal gene transfer, substantial interchanges of genes between organisms of different domains
• horizontal gene transfer due to transposable elements and fusion of different organisms (first eukaryote fusion of ancient bacteria with ancient archae)