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Phylogeny and the Tree of Life
1. Objectives
• understand & appreciate the major goal of systematics
• describe the lines of evidence used to reconstruct
phylogeny
• to compare advantages & disadvantages of the lines of
evidence
• explain the importance of distinguishing between
homology & analogy
2. Overview: Investigating the Tree of Life
• Evolutionary Biology – processes & history-Processes – natural selection & others – change in genetic composition
-History – phylogeny – evolutionary history of a spp. or group of related spp.
• Systematics - classify organisms & determine their evolutionary relationships (phylogenies)
• Major Goal – reconstruct the history of life on Earth
-evolutionary relationships – living & extinct spp.
2.1. Evidence used to reconstruct phylogenies
• Inferences
a) Fossil record
b) Homologies
- morphological / anatomical / phenotypic /
biochemical & molecular similarities
c) Molecular data
– AAs sequences
- nucleotide sequences in DNA & RNA
2.1.1. Fossil Record
• Preserved remnants or impressions
– organisms that lived in the past
• Fossils can be dated:
a) relatively – strata of the same age
– same fossils
b) Absolutely / numerically
– radioactive isotopes
-comparing 14C radioactivity of a fossil –
modern sample of organic matter
-absolute age of the fossil
Fig. 26-17
Eggs
Front limb
Hind limb
(a) Fossil remains of Oviraptorand eggs
(b) Artist’s reconstruction of the dinosaur’s posture
2.1.1. Fossil Record
• A fraction of existing fossils – discovered
• Species that:
a) Existed for a long time
b) Abundant & widespread
c) Had hard shells or skeletons
Fossils – can be used to construct phylogenies
only – determine their ages
2.1
.2. H
om
olo
gy
• Similarities due to shared ancestry
-anatomical, morphological, physiological, behavioural, molecular traits
• Comparative anatomy
e.g. Forelimbs of vertebrates –homologous structures
-same bones organized - same way - common ancestor
-they have a common decent – may differ - structure & function
Similarities in comparative embryology
Analogy
• Analogy – similarities due to convergent evolution
-not due to shared ancestry
-2 spp. from different lineages – morphological similarities
–similar env. pressures
-e.g. bat (wings) & bird (wings)
–analogous structures – flight
-bat’s wings are homologous to
other mammalian forelimbs
• Distinguishing homology from
analogy – critical for the construction
of phylogenies
Long snouts adapted for
eating ant in two
unrelated species-an
example of analogy or
convergent evolution
2.1.3. Molecular Data
• Chromosomes carry the hereditary information (genes)
• Sequences of nucleotides
in DNA
• DNA RNA Proteins
2.1.3. Molecular Data
• Nucleotides (DNA & RNA) & AAs (proteins)
• Speciation – mutation – changes in the base
pair sequences of DNA
-more closely related spp. – fewer changes – nucleotide sequences of their DNA
• DNA – codes for AA sequences in proteins
-more closely related spp. – fewer differences in their AA sequences of their proteins
2.1
.3. M
ole
cu
lar D
ata
• Molecular data – straightforward & numerical
-clarify anatomical variations & convergence
• Amenable – computer programs mathematical tools
-quick & accurate analyses
• Study closely related spp. – minor morphological differences – not directly affected by env. factors
• Abundant data – each AA or nucleotide
-separate independent character for analysis
• Comparisons – distantly related spp.
• DNA - heritable
Protein Comparisons
• Compare – primary structures of protein molsdirectly
• AAs – number & sequence of AAs in a protein
• Similar AA sequences – same protein – genetic similarity – evolutionary relationship
• E.g. cytochrome c – a protein found in all aerobic organisms
-aligning – AA sequences – of different spp. –
evolutionary links can be inferred
Amino acid sequence comparison of the human, mouse and rat atrial essential myosin light chains (ALC-1)
RNA & DNA Comparisons• All cells – ribosomes – protein synthesis
• Genes that codes for rRNA (ribosomal RNA)
– changed very slowly during evolution
• Comparative rRNA sequencing
– reliable indicator of similarity b/t spp.
• DNA similarities – DNA-DNA hybridization
-DNA double helix of each spp. separated
– single strands – combination
-the more closely related the two spp. – the better
the two strands will stick together
2.1.3. Molecular Data - Limitations
• 4 nucleotide bases – 4 alternative character states
• 20 AAs – 20 different character states
• 2 spp. –same nucleotide base sequence at the same point in their DNA molecule – their similarity – may be due to chance & not reflecting evolutionary relationship
• Systematists – find it difficult to verify that such molecular similarities – were inherited from a common ancestor