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