Chapter 27- Phylogenies and the History of Life
27.1 Tools for Studying History: Phylogenetic Trees
Phylogeny: evolutionary history of a group of organisms
Phylogenetic tree: shows ancestor-descendant relationships among populations or species, and clarifies
who is related to whom
-branch represents a population through time; the point where two branches diverge, called a node (or
fork) represents point in time when an ancestral group split into two or more descendants groups; and a
tip (terminal node), the end-point of a branch represents a group (a species or larger taxon) living today or
one that ended in extinction.
How Do Researches Estimate Phylogenies?
Phylogenetic trees are an extremely effective way of summarizing data on the evolutionary history of a
group of organisms.
Phenetic approach: computing a statistic that summarizes the overall similarity among populations based
on data. Ex. Gene sequences used to compute an overall genetic distance
Cladistic approach: relationships among species can be reconstructed by identifying shared derived
characters, or synapomorphies, in the species being studied.
Synapomorphy- a trait that certain groups of organisms have that exists in no others. Allow biologists to
recognize monophyletic groups- called clades or lineages. Ex. Fur and lactation are synapomorphies that
How can biologists distinguish between homology and homoplasy?
Traits can be similar in 2 species not because they were present in common ancestor but because similar
traits evolved independently in two distantly related groups.
Homology- occurs when traits are similar due to shared ancestry
Homoplasy- when traits are similar for reasons other than common ancestry.
Convergent evolution- occurs when natural selection favors similar solutions to the problems posed by a
similar way of making a living. Common cause of homoplasy. Results in analogous traits.
If similar traits found on distantly related lineages are similar due to common ancestry, then similar traits
should be found in many intervening lineages on the tree of life.
Parsimony: under parsimony, the most likely explanation or pattern is the one that implies the least
amount of change.
Convergent evolution and other causes of homoplasy should be rare compared with similarity due to
shared descent, so the tree that implies the fewest overall evolutionary changes should be the one that
most accurately reflects what really happened during evolution.
27.2 The Fossil Record
Provides direct evidence about what organisms that lived in the past looked like, where they lived, and
when they existed. Fossil- piece of physical evidence from an organism that lived in the past
Fossil record is the total collection of fossils that have been found throughout the world.
How do fossils form?
- All or part of an organism is buried
- Decomposition/ preserved intact/sediments accumulate on top of material create thin
carbonaceous film/remains decompose after being buried, remaining hole can fill with dissolved
minerals forming cast of the remains
Limitations of fossil record
- Habitat bias: organisms that live in areas where sediments are actively being deposited, ex
beaches mudflats and swamps, are much more likely to form fossils
- Taxonomic bias: slow decaying animals. Bones and teeth are much more likely to leave fossils
- Temporal bias: recent fossils are much more common than ancient fossils.
- Abundance bias: because fossilization is so improbable, the fossil record is weighted toward
Paleontologists- scientists that study fossils
Life’s Time Line
Radiometric dating- based n well-studied decay rates of certain radioactive isotopes
- Precambrian: earth’s formation 4.6 billion years ago (bya) and appearance of most animal groups
542 mya. Divided into Hadean, Archaean, Proterozoic eons. Life was unicellular. Oxygen was
virtually absent from the oceans and atmosphere for almost 2 billion years after the origin of life
- Phanerozoic eon: Interval between 542 mya and present. Divided into 3 eras. Eras divided into
- Paleozoic era: begins with appearance of many animal lineages, and ends with obliteration of
almost all multi-cellular life-forms at the end of the Permian period. Origin and initial
diversification of animals, land pants, fungi, appearance of land animals
- Mesozoic era: Age of Reptiles. Begins with the end-Permian extinction events and ends with
extinction of dinos and other groups at the boundary between Cretaceous period and Palaeogene
period. In terrestrial environments of the Mesozoic, gymnosperms were the dominant plants and
dinos were dominant vertebrates
- Cenozoic era: Divided into Palaeogene period and Neogene period.Age of Mammals.
Angiosperms dominant plants mammals dominant vertebrates
27.3 The Cambrian Explosion
-Aburst of diversification occurred soon after the first animals appeared in the fossil record about 570
mya. -About 565 mya, the first animals—sponges, jellyfish, simple worms—appear in the fossil record. Just 50
mya later, virtually every major group of animals had appeared. In relatively short time, creatures with
shells, exoskeletons, internal skeletons, legs, heads, tails, eyes, antennae, jaw-like mandibles, segmented
bodies, muscles, and brains had evolved.
The Doushantuo Microfossils: sponges in samples dated 580 mya. In samples dated 570 mya