Lesson 3: Phylogeny & Modern
Taxonomy
• Recall that the fundamental unit of
biodiversity is the species
• Today’s lesson focuses on:
– Methods to define a species
– Defining and using phylogenies to classify and
organize species
• Classification of species is a difficult task
partly because there is no one clear-cut way
to single handily define a species.
• Furthermore, as new species are being
identified, we need to find ways to
determine how closely related it is to other
species
• There are different ways to categorize a
species
Identifying Species:
Using the Species Concept
1. Morphological Species Concept
2. Biological Species Concept
3. Phylogenetic Species Concept
1. Morphological Species
Concept
• Focus on the morphology of an
organism
• Morphology refers to: body size, shape,
and other structural features
• Organisms are compared and scientists
decide whether similar organisms
represent different species
1. Morphological Species con’t
ADVANTAGE:
• Simple to use
• Most widely used by people in
general
DISADVANTAGE:
• Too much variation within a species
Problems with Morphological
based classification
2. Biological Species Concept
• Focus on similar characteristics AND
the ability of organisms to interbreed in
nature and produce viable, fertile
offspring
2. Biological Species con’t
ADVANTAGE:
• It is widely used by scientists
DISADVANTAGE:
• Can’t be applied for species that reproduce
asexually
• Uncertain for populations that are physically
separated and don’t have the opportunity to
breed naturally
• Can’t be applied to fossil species, which are no
longer reproducing
3. Phylogenetic Species Concept
• Focus on evolutionary relationships
among organisms
•A species is defined
as a cluster of
organisms that is
distinct from other
clusters and shows a
pattern of relationship
among organism
3. Phylogenetic Species con’t
ADVANTAGES:
• Can be applied to extinct species
• Considers information about
relationships among organisms learned
from DNA analysis
DISADVANTAGE:
• Evolutionary histories are not known for
all species
Phylogeny....we need to know a
bit about evolution
• Evolution: the scientific theory that describes
changes in species over time and their shared
ancestry.
• All living things are descended from a common
ancestor.
• When scientists say that two species are closely
related, it means that they share a common
ancestor in their recent, evolutionary, history.
Which are more closely related?
More About Phylogeny
•
•
Phylogeny refers to the evolutionary
history of a species
Phylogenies are determined through:
1. Developmental Traits (Embryology)
2. Structural Traits (Homology)
3. Molecular Traits (Genetics & Molecular
Biology)
Phylogeny con’t:
Organisms likely share a common
ancestry if:
• They show similar stages of
embryological development
• They have similar anatomical
structures, regardless of function
• They are genetically similar
Evidence of common ancestry:
Embryological Similarities
• It has been observed that the early stages of embryo
development are similar in closely related species
• All vertebrate embryos go through a state in which
they have gill pouches similar to those of fish and a
postanal tail.
History:
Recapitulation Theory:
• Theory proposed by embryologists in the 19th century
(Ernst Haeckel)
• The notion that “ontogeny recapitulates phylogeny” is
an overstatement. Some scientists proposed that the
stages of embryonic development was a replay of its
evolutionary history. This is generally not accepted;
however, what recapitulation does occur is a replay
of embryonic states, not a sequence of adultlike
states of ever more advanced vertebrates
Ontogeny: the development of an individual organism
Phylogeny: evolutionary history of the species
Recapitulates: replay
• If ontogeny recapitulates phylogeny, then the
embryonic development of a chick would look like the
above picture. This is not the case. Instead, we see it
more like below. The early embryos of the chick will
be similar to the early embryos of reptiles or fish at
points in its development, but it does not recapitulate
the adult forms of ancestors. .
Evidence of common ancestry:
Anatomical Similarities - Homology
• Homologous Structures: structures of
different organisms that are similar in
form
Evidence of Common Ancestry:
Molecular Similarities
• Evolutionary relationships among species are
reflected in DNA and proteins
• Two species that have matching gene and
protein sequences would share a common
ancestor
• Humans and
chimpanzee share 99.9%
identical DNA sequence
International Barcode of Life
Project
• Paul Herbert of the U of Guelph started the
IBL project in 2010 to catalogue every
species using a DNA barcode
• Applications of Barcoding
• iBOL
• http://ibol.org/
Traditional Method of
Classification
MODERN TAXONOMY:
Phylogenetic Tree
Phylogenetic Tree: a diagram depicting the
evolutionary relationships between different
species or groups (previously called
cladogram)
• Phylogenetic Trees differ from
Dichotomous keys because Dichotomous
keys focus on catagories based on
morphology (physical features) and
Phylogenetic trees focus on catagories
based on common ancestors (evolution).
Clades
Clades: a taxonomic group that includes a single
common ancestor and all its descendants. Each
clade on a phylogenetic tree can be thought of
as a branch on the tree of life.
Importance of Classification
• Drugs – narrow your search for pharmaceuticals to closely
related species that produce valuable proteins/chemicals
• Disease – trace the transmission of diseases; disease
spread more rapidly between species that share common
genetics
• Agriculture – increase crop yield and resistance by
cross-breeding related species; biological control of natural
predators, parasites, disease
• Conservation – ex: In 2001 African forest elephant,
Loxodonta cyclotis was declared a different species from African
bush elephant, Loxodonta africana; only Loxodonta africana
was protected
Thinking Question
In northwestern Ontario, there are two similar-looking
garter snakes: the red-sided garter snake and the eastern
garter snake. The two interbreed successfully in nature in
that part of Ontario, producing offspring that have a mix of
physical traits of the two. The eastern garter snake also
co-exists in southern Ontario with another very similar
snake, the eastern ribbon snake. However, these two
snakes are not known to interbreed successfully. Infer
whether these three snakes are the same species or not.
Explain your reasoning.
Complete the Tutorial
on
How to Read a Phylogenetic Tree