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

generalDISADVANTAGE:• 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’tADVANTAGE:• 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’tADVANTAGES:• 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)

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

Phylogeny con’t:

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.

Evidence of common ancestry:

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 speciesRecapitulates: replay

History:

• 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. .

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

Evidence of Common Ancestry:

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

Phylogenetic TreePhylogenetic Tree: a diagram depicting the

evolutionary relationships between different species or groups (previously called cladogram)

MODERN TAXONOMY:

• 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 Tutorialon

How to Read a Phylogenetic Tree