2.3: An overview of systematics Flashcards
Overview of systematics and tree building
**Systematics and Taxonomy **
What is systematics about?
What is taxonomy?
- Systematics is about making sense of biological diversity, includes taxonomy, how they are related in evolutionary terms and the naming of things and how we put things into different orders
- Taxonomy is one aspect of systematics – the naming of biological groups
- Week 4 we will be doing some of this
Carolus Linnaeus (1707—1778)
What did Linnaeus formulise?
What system did he devise?
What was his view of change in the living world?
- Taxonomy was first formulised by Linnaeus, the Swedish biologist devised the binomial system still used today for naming organisms. When he did this work, way before Darwin his view was that there was no change in the living world
- His view was that species do not change. He named things and put them in groups according to their similarities according to appearance
The hierarchical system of grouping in animal taxonomy
What is the taxonomic definition at individual level?
The only taxonomic definition is with the species level, biological species concept, over 20 definitions knocking around some of the newer ones are on phylogenetics and naming clades and specific trees
The three major schools of hierarchical classification
What are they?
What is phenetics?
What is cladistics? What standpoint does it take?
What is evolutionary taxonomy?
- PHENETICS (older) or aka numerical taxonomy, builds hierarchies on the basis of physical similarities, and therefore emphasises adaptation (and therefore the results of evolution). Measure stuff based on similarity, if a particular anatomical feature is under selection then that trait will look similar and you will put them together. Phenetics was popular in the 1950s Sokal – the dreaded statistics book, standard text book, developed this field now a days….
- CLADISTICS is concerned only with relatedness or phylogenetics (evolutionary history). Due to Hennig, 1950’s. Takes a strong evolutionary stand point only interested in taking evolutionary history of phylogenetics
- EVOLUTIONARY TAXONOMY is a mixture between the two above. A bit of a hybrid.
**Cladistics: some definitions **
**What is cladistics based on? What is often the case? **
**Cladistics involves building evolutionary trees… what is it based on?
Shorter branches =
Longer more remote branches = **
**What is a clade?
What is a phylogenetic tree? **
- CLADISTICS classifying organisms based on shared common ancestry (evolutionary origin), often they are going to look similar thus PHENETICS will give similar results
- CLADISTICS classifying organisms based on shared common ancestry (evolutionary origin), often they are going to look similar thus PHENETICS will give similar results
- Involves building evolutionary trees based on similarity between species; similar species are on shorter branches in close proximity on a tree, distantly related are on longer more remote branches
- A clade (from which the term is derived) is a lineage within a tree containing the descendants of a common ancestor
- A phylogenetic (evolutionary) tree is a branching diagram representing the history of a group of species/taxa (or a gene)
Phylogenetic Trees
Draw and label the parts of phylogenetic tree
Because of the mathematical description of trees – terminal nodes analogues to a leaf, INTERNAL NODE represents common ancestors, 3 descendent lineages
Clade if you include bottom left root
Has to be thought of in a hierarchal way
Cladistics
Uses _____ _______ ____ … ** (due to inheritance through common ancestry) this is how to define taxonomic groups**
What does classification (taxonomy) reflect?
Some groups will contain some but not all descendants - what is this called?
Does cladistics reject these groups?
Uses shared derived features (due to inheritance through common ancestry) this is how to define taxonomic groups
- Classification (taxonomy) reflects evolutionary history (phylogeny), this is the most sensible way to organise – but because of the historical context, where these names go back long time taxonomy doesn’t reflect history, some groups will contain some but not all the descendants - PARAPHYLETIC
- Rejects groups that contain some but not all the descendants of a common ancestor (PARAPHYLETIC groups).
Diversity of type, unity of pattern
Although these vertebrate species differ, the underlying pattern of the forelimb is fundamentally the same
At what level can this variation be understood?
Diversity of type, unity of pattern
Although these vertebrate species differ, the underlying pattern of the forelimb is fundamentally the same. We have all kinds of amazing adaptations of the forelimb but we can find similar bones that have been modified in different ways – diversity of type unity of pattern
This amazing variation is due/ can be understood on the level of developmental genes
Homology and analogy
Darwin cited these examples but didn’t understand the underlying molecular mechanisms
What does homology mean?
Homology - vertebrate forearms: the bat wing, mouse forearm, and human arm are homologous structures because all are composed of similar bones inherited from a [recent] common ancestor
Homology and analogy
Darwin cited these examples but didn’t understand the underlying molecular mechanisms
What does analogy mean?
Analogy: The wings of bats, butterflies, and birds evolved independently, not from a recent common ancestor. But they have a similar function, flight, and so are analogous. Things that have evolved to have the same function but have arisen independently and might superfacially look similar have a different evolutionary origin. We have to be able to differentiation analogy and homology to distingusish between trees but sometimes analogues hard to tease them out from each other
The utility of different character types in systematics
(The usefulness of different character types in systematics)
If a character is analogy is it useful?
If a character is primitive is it useful?
If a character is primitive is it useful?
Homologous always trying to find and quantify but again depending on the level we are looking at some can be primitive i.e shared by all are of no use
What we are looking for are more derived features some have a particular feature and others don’t this can be used to separate
Three kinds of similarity in phylogenetics
What are they?
Character state
Convergent or a HOMOPLASY
Primitive or a PLESIOMORPHY
Shared primitive chracters are SYMPLESIOMORPHIES
Derived or a APOMORPHY
Shared derived characters are SYNAPOMORPHIES
Characters that are unique to a group are AUTOMORPHIES
Different traits or features represented by common ancestor
Building a simple phylogeny using cladistic principles
If we take a primate and take a single feature e.g finger nails
Finger nails is a trait shared by all primates if you want to divide the group by finger nails it is useless but if you want to separate from other mammals then you can because others have claws
Brow ridge will separate the great apes
Classify the apes its difficult
Recent ancestors have very pronounced brow ridge – separates off the apes
Example of phylogenetic tree
Polarising characters
We need to distinguish derived from ancestral. There are two ways to do this:
What are the two ways that can be used to distinguish between derived and ancestral?
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- <!--[endif]-->Work out the ancestral – root the tree to give ancestry maybe by having….
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- <!--[endif]-->Assumptions something about the direction of evolution (e.g Dollo’s law – loss of features more likely)
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- <!--[endif]-->Intuitively you can have some thoughts about that
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- <!--[endif]-->The most common way is to choose an outgroup
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- <!--[endif]-->Assume something about relationships – choose an outgroup known to be outside(distantly related to group of interest) – possum as an outgroup (i.e more distantly related to) the group of interest.
Distinguishing convergence from shared, derived characters
What is the Principle of Parsimony
assume that the majority of characters are correct, or that minimum number of changes in character state have occurred
Cladistics
• Rejects groups that contain some but not all the descendants of a common ancestor (PARAPHYLETIC groups)
Monophyly and paraphyly
What does a monophyletic group contain?
What does a paraphyletic group contain?
a monophyletic group contains a common ancestor an all its descendants
a paraphyletic group contains a common ancestor and some but not all of its descendants
