Cladograms & Evolution – We are all related
Brontosaurus painting by Charles Knight
Phylogenetics orPhylogenetic systematics is a discipline and a field of biology that aims to identify and understand the evolutionary relationships among the many different kinds of life forms on earth, both living (extant) and dead (extinct). Cladistics uses an understanding of phylogenetics to look at the “Tree of Life” and uses a type of diagram called a Cladogram to represent branched “evolutionary relationships” or phylogenies.
Phylogenies are lists or diagrams that show the evolutionary paths taken by populations of organisms through many generations and over long periods of time.
Nomenclature is the devising or choosing of names for things, especially within a branch of science. Systematics or Taxonomy is the branch of biology that deals with classification and nomenclature of organisms or life forms. A binomial (or two word) nomenclature is used to label and identify known species, using a Genus name and a Species name. Please see this post here for in depth information about TAXONOMY.
This simple diagram shows Linnaeus’s system of Binomial Nomenclature for the Grizzly Bear, whose two word scientific name is Ursus arctos.
Evolutionary theory states that similarity among individuals or species is attributable to common descent or inheritance from a common ancestor. Charles Darwin was the first to recognize that the systematic hierarchy of classification (taxonomy) represented a rough approximation of evolutionary history.
In the 1950s the German entomologist Willi Hennig proposed that biological systematics should reflect the known evolutionary history of lineages as closely as possible, an approach he called phylogenetic systematics or evolutionary systematics.
Evolutionary systematics attempts to construct “trees” with illustrations of the actual evolution of one species or higher “taxon” (unit of biological classification) into another, with information on when and where a branch occurs. It oftens use a “spindle diagram” as shown below, demonstrating the phylogeny of the horse.
Cladistics or Cladistic analysis, however, ignores when and where a branch occurs, tries to use purely objective criteria, and defines each branch point by a fundamental character of evolutionary significance.
A Clade isa group of taxa that includes a common ancestor and all of the descendants of the taxa. A Clade is characterized by one or more derived characters present in the first member of the taxon, which is inherited by its descendants (unless secondarily lost), and not inherited by any other taxa. Clades form a nested heirarchy.
Cladogenesis, also called branching evolution, is the splitting of a gene pool into two or more separate pools, which each give rise to one or more new species.
Derived characters are advanced or specialised traits which only appear in some organisms, which then form a distinct group.
The operation of Cladistics relies upon the premise or assumption that life arose on earth only once, and therefore all organisms from the beginning of Life on earth are related in some way or other. We can therefore take any collection of organisms and determine a meaningful pattern of relationships, provided we have the right kind of information.
There are three basic assumptions in cladistics:
- Any group of organisms are related by descent from a common ancestor.
- There is a bifurcating pattern of cladogenesis.
- Change in characteristics occurs in lineages over time.
A lineage is any continuous line of descent. Cladogenesis is the evolutionary change and diversification resulting from the branching off of new taxa from common ancestral lineages. Bifurcating means to separate into two parts or branches.
Cladistics is a system of arranging taxa by analysis of evolutionarily derived characteristics so that the arrangement will reflect certain phylogenetic relationships.
The outcome of a cladistic analysis is a Cladogram – a tree-shaped diagram that represents a phylogenetic hypothesis, a hypothesis on evolutionary relationships.
A cladogram is a stylized diagram that looks like a series of Y’s or forks in a road. At each branch, or “Y” junction, derived characters of evolutionary origin are used to separate off one group from the rest.
A. ______ Segmented Body
B. ______ Legs
C. ______ Crushing mouthparts
D ______ 6 Legs
E. ______ Jumping Legs
F. ______ Wings
G ______ Double set of wings
H. ______ Curly Antennae
The above Cladogram is read by starting at the letter A. Time moves from earliest at A to the most recent time at H. All animals A and above have segmented bodies, and then all animals at B and afterward/later have got legs, and so on. At F the trait of having wings appears in time. The five animals before F (so from A to E) do not have wings. The letters represent the unique derived characters.
A node is a point of divergence, with two branching lines of descendance, indicating evolutionary divergence from a common ancestor. Where the vertical lines meet the diagonal line in the example above, there is a node. In the example above, there are 6 nodes along the diagonal line, plus another node (the seventh) at the junction of organisms D and E.
The length of the horizontal lines in a Cladogram between Nodes may differ, indicating the amount of mutation since the branching from the common ancestors. In other words, where the line lengths are different, the length of the horizontal lines indicates the amount of time that has passed since the last common ancestor between two groups or species.
Anagenesis or Phyletic Change is evolutionary change between the nodes, or the evolutionary process whereby one species evolves into another without any splitting of the phylogenetic tree. When enough mutations reach fixation in a population to significantly differentiate from an ancestral population, a new species name may be assigned. A key point is that the entire population is different from the ancestral population so that the ancestral population can be considered extinct.
Anagenesis involves evolution (and relplacement, not splitting) within a lineage, while Cladogenesis is an evolutionary splitting event in which each branch and its smaller branches forms a “clade”. It is a process of adaptive evolution that leads to the development of a greater variety of “sister” organisms.
Cladogenesis usually occurs when a few organisms end up in new, often distant areas or when environmental changes cause several extinctions, opening up ecological niches for the survivors.
Anagenesis, also called phyletic evolution, is the accumulation of changes that gradually transform a given species into a species with different characteristics.
Cladogenesis and Anagenesis are two complementary aspects of phylogenesis, as shown in the diagram below.
The evolution of the Peppered Moth, from the white Biston betularia typica to the black Biston betularia carbonaria is an example of Anagenesis, a single lineage being traced from ancestor to descendant.
Cladogenesis is most famously represented by Darwin’s finches, a single South American species having multiplied into several species after reaching the Galapagos Islands. Save this image and zoom in.
Cladogenesis and Anagenesis are achieved through Allopatric Speciation or Sympatric Speciation.
Speciation is the evolutionary process by which new biological species arise. This excellent article here from the University of Illinois goes into more depth about Allopatric and Sympatric Speciation for those who want to learn more about it.
A very important point about Cladistics is that Cladistics marked a radical change from the earlier evolutionary or phylogenetic systematics in one important way. It is no longer necessarily about which species or group directly evolves into which (ancestor-descendent relationship), but rather which species shares a common ancestor with which other species (called a “sibling” or “sister-group” relationship).
All species that share the same ancestor belong to the same clade, the same family group. Because birds share a common ancestor with different types of dinosaurs, they are included in the Dinosaur Clade. Therefore, cladistically speaking, birds are dinosaurs. See below.
Just to finish off, here are some more examples of Cladograms, followed by a good video by Scott Collins on how to construct a Cladogram.
Human beings ( Homo sapiens ) are the second “chimp”
Two clades are sisters if they have an immediate common ancestor. In the diagram above, lemurs and lorises are sister clades.
Not a Clade
A clade is all the living descendants of a single common ancestor.
In general, it is an objective of the branch of biology called cladistics to define each taxonomical unit (species, genus, family, and so on) so that it is a clade. We’ll see how this works in the diagram below.
__________ Alpha | | | ____ Beta Z___ | | | | | | V_____| | |____ Gamma _______ | | | | |_______________ Delta Lucky ____| | | |________________________ Epsilon
Let’s say Lucky is extinct, and the endpoints Alpha through Epsilon are all species alive today. What are the clades in this tree?
The whole thing at the highest level is a Clade with all the living descendants of Lucky. Another clade is Beta plus Gamma: all the living descendants of V. Alpha, Beta, and Gamma constitute another clade: all the living descendants of Z.
But Alpha plus Gamma by themselves do not form a clade. The most recent common ancestor of Alpha and Gamma is Z, but Beta is also a descendant of Z, so it must be in any clade including both Alpha and Gamma.
To watch Scott’s video on “How to construct a Cladogram”
click on the link below
NOW, HERE IS A FUN CHALLENGE FOR THE BRAVE-HEARTED
YOUR JOB IS TO WRITE A GRID FOR THE DERIVED CHARACTERS OF THE ALIENS BELOW
AND TO CONSTRUCT A CLADOGRAM
List the derived characters 1 to 5 in a column on the left and the life forms A to D in a row across the top to form your matrix. I will post the answer later – come back to this site and I promise you that within a week of this post, I will post the ANSWER.
[ Hint: there are 5 different characters represented by the Aliens – it is a good idea to watch the video for help ]
And that is finally the end – thank goodness !! I hope you have liked this post.