Phylogenetics - Wikipedia
In evolutionary biology the word “relationship” has a particular meaning, which is key to understanding and using the Tree of Life. In biology, phylogenetics /ˌfaɪloʊdʒəˈnɛtɪks, -lə-/ is the study of the evolutionary history and relationships among individuals or groups of organisms (e.g. A phylogeny is a hypothetical relationship between groups of organisms being compared. A phylogeny is often depicted using a phylogenetic tree, such as the.
Similarly, the root represents a series of ancestors leading up to the most recent common ancestor of all the species in the tree.
Which species are more related? In a phylogenetic tree, the relatedness of two species has a very specific meaning. Two species are more related if they have a more recent common ancestor, and less related if they have a less recent common ancestor. We can use a pretty straightforward method to find the most recent common ancestor of any pair or group of species. For instance, suppose that we wanted to say whether A and B or B and C are more closely related. To do so, we would follow the lines of both pairs of species backward in the tree.
Since A and B converge at a common ancestor first as we move backwards, and B only converges with C after its junction point with A, we can say that A and B are more related than B and C. For instance, we can't say whether A and B are more closely related than C and D.
So, we can only compare the timing of branching events that occur on the same lineage same direct line from the root of the treeand not those that occur on different lineages. Some tips for reading phylogenetic trees You may see phylogenetic trees drawn in many different formats.
Some are blocky, like the tree at left below. Others use diagonal lines, like the tree at right below. You may also see trees of either kind oriented vertically or flipped on their sides, as shown for the blocky tree. You may want to take a moment to convince yourself that this is really the case — that is, that no branching patterns or recent-ness of common ancestors are different between the two trees.
The identical information in these different-looking trees reminds us that it's the branching pattern and not the lengths of branches that's meaningful in a typical tree.
So just like the two trees above, which show the same relationships even though they are formatted differently, all of the trees below show the same relationships among four species: Figure 3 by Robert Bear et al.
phylogenetic | Definition of phylogenetic in English by Oxford Dictionaries
So far, all the trees we've looked at have had nice, clean branching patterns, with just two lineages lines of descent emerging from each branch point. In general, a polytomy shows where we don't have enough information to determine branching order.
Where do these trees come from? These nodes represent important changes in the lineage that gave rise to a new line. The Homininae represent a subfamily of the great apes. Through physiological and genetic markers, scientist have determined that orangutans diverged from this line a long time ago.
This makes gorillas, chimps, and humans much more closely related than any of the groups are to orangutans. The Homininae is known as a clade, or a group of lineages that share a common ancestor. Typically, the organisms in a clade share a synapomorphy. In the case of the Homininae, they share the characteristic of being able to run efficiently on the ground, a step towards bipedalism.
The orangutans do not share this ability, and spend most of their time climbing in the trees. In the above cladogramthe orangutan is considered the outgroup, as it is the least related organism, and is used to help distinguish the relatedness between the other groups.
If scientists wanted to include orangutans in the clade being discussed, they would use a cladogram, or phylogenetic tree, like the one below. This cladogram includes the genus Hylobates, or the gibbons.
Gibbons are even less related, as they diverged even further in the past than the apes. In this version of the phylogeny, the gibbons are represent the outgroup. Note that although the same phylogeny is represented, it can be drawn in very different ways.The Phylogenetic Tree Biology [HD Animation]
However it is drawn, the various nodes and lines represent a sequence of evolutionary events. As long as that sequence is the same, the two cladograms or phylogenetic trees represent the same phylogeny, or hypothesis. Scientist try to design a phylogeny that has the fewest changes, or nodes. Changes can only be produced by mutations in an organisms, or an event that separates a population.
Both of these events are relatively unlikely. The founder of phylogenetic systematics, Willi Hennig, suggested that the most probable phylogeny would be the most parsimonious phylogeny. Parsimonious phylogenies have the fewest changes, compared to other phylogenies. Until recent advances in genetic techniques, this was considered more factual. Now, it has been shown that reversals and other very unlikely mutations have indeed been the cause of various organisms appearing out of place in the phylogeny.
With additional information and genetic techniques, a clearer view of the relationships between organisms can be obtained. In conservation biologyscientist use phylogenies to help identify which organism to protect.
For example, part of the reason it has been difficult for scientist to protect the interests of the polar bear is that the polar bear is still closely related to the brown bear.
Evolutionarily, brown bears form three distinct lineages, of which the polar bear is a part. Polar bears are still able to successfully breed with brown bears. By contrast, the clouded leopard actually exists as 3 distinct ESUs. Populations of clouded leopards spread from Asia south, into islands like Borneo. As they spread, the populations were separated from one another and became genetically distinct. Each separate ESU is as differently related as lions and tigers. Thus, when making decisions about which animals to protect, scientists sometimes find it wiser to protect animals like the clouded leopard, which actually represent a lot of genetic diversity, rather than the polar bear, which is a relatively recent evolution of brown bears.
A phylogeny can also be used by geneticists to help identify populations that are at risk for various genetic diseases. Because genetic mutations are responsible for genetic disease, they exist in populations in the same way that inheritable characters do.