With labels, you know which is which. Try differentiating them without labels

You probably ever had those days when you and your mom went to the supermarket to prepare dinner, and you screwed up because you couldn't differentiate between sardine and mackerel, and thought "They're both fish, what's so different about them?" Well, apparently, they are different, and you'll be surprised that there are more organisms that looked very similar yet different. This is caused by evolution, a very slow gradual process of complexifying simple organisms into complex ones (Montogomery, S., 1997), an example is shown like the previous condition - tuna and mackerel are fishes, but there are more type of fishes other than those, like; sharks, and stingrays. Plus, this process is so widespread that it would be hard to explain by mere writings; hence, the "tree of life" was made to solve this problem. 

Tree of Life by Darwin in his notes of 36th page

Tree of Life
The "tree of life" or phylogenetic tree is a diagram that shows how an organism evolves into different types of modified species and how they are related to one and another in a branching manner; henceforth, resulting in a tree-like image (BBC, 2014) It was firstly made by Charles Darwin, who drew (the first) model of a phylogenetic tree on the 36th page of his notebook and well known for his book about evolution, "On the Origin of Species".

Evidence of Evolution
Organisms in phylogenetic tree are differentiated based on similarities and differences. However, they would need evidences to prove that the two or more different species they're comparing are related in an evolutionary way. 

Homology in pentadactyl limb structure

One good evidence is homology, a characteristic which shows similarity in structure between two or more species, for example; the skeletal structure between a monkey's hand, bird's wing, dolpin's fin and mole's leg are similar, although they're clearly different.

They have a pentadactyl limb with similar skeletal pattern. This characteristic isn't shown only in the following above, but also in almost all mammals. However, the function of the limbs for each species is modified based on their diet and habitat, like the monkey, its limbs are modified to hang and climb on to trees for fruits, while mole's are modified for digging underground to search for worms. Another evidence is the homologous traits shared by vertebrates during embryonic stage. 

Embryonic homology

As seen from the picture, every vertebrate has;
a tail posterior to the anus
dorsal, hollow nerve cord
muscle arrangements in sacromere
cartilaginous dorsal notochord
This evidence leads to hypothesis that human might share a common ancestor with other mammals (based on pentadactyl limb similarity), and other vertebrates (based on similarities during embryonic stages), and also states that evolution occured for one main purpose - to survive. 
The drive to survive not only evolved animals, but also plants and bacteria, by developing advantagous traits which will be passed down to the generation via gene transfer. There are two types of gene transfer - horizontal gene transfer and vertical gene transfer. Horizontal gene transfer (HGT), also known as lateral gene transfer, is the transfer of genetic materials between species, mostly seen in prokaryotes via transformation, transduction, and conjugation. It does occur in eukaryotes but largely unaffected (JO. A., 2005). Contrast to HGT, the vertical gene transfer transfers genetic materials from parents to their offsprings. In gene transfer, the donor cell passes down genetic materials to the recipient, resulting in recombination, which plays an important role in evolution by passing down advantagous genetic materials, making the fitter species to adapt and survive, while those that are unable to cope up will extinct; hence, recombination (new species) results in branching of the phylogenetic tree.

Cambrian Explosion
Another example of evolution is the Cambrian Explosion (took place around 542 million years ago), the sudden rapid appearance of complex organisms within around 10 million years, which is considered really fast; hence, results in an "explosion" of species. 

Cambrian Explosion

Not to mention, all animal body plans (distinctive Phylums like Molluscs and Chordata) appeared during this time, marking a tremendous change in the biosphere; conclusively, this explosion ultimately causes the phylogenetic tree to branch out rapidly from a single "trunk". (The Burgess Shale, 2011) 

Why Do Species Went Extinct?
Although there was a rapid introduction of new species during the Cambrian explosion, not much of them were able to survive, while those who survived kept evolving, and this pattern of "others didn't survive while the rest evolves" kept going until now. This pattern is defined by certain factors, such as;
Genetic variation between individuals
The DNA consists of genetic codes that code for different proteins which are needed for the body to undergo different functions. A random mutation on any single part of the genetic sequence could lead to either a better or worse protein that could either upgrade or degrade the individual itself. Hence, genetic variation between individuals increase, and as a result, those with positive mutation upgraded and passes down their genes, while those suffering with negative mutation are most likely to extinct. (Biotechnology Learning Hub, 2005)
Natural selection
Nature is a forever-changing battlefield, and in order to survive, one must adapt to it. If they succeed, they're most likely to pass down these advantagous genes to their offsprings, while those who can't keep up will extinct. Basically, natural selection is the "survival of the fittest". (Biotechnology Learning Hub, 2005)
Sexual recombination
When parents pass down their genes, the genetic codes are shuffled and results in recombination of genes. This could result into genetic variations, and like previously mentioned, this recombination will be tested for natural selection. (Biotechnology Learning Hub, 2005)
Genetic flow
Sometimes, when a group of species migrates away from its population, it changes the allelic frequency of its population. Likewise, gene flow occurs when a species of a group mates with another species of the different group, resulting in change of allelic frequencies of the group. (NFSTC Science Serving Justice, 2007)
Genetic drifting
In nature, there are some groups with allellic frequency which are so small that they're not represented with the groups with larger frequency. Genetic drifting occurs when allellic frequency of either sides increase or decrease, and this only happens by pure chance when there's a limited size of population. (NFSTC Science Serving Justice, 2007)
Overall, the phylogenic tree explains about the evolutionary pattern of living matters based on their differences and similarities, which are proven with evidence, such as; fossil evidence, anatomical evidence, genetic evidence, and other supporting evidence, and as far as I can say, the Tree of Life is nothing more than an imaginary tree that aids scientists in understanding how evolution occurs, which will keep branching in the future as science and technology develop.

So, know any infos I might've missed? Share them in the comments below!

References:
BBC. (2014) Tree of Life. [Online] Available from: http://www.bbc.co.uk/nature/life [Accessed: Tuesday, 2 December 2014]
Biotechnology Learning Hub. (2005) Factors that influence evolution [Online] Available from: http://biotechlearn.org.nz/focus_stories/evolved_enzymes/factors_that_influence_evolution [Accessed: Tuesday, 2 December 2014]
Darwin Online. (2009) Darwin's Tree of Life on the 36th Page. [image] Available from: http://darwin-online.org.uk/content/frameset?itemID=CUL-DAR121.-&viewtype=image&pageseq=1 [Accessed: Tuesday, 2 December 2014]
Evolution Explained. (2011) Early and Later Embryonic Stages of Vertebrates. [image] Available from: http://evolutionexplained.blogspot.sg/2011/12/evidences-comparative-embryology.html?m=0 [Accessed: Thursday, 4 December 2014]
JO., A. (2005) Lateral gene transfer in eukaryotes. [Online] Available from: http://www.ncbi.nlm.nih.gov/pubmed/15761667 [Accessed: Sunday, 30 November 2014]
Mann, J. C. (2005) Homology in Limbs of Mammals. [image] Available from: http://palaeos.com/evolution/glossary.html [Accessed: Wednesday, 3 December 2014]
Montgomery, S. (1997) What is Evolution?. [Online] Available from: http://darwin200.christs.cam.ac.uk/pages/index.php?page_id=j2 [Accessed: Monday, 1 December 2014]
NFSTC Science Serving Justice. (2007) Genetic Drifting and Natural Selection. [Online] Available from: http://www.nfstc.org/pdi/Subject07/pdi_s07_m01_02_d.htm [Accessed: Wednesday, 3 December 2014]
NFSTC Science Serving Justice. (2007) Migration and Gene Flow. [Online] Available from: http://www.nfstc.org/pdi/Subject07/pdi_s07_m01_02_c.htm [Accessed: Wednesday, 3 December 2014]
The Burgess Shale. (2011) The Cambrian Explosion. [Online] Available from: http://burgess-shale.rom.on.ca/en/science/origin/04-cambrian-explosion.php [Accessed: Tuesday, 2 December 2014]
The Burgess Shale. (2011) The Tree of Life after Cambrian Explosion. [image] Available from: http://burgess-shale.rom.on.ca/en/science/origin/04-cambrian-explosion.php [Accessed: Tuesday, 2 December 2014]