How are phylogenetic trees of life made up?

Where did you get this idea that a phylogenetic tree is based on one gene?

Nick Lane, The vital question, p. 123.

"Almost any tree of life you can look at from the modern era of molecular genetics will be based on a single gene, chosen carefully by the pioneer of molecular phylogenetics, Carl Woese -- the gene for small subunit ribosomal RNA."

Wikipedia, "16S ribosomal RNA": This gene is "...used in reconstructing phylogenies, due to the slow rates of evolution of this region of the gene."

I think I am misunderstanding something, but do not see what. Thanks for your interest.

Oh wow, I didn't know it was solely based on molecular difference. I guess the premise behind this is that  closely related organisms have a high degree of agreement in the molecular structure of these substances, while the molecules of organisms distantly related usually show a pattern of dissimilarity. According to Wikipedia, conserved sequences, such as mitochondrial DNA, are expected to accumulate mutations over time, and assuming a constant rate of mutation, provides a molecular clock for dating divergence. This makes sense then.

The article goes on to state: The most common approach is the comparison of homologous sequences for genes using sequence alignment techniques to identify similarity. Another application of molecular phylogeny is in DNA barcoding, wherein the species of an individual organism is identified using small sections of mitochondrial DNA or chloroplast DNA. Another application of the techniques that make this possible can be seen in the very limited field of human genetics, such as the ever-more-popular use of genetic testing to determine a child's paternity, as well as the emergence of a new branch of criminal forensics focused on evidence known as genetic fingerprinting.

I got an answer for this from Jerry Coyne on his web site, Why Evolution is True. Here it is:

"The answer is that it [the single gene] has to vary to make a tree, but not wildly, or you have a tree only good for closely related species and can’t identify the gene in other species. The ribosomal RNA genes tend to change very slowly, and so are good for deep phylogenies.

"Genes in mitochondrial DNA, on the other hand, change quickly, and are more useful for the closely related species (but there’s the possibility of mtDNA introgression, which screws up phylogenies)."

Thanks for your help.