Sounds like he is trying to apply a relatively narrow definition to broader meaning which is has no business defining. Homology is a term used to define structures which are made from the same morphogenic precursors in separate species. For instance, a bat wing is homologous to a human arm, in that the same structures are used, morphologically speaking, to create them, while a birds wing is only analogous to a butterfly wing, since they preform the same function, but are derived from different precursors. Homology does not define the relatedness of species. The human is not homologous to the chimp. I don't think, strictly speaking, that a human hand could be considered homologous to a chimp hand, they are both derived from the same precursors, but they are also both hands, they are the same structure.
Yikes -- wrong info! I came across this thread by a google search and felt the need to comment on this in case anyone else stumbles upon this thread. This explanation is so wrong.
Homologous by its very definition is any structure that shares common ancestry (but do not necessarily share the same function). In the event that homologous structures DO share the same function, they are known as
orthologous structures. On the other side of the spectrum, if two structures function similarly (as in the example of bird vs. insect wings) but are NOT ancestrally related, they are
analagous structures. The similarity between analagous structures come about because of similar environmental extremes select for certain features that are shared amongst a wide variety of species than enhances their ability to survive and reproduce and thereby, increase their fitness. (Analgous structures occur as a result of convergent evolution, while homologous structures occur due to divergent evolution). This is the essence of evolution by natural selection.
Note, homology has absolutely nothing to do with morphology as this person pointed out. A bat wing and the human are (same function, but different ancestral origin) are analgous structures, as are the bird and butterfly wing example. Homology DOES define the relatedness of species (consider what a phylogenic tree tells you).
On a genetic level, gene duplications during recombination and transpositions (protein "domain shuffling") give rise to protein families (an evolving ancestral protein that gives rises to similar functioning protein or protein subunits, example: different classes of hemoglobin), as well as Homologous Proteins (structures that have similar function but as a result in slight changes in structure and levels of gene expression -- are categorized under a unique species, example: Horse vs. Human Cytochrome C). This is accurate:
On the genetic level, yes, individual genes can be considered homologs if they code for a similar protein that has the same general functionality. There are two types of genetic homologs, paralogs and orthologs. In paralogs, the last common ancestor underwent gene duplication before speciation split the lineages of the two genes or proteins, e.g. human beta-globin is the paralog of both human alpha-globin and horse alpha-globin, while all three are paralogs of human myoglobin. In Orthologs, the last common ancestor split into two lineages without gene duplication. Horse alpha-globin is the ortholog of human alpha-globin and human myoglobin is an ortholog of horse myoglobin.
The wording quoted by the OP is a little confusing. The fact that most of our DNA is nearly identical is due to the fact that we share common ancestry but the differences are a result of mutation and chemical changes that occur naturally. Therefore, both chimps and humans are homologs of one another, both as a whole and on a molecular level (proteins, genes, etc). As far as our phenotypic differences, although we are genetically similar, the reason why human and chimps are phenotypically different is because in addition to slight variations in protein structure (though function is the same), on a genetic level, mutations in DNA can impart changes in gene expression, thus altering, when, where, and how much of the relevant protein would be transcribed. This results in differing behavioral and adaptive differences between both species.
