Structural and Developmental Homology
o Organisms show curious similarities in structure and development unrelated to function.
o The underlying design of many vertebrae forelimbs is similar even though their function
and appearance are different
o Such similarity makes sense if all vertebrae descended from a common ancestor, from
which they inherited the fundamental design of their limbs.
o What causes these similarities?
Darwin argued that descent from a common ancestor is the most logical
o Advances in molecular genetics have revealed other similarities among organisms.
Prominent among these is the genetic code.
o With a few minor exceptions, all organisms studied to date use the same nucleotide
triplets to specify the same amino acids to be incorporated into proteins.
However, an enormous number of alternative codes is theoretically possible,
some of which provide a given species advantages.
If using alternative codes is advantageous, then why do virtually all organisms
use the same one?
Darwin’s suggestion again answered this question. All organisms inherited their
genetic code from a common ancestor.
o One example can be found on chromosome 17 in the human genome. Read elaboration
on page 57.
o Another example concerns another kind of genetic quirk that might be considered as a
flaw: processed pseudogenes
These are nonfunctional copies of normal genes that originate when processed
mRNAs are accidentally reverse transcribed to DNA, then inserted back into the
genome at a new location. They are readily distinguished from their mother
genes because they lack both introns and promoters.
Since processed pseudogenes have no function, they tend to accumulate
mutations. The older a processed pseudogene, the more mutations it will have
Using Darwin’s thinking, if species are related from a common ancestor, then
older processed pseudogenes should be shared by a greater variety of species.
These pseudogenes are found in common with a great range of primates of
different life times and prove as evidence for the notion of common ancestry.
See Fig. 2.24 on page 58.
The Modern Concept of Homology
o Many biologist, now, define homology as similarity due to the inheritance of traits from
a common ancestor.
Read Box 2.2. on page 59.
Relationships among Species
o Read page 60
o The existence of closely related forms in island groups was a logical outcome of descent