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Lecture 4

BIOB51H3 Lecture 4: 4

Biological Sciences
Course Code
Maydianne Andrade

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Similarity of Characteristics
o ‘Likeness of similarities of animals have been recognized for centuries
o Homology (old definitions)
o Study of ‘likeness’
The puzzling thing people found that organisms have similarities
underlying their external traits
o “The same organ in different animals under every variety of form and function”
o Comes from the same embryonic precursors (stem) cells
o Similarities recognized before organisms were ever classified into evolutionary
o Homology
o Suggests common origin of structures (in some common ancestors)
o Examples of homology from 4 types of characteristics:
o Structural homology
Similar features/structures in similar relative positions from the same
embryonic precursors
Different functions in different organisms
(e.g. vertebrae forelimbs)
o Vestigial structures
Apparently functionless or reduced structures or organs that are found
that are similar those functional organs in related species
Formed from similar embryonic precursor cells found in the similar
relative positions
(e.g. vestigial hind limbs in snakes, semilunar fold in humans)
o Developmental Homology
Animals that we now classify together, despite having different adult
forms also have similar embryonic stages
We now know that this arises partly because there’s a similar genetic
developmental program
o Molecular homology
Not evidence that was available to Darwin
Most compelling evidence we have for common ancestry
I. Universal genetic code
The way in which base pairs code for amino acids is
common across all life
Other codes could do the jobs better in terms of accuracy
with fewer errors
If humans had a different code, this would remove the risk
of viruses that jump from other species (HIV, H1N1)
II. Large sections of DNA are similar in species classified together
Functional DNA is similar
Even true in functionless DNA (introns)
III. Chromosomal homology
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