BIO120H1 Chapter Notes - Chapter 1: Pelargonidin, Penstemon, Cyanidin
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Introduction, motivation, goals and advice
1. How do the characteristics of organisms achieve life's functions?
oThis question asks for answers that focus on the functioning of the whole
organism and also its organs, tissues, cells, organelles, and molecules.
2. Why do organisms have the characteristics they do?
oWhat evolutionary effects attributed to the organism's characteristics?
Reductionist explanation: an explanation that tells a clear, unambiguous story at the level of
genes and molecules
•This explanation is proximate- it doesn't explain the underlying cause hence doesn't
answer the why question.
•Example of a reductionist explanation:
oThe wildflower Penstemon strictus has blue flowers but its close relatives P.
Barbatus has red flowers. This is because they have differing biochemical pathways
for the synthesis of anthocyanin pigments differ between the two species.
Penstemon strictus makes an enzyme that sends a precursor compound into
biosynthetic pathways that produce the blue purple pigments cyanidin and
delphinidin. In P. Barbatus, that enzyme is lost; the precursor instead goes into a
pathway that makes a red anthocyanin, pelargonidin.
oThis explanation doesn't the following why question: why is one pathway active in
one species and not the other?
oThe proposed answer to the why question is the following.
•The evolutionary lineage leading to Penstemon barbatus had blue
flowers that were adapted for pollination by bees (like P. Stratus), but
encountered ecological conditions in which hummingbirds provided better
•In the hummingbird-rich environment, natural selection favoured red-
flowered mutant plants because the red-flowered plants were more attractive to
the hummingbirds. The genes that make red flowers spread through
populations, replacing the blue genes.
oThis "ultimate" explanation is untidy; this explanation is essentially a complex
hypothesis that weighs on a series of assumptions, most of which are non-trivial to
test because the events that are hypothesized has already happened in the past.
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