Transcript BIO 10 Lecture 2

BIO 10
Lecture 12
EVOLUTION:
FROM GENE TO PROTEIN TO PHENOTYPE
Definitions for Evolution
• “Any change in the frequency of alleles within a
gene pool from one generation to the next." Curtis and Barnes, Biology, 5th ed. 1989 Worth
Publishers, p.974
• “The process by which living things can
undergo modification over successive
generations.” (Krogh)
• A change in the digital information carried by
living organisms over time. (Dawkins)
Evolution …
• Happens to populations, not individuals
• Leads to populations being better adapted to
their surroundings over time
• Is ultimately driven by random mutations in
DNA
– Mutations give rise to new alleles
– A new allele can be lost from the population or its
frequency can change due to:
• Selective pressure
• Random genetic drift
• Other factors
• Ultimately, evolution happens
because changes in the DNA
sequence result in survival machines
that are either better or worse fit
– Where “fitness” is defined as
reproductive fitness – i.e. how
successful that organism is at passing
its genes to the next generation
– An organism that fails to pass on his or
her genes has zero fitness to an
evolutionary biologist, even if he/she is
otherwise a very robust organism
Sorry folks! You
have ZERO fitness if
you don’t have kids!!
• Evolution “works” because the DNA sequences
(alleles) carried by survival machines code for
proteins that increase or decrease the
relative reproductive fitness of those
survival machines.
• Over time, the alleles carried by the fittest
survival machines will increase in a population
• Random mutations create new alleles that are
“tested” by the survival machines that carry
them and are either culled or retained depending
on whether that survival machine reproduced
• Therefore, the only thing that matters in the
“game of life” is how many offspring an individual
produces
The Genetic Code
– GIven the genetic code, it is easy to see how
changes in the DNA sequence encoding a
polypeptide can change the phenotype of the
organism
– A single base-pair substitution
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•
•
•
UUA (phe)
UUC (lys)
Causes an amino acid change in a polypeptide
Creates a new allele
Could destroy the function of a protein or subtly
alter its function
• Will get passed on and increase in frequency if it
increases the reproductive fitness of its host
Short Review of Lecture 12
• There are many working definitions for evolution but the
most precise is that it is the change in allele frequencies in
a population over time
• Evolution is driven by random mutations.
• Mutations give rise to new alleles that can make the
organism who carries them
– more reproductively fit
– less reproductively fit
– No change
• New alleles must affect proteins in order to be acted upon
by natural selection
• This is accomplished because alleles are transcribed into
RNA and then translated into polypeptides via the genetic
code