BIO 10 BIO 10 Lecture 12Lecture 12BIO 10 BIO 10 Lecture 12Lecture 12

EVOLUTION: EVOLUTION:

FROM GENE TO PROTEIN TO PHENOTYPEFROM 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 • 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