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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 • • • • 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