Environment-dependent dependent expression of a genotype

Phenotypes are not always a direct reflection of genotypes Temperature-sensitive alleles: Siamese color pattern Nutritional effects: phenylketonuria

Siamese or “Himalayan” Different allele of the C locus that causes albinism. Temperature sensitive.

Phenylketonuria Nutritional defect: can’t metabolize phenylalanine. Can lead to severe physical and mental disorders in children, but only if they consume phenylalanine. Disease phenotype can be avoided by eliminating phenylalanine from the diet

Modern Homo Sapiens Regional-Continuity Model (Milford Wolpoff, UMich) Humans evolved more or less simultaneously across the entire Old World from several ancestral populations. Rapid-Replacement Model (Chris Stringer, NHM London) Humans evolved only once--in Africa from H. heidelbergensis ancestors--and then migrated throughout the Old World, replacing their archaic predecessors. Also called the “Out of Africa” and “Killer Ape” hypothesis.

1. Multi regional hypothesis Homo sapiens evolved out of an interbreeding network of H. erectus that had migrated through out much of the world - fossil record shows a gradual transition from archaic to modern human taking place at multiple sites outside of Africa.   Proposal - H erectus becomes gradually transformed into H. sapiens rather than being replaced by it.

2. Out of Africa hypothesis - H. erectus moved out of Africa and populated parts of Eurasia - population that remained behind evolved and gave rise to Homo sapiens (or via H. heidelbergensis as an ‘intermediate’ species) - from this single source, Homo sapiens migrated outward and displaced the H. erectus populations in Eurasia - this hypothesis predicts - only one speciation event occurred in a restricted African population - all humans should show a high degree of relatedness  

  Support for Out of Africa - African populations have more diversity in DNA than non-African population - mitochondrial DNA analysis indicates a common African ancestor - calculation indicates origin about 200,000 yr ago

What happened to Neandertals? H. neanderthalensis coexisted with H. sapiens for at least 20,000 years, perhaps as long as 60,000 years What happened? Neandertals interbred with H. sapiens Neandertals were killed off by H. sapiens H. sapiens drove Neandertals into extinction by competition

THE GENETIC CODE IS REDUNDANT Sequences of 3 bases in RNA code for a single amino acid There are 64 possible ‘triplets’ that can be formed from the 4 different bases, but there are only 20 amino acids (AA) In most cases, more than one type of triplet codes for a given AA For example, CAA and CAG both code for the same AA, glutamine A substitution of a G for the second A in CAA would constitute a neutral mutation, since the triplet would still code for glutamine (i.e., CAA becomes CAG). Thus, natural selection will not act directly to alter the frequency of the ‘mutant’ codon Such neutral mutations may serve as useful genetic markers