NATURAL SELECTION ALTERS THE EXPECTED POPULATION DISTRIBUTION Normal distribution

• AVERAGE IS FAVORED ex: baby birth weight

• ONE EXTREME SELECTED Ex: Peppered moths

• BOTH EXTREMES SELECTED ex: butterfly mimics Can lead to speciation Normal curve Disruptive selection

i.e. due to geographic factors VARIATION ALSO EXISTS BETWEEN POPULATIONS i.e. due to geographic factors Island Fox Urocyon littoralis Channel Islands, CA Ex: the average number of caudal (tail) vertebrae U. littoralis from separate islands are physically / genetically distinct.

Can be a gradual change, called a cline Ex: A ‘ring species Variants can/do interbreed where range overlaps But adjacent variants do not interbreed where the circle closes

Example: Ensatina salamanders

Not all variation is heritable Phenotype is influenced by environment as well In mammal species that colonize islands, big ones have a tendency to shrink while small ones are apt to enlarge mainland island island mainland

OTHERS FACTORS leading to EVOLUTION Genetic Drift Populations experience unique (and random) mutations, becoming less alike over time River divides the population

OTHERS FACTORS leading to EVOLUTION Genetic Drift

OTHERS FACTORS leading to EVOLUTION Founders Effect A few individuals are the founders of a new population; this group lacks all the variations of the original population

OTHERS FACTORS leading to EVOLUTION Founders Effect Summer 1995: at least 15 iguanas survived Hurricane Marilyn on a raft of uprooted trees eventually colonizing Anguilla. These were the first Iguana iguana, to reach the island. Will this population become distinct in time?

OTHERS FACTORS leading to EVOLUTION The Bottleneck Effect

OTHERS FACTORS leading to EVOLUTION The Bottleneck Effect The effect of loss of prairie habitat on greater prairie chickens in Illinois. The bottleneck led to reduced genetic variation, as measured by number of alleles per locus. The reduced genetic variation is also reflected in lowered egg hatching rates.

Gene Flow populations become more alike as genes are exchanged hybridization seeds, pollen seeds, pollen Genetic engineering hitch-hikers plasmid exchange