The Process of Speciation 16-3 The Process of Speciation

Isolating Mechanisms speciation: the formation of new species As new species evolve, populations become reproductively isolated from each other reproductive isolation: when members of two populations cannot interbreed and produce fertile offspring (3 types) behavioral isolation capable of interbreeding but have differences in courtship rituals and other behaviors geographic isolation two populations are separated by geographic barriers (rivers, mountains, or bodies of water) temporal isolation two or more species reproduce at different times

Testing Natural Selection in Nature The finches Darwin observed on the Galápagos Islands are examples of natural selection in action They all descended from a common ancestor… Natural selection shaped the beaks of different populations as they adapted to eating different kinds of foods Peter and Rosemary Grant (Princeton University) tested the hypothesis on a medium ground finch on Daphne Major Variation After making extensive measurements of many characteristics of the finches they concluded there was great variation of inheritable traits

Testing Natural Selection in Nature During periods of drought food became scarce and beak size would determine which birds were more likely to survive. Those with larger beaks had better chance of survival. Beak size also became important in type of food eaten and mating behavior. Average beak size in the population increased accordingly. Rapid evolution Changes in food supply caused rapid fluctuations in finch population. This documented example of natural selection provided evidence of evolution.

Speciation in Darwin’s Finches Evolution of the Galápagos finches occurred through a series of steps 1. founders arrive many years ago a few South American finches lost or blown off course arrived at one of the Galápagos Islands (Species A) 2. separation of populations members of the population on the first island crossed to another island and became isolated 3. changes in the gene pool each population became adapted to the local environments different environmental pressure (food source) produced finches with different beaks (Species B)

Speciation in Darwin’s Finches 4. reproductive isolation if finches from Species B cross back to first island they will not breed with Species A because of differing courtship behaviors and preferences 5. ecological competition the two species live together in the same environment on the first island extreme variations within the two species allow for improved survival through specialization for different food sources eventually Species B on the first island evolves into Species C 6. continued evolution isolation on different islands, genetic change, and reproductive isolation repeated itself many times to produce 13 different species of finches

One island in this cluster of three is seeded by a small colony of population A from the mainland population Its gene pool is now isolated – develops into population B Storms or other agents of dispersion spread species B to a second island The isolated colony evolves into population C …where it adapts and forms species D Species D is dispersed to the two islands of its ancestors …and evolves into a new species, E, on one of the islands… Later, individuals from species C recolonize the first island and cohabit with species B (reproductive barriers keep them distinct) A colony of species C also populates a third island