 Speciation is the process of forming a new species. A species is a group of beings that breed with one another and produce fertile offspring.

When the members of 2 populations cannot interbreed and produce fertile offspring, reproductive isolation has occurred.

 One type of isolating mechanism, behavioral isolation, occurs when 2 populations are capable of interbreeding but have differences in courtship rituals.

WESTERN MEADOWLARK EASTERN MEADOWLARK

 With geographic isolation, 2 populations are separated by geographic barriers such as rivers, mountains, or bodies of water.

ABERT SQUIRRELKAIBAB SQUIRREL

 Natural selection worked separately on each group and formed a distinct subspecies, the Kaibab squirrel.

 A 3 rd isolating mechanism is temporal isolation, which occurs when 2 or more species reproduce at different times.

The mechanisms of evolutionary change be observed in nature

 Peter and Rosemary Grant from Princeton University collaborated to band and measure finches on the Galapagos for over 20 years on the medium ground finch on Daphne Major.

DAPHNE MAJOR MEDIUM GROUND FINCH

 The Grants first identified and measured as many individual birds as possible on the island.

They found that birds with different sized beaks have different chances of survival during a drought. When food was scarce, birds with larger beaks had better survival.

Natural Selection Also, big beaked birds tend to mate with other big beaked birds, and the Grants therefore observed that average beak size in that finch population increased dramatically over time.

 By documenting natural selection in the wild, the Grants provided evidence of the process of evolution. The change in beak size only took a couple of decades.

 Speciation in the Galapagos finches occurred by founding of a new population, geographic isolation, changes in the new population’s gene pool, reproductive isolation, and ecological competition.

 Many years ago, a few species from the South American mainland-”species A”-flew or were blown to one of the Galapagos Islands.

 Later on, some birds from species A crossed to another island in the Galapagos group. Because species do not usually fly over open water, they rarely move from island to island and the finch populations became isolated from each other and no longer shared a common gene pool.

 Plants on the second island had sturdier seeds. Therefore, natural selection on the second island would favor individuals with larger, heavier beaks.  Over time, natural selection would have caused the population to evolve larger beaks, forming a separate population, “B”.

Reproductive Isolation Because of difference in beak size and difference in mating behavior, reproductive isolation occurs and the 2 populations become separate species.

 Species that are more different from each other are more specialized to eat certain foods, and do not have to compete with each other as much, which make individuals in both species have greater fitness.

Continued Evolution The process of isolation on different islands, genetic change, and reproductive isolation probably repeated itself many times across the entire Galapagos island chain, producing the 13 finch species found there today.

Studying Evolution Since Darwin Scientific evidence supports Darwin’s theory of evolution.

 The Grant’s work does have limitations. For example, while the Grants observed changes in size of the finches’ beaks, they did not observe the formation of a new species.

Unanswered Questions Evolution continues today, driving changes in the living world such as drug resistance in bacteria and viruses, and pesticide resistance in insects.

Unanswered Questions