Speciation
How can one species become two? Species: a group of organisms that can interbreed and produce viable offspring Speciation: the process of forming new species
Reproductive Isolation Reproductive isolation: when populations of the same species no longer interbreed with each other. Once this occurs that populations may evolve into different species What can cause reproductive isolation?
Barriers to Reproduction Two Types: Pre-zygotic Barriers Post-Zygotic Barriers What is a zygote?
Pre-zygotic Barriers
1. Behavioral Isolation Differences in mating rituals Two separate populations that are capable of interbreeding develop differences in courtship
Different species of bowerbird construct elaborate homes to woo females. The Satin bowerbird (left) builds a channel between upright sticks, and decorates with blue objects. The MacGregor s Bowerbird (right) builds a tall tower and decorates with bits of charcoal. Evolutionary changes in mating rituals contribute to speciation.
Different bowerbird nests
2. Geographic Isolation Barriers: keep species separate and allow them to evolve and adapt separately Differing habitats
Kaibab squirrel Lives on the north side of the Grand Canyon Abert squirrel Lives on the south side of the Grand Canyon These two squirrel populations became isolated over 10,000 years ago, preventing their inbreeding.
3. Temporal Isolation Mate at different times of the year Spring field crickets and fall field crickets mate in different seasons These orchids bloom on different days. The flowers that last only a day and must be pollinated on that day to produce seeds.
The eastern and the western spotted skunk mate at different times of year
Post-zygotic Barriers
Reproductive isolation Post zygotic barriers if members of different species try to mate, the sperm from one species and the egg from another are incompatible with each other Hybrid could be sterile (Hybrid Infertility) Hybrid could die (Hybrid death)
*Sometimes different species can interbreed* The liger is probably the largest cat in the world, usually bigger than either of its parent species. At an average weight of 900 pounds, ligers can be twice the size of male Siberian tigers, the largest non-extinct naturally occurring member of the cat family.
Example: Speciation in Darwin s Finches How might the founder effect and natural selection have produced reproductive isolation that could have led to speciation among Galapagos finches? Founding a new population (founders), geographic isolation, changes in population gene pool, behavioral isolation, ecological isolation.
Founders Arrive Many years ago, a few finches from South America species M arrived on one of the Galápagos islands, as shown in the figure. Because of the founder effect, the allele frequencies of this founding finch population could have differed from those in the South American population.
Geographic Isolation The islandʼs environment was different than the environment in South America Combinations of the founder effect, geographic isolation, and natural selection produces different species. Crossed to other islands
Changes in Gene Pools Over time, populations on each island adapted to local environments. Natural selection could have caused two distinct populations to evolve (A and B), each characterized by a new phenotype.
Behavioral Isolation If the two different species of birds did meet, they would not interbreed because finches mate with birds that have the same sized beaks The gene pools remain isolated.
Competition and Continued Evolution Birds that are most different from each other have the highest fitness. More specialized birds have less competition for food. Over time, species evolve in a way that increases the differences between them, and new species may evolve (C, D, and E).