Transcript Speciation
Speciation What is Speciation? How does speciation occur? The formation of a species; when two or more species are created from an ancestral group Occurs when a population can no longer interbreed with any other population Reproductive Barriers Reproductive barriers prevent organisms from two different species from producing viable, fertile offspring Prezygotic Barriers Prevent mating between species Do not allow the egg to be fertilized if members of different species attempt to mate Postzygotic Barriers Prevent the zygote from developing into a viable, fertile adult if the egg does become fertilized Prezygotic Barriers Habitat Isolation Species that live in different habitats within the same area will rarely (if ever) encounter one another Behavioral Isolation Special signals that attract mates are species-specific Ie. courtship rituals Prezygotic Barriers Temporal Isolation Species that breed during different times of the day, different seasons, or different years cannot mix their gametes Mechanical Isolation Anatomically incompatible species Gametic Isolation Sperm of one species can’t survive in the female reproductive tract of another species Ova may only recognize sperm of their own species Postzygotic Barriers Reduced Hybrid Viability Offspring dies during embryonic development Reduced Hybrid Fertility Even if hybrids survive, they are usually sterile Mule – horse & donkey Hybrid Breakdown Even if the first generation is fertile and viable, offspring of the first generation is infertile What are the two ways in which speciation can occur? 1. Allopatric Speciation 2. Sympatric Speciation Modes of Speciation Figure 24.5 (a) Allopatric speciation. A population forms a new species while geographically isolated from its parent population. (b) Sympatric speciation. A subset of a population forms a new species without geographic separation. Allopatric Speciation Happens due to physical separation – dispersal to new habitat, physical barrier Example: squirrel species on opposite rims of the Grand Canyon Figure 24.8 A. formosus A. nuttingi Atlantic Ocean Example: 15 sibling species of shrimp at the isthmus of Panama Isthmus of Panama Pacific Ocean A. panamensis A. millsae Sympatric Speciation Sympatric speciation occurs when new species arise within the same area of the ancestral population Reproductive isolation evolves without geographical isolation What are some factors that can result in sympatric speciation taking place? Occupation of a new ecological niche (food source, breeding grounds, habitat location) Polyploidy Sexual Selection Polyploidy in plant species Ex: strawberries, wheat, potatoes Sexual Selection in Cichlid species Females or males choose mates based on certain characteristics which can cause reproductive isolation Ex: Cichlid species in lake victoria Cichlids in Lake Victoria! What can be learned? What can be learned from this experiment? Species grown separately with separate food sources led to a reproductive barrier in fruit flies -> allopatric speciation! How quickly do new species evolve? Two theories: Gradualism Punctuated Equilibrium Figure 24.17 (a) Punctuated pattern Time (b) Gradual pattern Represent these two models in graphic form Gradualism Species are the product of slowly accumulating, small evolutionary changes Punctuated Equilibrium Species undergo long periods of very little change, followed by rapid, large evolutionary changes Adaptive Radiation What is Adaptive Radiation? When one species evolves into many species that occupy a variety of available niches Common after Mass extinctions or when organisms are isolated on islands Ex: mammals, galapagos finches 1. Bishop pines and Monterrey pines release their pollen at different times of year Pre or post reproductive barrier? Specific type? Allopatric or Sympatric? 2. To attract male fireflies, female fireflies give a species-specific sequence of flashes Pre or Post? Specific Type? Allopatric or Sympatric? 6. Eastern meadowlarks and western meadowlarks are almost identical morphologically, but hybrid offspring are largely infertile Pre or Post? Specific Type? Allopatric or Sympatric?