Process of Speciation

Process of Speciation

• How do natural selection and genetic drift create new species?– Speciation – formation of new species– Species – group of organisms that breed with

one another and produce fertile offspring – share a common gene pool

Isolating Mechanisms

• As new species evolve, populations become reproductively isolated from each other

• When members of two populations cannot interbreed and produce fertile offspring, reproductive isolation has occurred

• Three types of isolating mechanisms:– Behavioral isolation– Geographic isolation– Temporal isolation

Behavioral Isolation

• Two populations are capable of interbreeding but have differences in courtship rituals or other types of behavior – this prevents interbreeding– Ex. Birds using different “songs” to attract

members of their own species

Geographic Isolation

• Two populations are separated by geographic barriers such as rivers, mountains, bodies of water, or man-made barriers (roads, walls, etc)

• Does not guarantee formation of new species because lakes may be linked or a land bridge may be present between islands, allowing “mixing”

Temporal Isolation

• Two or more species reproducing at different times– Ex. Orchid species

release pollen on separate days

Testing Natural Selection in Nature

• Recall the finches Darwin observed while visiting the Galapagos Islands. The finches varied greatly in the sizes and shapes of their beaks and feeding habits

• Darwin first thought they were all different species of birds, but discovered that they were all finches and hypothesized that they had descended from a common ancestor. He proposed natural selection shaped the beaks as they adapted to different foods and feeding behaviors

Testing Natural Selection in Nature (continued)

• Darwin’s hypothesis was tested by Peter and Rosemary Grant. They measured finches on the islands– Variation – Grants’ had to identify and measure as many birds

as possible (they measured beak characteristics) and from their data, they concluded there was a great variation of traits (these traits are polygenic!)

– Natural selection – different sized beaks meant different chances of survival during droughts – largest beaks survived when food was scarce

– Rapid evolution – next generation of finches had larger beaks and they found that natural selection took place frequently and sometimes rapidly

Galapagos Island Finches

Speciation in Darwin’s Finches

• Hypothetical scenario for the evolution of all Galapagos finches from a single group of “founding” birds

Speciation in Darwin’s Finches (continued)

• 1) Founders Arrive– Many years ago, a few

finches from South America (Species A) flew or were blown by a storm to one of the islands where they managed to survive and reproduce (finches don’t normally fly over large bodies of open water).

Speciation in Darwin’s Finches (continued)

• 2) Separation of Population– Later on, some birds from

Species A crossed to another island in the Galapagos group. Because finches rarely move over open water, the finches were essentially isolated from each other, no longer sharing a gene pool

Speciation in Darwin’s Finches (continued)

• 3) Changes in the Gene Pool– Over time, populations on

each island became adapted to their local environments. Different plants on the 2 different islands caused different beak sizes to become apparent. Birds with bigger beak sizes on the second island survived better, creating a new species, Species B

Speciation in Darwin’s Finches (continued)

• 4) Reproductive Isolation– A few birds from

Species B cross back to the first island and do not breed with Species A because they choose their mates carefully and only breed with big-beak birds

Speciation in Darwin’s Finches (continued)

• 5) Ecological Competition– Species A and B on the first

island compete with each other for food. During the dry season, individuals that are most different from each other have the highest fitness and less competition. Over time, species evolve in a way that increases the differences. Species B birds on the first island may then evolve in Species C

Speciation in Darwin’s Finches (continued)

• 6) Continued Evolution– This whole process

repeated itself time and time again across all the Galapagos Islands. Now, there are 13 different finch species

Speciation in Darwin’s Finches (continued)