Process of Speciation

Process of Speciation

– In the 150 years since the publication of Darwin’s book On the Origin of Species by Means of Natural Selection, new discoveries and technological advances have given scientists a wealth of new information about the evolution of life.

– The diversity of life evolved through speciation, the process in which one species splits into two or more species.

Process of Speciation

• How do natural selection and genetic drift create new species?– The biological species concept defines a

species as “A group of populations whose members have the potential to interbreed with one another in nature to produce fertile offspring.”

Reproductive Barriers

• 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

• Two types of reproductive barriers:– Prezygotic barriers – prevent mating – temporal,

habitat, behavioral, mechanical, gametic– Postzygotic barriers – hybrid zygotes

Prezygotic Barriers – Temporal Isolation

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

release pollen on separate days

Prezygotic Barriers – Habitat Isolation

• Two populations are separated by living in different habitats– Ex. One species of snakes living in water,

while another living on land

Prezygotic Barriers – 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

Prezygotic Barriers – Mechanical Isolation

• Egg producing and sperm producing structures of different specis are anatomically incompatible– Ex. Snail shells spiral in opposite directions

Prezygotic Barriers – Gametic Isolation

• Gametes are incompatible – they won’t fuse together– Ex. Sea urchins

Postzygotic Barriers

– Postzygotic barriers operate if• interspecies mating occurs and • hybrid zygotes form.

– Postzygotic barriers include• reduced hybrid viability,• reduced hybrid fertility, and• hybrid breakdown.

Mechanisms of Speciation

– A key event in the potential origin of a species occurs when a population is somehow cut off from other populations of the parent species.

– Species can form by• allopatric speciation, due to geographic isolation,

or• sympatric speciation, without geographic isolation.

Mechanisms of Speciation

Allopatric speciation Sympatric speciation

Allopatric Speciation

– Geologic processes can• fragment a population into two or more isolated

populations• contribute to allopatric speciation.

– Speciation occurs with the evolution of reproductive barriers between

• the isolated population and• its parent population.

– Even if the two populations should come back into contact at some later time, the reproductive barriers will keep them as separate species.

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)

Sympatric Speciation

– Sympatric speciation occurs in populations that live in the same geographic area.

– An accident during cell division that results in an extra set of chromosomes is a common route to sympatric speciation in plants.

Sympatric Speciation

– Many domesticated plants are the result of sympatric speciation, including

• oats,

• potatoes,

• bananas,

• peanuts,

• apples,

• coffee, and

• wheat.

What is the Pace of Speciation?

– There are two contrasting patterns for the pace of evolution:

1. the gradual pattern, in which big changes (speciations) occur by the steady accumulation of many small changes, and

2. the punctuated equilibria pattern, in which there are– long periods of little apparent change (equilibria)

interrupted (punctuated) by– relatively brief periods of rapid change.

What is the Pace of Speciation?

Punctuatedpattern

Gradualpattern

Time