Chapter 17

EVOLUTION:MACROEVOLUTION & ORIGIN OF

SPECIES

I. MacroevolutionA. large-scale change at or above species levelB. includes development of new speciesC. fundamentally linked to microevolution

• continued microevolution often leads to development of new sp.

II. What is a Species?A. many different definitionsB.Ernst Mayr’s “Biological Species Concept”

1. group of interbreeding natural populations2. produce fertile offspring3. reproductively isolated from other such groups4. problems?

C. subspecies• geographical variety of the same species

III. SpeciationA. formation of a new species

1. key factors are geography and environmental conditions 2. ultimate goal is reproductive isolation

B. allopatric speciation1. large, continuous pop. geographical change pop. split into

two groups geographic isolation env. conditions different for each group two groups evolve separately due to nat. sel., etc. reprod. isol.

2. many processes can lead to geographic isolation

a. natural environmental changeb. human causesc. individuals leaving on their own

i. dispersalii. founder effect

A schematic view of allopatric speciation

Ensatina eschscholtzi picta

Ensatina eschscholtzi platensis

1

2

3

Members of a northern ancestral population migrated southward.

Ensatina eschscholtzioregonensis

Subspecies are separated by California’s Central Valley. Some interbreeding between populations does occur.

Ensatina eschscholtzixanthoptica Ensatina eschscholtzi

croceater

Ensatina eschscholtzieschscholtzii

Ensatina eschscholtziklauberiEvolution has occurred, and in

the south, subspecies do not interbreed even though they live in the same environment.

CentralValleyBarrier

Fig. 17.8 Allopatric speciation in salamanders

Fig. 17.9 Allopatric speciation among sockeye salmon. In Lake Washington, salmon that matured (a) at Pleasure Point Beach do not reproduce with those that matured in (b) Cedar River. The females from Cedar River are noticeably larger and the males are more slender than those from Pleasure Point Beach, and these shapes help them reproduce in the river.

C. sympatric speciation1. occurs in pops. occupying the same area

• no geographic isolation, but reproductive isolation still occurs

2. most common in plants• hybridization, polyploidy, etc.

A schematic view of sympatric speciation

Fig. 17.12. An example of sympatric speciation – polyploidy in plants. Reproduction between two species of Clarkia results in a sterile hybrid. Doubling of the chromosome number results in a fertile third Clarkia species that can reproduce only with itself.

IV. Reproductive Isolating MechanismsA. means by which species maintain their integrityB. prevent two species from interbreeding to produce fertile

offspringC. prezygotic barriers

1. occur before a zygote is formed

2. habitat isolation3. behavioral isolation4. temporal isolation5. mechanical isolation6. gamete isolation

D. postzygotic barriers1. take place only if all

prezygotic barriers are bypassed2. zygote mortality3. F2 fitness4. hybrid sterility

Fig. 17.5 Temporal isolation

Fig. 17.4 Reproductive isolating mechanisms