Chapter 17

EVOLUTION:MACROEVOLUTION & ORIGIN OF

SPECIES

I. MacroevolutionA. large-scale change at or above species level

B. includes development of new species

C. fundamentally linked to microevolution• continued microevolution often leads to development

of new sp.

II. What is a Species?A. many different definitions

B.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 speciation

1. 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 eschscholtziklauberi

Evolution 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 integrity

B. prevent two species from interbreeding to produce fertile offspring

C. prezygotic barriers1. occur before a zygote

is formed

2. habitat isolation

3. behavioral isolation

4. temporal isolation

5. mechanical isolation

6. gamete isolation

D. postzygotic barriers1. take place only if all

prezygotic barriers are bypassed

2. zygote mortality

3. F2 fitness

4. hybrid sterility

Fig. 17.5 Temporal isolation

Fig. 17.4 Reproductive isolating mechanisms