Mechanisms of Evolution

Macroevolution

Speciation

Sympatric (“Same Country”) Speciation

• In sympatric speciation– Speciation takes place in geographically

overlapping populations

• Sympatric speciation: A new species originates in within the range of the parent population.

• Reproductive isolation can evolve without geographical isolation. 

– This can occur quickly (in one generation) if a genetic change results in a reproductive barrier between the mutants and the parent population. 

Polyploidy

• Polyploidy

– Is the presence of extra sets of chromosomes in cells due to accidents during cell division

– Has caused the evolution of some plant species

• Speciation by the formation of polyploids

• Many plant species have originated from improper cell division that results in extra sets of chromosomes. This is a mutant condition called polyploidy.

• Depending on the origin of the extra set of chromosomes, polyploids are either: autopolyploids or allopolyploids.

• Autopolyploid: An organism that has more than two sets of chromosome, all derived from a single parent species. See Figure 24.8.

• 1. Nondisjunction in the germ cell line (in either mitosis or meiosis) results in diploid gametes.

• 2. Self-fertilization would double the chromosome number to tetraploid.

• 3. Tetraploids can self-pollinate or mate with other tetraploids.• 4. The mutants cannot interbreed with diploids of the parent

population because triploid hybrids would be sterile due to unpaired chromosomes interfering with meiosis.

Figure 24.8

2n = 64n = 12

2n

4n

Failure of cell divisionin a cell of a growing diploid plant afterchromosome duplicationgives rise to a tetraploidbranch or other tissue.

Gametes produced by flowers on this branch will be diploid.

Offspring with tetraploid karyotypes may be viable and fertile—a new biological species.

• An instantaneous special genetic event could thus produce a postzygotic barrier which isolates the gene pool of the polyploid in just one generation.

• Allopolyploid: A polyploid hybrid with parents of two different species. See Figure 24.9.

• 1. More common than autopolyploidy.• 2. Potential evolution of an allopolyploid begins when two different

species interbreed and a hybrid is produced.• 3. Such interspecific hybrids are usually sterile: The haploid sets

of chromosomes from each species cannot pair with the chromosomes from the other species.

• 4. These sterile hybrids may propagate asexually.

Figure 24.9

Meiotic error;chromosomenumber notreduced from2n to n

Unreduced gametewith 4 chromosomes

Hybrid with7 chromosomes

Unreduced gametewith 7 chromosomes Viable fertile hybrid

(allopolyploid)

Normal gameten = 3

Normal gameten = 3

Species A 2n = 4

Species B 2n = 6

2n = 10

• Two mechanisms can transform sterile allopolyploids into fertile polyploids.

• 1. Mitotic nondisjunction in the reproductive tissue of the hybrid may double chromosome number. – The hybrid clone will then be able to produce

gametes by meiosis because each chromosome will have a homologue.

– This would produce a new species of interbreeding individuals, reproductively isolated from both parent species.

• 2. Meiotic nondisjunction in one species produces an unreduced (diploid) gamete (24.9). – This abnormal gamete fuses with a normal haploid gamete of a

second species and produces a triploid hybrid.– In the sterile triploid clone, meiotic nondisjunction again produces

an unreduced gamete (triploid).– Combination of this triploid gamete with a normal haploid gamete

from the second parent species would result in a fertile hybrid with homologous pairs of chromosomes.

Figure 24.9

Meiotic error;chromosomenumber notreduced from2n to n

Unreduced gametewith 4 chromosomes

Hybrid with7 chromosomes

Unreduced gametewith 7 chromosomes Viable fertile hybrid

(allopolyploid)

Normal gameten = 3

Normal gameten = 3

Species A 2n = 4

Species B 2n = 6

2n = 10

• Speciation by polyploidy (especially allopolyploidy) has been important in plants. (relatively rare in animals)

• Some allopolyploids are very vigorous because they contain the best qualities of both parent species.

• The accidents required to produce these new plant species (interspecific hybridization coupled with nondisjunction) have occurred often enough that between 25% and 50% of all plant species are polyploids.

Sympatric speciation may also occur animals, but the mechanisms are different.

• A group of animals may become isolated within the range of a parent population if genetic factors cause them to become fixed on resources not used by the parent population as a whole.

Resource Fixation

• Example:• 1. A particular species of wasp pollinates a

particular species of figs. The wasps mate and lay their eggs in the figs.

• 2. A genetic change causes some wasps to select a different fig species. This would segregate mating individuals of the new phenotype from the parent population.

• 3. Divergence could then occur after such an isolation.

Sympatric speciation may also occur animals, but the mechanisms are different.

• Sympatric speciation could result from balanced polymorphism combined with assortative mating (sexual selection).

Polmorphism and assortative mating

• • Example: if birds in a population that is dimorphic for beak size began to selectively mate with birds of the same morph, speciation could occur over time.

• Also blue/white snowgeese example

Sympatric speciation may also occur animals, but the mechanisms are different.

• Sympatric speciation– Can also result from the appearance of new

ecological niches

Habitat Differentiation and Sexual Selection • In cichlid fish

– Sympatric speciation has resulted from nonrandom mating due to sexual selection

Figure 24.10

Researchers from the University of Leiden placed males and females of Pundamilia pundamilia and P. nyererei together in two aquarium tanks, one with natural light and one with a monochromatic orange lamp. Under normal light, the two species are noticeably different in coloration; under monochromatic orangelight, the two species appear identical in color. The researchers then observed the mating choices of the fish in each tank.

EXPERIMENT

P. nyererei

Normal lightMonochromatic

orange light

P. pundamilia

Under normal light, females of each species mated only with males of their own species. But under orange light, females of each species mated indiscriminately with males of both species. The resulting hybrids were viable and fertile.

RESULTS

The researchers concluded that mate choice by females based on coloration is the main reproductive barrier that normally keeps the gene pools of these two species separate. Since the species can still interbreed when this prezygotic behavioral barrier is breached in the laboratory, the genetic divergence between the species is likely to be small. This suggests that speciation in nature has occurred relatively recently.

CONCLUSION

Allopatric and Sympatric Speciation: A Summary

• In allopatric speciation– A new species forms while geographically

isolated from its parent population

• In sympatric speciation– The emergence of a reproductive barrier

isolates a subset of a population without geographic separation from the parent species

Allopatric vs. Sympatric speciation.

• Both allopatric speciation and sympatric speciation have important roles in plant evolution, but in animals, allopatric speciation is much more common.

Adaptive Radiation

• Adaptive radiation– Is the evolution of diversely adapted species from a

common ancestor. Generally occurs when new environments open up to colonization.

Figure 24.11

Adaptive Radiation

• The Galapagos are volcanic islands close enough to the coast of South America to allow for occasional colonization, yet far enough to prevent significant gene flow.

• They are relatively young islands, and they provided new uninhabited environments waiting to be exploited by colonizing species.

• As organisms adapted to the unique environments on the islands, many endemic species diverged.

Adaptive Radiation• Another example is the Hawaiian Archipelago, volcanic islands 3500 km

from the mainland.

• 1. Hawaii is the youngest (<one million years old), largest island and has active volcanoes.

• 2. The islands grow progressively older in a northwesterly direction away from Hawaii. 

• 3. As each island was formed and cooled, flora and fauna carried by ocean and wind currents from other islands and continents became established. 

• 4. The physical diversity of each island provided many environmental opportunities for evolutionary divergence. 

• 5. Multiple invasions and allopatric speciation have caused so much adaptive radiation that there are thousands of species endemic to the islands.

• The Hawaiian archipelago– Is one of the world’s great showcases of

adaptive radiation

Figure 24.12

Dubautia laxa

Dubautia waialealae

KAUA'I5.1

millionyears O'AHU

3.7millionyears

LANAI

MOLOKA'I

1.3 million years

MAUI

HAWAI'I0.4

millionyears

Argyroxiphium sandwicense

Dubautia scabra Dubautia linearis

N