• Chapter 24 ~ Chapter 24 ~ The Origin of The Origin of SpeciesSpecies

What is a species?What is a species?• Morphological species Morphological species

concept (Linnaeus): concept (Linnaeus): grouping based on how organisms look

• Biological species concept Biological species concept (Mayr):(Mayr): a population or group a population or group of populations whose members of populations whose members have the potential to interbreed have the potential to interbreed and produce viable, fertile and produce viable, fertile offspringoffspring

If species are defined by If species are defined by reproductive ability...reproductive ability...

• Group Group isolationisolation leads to a leads to a stop in the exchange of stop in the exchange of genetic informationgenetic information

• Gene flowGene flow is commonly is commonly the force that causes a the force that causes a mixing of alleles between mixing of alleles between populationpopulation

• Genetic divergenceGenetic divergence is a is a build up of differences build up of differences between separated between separated populationspopulations

Reproductive Isolation (isolation of gene pools)Reproductive Isolation (isolation of gene pools)

• Reproductive Isolating Mechanism:Reproductive Isolating Mechanism: Force that prevents successful mating between populations

• Prezygotic barriers:Prezygotic barriers: impede mating between species or impede mating between species or hinder the fertilization of the ovahinder the fertilization of the ova– Ecological/Habitat (garter snakes; water/terrestrial)Ecological/Habitat (garter snakes; water/terrestrial)– Temporal (eastern spotted skunks; seasonal mating)Temporal (eastern spotted skunks; seasonal mating)– Behavioral (blue-footed boobies; mate signaling)Behavioral (blue-footed boobies; mate signaling)– Mechanical (monkey flowers; pollination anatomy)Mechanical (monkey flowers; pollination anatomy)– Gametic (sea urchins; egg coat receptors)Gametic (sea urchins; egg coat receptors)

Reproductive IsolationReproductive Isolation

• Postzygotic barriers: Postzygotic barriers: fertilization occurs, but the hybrid fertilization occurs, but the hybrid zygote does not develop into a viable, fertile adultzygote does not develop into a viable, fertile adult– Reduced hybrid viability (salamanders; zygotes Reduced hybrid viability (salamanders; zygotes

fail to develop or reach sexual maturity)fail to develop or reach sexual maturity)– Reduced hybrid fertility (mule; horse x donkey; Reduced hybrid fertility (mule; horse x donkey;

cannot backbreed)cannot backbreed)– Hybrid breakdown (rice and cotton; 2nd Hybrid breakdown (rice and cotton; 2nd

generation hybrids are sterile)generation hybrids are sterile)

Prezygotic and postzygotic barriersPrezygotic and postzygotic barriersPrezygotic barriers impede mating or hinder fertilization if mating does occur

Individualsof differentspecies

Matingattempt

Habitat isolation

Temporal isolation

Behavioral isolation

Mechanical isolation

HABITAT ISOLATION TEMPORAL ISOLATION BEHAVIORAL ISOLATION MECHANICAL ISOLATION

(b)

(a)(c)

(d)

(e)

(f)

(g)

Viablefertile

offspring

Reducehybrid

viability

Reducehybridfertility

Hybridbreakdown

Fertilization

Gameticisolation

GAMETIC ISOLATION REDUCED HYBRID VIABILITY

REDUCED HYBRID FERTILITY HYBRID BREAKDOWN

(h)(i)

(j)

(k)

(l)

(m)

Limitations of the Biological Limitations of the Biological Species ConceptSpecies Concept

• The biological species concept cannot The biological species concept cannot be applied tobe applied to– Asexual organismsAsexual organisms

– FossilsFossils

– Organisms about which little is known Organisms about which little is known regarding their reproductionregarding their reproduction

Modes of speciation Modes of speciation (based on how gene flow is interrupted)(based on how gene flow is interrupted)

• Allopatric:Allopatric: populations populations segregated by a segregated by a geographical/ecological geographical/ecological barrier; can result in barrier; can result in adaptive radiation (island adaptive radiation (island species- species- archipelagoarchipelago))

• Sympatric:Sympatric: reproductively reproductively isolated subpopulation isolated subpopulation within the range of its parent within the range of its parent population; population; polyploidy polyploidy (change in chromosome (change in chromosome number) in wheatnumber) in wheat

Allopatric SpeciationAllopatric Speciation• Gene flow is interrupted or reduced when a population Gene flow is interrupted or reduced when a population

is divided into two or more geographically isolated is divided into two or more geographically isolated subpopulationssubpopulations

• One or both populations may undergo evolutionary One or both populations may undergo evolutionary change during the period of separationchange during the period of separation

A. harrisi A. leucurus

Harris’s antelope squirrel and white-tailed antelope squirrels separated by the grand canyon

Sympatric Speciation in cichlids due to Sympatric Speciation in cichlids due to sexual selection or feeding preferencessexual selection or feeding preferences

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 orange light, 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

• In allopatric speciationIn allopatric speciation– A new species forms while A new species forms while

geographically isolated from its parent geographically isolated from its parent populationpopulation

• In sympatric speciationIn sympatric speciation– The emergence of a reproductive barrier The emergence of a reproductive barrier

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

Modes of speciationModes of speciation

• Parapatric SpeciationParapatric Speciation--– neighboring populations neighboring populations

become distinct while become distinct while sharing a common border.sharing a common border.

– Hybrid zoneHybrid zone- region - region between the two species between the two species where some hybridization where some hybridization still occursstill occurs Hybrid zone between two

oak species on Australia

Evolutionary TreesEvolutionary Trees• Evolutionary

Trees- summarize information about the appearance of new species

• Extinctions (irreversible losses of species) are recorded and shown on these “Trees of Life”

Recent(11,500 ya)

Pleistocene(1.8 mya)

Pliocene(5.3 mya)

Miocene(23 mya)

Oligocene(33.9 mya)

Eocene(55.8 mya)

EquusHippidion and other genera

Nannippus

Pliohippus

NeohipparionHipparion

Sinohippus MegahippusCallippus

Archaeohippus

Merychippus

Parahippus

HypohippusAnchitherium

Miohippus

Mesohippus

Epihippus

Orohippus

Paleotherium

Propalaeotherium

Pachynolophus

GrazersBrowsers

Key

Hyracotherium

Gradual vs. Punctuated equilibriaGradual vs. Punctuated equilibria• Speed Speed of speciation:of speciation:

gradualgradual vs. vs. punctuatedpunctuated speciation; Niles speciation; Niles Eldredge and Stephen Eldredge and Stephen Jay Gould (1972); Jay Gould (1972); helped explain the non-helped explain the non-gradual appearance of gradual appearance of species in the fossil species in the fossil recordrecord

• Adaptive radiationAdaptive radiation– Is the evolution of diversely adapted Is the evolution of diversely adapted

species from a common ancestor upon species from a common ancestor upon introduction to new environmental introduction to new environmental opportunitiesopportunities

Pisonia seeds cling to the feathers of this black noddy tern which will soon migrate elsewhere

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

adaptive radiationadaptive radiation

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 scabraDubautia linearis

N

5 million years ago an ancestral tarweed arrived on Hawaii and quickly diverged into many different silversword species