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The Origin of SpeciesThe Origin of SpeciesChapter 23Chapter 23

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Why don’t we call them the same species?

Santa Cruz Island

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Why donWhy don’’t we call them different species? t we call them different species?

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Ernst Mayr (Harvart Univ.)

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Biological Species Concept

Ernst Ernst MayrMayr ……..“…“…groups of actually or potentially interbreeding groups of actually or potentially interbreeding natural populations which are reproductively isolated natural populations which are reproductively isolated from other such groups.from other such groups.””

Individuals that cannot produce fertile offspring Individuals that cannot produce fertile offspring are termed are termed reproductively isolatedreproductively isolated, and thus , and thus members of different species.members of different species.

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Species are based on Species are based on interfertilityinterfertility, not physical , not physical similarity.similarity.For example, the eastern and western meadowlarks may For example, the eastern and western meadowlarks may have similar shapes and coloration, but differences in have similar shapes and coloration, but differences in song help prevent interbreeding between the two song help prevent interbreeding between the two species.species.

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On the other handOn the other hand, , humans havehumans haveconsiderable considerable diversity,diversity,but we all belong to but we all belong to thethe same species same species because ofbecause of our our capacity to capacity to interbreedinterbreed..

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The Process of Speciation

Speciation is a twoSpeciation is a two--part process:part process:identical populations must divergeidentical populations must divergereproductive isolation must evolve to maintain the reproductive isolation must evolve to maintain the differencesdifferences

Reproductive isolation prevents populations Reproductive isolation prevents populations evolved inevolved into different species from to different species from interbreeding, even if their ranges overlap.interbreeding, even if their ranges overlap.

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The process of speciation is the

process of reproductive

isolation !

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How do 2 populations become How do 2 populations become reproductively isolated?reproductively isolated?

Reproductive barriers can be categorized Reproductive barriers can be categorized as as prezygoticprezygotic or or postzygoticpostzygotic, depending , depending on whether they function before or after on whether they function before or after the formation of zygotes.the formation of zygotes.

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Reproductive isolating mechanisms

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Reproductive isolating mechanisms

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Geographic isolationGeographic isolation

It seems geographically separated It seems geographically separated populations are more likely to evolve populations are more likely to evolve substantial differences leading to substantial differences leading to speciation.speciation.

This is called This is called AllopatricAllopatric speciationspeciation

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Geographic IsolationGeographic Isolation

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Prezygotic Isolating MechanismsPrezygotic Isolating Mechanisms

Prezygotic isolating mechanisms prevent the Prezygotic isolating mechanisms prevent the formation of zygotes.formation of zygotes.

Ecological isolation (Habitat isolation)Ecological isolation (Habitat isolation)Temporal isolationTemporal isolationBehavioral isolationBehavioral isolationMechanical isolationMechanical isolationPrevention of gamete fusionPrevention of gamete fusion

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Habitat isolationHabitat isolation

Two species occur in the same general Two species occur in the same general area, but utilize different portions of the area, but utilize different portions of the environment and thus do not hybridize environment and thus do not hybridize because they do not encounter each because they do not encounter each other.other.

Can you think of an example?Can you think of an example?

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Habitat isolation

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Temporal isolationTemporal isolation

Two species that breed during different Two species that breed during different times of day, different seasons, or times of day, different seasons, or different years cannot mix gametes.different years cannot mix gametes.

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Temporal isolation

Eastern spotted skunk(mate in late winter)

Western spotted skunk(mate in late summer)

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Behavioral isolation

Many species, e.g. birds, differ in Many species, e.g. birds, differ in courtship rituals which tends to keep courtship rituals which tends to keep species distinct.species distinct.

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Behavioral isolation

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Behavioral isolation

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Behavioral isolation

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Mechanical isolation

Closely related species may attempt to Closely related species may attempt to mate but fail because they are anatomically mate but fail because they are anatomically incompatible and transfer of sperm is not incompatible and transfer of sperm is not possible.possible.

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Mechanical isolation

Large landing platform

Small landing platform

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Gametic isolation

GameticGametic isolation occurs when gametes isolation occurs when gametes of two species do not form a zygote of two species do not form a zygote because of incompatibilities preventing because of incompatibilities preventing fusion fusion of sperms and eggsof sperms and eggs..

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Oops! can’t get in.

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Postzygotic Isolating Mechanisms

PPostzygoticostzygotic barriers prevent the hybrid barriers prevent the hybrid zygote from developing into a viable, fertile zygote from developing into a viable, fertile adult.adult.

These barriers include These barriers include reduced hybrid viability, reduced hybrid viability, reduced hybrid fertilityreduced hybrid fertilityhybrid breakdown.hybrid breakdown.

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Reduced hybrid viabilityReduced hybrid viability. Genetic . Genetic incompatibility between the two species incompatibility between the two species may abort the development of the hybrid at may abort the development of the hybrid at some embryonic stage or produce frail some embryonic stage or produce frail offspring.offspring.

This is true for the occasional hybrids This is true for the occasional hybrids between frogs in the genus between frogs in the genus RanaRana, which , which do not complete development and those do not complete development and those that do are frail.that do are frail.

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Reduced hybrid viabilityReduced hybrid viability in Leopard Frogsin Leopard Frogs

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Reduced hybrid fertilityReduced hybrid fertility. Even if the . Even if the hybrid offspring are vigorous, the hybrids hybrid offspring are vigorous, the hybrids may be infertile.may be infertile.

For example, while a mule, the hybrid For example, while a mule, the hybrid product of mating between a horse and product of mating between a horse and donkey, is a robust organism, it cannot donkey, is a robust organism, it cannot mate with either horses or donkeys.mate with either horses or donkeys.

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TiglonTiglon

LionLionTigerTiger

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Hybrid breakdownHybrid breakdown. In some cases, first . In some cases, first generation hybrids are viable and fertile.generation hybrids are viable and fertile.

However, when they mate with either However, when they mate with either parent species or with each other, the parent species or with each other, the next generation are feeble or sterile.next generation are feeble or sterile.For example,For example, different cotton species different cotton species can produce fertile hybrids, but can produce fertile hybrids, but breakdown occurs in the next generation breakdown occurs in the next generation when offspring of hybrids die as seeds or when offspring of hybrids die as seeds or grow into weak and defective plants.grow into weak and defective plants.

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Reproductive Reproductive barriersbarrierscan occur beforecan occur beforemating, betweenmating, betweenmating andmating andfertilization, orfertilization, orafter fertilization.after fertilization.

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Extinct species Extinct species are separated into species based on are separated into species based on morphologymorphology ((fossilsfossils).).

There is There is high levels of hybridization among some species high levels of hybridization among some species groupsgroups..

ItIt’’s difficult to apply to species that do not occur together s difficult to apply to species that do not occur together in naturein nature..

In addition, many species (e.g., bacteria) reproduce entirely In addition, many species (e.g., bacteria) reproduce entirely asexually and are assigned to species based mainly on asexually and are assigned to species based mainly on structural and biochemical characteristics.structural and biochemical characteristics.

TThe biological species concept has limithe biological species concept has limitsswhen applied to species in naturewhen applied to species in nature

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No single definition of species No single definition of species may be universally applicable.may be universally applicable.

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How do speciation get started?How does a population diverge

in the first place?

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Geographic IsolationGeographic Isolation

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Random changes may initiate Random changes may initiate reproductive isolationreproductive isolation

Given long enough periods of time, any two Given long enough periods of time, any two isolated populations will diverge due to isolated populations will diverge due to genetic driftgenetic drift. If there is a chance divergence . If there is a chance divergence in reproductive mechanism, then itin reproductive mechanism, then it’’s the s the start of reproductive isolation.start of reproductive isolation.

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AllopatricAllopatric speciation in the vials: speciation in the vials: Females from populations raised on starch medium Females from populations raised on starch medium preferred males raised on starch medium over males preferred males raised on starch medium over males raised on maltose medium after several generations raised on maltose medium after several generations of isolation.of isolation.

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Natural selection can enhance speciationNatural selection can enhance speciation

Initially incomplete isolating mechanisms are Initially incomplete isolating mechanisms are reinforced by natural selection favoring the reinforced by natural selection favoring the perfection of reproductive isolation.perfection of reproductive isolation.

Hybrids are often selected against Hybrids are often selected against a case of disruptive selection.a case of disruptive selection.

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Piedflycatcher

Collaredflycatcher

Reinforcement of reproductive isolation

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NEW GUINEA

Mainland populations

Exercise: The likelihood of allopatricspeciation increases when a population is both small and isolated. WHY?

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Not all speciation are allopatric

Geographic isolation (Geographic isolation (allopatricallopatric speciation) is speciation) is not the only way to initiate speciation.not the only way to initiate speciation.

Sympatric speciation Sympatric speciation ---- instantaneous speciation instantaneous speciation through through polyploidypolyploidy

individual is born that is reproductively individual is born that is reproductively isolated from other members of species isolated from other members of species because of polyploidy because of polyploidy -- more than 2 sets of more than 2 sets of chromosomes.chromosomes.

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An individual can have more that two sets of An individual can have more that two sets of chromosomes from a single species if a failure in chromosomes from a single species if a failure in meiosis results in a meiosis results in a tetraploidtetraploid (4(4nn) individual.) individual.This mutant can This mutant can only only reproduce with itself (selfreproduce with itself (self--pollination) or with other pollination) or with other tetraploidstetraploids..

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Hugo de Hugo de VriesVries produced a new primrose produced a new primrose species, the species, the tetraploidtetraploid OenotheriaOenotheria gigasgigas, from the , from the diploid diploid OenotheraOenothera lamarckianalamarckiana..

The two speciesThe two species could not interbreed.could not interbreed.

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Many plants important for agriculture are Many plants important for agriculture are the products of polyploidy.the products of polyploidy.

For example, oats, cotton, potatoes, For example, oats, cotton, potatoes, tobacco, and wheat.tobacco, and wheat.

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Allopatric Sympatric

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All of the 500 or so endemic species of All of the 500 or so endemic species of DrosophilaDrosophilain the Hawaiian archipelago descended from a in the Hawaiian archipelago descended from a common ancestor that reached Kauai over 5 common ancestor that reached Kauai over 5 million years ago. million years ago.

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The evolution of many diverselyThe evolution of many diversely--adapted species adapted species from a common ancestor is called an from a common ancestor is called an adaptive adaptive radiationradiation..

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Multiple invasions and Multiple invasions and allopatricallopatric speciation speciation cancanignite an explosion of ignite an explosion of adaptive radiationadaptive radiation

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Adaptive radiation of DarwinAdaptive radiation of Darwin’’s finchess finches

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Geospizafuliginosa

Geospizaconirostris

Geospizadifficilis

Groundand

cactusfinches

Geospizafortis

Geospizamagnirostris

Geospizascandens

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CamarhynchusparvulusCamarhynchuspsittacula

CamarhynchuspauperCactospizaheliobatesCactospizapallida(woodpecker finch)Platyspizacrassirostris

Treefinches

Vegetariantree finch

Warblerfinches

CerthideafuscaCerthideaolivacea

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Fish eater

Snail eater

Algae scraper

Zooplankton eater

Leaf eater

Second set of jawsInsect eater

Adaptive radiation of cichlid fishes in Lake Victoria

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Glaciers linkalpine zonesinto onecontinuousrange.

Mountainpopulationsbecome isolated,permittingdivergence andspeciation.

Alpine zonesare reconnected.Separatelyevolved speciescome back intocontact.

Glaciers recede Glaciation

Adaptive radiation is favored by periodic isolation events

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Periodic glaciation has favored the adaptive radiation in alpine buttercup, 14 species.

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DarwinDarwin —— GradulismGradulism

IInfinitesimally small changes accumulating in a nfinitesimally small changes accumulating in a slow, steady way.slow, steady way.

How fast doesHow fast does sspeciationpeciation occur?occur?

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FFossil recordossil records show that the change of species s show that the change of species are are in stasis in stasis over eover enormous lengths of time normous lengths of time (species undergo little change)(species undergo little change), then followed , then followed by by abrupt abrupt occurrenceoccurrence of of new structures, and new structures, and many new many new specispecieses..

How fast doesHow fast does sspeciationpeciation occur?occur?

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Punctuated equilibrium

Jay Gould and Niles Jay Gould and Niles EldredgeEldredge proposeproposed d ----punctuated equilibriumpunctuated equilibrium

Speciation normally proceeds in spurts, Speciation normally proceeds in spurts, with long periods of little speciation in with long periods of little speciation in between.between.

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Rate of Speciation

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GRADUALISM

PUNCTUATEDEQUALIBRIUM

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Speciation and Extinction Through TimeSpeciation and Extinction Through TimeThere have been 5 major mass extinctions There have been 5 major mass extinctions interspersed within relatively consistent extinction interspersed within relatively consistent extinction patterns. However, biological diversity tends to patterns. However, biological diversity tends to rebound after mass extinctions.rebound after mass extinctions.

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Human impact the process of evolutionHuman impact the process of evolution

Human alter environmentsHuman alter environmentse.g., global warminge.g., global warming

Human decreased population sizesHuman decreased population sizesincreased genetic driftincreased genetic driftincreased extinctionincreased extinction

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Behavioral BiologyBehavioral BiologyChapter 52Chapter 52

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Once upon a summer time,….

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Why did the wren sing?Why did the wren sing?Exercise:Exercise:

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Why did the wren sing?Why did the wren sing?The wren heard the tape through itThe wren heard the tape through it’’s neural s neural network, the perception of a song increased the network, the perception of a song increased the sex hormone in the blood, which bind to the sex hormone in the blood, which bind to the receptors in the brain, and triggered the receptors in the brain, and triggered the production of songs.production of songs.

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Why did the wren sing?Why did the wren sing?The wren sang to defend a territory from The wren sang to defend a territory from other males and to attract females with which other males and to attract females with which to reproduce.to reproduce.

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Proximate Proximate reasonsreasons

Neural pathway & hormone

Survival & reproduction

Ultimate Ultimate reasonsreasons

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What is Behavior?What is Behavior?

The way an organism respond actively to The way an organism respond actively to stimuli in its environment.stimuli in its environment.

Generally, a behavior should increase the Generally, a behavior should increase the fitness of an individual. If not, natural fitness of an individual. If not, natural selection will select against the behavior.selection will select against the behavior.

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Was the singing of a male wren an Was the singing of a male wren an innate or learned behavior?innate or learned behavior?

Exercise: natureExercise: nature--versusversus--nurturenurture

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Early ethologists believe that behaviors are largely innate.

FrischFrisch LorenzLorenz TinbergenTinbergen

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Egg-rolling behavior

(FAP)

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A A sequence of behavioral acts that is sequence of behavioral acts that is essentially unchangeable and usually essentially unchangeable and usually carried to completion once initiated.carried to completion once initiated.

The FAP usually occurs in a series of The FAP usually occurs in a series of actions the same way every time.actions the same way every time.

Fixed action pattern (FAP)Fixed action pattern (FAP)

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Oh, I hate the red bellies !Oh, I hate the red bellies !

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Innate BehaviorsInnate Behaviors

Behaviors that are developmentally fixed, usually Behaviors that are developmentally fixed, usually based on preset pathways in the nervous system based on preset pathways in the nervous system and thus are likely genetically programmed.and thus are likely genetically programmed.

The range of environmental differences The range of environmental differences among individuals does not appear to alter among individuals does not appear to alter the behavior.the behavior.

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Kinesis and taxisKinesis and taxis: : the simplest mechanisms of the simplest mechanisms of movement.movement.

Kinesis is a change in activity rate in response to a Kinesis is a change in activity rate in response to a stimulus.stimulus.Taxis is an automatic, oriented movement to or away Taxis is an automatic, oriented movement to or away from a stimulus.from a stimulus.

For example, For example, phototaxisphototaxis..

Innate Behaviors ?Innate Behaviors ?

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Evidence that behaviors have strong Evidence that behaviors have strong genetic bases.genetic bases.

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Evidence 1Evidence 1–– Identical twinsIdentical twinsIdentical human twins, separated at birth, develop Identical human twins, separated at birth, develop behavioral similarities, even though they were behavioral similarities, even though they were raised in very different environments.raised in very different environments.

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P

F1

F2

F5

F7

Errors made in the maze9 39 64 114 214

Quicker ratsSlower rats

Evidence 2 Evidence 2 –– artificial selectionartificial selectionBehavioral traits pass from one generation to the Behavioral traits pass from one generation to the next, thus are subject to selection.next, thus are subject to selection.

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Evidence 3 Evidence 3 ––

hybrid hybrid behaviorbehavior

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Evidence 4 Evidence 4 –– Single gene defectSingle gene defect

Fos B mutant

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Certainly, behaviors have genetic basis. Certainly, behaviors have genetic basis. However, learning influences behaviorHowever, learning influences behavior

Learning occurs when animals alter their Learning occurs when animals alter their behaviors as a result of previous experience.behaviors as a result of previous experience.

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How does How does learning influences influences behavior ?behavior ?

Associative learningAssociative learningNonNon--associative learningassociative learning

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Associative learning (Conditioning)Associative learning (Conditioning)

An association between two stimuli (classical An association between two stimuli (classical conditioning), or between a stimulus and a conditioning), or between a stimulus and a response (operant conditioning) is response (operant conditioning) is developed.developed.

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Classic Conditioning – associate two stimuli

FOOD

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Pavlov and the poor dogPavlov and the poor dog

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Operant Conditioning– associate a stimulus & a response

OUCH!

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Food-and-light or food-and-shock

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Classic or operant conditioning?

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If Willy is not going to touch a porcupine If Willy is not going to touch a porcupine anymore, is it a classic or operant conditioning?anymore, is it a classic or operant conditioning?