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

Macroevolution: Macroevolution: the origin of new taxonomic groupsthe origin of new taxonomic groups

• Speciation: Speciation: the origin of new speciesthe origin of new species

• 1-1- Anagenesis Anagenesis (phyletic evolution): (phyletic evolution): accumulation of heritable changesaccumulation of heritable changes

• 2- Cladogenesis (branching evolution): budding of new species from a parent species that continues to exist (basis of biological diversity)

What is a species?What is a species?

• Biological species concept Biological species concept (ErnstMayr):(ErnstMayr): a population or a population or group of populations whose group of populations whose members have the potential to members have the potential to interbreed and produce viable, interbreed and produce viable, fertile offspring (genetic exchange fertile offspring (genetic exchange is possible and that is genetically is possible and that is genetically isolated from other populations)isolated from other populations)

How and why do new species originateHow and why do new species originate??

• Species are created by a series of Species are created by a series of evolutionary processesevolutionary processes– populations become populations become isolatedisolated

• geographically isolated geographically isolated • reproductively isolatedreproductively isolated

– isolated populations isolated populations evolve independentlyevolve independently

Reproductive Isolation (isolation of gene pools), IReproductive Isolation (isolation of gene pools), I

• Prezygotic barriers:Prezygotic barriers: impede mating impede mating between species or hinder the fertilization between species or hinder the fertilization of the ovaof the ova

• Habitat (snakes; water/terrestrial)Habitat (snakes; water/terrestrial)

• Behavioral (fireflies; mate signaling)Behavioral (fireflies; mate signaling)

• Temporal (salmon; seasonal mating)Temporal (salmon; seasonal mating)

• Mechanical (flowers; pollination Mechanical (flowers; pollination anatomy)anatomy)

• Gametic (frogs; egg coat receptors)Gametic (frogs; egg coat receptors)

Habitat isolationHabitat isolation• Species occur in same region, but occupy different Species occur in same region, but occupy different

habitats so habitats so rarelyrarely encounter each other encounter each other– reproductively isolatedreproductively isolated

2 species of garter snake, Thamnophis, occur in same area, but one lives in water & other is terrestrial

2 species of garter snake, Thamnophis, occur in same area, but one lives in water & other is terrestrial

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lions & tigers could hybridize, but they live in different habitats: lions in grasslands tigers in rainforest

lions & tigers could hybridize, but they live in different habitats: lions in grasslands tigers in rainforest

Temporal isolationTemporal isolation• Species that breed during different times of day, different Species that breed during different times of day, different

seasons, or different years cannot mix gametesseasons, or different years cannot mix gametes– reproductive isolationreproductive isolation

Eastern spotted skunk (L) & western spotted skunk (R) overlap in range but eastern mates in late winter & western mates in late summer

Eastern spotted skunk (L) & western spotted skunk (R) overlap in range but eastern mates in late winter & western mates in late summer

Behavioral isolationBehavioral isolation• Unique behavioral patterns & rituals isolate speciesUnique behavioral patterns & rituals isolate species

– identifies members of species identifies members of species – attract mates of same species attract mates of same species ��

• courtship rituals, mating callscourtship rituals, mating calls• reproductive isolationreproductive isolation

Blue footed boobies mate only after a courtship display unique to their species

Blue footed boobies mate only after a courtship display unique to their species

Mechanical isolationMechanical isolation• Morphological differences can prevent successful Morphological differences can prevent successful

matingmating– reproductive isolationreproductive isolation

Even in closely related species of plants, the flowers often have distinct appearances that attract different pollinators. These 2 species of monkey flower differ greatly in shape & color, therefore cross-pollination does not happen.

Even in closely related species of plants, the flowers often have distinct appearances that attract different pollinators. These 2 species of monkey flower differ greatly in shape & color, therefore cross-pollination does not happen.

Plants

sympatric speciation?

Mechanical isolationMechanical isolation

• For many insects, male & For many insects, male & female sex organs of female sex organs of closely related species do closely related species do not fit together, preventing not fit together, preventing sperm transfersperm transfer– lack of “fit” between sexual organs: lack of “fit” between sexual organs:

hard to imagine for us… but a big issue for insects with different hard to imagine for us… but a big issue for insects with different shaped genitals!shaped genitals!

Damsel fly penises

Animals

Gametic isolationGametic isolation• Sperm of one species may not be able to fertilize eggs of Sperm of one species may not be able to fertilize eggs of

another speciesanother species– mechanismsmechanisms

• biochemical barrierbiochemical barrier so sperm cannot penetrate egg so sperm cannot penetrate egg– receptor recognition: lock & key between egg & spermreceptor recognition: lock & key between egg & sperm

• chemical incompatibilitychemical incompatibility – sperm cannot survive in female reproductive tractsperm cannot survive in female reproductive tract

Sea urchins release sperm & eggs into surrounding waters where they fuse & form zygotes. Gametes of different species— red & purple —are unable to fuse.

Sea urchins release sperm & eggs into surrounding waters where they fuse & form zygotes. Gametes of different species— red & purple —are unable to fuse.

Gametic isolationGametic isolation• Sperm of one species may not be able to fertilize eggs of Sperm of one species may not be able to fertilize eggs of

another speciesanother species– mechanismsmechanisms

• biochemical barrierbiochemical barrier so sperm cannot penetrate egg so sperm cannot penetrate egg– receptor recognition: lock & key between egg & spermreceptor recognition: lock & key between egg & sperm

• chemical incompatibilitychemical incompatibility – sperm cannot survive in female reproductive tractsperm cannot survive in female reproductive tract

Sea urchins release sperm & eggs into surrounding waters where they fuse & form zygotes. Gametes of different species— red & purple —are unable to fuse.

Sea urchins release sperm & eggs into surrounding waters where they fuse & form zygotes. Gametes of different species— red & purple —are unable to fuse.

Reproductive Isolation, IIReproductive Isolation, II• Postzygotic barriersPostzygotic barriers: : fertilization occurs, but fertilization occurs, but

the hybrid zygote does not develop into a the hybrid zygote does not develop into a viable, fertile adultviable, fertile adult

• Reduced hybrid viability (frogs; zygotes Reduced hybrid viability (frogs; zygotes fail to develop or reach sexual maturity)fail to develop or reach sexual maturity)

• Reduced hybrid fertility (mule; horse x Reduced hybrid fertility (mule; horse x donkey; cannot backbreed)donkey; cannot backbreed)

• Hybrid breakdown (cotton; 2nd Hybrid breakdown (cotton; 2nd generation hybrids are sterile)generation hybrids are sterile)

Reduced hybrid viabilityReduced hybrid viability• Genes of different parent species may Genes of different parent species may

interact & impair the hybrid’s interact & impair the hybrid’s developmentdevelopment

Species of salamander genus, Ensatina, may interbreed, but most hybrids do not complete development & those that do are frail.

Species of salamander genus, Ensatina, may interbreed, but most hybrids do not complete development & those that do are frail.

Mules are vigorous, but sterile

Reduced hybrid fertilityReduced hybrid fertility• Even if hybrids are vigorous Even if hybrids are vigorous

they may be sterilethey may be sterile– chromosomes of parents may differ in number or chromosomes of parents may differ in number or

structure & meiosis in hybrids may fail to produce structure & meiosis in hybrids may fail to produce normal gametesnormal gametes

Donkeys have 62 chromosomes(31 pairs)

Horses have 64 chromosomes(32 pairs) Mules have 63 chromosomes!

Hybrid breakdownHybrid breakdown• Hybrids may be fertile & viable in first Hybrids may be fertile & viable in first

generation, but when they mate offspring generation, but when they mate offspring are feeble or sterileare feeble or sterile

In strains of cultivated rice, hybrids are vigorous but plants in next generation are small & sterile.On path to separate species.

In strains of cultivated rice, hybrids are vigorous but plants in next generation are small & sterile.On path to separate species.

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

• AllopatricAllopatric:: “other “other

country”country” populations populations

segregated by a geographical segregated by a geographical barrier; can result in adaptive barrier; can result in adaptive radiation (island species)radiation (island species)

• SympatricSympatric:: ““samesame countrycountry reproductively isolated reproductively isolated subpopulation in the midst of subpopulation in the midst of its parent population (change its parent population (change in genome); polyploidy in in genome); polyploidy in plants; cichlid fishesplants; cichlid fishes

Niles EldredgeCurator

American Museum of Natural History

Rate of SpeciationRate of Speciation• Current debateCurrent debate::

Does speciation happen gradually or Does speciation happen gradually or rapidly rapidly – GradualismGradualism

• Charles DarwinCharles Darwin• Charles LyellCharles Lyell

– Punctuated equilibriumPunctuated equilibrium• Stephen Jay GouldStephen Jay Gould• Niles EldredgeNiles Eldredge

GradualismGradualism • Gradual Gradual

divergence over divergence over long spans of timelong spans of time– assume that big assume that big

changes occur as changes occur as the accumulation the accumulation of many small of many small onesones

Punctuated EquilibriumPunctuated Equilibrium• Rate of speciation is not Rate of speciation is not

constantconstant– rapid bursts of change rapid bursts of change

– long periods of little or long periods of little or no changeno change

– species undergo rapid species undergo rapid change when they 1change when they 1stst bud from parent bud from parent populationpopulation Time

Stephen Jay Gould Stephen Jay Gould (1941-2002)(1941-2002)• Harvard paleontologist & Harvard paleontologist &

evolutionary biologistevolutionary biologist– punctuated equilibriumpunctuated equilibrium– prolific authorprolific author

• popularized evolutionary thoughtpopularized evolutionary thought

Evolution is not goal-orientedEvolution is not goal-orientedAn evolutionary An evolutionary trendtrend does does notnot mean that evolution is mean that evolution is goal-orientedgoal-oriented. .

Surviving species Surviving species do not represent do not represent the peak of the peak of perfection. There perfection. There is compromise & is compromise & random chance random chance involved as wellinvolved as well

Remember that for Remember that for humans as well!humans as well!

Evolution is not the survival of the fittest. Rather it is the survival of the just good enough.

Categories of EvolutionCategories of Evolution

• Convergent evolutionConvergent evolution - acquisition of - acquisition of the same biological trait in unrelated the same biological trait in unrelated lineages. lineages.

• Leads to analogous structuresLeads to analogous structures

Parallel evolutionParallel evolution is the development of a similar trait in is the development of a similar trait in related, but distinct, species descending from the same related, but distinct, species descending from the same

ancestor, but from different clades.ladesancestor, but from different clades.lades

Divergent evolution - Divergent evolution - accumulation of differences between groups which accumulation of differences between groups which can lead to the formation of new species, usually a result of diffusion of the same can lead to the formation of new species, usually a result of diffusion of the same

species adapting to different environmentsspecies adapting to different environments