Species ConceptsSpecies Concepts

Level 1 Biological DiversityLevel 1 Biological Diversity

Jim ProvanJim Provan

Campbell: Chapter 24Campbell: Chapter 24

Macroevolution and speciationMacroevolution and speciation

Evolutionary theory must explainEvolutionary theory must explain macroevolutionmacroevolution, , the origin of new taxonomic groupsthe origin of new taxonomic groupsSpeciationSpeciation, or the origin of new species, is central to , or the origin of new species, is central to macroevolution since all higher taxa originate with a macroevolution since all higher taxa originate with a new species which is novel enough to be the first new species which is novel enough to be the first membermemberFossil record provides evidence for two patterns of Fossil record provides evidence for two patterns of speciation:speciation:

AnagenesisAnagenesis ((phyletic evolutionphyletic evolution)) – transformation of an – transformation of an unbranched lineage of organisms to a different state (the unbranched lineage of organisms to a different state (the new species)new species)CladogenesisCladogenesis ((branching evolutionbranching evolution) – budding of one or ) – budding of one or more species from a parent species that continues to existmore species from a parent species that continues to exist

Anagenesis and cladogenesisAnagenesis and cladogenesis

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What is a species?What is a species?

SpeciesSpecies = Latin for = Latin for “kind” or “appearance”“kind” or “appearance”

Linnaeus described Linnaeus described species in terms of their species in terms of their morphologymorphology

Modern taxonomists Modern taxonomists also consider genetic also consider genetic makeup and functional makeup and functional and behavioural and behavioural differences when differences when describing speciesdescribing species

The biological species concept The biological species concept emphasizes reproductive emphasizes reproductive

isolationisolation

In 1942, Ernst Mayr proposed theIn 1942, Ernst Mayr proposed the biological biological species conceptspecies conceptAA biological speciesbiological species is defined as a population is defined as a population or group of populations whose members have or group of populations whose members have the potential to interbreed and produce viable, the potential to interbreed and produce viable, fertile offspring but cannot do so with fertile offspring but cannot do so with members of other speciesmembers of other species

The species is the largest unit of population in which The species is the largest unit of population in which gene flow is possiblegene flow is possibleIt is defined by reproductive isolation from other It is defined by reproductive isolation from other species in natural environments (hybrids may be species in natural environments (hybrids may be possible in the lab or in zoos)possible in the lab or in zoos)

Gene pools of biological species are Gene pools of biological species are isolated by pre- and post-zygotic isolated by pre- and post-zygotic

barriersbarriers

Any factor that impedes two species from Any factor that impedes two species from producing viable, fertile offspring contributes to producing viable, fertile offspring contributes to reproductive isolation:reproductive isolation:

Most species sequestered from others by multiple barriersMost species sequestered from others by multiple barriersReproductive barriers prevent interbreeding between Reproductive barriers prevent interbreeding between closely related speciesclosely related species

Various barriers classified by whether they Various barriers classified by whether they function before or after zygote formation:function before or after zygote formation:

Pre-zygotic barriersPre-zygotic barriers impede mating between species of impede mating between species of hinder fertilisation of the ova by sperm from another hinder fertilisation of the ova by sperm from another speciesspeciesIf fertilisation does occur,If fertilisation does occur, post-zygotic barrierspost-zygotic barriers prevent prevent the hybrid zygote from developing into a viable, fertile the hybrid zygote from developing into a viable, fertile adultadult

Pre-zygotic barriers: habitat Pre-zygotic barriers: habitat isolationisolation

Two species living in Two species living in different habitats may different habitats may not encounter each not encounter each other:other:

Two species of garter Two species of garter snake (snake (ThamnophisThamnophis) occur ) occur in the same area but one in the same area but one species lives in water and species lives in water and the other is terrestrialthe other is terrestrial

Since they live in separate Since they live in separate habitats, the two seldom habitats, the two seldom come into contact as they come into contact as they are ecologically isolatedare ecologically isolated

Pre-zygotic barriers: behavioural Pre-zygotic barriers: behavioural isolationisolation

Species-specific signals and elaborate behaviour to Species-specific signals and elaborate behaviour to attract mates e.g different flashing patterns in attract mates e.g different flashing patterns in firefliesfirefliesMany animals recognise mates by sensing Many animals recognise mates by sensing pheromones:pheromones:

Female Gypsy moths emit a volatile compound to which Female Gypsy moths emit a volatile compound to which olfactory organs of male gypsy moths are specifically tunedolfactory organs of male gypsy moths are specifically tunedMales of other moth species do not recognise this chemical Males of other moth species do not recognise this chemical as a sexual attractantas a sexual attractant

Other behavioural isolating mechanisms:Other behavioural isolating mechanisms:Eastern and western meadowlarks only recognise songs of Eastern and western meadowlarks only recognise songs of the same speciesthe same speciesSpecific courtship ritualsSpecific courtship rituals

Other pre-zygotic isolating Other pre-zygotic isolating mechanismsmechanisms

Temporal isolation:Temporal isolation:Two species that breed at different times of the day, Two species that breed at different times of the day, seasons or years cannot mix gametesseasons or years cannot mix gametesBrown trout breed in the autumn whereas rainbow trout Brown trout breed in the autumn whereas rainbow trout living in the same streams breed in the springliving in the same streams breed in the spring

Mechanical isolation:Mechanical isolation:Anatomical incompatibility may prevent sperm transferAnatomical incompatibility may prevent sperm transferClasping appendages in dragonfliesClasping appendages in dragonfliesFloral anatomy corresponding to specific pollinatorFloral anatomy corresponding to specific pollinator

Gametic isolation:Gametic isolation:Sperm of one species may not survive internal Sperm of one species may not survive internal environment of female reproductive tract in another environment of female reproductive tract in another speciesspeciesLack of gamete recognition in external-fertilising speciesLack of gamete recognition in external-fertilising species

Post-zygotic isolating Post-zygotic isolating mechanismsmechanisms

Reduced hybrid viability:Reduced hybrid viability:Genetic incompatibility may abort development at Genetic incompatibility may abort development at embryonic stageembryonic stageSeveral species of the frog Several species of the frog RanaRana live in the same live in the same habitats but hybrids do not complete developmenthabitats but hybrids do not complete development

Reduced hybrid fertility:Reduced hybrid fertility:Species mate and hybrid is viable but sterile e.g. muleSpecies mate and hybrid is viable but sterile e.g. muleIf chromosome numbers are different, meiosis cannot If chromosome numbers are different, meiosis cannot produce normal gametesproduce normal gametes

Hybrid breakdown:Hybrid breakdown:First generation hybrids are fertile but subsequent First generation hybrids are fertile but subsequent generations are defectivegenerations are defective

Reproductive barriers – a Reproductive barriers – a summarysummary

The biological species concept is The biological species concept is not always applicablenot always applicable

The biological species concept cannot be The biological species concept cannot be applied to organisms that are completely applied to organisms that are completely asexual e.g. some protists and fungi, some asexual e.g. some protists and fungi, some plants (bananas), many bacteria:plants (bananas), many bacteria:

Asexual reproduction effectively produces a series of Asexual reproduction effectively produces a series of clonesclones

Asexual organisms can only be assigned to species by Asexual organisms can only be assigned to species by grouping clones with the same morphology / grouping clones with the same morphology / biochemistrybiochemistry

Cannot be applied to extinct organisms Cannot be applied to extinct organisms represented only by fossils (obviously): must be represented only by fossils (obviously): must be classified morphologicallyclassified morphologically

The biological species concept is The biological species concept is not always applicablenot always applicable

Four phenotypically distinct populations Four phenotypically distinct populations ((subspeciessubspecies) of deer mouse () of deer mouse (Peromyscus Peromyscus maniculatismaniculatis) are geographically isolated in the ) are geographically isolated in the Rocky MountainsRocky Mountains

Populations overlap at certain locations and some Populations overlap at certain locations and some interbreeding occurs: same species by BSC criteriainterbreeding occurs: same species by BSC criteria

Two subspecies (Two subspecies (P. m. P. m. ssp. ssp. artemisiaeartemisiae and and P. m. P. m. ssp. ssp. nebrascensisnebrascensis) do not interbreed, but can breed with ) do not interbreed, but can breed with other neighbouring subspeciesother neighbouring subspecies

Very limited gene flow between the two does occur, Very limited gene flow between the two does occur, even though it is via populations of other subspecieseven though it is via populations of other subspecies

Other species conceptsOther species concepts

The The morphological species conceptmorphological species concept defines species defines species based on measurable physical featuresbased on measurable physical featuresIn theIn the recognition species conceptrecognition species concept, a species is , a species is defined by a set of characteristics that maximise defined by a set of characteristics that maximise successful matingsuccessful matingTheThe cohesion species conceptcohesion species concept relies on mechanisms relies on mechanisms that maintain species as discrete phenotypic entitiesthat maintain species as discrete phenotypic entitiesTheThe ecological species conceptecological species concept defines species on defines species on the basis of where they live and what they do the basis of where they live and what they do (adaptation)(adaptation)TheThe evolutionary species conceptevolutionary species concept defines species in defines species in terms of ancestral and descendent populations that terms of ancestral and descendent populations that are evolving independently of other such groupsare evolving independently of other such groups