the origin of new species, is at the 

focal point of evolutionary theory

refers to broad patterns of 

evolutionary change above the species level

species is a group of populations whose members 

have the potential to interbreed in nature and 

produce viable, fertile offspring; they do not breed 

successfully with other populations

holds the 

phenotype of a population together

aretheoffspringofcrossesbetweendifferent

species

Prezygotic barriers block fertilization from 

occurring by

Impeding different species from attempting to 

mate 

– Preventing the successful completion of 

mating 

– Hindering fertilization if mating is successful

 twospeciesliveinthesame

generalareabutnotinthesamekindofplace

twospeciesbreedatdifferenttimes(seasons,

timesofday,years).

littleornomaterecognitionbetweenfemalesand

malesofdifferentspecies

femaleandmalesex

organsarenotcompatible

mosthybridoffspring

donotsurvive

hybridoffspring

arevigorousbutsterile.

Morphologicalspecies

concept

canbeappliedto

– asexualorganismsand

– fossils

Definesaspeciesbyitsecologicalrole

ornicheand

– focusesonuniqueadaptationsto

particularrolesinabiological

community.

definesaspeciesasthesmallest

groupofindividualsthatsharesa

commonancestorandthus

• formsonebranchofthetreeoflife.

geneflowis interrupted

orreducedwhen a populationisdividedinto

geographicallyisolatedsubpopulations

Thedefinitionofbarrierdependsontheabilityofa populationtodisperse

if the species have a great reproductive barrier, they could diverge into 2 species

Regionswithmany geographic barrierstypicallyhave

morespecies thandoregionswithfewerbarriers

• Reproductiveisolationbetweenpopulationsgenerally

increasesasthedistancebetweenthemincreases

speciation takes place 

in geographically overlapping populations

isthepresenceofextrasetsofchromosomes

duetoaccidentsduringcelldivision

more common in plants than animals

isanindividualwithmorethantwo

chromosomesets,derivedfromonespecies

is a species with multiple 

sets of chromosomes derived from different 

species

Sympatricspeciationcanalsoresultfromthe

appearanceofnewecologicalniches

Inallopatricspeciation, geographic isolationrestricts

geneflowbetweenpopulations

• Reproductiveisolation maythenarisebynatural

selection,geneticdrift,or sexualselectioninthe

isolated populations

• Even if contact isrestoredbetweenpopulations,

interbreeding isprevented

is a regionin whichmembersofdifferent

speciesmateandproducehybrids

Hybridsoftenhavereducedfitnesscomparedwith

parentspecies

barriersoccurswhenhybridsare

lessfitthantheparentspecies

Ifhybrids are as fit asparents,therecanbe

substantialgeneflowbetweenspecies

• Ifgeneflowisgreatenough,theparentspecies

canfuseintoasinglespecies

continued formation of hybrids

Extensivegeneflowfromoutside the hybrid zonecan

overwhelm selectionforincreasedreproductive

isolationinside thehybridzone

Thepunctuated patternin the fossil recordand

evidencefromlabstudies suggestthatspeciationcan

be rapid

is the cumulative effect of many 

speciation and extinction events