NOTES CH 24: The Origin Of Species

NOTES – CH 24:The Origin of Species

SpeciesHummingbirds of Costa Rica

SPECIES: a group of individuals that matewith one another and produce fertileoffspring; typically members of a speciesappear similar(exceptions: males vs. females; young vs.mature)

*the “biological” species concept definesa species as “a population or group ofpopulations whose members have thepotential to interbreed to produceviable, fertile offspring.”Same species of ant!!!2 different species ofmeadowlarks!

Classification of species Carolus Linnaeus: Swedish biologist;developed system of classification in the1700’s based on appearances(“morphological” definition of species)

Levels of amilyGenusSpeciesExample – anidaeCanisCanis familiaris

*SPECIATION: the process by which onespecies splits into two species, whichthereafter evolve as distinct lineages (i.e.they cease to interbreed!!)

SPECIATION 2 critical requirements for speciation tooccur are:(1) separation of the gene pool of theancestral species into two separate genepools (gene flow sufficiently reduced),and(2) over time, allele and gene frequenciesmay change due to natural selection(genetic divergence!)

Two Modes of Speciation:1) ALLOPATRIC SPECIATION:(a.k.a. “geographic speciation”);a population is separatedgeographically, either by a naturalbarrier or when some individualsleave a population and found a newpopulation Example: finches of the GalapagosIslands

Two Modes of Speciation:2) SYMPATRIC SPECIATION: a gene pool isdivided without geographicseparation HOW? POLYPLOIDY: an increase in the# of chromosomes (common in plants) precise selection of habitat or mating site byindividuals (animals)

Sympatric speciationDivergence occurs despite lack of geographic isolationPOLYPLOIDS:Organisms with extra sets of chromosomes;can result in sympatric speciation in onegenerationThe origin of a polyploid individual is usuallya mistake in meiosis or mitosisAutopolyploidy vs. allopolyploidy

Sympatric speciationAn autopolyploid an individual thathas two chromosome sets, all from asingle speciesFailure of cell divisionin a cell of a growingdiploid plant produces atetraploid branch.Gametes producedby flowers on thisbranch will be diploid.If offspring are viableand fertile, it is a newbiological species.2n2n 64n 124n

Sympatric speciationAn allopolyploid an individual with two chromosome sets, derived fromdifferent speciesUnreduced gametewith 4 chromosomesHybrid with7 chromosomesSpecies A2n 4Unreduced gametewith 7 chromosomesViable fertilehybridMeiotic error;chromosomenumber notreduced from2n to n2n 10Normal gameten 3Species B2n 6Figure 24.9Normal gameten 3

Polyploidy isespecially commonin plants

Allopatric speciationDivergence occurs in geographic isolation

Sympatric speciationDivergence occurs despite lack of geographic isolation

Allopatric vs. sympatric speciation

REPRODUCTIVE ISOLATING MECHANISMS: Prezygotic Barriers (operate beforemating) Habitat Isolation Behavioral Isolation Temporal Isolation Mechanical Isolation Gametic Isolation

Mechanical Isolation

GalapagosIslandsRed-footed boobyBlue-footed boobies

Dog-day(17 year)Periodical(12 year)

headacrosometailmidpieceovum

REPRODUCTIVE ISOLATING MECHANISMS: Postzygotic Barriers (operate after mating): Hybrid zygote abnormality Hybrid Infertility (Reduced Hybrid Fertility) Low Hybrid Viability (Reduced HybridViability) Absence or Sterility of one sex Hybrid Breakdown

x sterile

Reproductive lsof differentspeciesHABITAT ingattemptTEMPORAL ISOLATIONBEHAVIORAL ISOLATIONMECHANICAL ISOLATION

Reproductive lizationGAMETIC ISOLATIONREDUCED nViablefertileoffspringREDUCED HYBRID FERTILITY HYBRID BREAKDOWN

The Tempo of Speciation *the fossil record reveals that at certain timesin some lineages, speciation rates haveexceeded extinction rates the result:ADAPTIVE RADIATION

ADAPTIVE RADIATION: gives rise toa large number of daughter species-likely to occur when apopulation colonizes anenvironment that hasrelatively few species-often evident on islands (islands havemany ecological opportunities for newspecies)

Case study ofadaptive radiation:the HAWAIIANISLANDS!*1,000 species of flowering plants (more than90% of these are “endemic” – found nowhereelse)*10,000 species of insects (believed to haveevolved from only 400 immigrant species)*1,000 species of land snails

Case study of adaptive radiation:the HAWAIIAN ISLANDS!*more than 100 bird species(believed to have evolvedfrom only 7 immigrant species)*no amphibians or reptiles (until humansintroduced them)*only one mammal species: a bat(until humans introduced more)

Tempo of Evolution: Gradualismvs. Punctuated Equilibrium Gradualism Model: species descendedfrom a common ancestor gradually divergemore and more in morphology as theyacquire unique adaptions Punctuated Equilibrium Model: a newspecies changes most as it buds from aparent species, and then changes little forthe rest of its existence

Tempo of EvolutiontimeGradualismPunctuatedEquilibrium

Evolutionary NoveltiesEvolutionarynovelties usuallyarise asmodifications ofexisting traits

Evolution is not “goal oriented”Evolution is a genetic response to the interactionbetween the individuals of a population and theircurrent environment (which includes otherindividuals)Natural selection can only act on the phenotypicvariation present, and selection can only causeevolutionary change if phenotypic variationresults (at least in part) from genetic variation

Evolution is not “goal oriented”Therefore, even if female tigers in a populationwould prefer blue males, blue tigers will neverevolve if the population lacks variation thatincludes blue furYES! But heexists onlyin my dreams.He’s not blue,but he’s betterthan nothing!

(“morphological” definition of species) . Phylum Chordata (subphylum: Vertebrata) Class Mammalia Order Carnivora Family Canidae Genus Canis Species Canis familiaris *SPECIATION: the process by which one species splits into two species, which thereafter evolve