Chapter 19: Speciation and Macroevolution

Biological Species ConceptSpecies consist of 1+ populations whose members are capable of interbreeding in nature to produce fertile offspring and do not interbreed with members of different speciesSexual reproductionReproductive isolation

Fig. 24-2(a) Similarity between different species(b) Diversity within a species

Reproductive Isolating MechanismsPrevent interbreeding between 2 speciesPreserve genetic integrityGene flow is prevented

Reproductive barriers - prezygoticPrezygotic prevent fertilizationInterspecific zygote never madeTypes:TemporalHabitatBehavioralMechanicalgametic

TemporalDifferent timesDay, season, yearEx: Fruit flies afternoon vs. morningFrogs late March vs. mid-April

Fig. 24-4e(c)Eastern spotted skunk(Spilogale putorius)

Fig. 24-4f(d)Western spotted skunk(Spilogale gracilis)

HabitatSame geographical area, different habitatEx: FlycatchersOpen woods/farmsDeciduous forestWet thicketsConiferous forestBrushy pastures/ willow thickets

Fig. 24-4c(a)Water-dwelling Thamnophis

Fig. 24-4d(b)Terrestrial Thamnophis

Behavioral Courtship (signals before mating)Aka sexual isolationEx:Nest decoration, dance, song, vocalizations

Fig. 24-4g(e)Courtship ritual of blue-footed boobies

Mechanical Incompatible structures of genital organsEx:Flowers adapted for different insect pollinators

Fig. 24-4h(f)Bradybaena with shellsspiraling in oppositedirections

GameticEgg and sperm incompatible after mating Ex:Aquatic animals release egg and sperm at once; egg and sperm protein bind to each other

Fig. 24-4k(g)Sea urchins

Reproductive barriers - PostzygoticPrevent gene flow when fertilization occursHybrid inviabilityHybrid sterilityHybrid breakdown

Fig. 24-4l(h)Ensatina hybrid

Hybrid inviabilityIncreased likelihood of reproductive failure after fertilizationSpontaneous abortion genes do not interact properly

Hybrid sterilityInterspecific hybrid lives but cant reproduceIncompatible courtship w/ either parent speciesGametes of hybrid abnormal during meiosisDifferent chromosome #sFemale horse 64 Male donkey 62Mule - 63

Fig. 24-4m(i)Donkey

Fig. 24-4n( j)Horse

Fig. 24-4o(k)Mule (sterile hybrid)

Hybrid breakdownInability of a hybrid to reproduce due to some defectF2sEx: 2 sunflower species 80% F2 cant reproduce

Fig. 24-4p(l)Hybrid cultivated rice plants withstunted offspring (center)

Reproductive isolation is the Key to SpeciationSpeciation = evolution of a new species2 patterns1) Anagenic2) Cladogenic

Anagenesis(phyletic evolution)Relatively small, progressive evolutionary changes in a single lineage over long periodsEnough time conversion of 1 species to anotherSequence of species occurs over time without an increase in the number of species

Cladogenesis(branching evolution)2+ populations of an ancestral species split and diverge, eventually forming 2+ new speciesClade = cluster of species derived from a single common ancestorOver time increase species richness

When has speciation occurred?Population is sufficiently different from its ancestral species that no genetic exchange can occur between them2 ways:Allopatric Sympatric

Fig. 24-5(a) Allopatric speciation(b) Sympatric speciation

Allopatric SpeciationOccurs when 1 population becomes geographically separated from the rest of the species and then evolves by natural selection and/or genetic driftMost commonGeographic isolation by:Changing of Rivers, glaciers, mountains, land bridges, lakesBirds vs. ratsSmall population migrates or is dispersedColonize new areaIsolated gene pool microevolution new species

Fig. 24-6A. harrisiA. leucurus

Sympatric SpeciationNew species evolves within the same geographical region as the parent species2 ways:Change inPloidy Ecology

Ploidy Polyploidy - 2+ chromosome setsPlants rapid speciationAutopolyploid multiple sets chromosomes from a single speciesAllopolyploidy multiple sets of chromosomes from 2+ speciesAllopolyploid diff # chromosomes from parents = new species1) extinct2)coexist3)replace parent species

Fig. 24-10-32n = 64n = 12Failure of celldivision afterchromosomeduplication givesrise to tetraploidtissue.2nGametesproducedare diploid..4nOffspring withtetraploidkaryotypes maybe viable andfertile.

Fig. 24-11-4Species A2n = 6Normalgameten = 3MeioticerrorSpecies B2n = 4Unreducedgametewith 4chromosomesHybridwith 7chromosomesUnreducedgametewith 7chromosomesNormalgameten = 3Viable fertilehybrid(allopolyploid)2n = 10

Allopatric and Sympatric Speciation: A ReviewIn allopatric speciation, geographic isolation restricts gene flow between populationsReproductive isolation may then arise by natural selection, genetic drift, or sexual selection in the isolated populationsEven if contact is restored between populations, interbreeding is prevented

In sympatric speciation, a reproductive barrier isolates a subset of a population without geographic separation from the parent speciesSympatric speciation can result from polyploidy, natural selection, or sexual selection

EcologyParasitic insectsEx: fruit maggot fliesSwitched host from hawthorn tree fruits to domestic applesMutation disruptive selection different ecological opportunity

Evolutionary Change rapid or gradual?2 models

Punctuated Equilibrium fossil record accurately reflects evolution as it actually occursLong periods of stasis are punctuated by short periods of rapid speciation triggered by changes in the environmentSpeciation in spurtsShort periods evolution, long periods stabilityAccounts for abrupt appearance of new species with few intermediate forms

Fig. 24-17(a) Punctuated pattern(b) Gradual patternTime

Gradualism traditional view of evolutionEvolution proceeds continuously over long periodsRarely observed, fossil record incompletePopulations slowly diverge from 1 another by the gradual accumulation of adaptive characteristics within each population

Macroevolution Dramatic changes that occur over long time spans in evolutionAttempts to explain large phenotypic changes (novelties)Important aspectsEvolutionary noveltiesAdaptive radiationMass extinction

Macroevolution

Adaptive radiationEvolutionary diversification of many related species from 1 or a few ancestors in a short periodAdaptive zones: new ecological opportunities that were not exploited by an ancestor Islands common fewer species thereEx: Darwins finches, honeycreeper birds, silversword plants

Extinction End of lineage; last member of species diesPermanentMakes adaptive zones vacantBackground extinctionContinuous, low-levelMass extinctionNumerous species die at onceAdaptive radiation follows

Extinction videoCauses of mass extinctionClimate change / Earths temp.CatastrophesComet/asteroid dust (block light) food chain disrupted, drop in temp.Competition Humans animal / plant habitats

Microevolution vs. MacroevolutionGeneticsWell suited surviveChance eventslucky to surviveRight place, right time

*Figure 24.2 The biological species concept is based on the potential to interbreed rather than on physical similarity*Figure 24.4 Reproductive barriers*Figure 24.4 Reproductive barriers*Figure 24.4 Reproductive barriers*Figure 24.4 Reproductive barriers

*Figure 24.4 Reproductive barriers*Figure 24.4 Reproductive barriers*Figure 24.4 Reproductive barriers*Figure 24.4 Reproductive barriers*Figure 24.4 Reproductive barriers*Figure 24.4 Reproductive barriers*Figure 24.4 Reproductive barriers*Figure 24.4 Reproductive barriers*Fig 24.5 Two main modes of speciation*Figure 24.6 Allopatric speciation of antelope squirrels on opposite rims of the Grand Canyon*Fig 24.10 Sympatric speciation by autopolyploidy in plants

*Figure 24.11 One mechanism for allopolyploid speciation in plants

***Figure 24.17 Two models for the tempo of speciation