Origin of SpeciesChapter 24

Darwin

Natural selectionOne species evolves due to adaptation to it’s environment Adaptation does not explain why one species becomes another

Speciation

One species gives rise to many descendant species

Galápagos giant tortoise, another species unique to the islands

Fig. 25-25

Recent(11,500 ya)

NeohipparionPliocene(5.3 mya)

Pleistocene(1.8 mya)

Hipparion

Nannippus

Equus

Pliohippus

Hippidion and other genera

Callippus

Merychippus

Archaeohippus

Megahippus

Hypohippus

Parahippus

Anchitherium

Sinohippus

Miocene(23 mya)

Oligocene(33.9 mya)

Eocene(55.8 mya)

Miohippus

Paleotherium

Propalaeotherium

Pachynolophus

Hyracotherium

Orohippus

Mesohippus

Epihippus

Browsers

Grazers

Key

Species

Latin meaning “kind” or “appearance.”Morphological differences have been used to distinguish species. Differences in body function, biochemistry, behavior, and genetic makeup

Biological species concept

Defines species as“groups of actually or potentially interbreeding natural populations which are reproductively isolated from other such groups”Species composed of populations that can mate & produce offspring

Species

Homo sapiensDiversityBelong to same speciesCapacity to interbreed.

Species

Species are based on inter-fertility, Not physical similarity.Eastern & western meadowlarks Have similar shapes & colorationDifferences in song prevent interbreeding between species.

Reproductive isolation

Members of a populationCannot mate Or produce fertile offspringWhat causes reproductive isolation?What helps species to retain their identities?

Reproductive isolation mechanisms

Prevent genetic exchange1. Prezygotic: “before zygote” preventing formation of zygotes2. Postzygotic: “after zygote”preventing proper development of zygotes once formed

Prezygotic barriers

Postzygotic barriers

Habitatisolatio

n

Temporal

isolation

Behavioral

isolation

Mechanical

isolation

Gameticisolation

Reduced

hybridviability

Reduced

hybridfertility

Hybridbreakdow

n

Indivi-duals

ofdiffere

ntspecies

MATINGATTEMP

T

FERTILI-ZATION

VIABLE,FERTILE

OFF-SPRING

(l)

(i)

(j)

(h)

(g)

(f)(e)

(c)

(a)

(b)

(d)

(k)

Prezygotic isolating mechanisms

1. Ecological (Habitat) isolation2. Behavioral isolation3. Temporal isolation4. Mechanical isolation5. Gamete isolation

Ecological isolation

Species utilize different parts of the environment May not encounter each otherExamples: Lions & tigers in IndiaLions: open fields, in pridesTigers: forests, hunt in isolation

Ecological isolation

ToadThese species can interbreedUse different parts of the woods to breed

Behavioral isolation

Differ in courtshipsMating dances differMallard & Pintail ducks

Behavioral isolation

Blue-footed boobiesCourtship dance

Behavioral isolation

Pheromones:Chemical signals used when matingLacewings: Move their abdomen to create a mating songVaries among species

Temporal isolation

Mating or breeding times are differentFlowering times are differentWild lettuceFrogs: genus Rana 5 species live close together differ in mating seasons

Temporal isolation

Western spotted skunkLate summer

Eastern spotted skunkLate winter

Mechanical isolation

Structural differences prevent matingPlant structures & pollenInsects

Gamete isolation

Sperm from one species unable to fertilize egg of anotherPlants different shaped pollen tubes Difficult to form a hybrid

Postzygotic isolation

1. Hybrid mating may occur Genetic pairings can not function in embryo stage2. Offspring are inferior Will die in nature3. Sterile offspring

Postzygotic isolation

Examples:Leopard frogs problems with developing eggs Mule formed from female horse & male donkeyMule is sterile

Fig. 24-4o

(k)

Mule (sterile hybrid)

How does reproductive isolation arise?

1. By chanceA population moves to a new habitatAdaptMate within the new population2. Natural selectionSelect individuals that are able to reproduce with greater success

Speciation

1. Identical populations must diverge2. Reproductive isolation must evolve to maintain these differences

Mechanisms of sepciation

1. Allopatric speciation (other)2. Sympatric speciation (together)

Fig. 24-5

(a) Allopatric speciation (b) Sympatric speciation

Allopatric speciation

Populations separated by geographical locationMore likely to develop into new speciesUnable to reproduce with parent population

Allopatric speciation

Geographic barriersIslandMountainLakeSize & mobility of the animal

Antelope squirrels

Allopatric speciation

Ernst MayrFirst demonstrated that geographic isolation leads to speciationNew Guinea Papuan kingfisherIsolated species are more distinctive

Allopatric speciation

Hawaiian Islands

Adaptive radiation

Organisms form new speciesFill nichesNo competition

Sympatric species

Distinct species live in a single locationUse different parts of the habitatBehave separately (mating calls or chemicals)

Sympatric speciation

1. Instantaneous speciation2. Disruptive selection

Instantaneous speciation

Polyploidy: Individual has more than two copies of chromosomesAutopolyploidy: All chromosomes from one speciesAllopolyploidy: Two species hybridize

Instantaneous

Tetraploids (4 sets of chromosomes) Self pollinate or mate with another tetraploidOver time become fertile Established a new species

Polyploidy

Hugo de Vries---primrose

Polyploidy

More common in plantsPlants such as wheat, cotton, sugarcane Some animals such as insects, fish & salamanders

Disruptive selection

Two distinct phenotypes evolve into separate speciesLake Victoria Cichlid fishes

Hybrid zones

Region where different species mateCharacteristics are a combination of features of both populationsHybrid is formed

Fig. 24-13b

Fire-bellied toad, Bombina bombina

Fig. 24-13a

Yellow-bellied toad,Bombina variegata

Hybrid zones

1. Increases reproductive barriers Maintains 2 species2. Two species fuse3. Occasional hybrid is still formed

Sexual preferenceEnvironment

Gene flowPopulatio

nBarrier togene flow

Isolatedpopulationdiverges.

Hybridzone

Hybridindividual

Possibleoutcomes:

Stability

Fusion

Reinforcement

Grizzly bear (U. arctos)

▶

▶

Polar bear (U. maritimus)

Hybrid“grolar bear”

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Pace of evolution

1. Gradualism2. Punctuated equilibrium

Pace of evolution

GradualismChanges occur slowly over timeAccumulation of small changes over time

Gradualism

Pace of evolution

The fossil recordMany species appear as new forms rather suddenly (in geologic terms)Persist essentially unchangedThen disappear from the fossil record.

Punctuated equilibrium

Pace of evolution

Punctuated equilibriumSpecies experience long periods of stasisBursts of evolutionary change