BIOL 102 General Biology II - Chp 24: The Origin of Species

The Origin of The Origin of SpeciesSpecies

BIOL BIOL 102: 102: General Biology IIGeneral Biology II

Chapter Chapter 2424

Rob Rob SwatskiSwatski Associate Professor Associate Professor of Biologyof Biology

HACCHACC--YorkYork 1

SpeciationSpeciation

Explains how new species originate &

how populations evolve

Microevolution: adaptations that evolve within a

population’s gene pool

Macroevolution: evolutionary change

above the species level

Macroevolution is the cumulative effect of many speciation &

extinction events MicroevolutionMicroevolution 2

MacroevolutionMacroevolution

3

Macroevolution of TechnologyMacroevolution of Technology

4

How is How is Phylogeny Phylogeny

Determined?Determined?

Morphology

Physiology

Biochemistry

DNA sequences

5

Biological Biological Species Species ConceptConcept

A group of populations whose members have

the potential to interbreed in nature …

... & produce viable, fertile offspring

They do not breed successfully with other

populations

Gene flow between populations holds the phenotype together

6

Similarity between two Similarity between two differentdifferent speciesspecies

Eastern meadowlark Western meadowlark

7

Diversity Diversity within within a species a species

The biological species concept The biological species concept is is notnot based on physical similarity based on physical similarity 8

Reproductive Reproductive IsolationIsolation

Biological factors (barriers) that

prevent 2 different species from mating

& producing offspring (hybrids)

Reproductive isolation is classified by whether factors act before or after

fertilization

Prezygotic barriers & Postzygotic barriers

9

Habitat

Isolation Temporal

Isolation

Behavioral

Isolation

Mechanical

Isolation

Gametic

Isolation

Reduced Hybrid

Viability

Reduced Hybrid

Fertility

Hybrid

Breakdown

Individuals of

different species

MATING ATTEMPT FERTILIZATION

VIABLE, FERTILE

OFFSPRING

(a) (c) (e)

(d)

(b)

(g)

(k)

(h) (i)

(j)

(l) (f)

PrezygoticPrezygotic BarriersBarriers

PostzygoticPostzygotic BarriersBarriers

10

PrezygoticPrezygotic BarriersBarriers Block fertilization by:Block fertilization by:

Impeding different species from

attempting to mate

Preventing the successful

completion of mating

Hindering fertilization if

mating is successful

11

Habitat

Isolation

Temporal

Isolation

Behavioral

Isolation

Mechanical

Isolation

Gametic

Isolation

Individuals of

different species

MATING ATTEMPT

FERTILIZATION

(a) (c) (e) (f)

(b)

(g)

(d)

PrezygoticPrezygotic BarriersBarriers

12

Habitat Habitat IsolationIsolation

Two species rarely or never

encounter each other …

… because they occupy different

habitats …

… but are not isolated by

physical barriers

Ex: sticklebacks & garter snake

species 13

St. Lawrence SeawaySt. Lawrence Seaway

14

Water-dwelling garter snake Thamnophis Terrestrial Thamnophis

15

Temporal Temporal IsolationIsolation

Species that breed at different times of the day,

… seasons or years …

… & cannot mix their gametes

Ex: toads & spotted skunks

16

Bufo fowleri

Bufo americanus

17

Eastern spotted skunk (Spilogale putorius)

- mates in late winter

Western spotted skunk (Spilogale gracilis)

- mates in late summer

18

Behavioral Behavioral IsolationIsolation

Behaviors unique to a species are

effective barriers

Courtship rituals

Displays

Vocalizations

19

BlueBlue--Footed Booby Footed Booby Courtship RitualCourtship Ritual

20

Bush Cricket Mating Bush Cricket Mating DisplayDisplay

21

VocalizationVocalization

22

Mechanical Mechanical IsolationIsolation

Morphological differences can

prevent successful mating

Anatomical structures evolve

differently

Body plans are misaligned

23

Genital openings are not in alignment

Bradybaena species with shells spiraling in opposite directions

24

Spider Spider copulatorycopulatory organsorgans

25

GameticGametic IsolationIsolation

Sperm from one species cannot fertilize eggs of another species

Proteins on gamete surfaces cannot bind to

each other

Ex: sea urchins

26

PostzygoticPostzygotic BarriersBarriers Prevent hybrid zygote from developing into a viable,

fertile adult

Reduced hybrid

viability

Reduced hybrid fertility

Hybrid breakdown

27

Reduced Hybrid

Viability

Reduced Hybrid

Fertility

Hybrid

Breakdown

FERTILIZATION

VIABLE, FERTILE

OFFSPRING

(k)

(h) (i)

(j)

(l)

PostzygoticPostzygotic BarriersBarriers

28

Reduced Reduced Hybrid Hybrid

ViabilityViability

Genes of different parent species may

interact & impair hybrid development

May lead to incomplete development of young

Results in weak, frail, & infertile offspring

Ex: salamander (Ensatina) hybrids

29

Reduced Reduced Hybrid Hybrid FertilityFertility

Hybrid development may also be impaired in

other ways

Even if hybrids are vigorous, they may be

sterile

Meiosis in hybrids does not produce normal

gametes

Ex: donkeys & horses have different #’s of

chromosomes 30

Mule Mule (sterile hybrid)(sterile hybrid)

31

Hybrid Hybrid BreakdownBreakdown

Some 1st generation hybrids can be

fertile…

… but, when they mate with another

species (or with either parent

species) …

… offspring of the next generation will be feeble or sterile

Offspring carry too many recessive

alleles 32

Hybrid Rice CultivarsHybrid Rice Cultivars

33

Limitations of Limitations of the Biological the Biological

Species Species ConceptConcept

The biological species concept

cannot be applied to:

Fossils

Asexual organisms

Prokaryotes

Why Not?Why Not?

Why not?Why not? 34

One Species … or Two?One Species … or Two?

35

Grizzly bear (U. arctos)

Polar bear (U. maritimus)

Hybrid “grolar bear” 36

Alternative Definitions of Alternative Definitions of “Species”“Species”

Emphasize the unity within a species, not the separateness between different species

Morphological species concept

Ecological species concept

Phylogenetic species concept

37

Morphological Morphological Species Species ConceptConcept

Defines a species by it’s anatomical

features

Applies to both asexual & sexual

species

… but relies on subjective criteria

Which structures distinguish a

species? 38

39

Ecological Ecological Species Species ConceptConcept

Defines a species by its ecological niche

Also applies to both asexual & sexual

species

Emphasizes the role of disruptive

selection

Ex: barnacle species

40

41

PhylogeneticPhylogenetic Species Species ConceptConcept

Defines a species as the smallest group of

individuals on a phylogenetic tree

Also applies to both asexual & sexual

species

It can be difficult to determine the

degree of difference required for separate

species

Ex: slime molds 42

43

SpeciationSpeciation

Allopatric Speciation

Speciation with geographic isolation

(“other country”)

Sympatric speciation

Speciation without geographic isolation

(“same country”) 44

Allopatric speciation Sympatric speciation 45

AllopatricAllopatric SpeciationSpeciation

Gene flow is interrupted or

reduced …

… when a population is divided into

geographically isolated

subpopulations

Ex: habitat fragmentation

46

Habitat Fragmentation Habitat Fragmentation in the Amazon Rainforestin the Amazon Rainforest

47

48

A. harrisi A. leucurus

Harris’s antelope squirrel

(South rim)

White-tailed antelope squirrel

(North rim)

49

Mechanisms of Mechanisms of AllopatricAllopatric SpeciationSpeciation

Mutation Natural

Selection Genetic Drift

50

(a) Under high predation (b) Under low predation

Reproductive isolation as a by-product of selection: Mosquitofish 51

Evidence of Evidence of AllopatricAllopatric SpeciationSpeciation

Regions with many geographic

barriers …

… usually have more species

than regions with fewer barriers

Ex: tropical rain forest frogs

52

A. formosus

Atlantic Ocean

A. nuttingi

Isthmus of PanamaIsthmus of Panama

Pacific Ocean

A. panamensis A. millsae

Allopatric speciation in snapping shrimp

(Alpheus) – sibling species

53

Geographic distance (km)

De

gre

e o

f re

pro

du

ctiv

e is

ola

tio

n

0 0

50 100 150 250 200 300

0.5

1.0

1.5

2.0 Dusky salamandersDusky salamanders

Reproductive isolation increases as the distance between populations increases

54

Sympatric SpeciationSympatric Speciation Occurs in geographically overlapping populations

Polyploidy Habitat

Differentiation Sexual

Selection Hybrid Zones

55

PolyploidyPolyploidy

The presence of extra sets of

chromosomes due to accidents of cell

division

Much more common in plants

than in animals (flatworms, leeches,

salamanders, tree frogs)

Occurs in many important crops:

oats, cotton, potatoes, tobacco, &

wheat

Autopolyploidy & Allopolyploidy

56

57

2n = 6 4n = 12

Failure of cell division after chromosome

duplication gives rise to tetraploid

cells

2n

Gametes produced

are diploid.

4n

Offspring with tetraploid

karyotypes may be viable & fertile

& become new species

AutopolyploidyAutopolyploidy A new species with 2 or more chromosome sets, derived from

one species

58

Species A 2n = 6

Normal gamete

n = 3

Meiotic error

Species B 2n = 4

Unreduced gamete with 4

chromosomes Hybrid with 7

chromosomes

Unreduced gamete with 7

chromosomes

Normal gamete

n = 3

New viable fertile

hybrid species* (allopolyploid)

2n = 10

*The 2n chromosome # of the

new species = the sum of the 2n chromosome #’s of the

parent species

AllopolyploidyAllopolyploidy A new species with multiple sets of chromosomes, derived

from 2 different species

59

Habitat Habitat DifferentiationDifferentiation

Sympatric speciation can also result from the appearance of

new ecological niches

Ex: North American maggot fly

Lives on native hawthorn trees &

more recently introduced apple trees

60

Apples (new habitat) Hawthorns (original habitat)

61

Sexual Sexual SelectionSelection

Can also drive sympatric speciation

Selection for mates of different colors led to

speciation

Ex: cichlids in Lake Victoria

62

63

EXPERIMENTEXPERIMENT

Normal light Monochromatic

orange light

Pundamilia pundamilia

P. nyererei

Females mated with males of both species

Females only mated with males of their own species

64

Hybrid Hybrid ZonesZones

Regions where members of different

species mate & produce hybrids

Provide opportunities to

study factors that cause reproductive

isolation

Ex: swordtails (Xiphophorus)

65

66

Patterns Patterns Within Hybrid Within Hybrid

ZonesZones

Hybrids can occur in a single band where

adjacent species meet

Hybrids often have reduced fitness compared

with parent species

Hybrid zone distribution can be more complex if

parent species are found in multiple habitats

within the same region

67

68

Fire-bellied toad range

Hybrid zone

Yellow-bellied toad range

Fre

qu

en

cy o

f

B. vari

eg

ata

-sp

ecif

ic a

llele

Yellow-bellied

toad range

Hybrid

zone

Fire-bellied

toad range

Distance from hybrid zone center (km)

40

0.99

0.9

0.5

0.1

0.01

30 20 10 0 10 20

69

Possible Outcomes of Possible Outcomes of Hybrids Over TimeHybrids Over Time

Reproductive barriers become stronger

Reproductive barriers become weaker

Hybrid individuals continue to form

70

Gene flow

Population (5 individuals

are shown)

Barrier to gene flow

Isolated population diverges

Hybrid zone

PossiblePossible outcomes:outcomes:

Reinforcement

OR

OR

Fusion

Stability

Hybrid

71

ReinforcementReinforcement

Occurs when hybrids are less fit than their

parent species

Over time, hybrids gradually stop

forming

Reproductive barriers become

stronger

72

Sympatric male pied flycatcher

Allopatric male pied flycatcher

73

Pied flycatchers

Collared flycatchers

28

24

20

16

12

8

4

0

(none)

Nu

mb

er

of

fem

ale

sN

um

be

r o

f fe

mal

es

Females matingFemales mating with males from:with males from:

Own species

Other species

Sympatric malesSympatric males

Own species

Other species

AllopatricAllopatric malesmales 74

Pundamilia nyererei Pundamilia pundamilia

Pundamilia “turbid water,” hybrid offspring from a location

with turbid water

FusionFusion

Occurs when hybrids have the same fitness as both parent species

There can be significant gene flow

between species

Reproductive barriers weaken

If gene flow is high enough, parent

species can fuse into a single species

75

StabilityStability

Continued production of hybrid individuals

Extensive gene flow from outside the hybrid zone

can overwhelm selection for increased repro isolation inside the

hybrid zone

The resulting 1st or 2nd generation hybrid

offspring are less fit than either parent species

76

SpeciationSpeciation

Can occur slowly or rapidly

Can result from changes in few or many

genes

How long does it take for new species to

form?

How many genes need to differ between

species?

77

Speciation Patterns Can Speciation Patterns Can Be Studied Using:Be Studied Using:

The Fossil Record

Morphological Data

Molecular Data

78

Patterns in Patterns in the Fossil the Fossil

RecordRecord Species may appear

suddenly, persist essentially unchanged for some time, & then apparently disappear

Niles Eldredge & Stephen Jay Gould

coined the term punctuated

equilibrium to describe periods of

apparent stasis punctuated by sudden

change

This contrasts with the gradual change model

79

(a) PunctuatedPunctuated

patternpattern

Time

(b) GradualGradual

patternpattern

80

Speciation Speciation RatesRates

The punctuated pattern in the fossil record & lab

studies suggests that speciation can be rapid

The interval between speciation events can

range from:

4000 years (cichlids) to…

40 million years (some beetles)

81

Rapid Speciation of Rapid Speciation of Helianthus Helianthus anomalusanomalus

82

H. annuus

gamete H. petiolarus

gamete

F1 experimental hybrid

(4 of the 2n = 34

chromosomes are shown)

EXPERIMENT

RESULTS

Chromosome 1

H. anomalus

Chromosome 2

H. anomalus

Experimental hybrid

Experimental hybrid

83

Studying the Studying the Genetics of Genetics of SpeciationSpeciation

Genomics enables researchers to identify

specific genes involved in some cases of speciation

Speciation might require the change of only one

allele …

… or many alleles

84

SingleSingle--Gene SpeciationGene Speciation

85

(a) Typical Mimulus lewisii (b) M. lewisii with an M. cardinalis flower-color allele

A Locus That Influences Pollinator Choice:A Locus That Influences Pollinator Choice: Monkey flowersMonkey flowers

86

(c) Typical Mimulus cardinalis (d) M. cardinalis with an M. lewisii flower-color allele 87

88

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