Overview: The Origin of Species 24 That “Mystery of Mysteries” In the Galápagos Islands Darwin...

Overview: The Origin of Species 24That “Mystery of Mysteries”

• In the Galápagos Islands Darwin discovered plants and animals found nowhere else on Earth.

• Why?

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Figure 24.1

• Speciation, the origin of new species, is at the focal point of evolutionary theory

• Evolutionary theory must explain how new species originate and how populations evolve

• Microevolution consists of changes in allele frequency in a population over time

• Macroevolution refers to broad patterns of evolutionary change above the species level

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The biological species concept emphasizes reproductive isolation

• Species is a Latin word meaning “kind” or “appearance”• Biologists compare morphology, physiology, biochemistry,

and DNA sequences when grouping organisms• The biological species concept states that a 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

• Gene flow between populations holds the phenotype of a population together

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Figure 24.2

(a) Similarity between different species

(b) Diversity within a species

Reproductive Isolation

• Reproductive isolation is the existence of biological factors (barriers) that impede two species from producing viable, fertile offspring

• Hybrids are the offspring of crosses between different species

• Reproductive isolation can be classified by pre-zygotic barriers (act before) or post-zygotic barriers (after fertilization)

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Figure 24.3_b

Pre-zygotic barriersHabitat

IsolationTemporalIsolation

BehavioralIsolation

MechanicalIsolation

GameticIsolation

Individuals of

differentspecies

MATINGATTEMPT FERTILIZATION

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

(b)

(g)

(d)

• 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

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• Habitat isolation: Two species encounter each other rarely, or not at all, because they occupy different habitats, even though not isolated by physical barriers

• Temporal isolation: Species that breed at different times of the day, different seasons, or different years cannot mix their gametes

• Behavioral isolation: Courtship rituals and other behaviors unique to a species are effective barriers

• Mechanical isolation: Morphological differences can prevent successful mating• Gametic Isolation: Sperm of one species may not be able to fertilize eggs of another species

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Reduced HybridViability

Reduced HybridFertility

HybridBreakdown

FERTILIZATIONVIABLE,FERTILE

OFFSPRING

Post-zygotic barriers

(k)

(h) (i)

(j)

(l)

Figure 24.3_c

• Postzygotic barriers prevent the hybrid zygote from developing into a viable, fertile adult:

– Reduced hybrid viability

– Reduced hybrid fertility

– Hybrid breakdown

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• Reduced hybrid viability: Genes of the different parent species may interact and impair the hybrid’s development

• Reduced hybrid fertility: Even if hybrids are vigorous, they may be sterile Horse vs. Donkey vs. Mule

• Hybrid breakdown: Some first-generation hybrids are fertile, but when they mate with another species or with either parent species, offspring of the next generation are feeble or sterile

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Limitations of the Biological Species Concept

• The biological species concept cannot be applied to fossils or asexual organisms (including all prokaryotes)

• The biological species concept emphasizes absence of gene flow

• However, gene flow can occur between distinct species

– For example, grizzly bears and polar bears can mate to produce “grolar bears”

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Figure 24.4 Grizzly bear (U. arctos)

Polar bear (U. maritimus)

Hybrid “grolar bear”

Concept 24.2: Speciation can take place with or without geographic separation

• Speciation can occur in two ways:– Allopatric speciation

– Sympatric speciation

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Figure 24.5

(a) (b)Allopatric speciation.A population forms anew species whilegeographically isolatedfrom its parent population.

Sympatric speciation.A subset of a populationforms a new specieswithout geographicseparation.

Allopatric (“Other Country”) Speciation

• In allopatric speciation, gene flow is interrupted or reduced when a population is divided into geographically isolated subpopulations

– For example, the flightless cormorant of the Galápagos likely originated from a flying species on the mainland

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The Process of Allopatric Speciation

• The definition of barrier depends on the ability of a population to disperse

– For example, a canyon may create a barrier for small rodents, but not birds, coyotes, or pollen

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Figure 24.6

A. harrisii A. leucurus

• Separate populations may evolve independently through mutation, natural selection, and genetic drift

• Reproductive isolation may arise as a result of genetic divergence

– For example, mosquitofish in the Bahamas comprise several isolated populations in different ponds

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

Evidence of Allopatric Speciation

• 15 pairs of sibling species of snapping shrimp (Alpheus) are separated by the Isthmus of Panama

• These species originated 9 to 13 million years ago, when the Isthmus of Panama formed and separated the Atlantic and Pacific waters

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Figure 24.8

A. formosus

Atlantic Ocean

A. nuttingi

Isthmus of Panama

Pacific Ocean

A. panamensis A. millsae

• Regions with many geographic barriers typically have more species than do regions with fewer barriers

• Reproductive isolation between populations generally increases as the distance between them increases

– For example, reproductive isolation increases between dusky salamanders that live further apart

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Figure 24.9

Deg

ree

of

rep

rod

uct

ive

iso

lati

on

Geographic distance (km)0 50 100 150 200 250 300

2.0

1.5

1.0

0.5

0

• Barriers to reproduction are intrinsic; separation itself is not a biological barrier

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Figure 24.10 EXPERIMENT

RESULTS

Initial populationof fruit flies(Drosophila

pseudoobscura)

Some flies raisedon starch medium

Mating experimentsafter 40 generations

Some flies raised onmaltose medium

Female

Starch Maltose

Mal

eM

alto

seS

tarc

h

Number of matingsin experimental group

22 9

8 20

FemaleStarch

population 1

Mal

eS

tarc

hp

op

ula

tio

n 2

Number of matingsin control group

18 15

12 15

Starchpopulation 2

Sta

rch

po

pu

lati

on

1

Sympatric (“Same Country”) Speciation

• In sympatric speciation, speciation takes place in geographically overlapping populations

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Polyploidy

• Polyploidy is the presence of extra sets of chromosomes due to accidents during cell division

• Polyploidy is much more common in plants than in animals

• An autopolyploid is an individual with more than two chromosome sets, derived from one species

• An allopolyploid is a species with multiple sets of chromosomes derived from different species

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Figure 24.11-4

Species A2n = 6

Species B2n = 4

Normalgameten = 3

Meiotic error;chromosome number notreduced from 2n to n

Unreduced gametewith 4 chromosomes

Hybrid with7 chromosomes

Unreduced gametewith 7 chromosomes

Normalgameten = 3

New species:viable fertile hybrid(allopolyploid) 2n = 10

• Many important crops (oats, cotton, potatoes, tobacco, and wheat) are polyploids

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Habitat Differentiation

• Sympatric speciation can also result from the appearance of new ecological niches

• For example, the North American maggot fly can live on native hawthorn trees as well as more recently introduced apple trees

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Sexual Selection

• Sexual selection can drive sympatric speciation• Sexual selection for mates of different colors has

likely contributed to speciation in cichlid fish in Lake Victoria

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Figure 24.12

Normal lightMonochromatic

orange light

P. pundamilia

P. nyererei

EXPERIMENT

Allopatric and Sympatric Speciation: A Review

• In allopatric speciation, geographic isolation restricts gene flow between populations

• Reproductive isolation may then arise by natural selection, genetic drift, or sexual selection in the isolated populations

• Even if contact is restored between populations, interbreeding is prevented

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• In sympatric speciation, a reproductive barrier isolates a subset of a population without geographic separation from the parent species

• Sympatric speciation can result from polyploidy, natural selection, or sexual selection

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Concept 24.3: Hybrid zones reveal factors that cause reproductive isolation

• A hybrid zone is a region in which members of different species mate and produce hybrids

• Hybrids are the result of mating between species with incomplete reproductive barriers

• A hybrid zone can occur in a single band where adjacent species meet

– For example, two species of toad in the genus Bombina interbreed in a long and narrow hybrid zone

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Figure 24.13

EUROPE

Yellow-belliedtoad, Bombinavariegata

Fire-belliedtoad range

Hybrid zone

Yellow-belliedtoad range

Fire-bellied toad, Bombina bombina

Fre

qu

en

cy

of

B.

va

rie

ga

ta-s

pe

cif

ic a

lle

le

Yellow-belliedtoad range

Hybridzone

Fire-belliedtoad range

Distance from hybrid zone center (km)40

0.99

0.9

0.5

0.1

0.01

30 20 10 0 10 20

• Hybrids often have reduced fitness compared with parent species

• The distribution of hybrid zones can be more complex if parent species are found in patches within the same region

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Hybrid Zones over Time

• When closely related species meet in a hybrid zone, there are three possible outcomes:

– Reinforcement

– Fusion

– Stability

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Figure 24.14-4

Gene flow

PopulationBarrier togene flow

Isolatedpopulationdiverges

Hybridzone

Hybridindividual

Possibleoutcomes:

Reinforcement

OR

OR

Fusion

Stability

Reinforcement: Strengthening Reproductive Barriers

• The reinforcement of barriers occurs when hybrids are less fit than the parent species

• Over time, the rate of hybridization decreases• Where reinforcement occurs, reproductive barriers

should be stronger for sympatric than allopatric species

– For example, in populations of flycatchers, males are more similar in allopatric populations than sympatric populations

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Figure 24.15

Females choosing betweenthese males:

Females choosing betweenthese males:

Sympatric pied male

Sympatric collared male

Allopatric pied male

Allopatric collared male

(none)

Female mate choice Female mate choice

Ownspecies

Ownspecies

Otherspecies

Otherspecies

Nu

mb

er o

f fe

mal

es

28

24

20

16

12

8

4

0

Fusion: Weakening Reproductive Barriers

• If hybrids are as fit as parents, there can be substantial gene flow between species

• If gene flow is great enough, the parent species can fuse into a single species

• For example, researchers think that pollution in Lake Victoria has reduced the ability of female cichlids to distinguish males of different species

• This might be causing the fusion of many species

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Figure 24.16

Pundamilia nyererei Pundamilia pundamilia

Pundamilia “turbid water,”hybrid offspring from a locationwith turbid water

Stability: Continued Formation of Hybrid Individuals

• Extensive gene flow from outside the hybrid zone can overwhelm selection for increased reproductive isolation inside the hybrid zone

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Concept 24.4: Speciation can occur rapidly or slowly and can result from changes in few or many genes

• Many questions remain concerning how long it takes for new species to form, or how many genes need to differ between species

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The Time Course of Speciation

• Broad patterns in speciation can be studied using the fossil record, morphological data, or molecular data

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Patterns in the Fossil Record

• The fossil record includes examples of species that appear suddenly, persist essentially unchanged for some time, and then apparently disappear

• Niles Eldredge and Stephen Jay Gould coined the term punctuated equilibria to describe periods of apparent stasis punctuated by sudden change

• The punctuated equilibrium model contrasts with a model of gradual change in a species’ existence

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Figure 24.17

(a) Punctuatedpattern

Time

(b) Gradualpattern

Speciation Rates

• The punctuated pattern in the fossil record and evidence from lab studies suggest that speciation can be rapid

– For example, the sunflower Helianthus anomalus originated from the hybridization of two other sunflower species

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Figure 24.18

Figure 24.19

H. annuusgamete

H. petiolarusgamete

F1 experimental hybrid(4 of the 2n = 34chromosomes are shown)

EXPERIMENT

RESULTS

Chromosome 1

H. anomalus

Chromosome 2

H. anomalus

Experimental hybrid

Experimental hybrid

• The interval between speciation events can range from 4,000 years (some cichlids) to 40 million years (some beetles), with an average of 6.5 million years

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Studying the Genetics of Speciation

• A fundamental question of evolutionary biology persists: How many genes change when a new species forms?

• Depending on the species in question, speciation might require the change of only a single allele or many alleles

– For example, in Japanese Euhadra snails, the direction of shell spiral affects mating and is controlled by a single gene

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• In monkey flowers (Mimulus), two loci affect flower color, which influences pollinator preference

• Pollination that is dominated by either hummingbirds or bees can lead to reproductive isolation of the flowers

• In other species, speciation can be influenced by larger numbers of genes and gene interactions

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TypicalMimuluslewisii

(a)

TypicalMimuluscardinalis

(c)

M. lewisii with anM. cardinalis flower-colorallele

(b)

M. cardinalis with anM. lewisii flower-colorallele

(d)

Figure 24.20

From Speciation to Macroevolution

• Macroevolution is the cumulative effect of many speciation and extinction events

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Figure 24.UN01Celldivisionerror

2n = 6 Tetraploid cell4n = 12

2n

2n New species(4n)Gametes produced

by tetraploids

Figure 24.UN02

Original population

Allopatric speciation Sympatric speciation

Figure 24.UN03

Ancestral species:

Product:

Triticummonococcum(2n = 14)

WildTriticum(2n = 14)

WildT. tauschii(2n = 14)

T. aestivum(bread wheat)(2n = 42)

Figure 24.UN04