Chapter 24 The Origin of Species Part C

Chapter 24The Origin of

SpeciesPart C

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

Allopatric and Sympatric Speciation: A Review

• 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

Animals• Don't form polyploids and will

use other mechanisms.

Concept 24.3: Hybrid zones provide opportunities to study factors that cause reproductive isolation

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

Patterns Within Hybrid Zones

• A hybrid zone can occur in a single band where adjacent species meet• Hybrids often have reduced

fitness compared with parent species• The distribution of hybrid zones

can be more complex if parent species are found in multiple habitats within the same region

EUROPE

Fire-belliedtoad range

Hybrid zone

Yellow-belliedtoad rangeYellow-bellied toad,

Bombina variegata

Fire-bellied toad,Bombina bombina

Alle

le fr

eque

ncy

(log

scal

e)

Distance from hybrid zone center (km)40 30 20 2010 100

0.01

0.1

0.5

0.9

0.99

A narrow hybrid zone for B. variegata and B. bombina in Europe

Hybrid Zones over Time

• When closely related species meet in a hybrid zone, there are three possible outcomes:–Strengthening of reproductive barriers–Weakening of reproductive barriers–Continued formation of hybrid individuals

Fig. 24-14-1

Gene flow

Population(five individualsare shown)

Barrier togene flow

Formation of a hybrid zone and possible outcomes for hybrids over time

Fig. 24-14-2

Gene flow


Barrier togene flow

Isolated populationdiverges


Fig. 24-14-3

Gene flow


Barrier togene flow


Hybridzone

Hybrid


Fig. 24-14-4

Gene flow


Barrier togene flow


Hybridzone

Hybrid

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

Fig. 24-15

Sympatric malepied flycatcher

Allopatric malepied flycatcher

Pied flycatchers

Collared flycatchers

Num

ber o

f fem

ales

(none)

Females matingwith males from:

Ownspecies

Otherspecies

Sympatric males

Ownspecies

Otherspecies

Allopatric males

0

4

8

12

16

20

24

28

Reinforcement of barriers to

reproduction in closely related

species of European flycatchers

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

Fig. 24-16

Pundamilia nyererei Pundamilia pundamilia

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

The breakdown of reproductive

barriers

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

• In cases where hybrids have increased fitness, local extinctions of parent species within the hybrid zone can prevent the breakdown of reproductive barriers

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

• Evolution has two speeds of change:–Gradualism or slow change–Rapid bursts of speciation

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

The Time Course of Speciation

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

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

Patterns in the Fossil Record

• Niles Eldredge and Stephen Jay Gould coined the term punctuated equilibrium 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

Fig. 24-17

(a) Punctuated pattern

(b) Gradual pattern

Time

Two models for the tempo of speciation

Gradualism Evolution• Darwinian style evolution.• Small gradual changes over

long periods time.

Gradualism Predicts:• Long periods of time are

needed for evolution.• Fossils should show

continuous links.

Problem• Gradualism doesn’t fit the

fossil record very well. • (too many “gaps”).

Punctuated Evolution• theory that deals with the

“pacing” of evolution.• Elridge and Gould – 1972.

Predictions• Speciation can occur over a

very short period of time • (1 to 1000 generations).• Fossil record will have gaps or

missing links.

Predictions• New species will appear in

the fossil record without connecting links or intermediate forms.• Established species will show

gradual changes over long periods of time.

Speciation Rates

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

• The interval between speciation events can range from 4,000 years (some cichlids) to 40,000,000 years (some beetles), with an average of 6,500,000 years

Possible Mechanism• Adaptive Radiation, especially

after mass extinction events allow new species to originate.• Saturated environments favor

gradual changes in the current species.

Comment• Punctuated Equilibrium is the

newest ”Evolution Theory”.• Best explanation of fossil

record evidence to date.

(a) The wild sunflower Helianthus anomalus

Rapid speciation in a sunflower hybrid zone

Studying the Genetics of Speciation

• The explosion of genomics is enabling researchers to identify specific genes involved in some cases of speciation

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

Fig. 24-19

Single-gene speciation

From Speciation to Macroevolution

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

Origin of Evolutionary Novelty

• How do macroevolution changes originate?• Several ideas discussed in

textbook (read them)–Exaptation–Heterochrony–Homeosis

Another idea• Mutations in developmental

or control genes (Chapter 21)• Looking very promising as a

source of macroevolution

Exaptation

• When a structure that was adapted for one context is co-opted for another function.• Ex. – feathers and flying

Heterochrony• Changes in the timing or rate

of development.–Allometric Growth–Paedomorphsis

1. Allometric Growth – changes in the relative rates of growth of various parts of the body.• Ex. – skull growth in primates

2. Paedomorphosis – when an adult retains features that are present in the juvenile form.

Ex. – gills in adult salamanders

Ex - HomeosisChanges in the basic body

design or arrangement of body parts.

Ex. – Hox gene clusters that gave rise to vertebrates from invertebrates.

Gene Duplications• Allow genes to be used for

other functions such as in the previous slide.• Many other examples are

known.

Future of Evolution ?• Look for new theories and

ideas to be developed, especially from new fossil finds and from molecular (DNA) evidence.

Evolutionary Trends• Evolution is not goal oriented.

It does not produce “perfect” species.• Remember – species survive

because of their adaptations. They don’t adapt to survive.

Summary• Be able to discuss the main

theories of what is a “species”.• Know various reproductive

barriers and examples.

Summary• Know allopatric and

sympatric speciation.• Be able to discuss gradualism

and punctuated equilibrium theories.

Summary• Recognize various ideas

about the origin of evolutionary novelties.

You should now be able to:

1.Define and discuss the limitations of the four species concepts

2.Describe and provide examples of prezygotic and postzygotic reproductive barriers

3.Distinguish between and provide examples of allopatric and sympatric speciation

You should now be able to:

4. Explain how polyploidy can cause reproductive isolation

5. Define the term hybrid zone and describe three outcomes for hybrid zones over time

End of Chapter 24

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Chapter 24 The Origin of Species Part C