Chapter 24 The Origin of Species - PowerPoint Presentation

Slide1

Chapter 24

The Origin of Species

Part CSlide2

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 Slide3

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 selectionSlide4

Animals

Don't form polyploids

and will use other mechanisms.Slide5

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 hybridsSlide6

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 regionSlide7

EUROPE

Fire-bellied

toad range

Hybrid zone

Yellow-bellied

toad range

Yellow-bellied toad,

Bombina variegata

Fire-bellied toad,

Bombina bombina

Allele frequency (log scale)

Distance from hybrid zone center (km)

40

30

20

20

10

10

0

0.01

0.1

0.5

0.9

0.99

A narrow hybrid zone for

B.

variegata

and

B.

bombina

in EuropeSlide8

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 individualsSlide9

Fig. 24-14-1

Gene flow

Population

(five individuals

are shown)

Barrier to

gene flow

Formation of a hybrid zone and possible outcomes for hybrids over timeSlide10

Fig. 24-14-2

Gene flow

Population

(five individuals

are shown)

Barrier to

gene flow

Isolated population

diverges

Formation of a hybrid zone and possible outcomes for hybrids over timeSlide11

Fig. 24-14-3

Gene flow

Population

(five individuals

are shown)

Barrier to

gene flow

Isolated population

diverges

Hybrid

zone

Hybrid

Formation of a hybrid zone and possible outcomes for hybrids over timeSlide12

Fig. 24-14-4

Gene flow

Population

(five individuals

are shown)

Barrier to

gene flow

Isolated population

diverges

Hybrid

zone

Hybrid

Possible

outcomes:

Reinforcement

OR

OR

Fusion

Stability

Formation of a hybrid zone and possible outcomes for hybrids over timeSlide13

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

speciesSlide14

Fig. 24-15

Sympatric male

pied flycatcher

Allopatric male

pied flycatcher

Pied flycatchers

Collared flycatchers

Number of females

(none)

Females mating

with males from:

Own

species

Other

species

Sympatric males

Own

species

Other

species

Allopatric males

0

4

8

12

16

20

24

28

Reinforcement of barriers to reproduction in closely related species of European flycatchersSlide15

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 speciesSlide16

Fig. 24-16

Pundamilia nyererei

Pundamilia pundamilia

Pundamilia “turbid water,”

hybrid offspring from a location

with turbid water

The breakdown of reproductive barriersSlide17

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 barriersSlide18

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 speciationSlide19

Many questions remain concerning how long it takes for new species to form, or how many genes need to differ between speciesSlide20

The Time Course of Speciation

Broad patterns in speciation can be studied using the fossil record, morphological data, or molecular dataSlide21

Patterns in the Fossil Record

The fossil record includes examples of species that appear suddenly, persist essentially unchanged for some time, and then apparently disappearSlide22

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’ existenceSlide23

Fig. 24-17

(a) Punctuated pattern

(b) Gradual pattern

Time

Two models for the tempo of speciationSlide24

Gradualism Evolution

Darwinian style evolution.Small gradual changes over long periods time

.Slide25

Gradualism Predicts:

Long periods of time are needed for evolution.Fossils should show continuous links.Slide26

Problem

Gradualism doesn’t fit the fossil record very well.

(too many “gaps”).Slide27

Punctuated Evolution

theory that deals with the “pacing” of evolution.Elridge

and Gould – 1972.Slide28

Predictions

Speciation can occur over a very

short period of time

(1 to 1000 generations).

Fossil record will have gaps or missing links.Slide29
Slide30

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.Slide31

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 yearsSlide32

Possible Mechanism

Adaptive Radiation, especially after mass extinction events allow new species to originate.

Saturated environments favor gradual changes in the current species.Slide33

Comment

Punctuated Equilibrium is the newest ”Evolution Theory”.Best explanation of fossil record evidence to date.Slide34

(a) The wild sunflower

Helianthus

anomalus

Rapid speciation in a sunflower hybrid zoneSlide35

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 allelesSlide36

Fig. 24-19

Single-gene speciationSlide37

From Speciation to Macroevolution

Macroevolution is the cumulative effect of many speciation and extinction eventsSlide38

Origin of Evolutionary Novelty

How do macroevolution changes originate?

Several ideas discussed in textbook (read them)

Exaptation

Heterochrony

HomeosisSlide39

Another idea

Mutations in developmental or control genes (Chapter 21)Looking very promising as a source of macroevolutionSlide40

Exaptation

When a structure that was adapted for one context is co-opted for another function.

Ex. – feathers and flyingSlide41

Heterochrony

Changes in the timing or rate of development.

Allometric

Growth

PaedomorphsisSlide42

1

. Allometric

Growth – changes in the relative rates of growth of various parts of the body.

Ex. – skull growth in primatesSlide43

2.

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

Ex. – gills in adult salamandersSlide44
Slide45

Ex -

Homeosis

Changes in the basic body design or arrangement of body parts.

Ex. –

Hox

gene clusters that gave rise to vertebrates from invertebrates.Slide46
Slide47

Gene Duplications

Allow genes to be used for other functions such as in the previous slide.Many other examples are known.Slide48

Future of Evolution ?

Look for new theories and ideas to be developed, especially from new fossil finds and from molecular (DNA) evidence.Slide49

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.Slide50
Slide51

Summary

Be able to discuss the main theories of what is a “species”.Know various reproductive barriers and examples.Slide52

Summary

Know allopatric

and sympatric speciation.

Be able to discuss gradualism and punctuated equilibrium theories.Slide53

Summary

Recognize various ideas about the origin of evolutionary novelties.Slide54

You should now be able to:

Define and discuss the limitations of the four species concepts

Describe and provide examples of

prezygotic

and

postzygotic

reproductive barriers

Distinguish between and provide examples of

allopatric

and sympatric speciationSlide55

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 timeSlide56

End of Chapter 24