Chapter 24The Origin of Species

What You Need to Know:• The difference between microevolution and

macroevolution.• The biological concept of a species.• Prezygotic and postzygotic barriers that

maintain reproductive isolation in natural populaitons.

• How allopatric and sympatric speciation are similar and different.

• How autopolyploid or an allopolyploid chromosomal change can lead to sympatric speciation.

• How punctuated equilibrium and gradualism describe two different tempos of speciation.

Do Now # 8 Problem 1:• In humans, having the Rh blood antigen is

dominant over not having the Rh antigen. In the United States, 84% of the population is Rh positive. How many individuals are homozygous. (Assume it is Hardy-Weinberg)

Homework 7 Problem 2:• One in every 1700 Caucasian babies that

are born have cystic fibrosis. Cystic fibrosis is an autosomal recessive disorder (c).

A. What is the frequency of the recessive allele?

B. How many of every 1700 babies are heterozygous?

Speciation = origin of species

•Microevolution: changes within a single gene pool

•Macroevolution: evolutionary change above the species level▫cumulative effects of speciation over long

periods of time

•Species = population or group of populations whose members have the potential to interbreed in nature and produce viable, fertile offspring▫Reproductively compatible

•Reproductive isolation = barriers that prevent members of 2 species from producing viable, fertile hybrids

Prezygotic Barriers:▫Impede

mating/fertilizationTypes:

▫Habitat isolation▫Temporal isolation▫Behavioral isolation▫Mechanical

isolation▫Gametic isolation

Postzygotic Barriers:▫Prevent hybrid

zygote from developing into viable adult

Types:▫Reduced hybrid

viability▫Reduced hybrid

fertility▫Hybrid breakdown

REDUCED HYBRIDVIABILITY

REDUCED HYBRIDFERTILITY

HYBRID BREAKDOWN

Types of Reproductive Barriers

REDUCED HYBRIDVIABILITY

REDUCED HYBRIDFERTILITY

HYBRID BREAKDOWN

Types of Reproductive Barriers

Other definitions of species:

•Morphological – by body shape, size, and other structural features

•Ecological – niche/role in community

•Phylogenetic – share common ancestry, branch on tree of life

Two main modes of speciation

Two main modes of speciation:Allopatric Speciation

“other” “homeland”

Geographically isolated populations

• Caused by geologic events or processes

• Evolves by natural selection & genetic drift

Eg. Squirrels on N/S rims of Grand Canyon

Sympatric Speciation

“together” “homeland”

Overlapping populations within home range

Gene flow between subpopulations blocked by:

• polyploidy• sexual selection• habitat differentiation

Eg. polyploidy in crops (oats, cotton, potatoes, wheat)

Allopatric speciation of antelope squirrels on opposite rims of the Grand

Canyon

2n = 6 4n = 12 4n2n

Autopolyploid Speciation

Sympatric Speciation by Polyploidy•Autopolyploid: extra sets of chromosomes

▫Failure of cell division (2n 4n)

▫Eg. Strawberries are 4n, 6n, 8n, 10n (decaploid)!

•Allopolyploid: 2 species produce a hybrid▫Species A (2n=6) + Species B (2n=4)

Hybrid (2n=10)

Allopolyploidy

Allopolyploidy

•Many new species arise from a single common ancestor

•Occurs when: A few organisms make way to new,

distant areas (allopatric speciation) Environmental change extinctions

new niches for survivors•Eg. Hawaiian archiepelago

Founding

Parents

KAUAI5.1

millionyears OAHU

3.7millionyears

HAWAII0.4

millionyears

1.3millionyears

MAUIMOLOKAI

LANAI Argyroxiphium sandwicense

Dubautia linearisDubautia scabra

Dubautia waialealae

Dubautia laxa

N

Adaptive Radiation: Hawaiian plants descended from ancestral tarweed from North America 5 million years ago

Hybrid Zones• Incomplete reproductive barriers•Possible outcomes: reinforcement, fusion, stability

Hybrid zones

• Where divergent allopatric populations come back and interbreed

• Biologists look for patterns to study reproductive isolation

Fig. 24-13

EUROPE

Fire-belliedtoad range

Hybrid zone

Yellow-belliedtoad rangeYellow-bellied toad,

Bombina variegata

Fire-bellied toad,Bombina bombina

Alle

le f

req

uen

cy (

log

sca

le)

Distance from hybrid zone center (km)

40 30 20 2010 100

0.01

0.1

0.5

0.9

0.99

Reinforcement: Strengthening Reproductive Barriers

• The reinforcement of barriers occurs when hybrids are less fit than the parent species that is reproduce less successfully.

• 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

Nu

mb

er o

f fe

mal

es

(none)

Females matingwith males from:

Ownspecies

Otherspecies

Sympatric males

Ownspecies

Otherspecies

Allopatric males

0

4

8

12

16

20

24

28

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

Stability: Continued Formation of Hybrid Individuals

• Hybrids continue to be produced between the two species in the area of their overlap, but the gene pools of both parent species remain distinct.

Gradualism• Common ancestor• Slow, constant

change

Punctuated Equilibium• Eldridge & Gould• Long period of stasis

punctuated by short bursts of significant change

Tempo of Evolution