Chapter 24 The Origin of Species Part B

Chapter 24The Origin of

SpeciesPart B

How do species occur?Concept 24.2: Speciation can take place with or without geographic

separation

• Speciation can occur in two ways:–Allopatric speciation–Sympatric speciation

Both work through a block of gene flow between two populations.

Example• Pupfish populations in Death

Valley.• Generally happens when a

specie’s range shrinks for some reason.

Fig. 24-5

(a) Allopatric speciation (b) Sympatric speciation

Allopatric Speciation• Allopatric = other homeland• Ancestral population split by

a geographical feature.• Comment – the size of the

geographical feature may be very large or small.

The Process of Allopatric Speciation

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

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

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

Fig. 24-6

A. harrisi A. leucurus

Evidence of Allopatric Speciation

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

Fig. 24-7

Mantellinae(Madagascar only):100 species

Rhacophorinae(India/SoutheastAsia): 310 species

Other Indian/Southeast Asianfrogs

Millions of years ago (mya)1 2 3

1 2 3

100 80 60 40 20 0

88 mya 65 mya 56 mya

India

Madagascar

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

Fig. 24-8

Geographic distance (km)

Degr

ee o

f rep

rodu

ctive

isol

ation

00

50 100 150 250200 300

0.5

1.0

1.5

2.0

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

Fig. 24-9a

EXPERIMENT

Initial population

Some fliesraised on

starch medium Mating experimentsafter 40 generations

Some fliesraised on

maltose medium

Fig. 24-9b

RESULTS

FemaleFemale

StarchStarch Starch

Maltose population 1 population 2

Mal

e Star

chM

alto

se Mal

eSt

arch

Star

chpo

pula

tion

1po

pula

tion

2

22

8 20

9 18

12

15

15

Mating frequenciesin experimental group

Mating frequenciesin control group

Conditions Favoring Allopatric Speciation

1. Founder's Effect - with the peripheral isolate.

2. Genetic Drift – gives the isolate population variation as compared to the original population.

Conditions Favoring Allopatric Speciation

3. Selection pressure on the isolate differs from the parent population. (environment is different on the edges)

Result• Gene pool of isolate changes

from the parent population and new species can form.

Sympatric Speciation• Sympatric = same homeland• New species arise within the range

of parent populations.• Can occur in a single generation.• In sympatric speciation,

speciation takes place in geographically overlapping populations

Plants• Polyploids may cause new

species because the change in chromosome number creates postzygotic barriers.

Polyploidy

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

individual with more than two chromosome sets, derived from one species

Polyploid Types1. Autopolyploid - when a

species doubles its chromosome number from 2N to 4N.

2. Allopolyploid - formed as a polyploid hybrid between two species.–Ex: wheat

Fig. 24-10-1

2n = 6 4n = 12

Failure of celldivision afterchromosomeduplication givesrise to tetraploidtissue.

Autopolyploid

Fig. 24-10-2

2n = 6 4n = 12

Failure of celldivision afterchromosomeduplication givesrise to tetraploidtissue.

2n

Gametesproducedare diploid..

Autopolyploid

Fig. 24-10-3

2n = 6 4n = 12

Failure of celldivision afterchromosomeduplication givesrise to tetraploidtissue.

2n

Gametesproducedare diploid..

4n

Offspring withtetraploidkaryotypes maybe viable andfertile.

Autopolyploid

Autopolyploid

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

Fig. 24-11-1

Species A2n = 6

Normalgameten = 3

Meioticerror

Species B2n = 4

Unreducedgametewith 4chromosomes

allopolyploid

Fig. 24-11-2

Species A2n = 6

Normalgameten = 3

Meioticerror

Species B2n = 4

Unreducedgametewith 4chromosomes

Hybridwith 7chromosomes

allopolyploid

Fig. 24-11-3

Species A2n = 6

Normalgameten = 3

Meioticerror

Species B2n = 4

Unreducedgametewith 4chromosomes

Hybridwith 7chromosomes

Unreducedgametewith 7chromosomes

Normalgameten = 3

allopolyploid

Fig. 24-11-4

Species A2n = 6

Normalgameten = 3

Meioticerror

Species B2n = 4

Unreducedgametewith 4chromosomes

Hybridwith 7chromosomes

Unreducedgametewith 7chromosomes

Normalgameten = 3

Viable fertilehybrid(allopolyploid)2n = 10

allopolyploid

Allopolyploid

• Polyploidy is much more common in plants than in animals• Many important crops (oats,

cotton, potatoes, tobacco, and wheat) are polyploids

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

Sexual Selection• Sexual selection can drive

sympatric speciation• Sexual selection for mates of

different colors has likely contributed to the speciation in cichlid fish in Lake Victoria

Fig. 24-12

EXPERIMENT

Normal lightMonochromatic

orange light

P.pundamilia

P. nyererei

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

• 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

Adaptive Radiation• Rapid emergence of several

species from a common ancestor ( often Allopatric speciation)

• Common in island and mountain top populations or other “empty” environments.

Mechanism• Resources are temporarily

infinite.• Most offspring survive.• Result - little Natural

Selection and the gene pool can become very diverse.

When the Environment Saturates

• Natural Selection resumes.• New species form rapidly if

isolation mechanisms work.• Examples–Galapagos – Finches–Usambaras Mountains – African violets

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