The Origin of Species

Page
4

0

88 mya

65 mya

56 mya

India

Madagascar

Slide 24

Sympatric (“Same Country”) Speciation

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

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.

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

Polyploidy is common in plants. Many important crops (oats, cotton, potatoes, tobacco, and wheat) are polyploids.

Slide 25

Sympatric Speciation via Polyploidy is Common in Plants

2n = 6

4n = 12

Failure of cell

division after

chromosome

duplication gives

rise to tetraploid

tissue.

2n

Gametes

produced

are diploid

4n

Offspring with

tetraploid

karyotypes may

be viable and

fertile.

Slide 26

Sympatric Speciation - Polyploidy --> Allopolyploid

Species A

2n = 6

Normal

gamete

n = 3

Meiotic

error

Species B

2n = 4

Unreduced

gamete

with 4

chromosomes

Hybrid

with 7

chromosomes

Unreduced

gamete

with 7

chromosomes

Normal

gamete

n = 3

Viable fertile

hybrid

(allopolyploid)

2n = 10

Slide 27

Sympatric Speciation: Habitat Differentiation and Sexual Selection

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 can drive sympatric speciation.

Sexual selection for mates of different colors has likely contributed to the speciation in cichlid fish in Lake Victoria.

Slide 28

Allopatric and Sympatric Speciation May Cause Reproductive Isolation: A Review

In allopatric speciation, geographic separation 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.

Slide 29