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Chapter 24: The Origin of Species. Figure 24.1 The flightless cormorant (Nannopterum harrisi), one of many new species that have originated on the isolated Galápagos Islands. (b) Cladogenesis. (a) Anagenesis. Figure 24.2 Two patterns of evolutionary change.

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Chapter 24: The Origin of Species

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Chapter 24 the origin of species

Chapter 24:

The Origin of Species


Chapter 24 the origin of species

Figure 24.1 The flightless cormorant (Nannopterum harrisi), one of many new species that have originated on the isolated Galápagos Islands


Figure 24 2 two patterns of evolutionary change

(b) Cladogenesis

(a) Anagenesis

Figure 24.2 Two patterns of evolutionary change


Chapter 24 the origin of species

Similarity between different species. The eastern meadowlark (Sturnella magna, left) and the western meadowlark (Sturnella neglecta, right) have similar body shapes and colorations. Nevertheless, they are distinct biological species because their songs and other behaviors are different enough to prevent interbreeding should they meet in the wild.

(a)

(b)

Diversity within a species. As diverse as we may be in appearance, all humans belong to a single biological species (Homo sapiens), defined by our capacity to interbreed.

Figure 24.3 The biological species concept is based on the potential to interbreed rather than on physical similarity


Figure 24 4 reproductive barriers

Prezygotic barriers impede mating or hinder fertilization if mating does occur

Behavioral isolation

Habitat isolation

Temporal isolation

Mechanical isolation

Individualsof differentspecies

Matingattempt

HABITAT ISOLATION

MECHANICAL ISOLATION

TEMPORAL ISOLATION

BEHAVIORAL ISOLATION

(g)

(b)

(d)

(e)

(f)

(a)

(c)

Figure 24.4 Reproductive Barriers


Chapter 24 the origin of species

Gameticisolation

Reducehybridfertility

Reducehybridviability

Hybridbreakdown

Viablefertileoffspring

Fertilization

REDUCED HYBRID VIABILITY

GAMETIC ISOLATION

HYBRID BREAKDOWN

REDUCED HYBRID FERTILITY

(k)

(j)

(m)

(l)

(i)

(h)


Figure 24 5 two main modes of speciation

(a)

(b)

Sympatric speciation. A smallpopulation becomes a new specieswithout geographic separation.

Allopatric speciation. A population forms a new

species while geographically isolated from its parent population.

Figure 24.5 Two main modes of speciation


Figure 24 6 allopatric speciation of antelope squirrels on opposite rims of the grand canyon

A. harrisi

A. leucurus

Figure 24.6 Allopatric speciation of antelope squirrels on opposite rims of the Grand Canyon


Figure 24 7 can divergence of allopatric fruit fly populations lead to reproductive isolation

EXPERIMENT

Diane Dodd, of Yale University, divided a fruit-fly population, raising some populations on a starch medium and others on a maltose medium. After many generations, natural selection resulted in divergent evolution: Populations raised on starch digested starch more efficiently, while those raised on maltose digested maltose more efficiently. Dodd then put flies from the same or different populations in mating cages and measured mating frequencies.

Initial population of fruit flies(DrosphilaPseudoobscura)

Some fliesraised on starch medium

Some fliesraised on maltose medium

Mating experimentsafter several generations

Figure 24.7 Can divergence of allopatric fruit fly populations lead to reproductive isolation?


Chapter 24 the origin of species

RESULTS

When flies from “starch populations” were mixed with flies from “maltose populations,”the flies tended to mate with like partners. In the control group, flies taken from different populations that were adapted to the same medium were about as likely to mate with eachother as with flies from their own populations.

Female

Different populations

Same population

FemaleStarch Maltose

Same population

18

22

15

9

MaleMaltose Starch

Male

Different populations

8

12

15

20

Mating frequencies in experimental group

Mating frequencies in control group

CONCLUSION

The strong preference of “starch flies” and “maltose flies” to mate with like-adapted flies, even if they were from different populations, indicates that a reproductive barrier is forming between the divergent populations of flies. The barrier is not absolute (some mating between starch flies and maltose flies did occur) but appears to be under way after several generations of divergence resulting from the separation of these allopatric populations into different environments.


Figure 24 8 sympatric speciation by autopolyploidy in plants

Failure of cell divisionin a cell of a growing diploid plant afterchromosome duplicationgives rise to a tetraploidbranch or other tissue.

Offspring with tetraploid karyotypes may be viable and fertile—a new biological species.

Gametes produced by flowers on this branch will be diploid.

2n

2n = 6

4n

4n = 12

Figure 24.8 Sympatric speciation by autopolyploidy in plants


Figure 24 9 one mechanism for allopolyploid speciation in plants

Unreduced gamete

with 4 chromosomes

Unreduced gamete

with 7 chromosomes

Viable fertile hybrid

(allopolyploid)

Hybrid with

7 chromosomes

Meiotic error;

chromosome

number not

reduced from

2n to n

Species A

2n = 4

2n = 10

Normal gamete

n = 3

Normal gamete

n = 3

Species B

2n = 6

Figure 24.9 One mechanism for allopolyploid speciation in plants


Figure 24 10 does sexual selection in cichlids result in reproductive isolation

EXPERIMENT

Researchers from the University of Leiden placed males and females of Pundamilia pundamilia and P. nyererei together in two aquarium tanks, one with natural light and one with a monochromatic orange lamp. Under normal light, the two species are noticeably different in coloration; under monochromatic orangelight, the two species appear identical in color. The researchers then observed the mating choices of the fish in each tank.

Monochromatic

orange light

Normal light

P. pundamilia

P. nyererei

Under normal light, females of each species mated only with males of their own species. But under orange light, females of each species mated indiscriminately with males of both species. The resulting hybrids were viable and fertile.

RESULTS

The researchers concluded that mate choice by females based on coloration is the main reproductive barrier that normally keeps the gene pools of these two species separate. Since the species can still interbreed when this prezygotic behavioral barrier is breached in the laboratory, the genetic divergence between the species is likely to be small. This suggests that speciation in nature has occurred relatively recently.

CONCLUSION

Figure 24.10 Does sexual selection in cichlids result in reproductive isolation?


Figure 24 13 two models for the tempo of speciation

Time

(b)

(a)

Gradualism model. Species

descended from a common

ancestor gradually diverge

more and more in their

morphology as they acquire

unique adaptations.

Punctuated equilibrium

model. A new species

changes most as it buds

from a parent species and

then changes little for the

rest of its existence.

Figure 24.13 Two models for the tempo of speciation


Figure 24 15 allometric growth

(a) Differential growth rates in a human. The arms and legs

lengthen more during growth than the head and trunk, as can be seen in this conceptualization of an individual at different ages all rescaled to the same height.

Newborn 2 5 15 Adult

Chimpanzee fetus

Chimpanzee adult

(b) Comparison of chimpanzee and human skull growth. The fetal skulls of humans and chimpanzees are similar in shape. Allometric growth transforms the rounded skull and vertical face of a newborn chimpanzee into the elongated skull and sloping face characteristic of adult apes. The same allometric pattern of growth occurs in humans, but with a less accelerated elongation of the jaw relative to the rest of the skull.

Human adult

Human fetus

Figure 24.15 Allometric growth

Age (years)


Figure 24 16 heterochrony and the evolution of salamander feet in closely related species

(a)

Ground-dwelling salamander. A longer time

peroid for foot growth results in longer digits and

less webbing.

(b)

Tree-dwelling salamander. Foot growth ends

sooner. This evolutionary timing change accounts

for the shorter digits and more extensive webbing,

which help the salamander climb vertically on tree

branches.

Figure 24.16 Heterochrony and the evolution of salamander feet in closely related species


Figure 24 17 paedomorphosis

Figure 24.17 Paedomorphosis


Figure 24 20 the branched evolution of horses

Recent

(11,500 ya)

Equus

Hippidion and other genera

Pleistocene

(1.8 mya)

Nannippus

Pliohippus

Neohipparion

Pliocene

(5.3 mya)

Hipparion

Megahippus

Sinohippus

Callippus

Archaeohippus

Miocene

(23 mya)

Merychippus

Hypohippus

Anchitherium

Parahippus

Miohippus

Oligocene

(33.9 mya)

Mesohippus

Paleotherium

Epihippus

Propalaeotherium

Eocene

(55.8 mya)

Pachynolophus

Orohippus

Key

Grazers

Hyracotherium

Browsers

Figure 24.20 The branched evolution of horses


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