Chapter 24: The Origin of Species

1. Chapter 24: The Origin of Species • What is a species? • A population whose members can interbreed in nature and • produce viable, fertile offspring • aka….reproductive isolation • What kinds of barriers keep different species isolated so they cannot mate? • Figure 24.4 • Pre–zygotic barriers – before mating &/or zygote is formed • Post–zygotic barriers – after zygote is formed

2. 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

3. Gameticisolation Reducehybridfertility Reducehybridviability Hybridbreakdown Viablefertileoffspring Fertilization REDUCED HYBRID VIABILITY GAMETIC ISOLATION HYBRID BREAKDOWN REDUCED HYBRID FERTILITY (k) (j) (m) (l) (i) (h)

4. Chapter 24: The Origin of Species • What is a species? • What kinds of barriers keep different species isolated so they cannot mate? • How are new species created? • Allopatric speciation • when a geographic barrier isolates a population blocks gene flow • ex. mountain range emerging, new river dividing a field, island • Sympatric speciation • intrinsic factors such as chromosomal changes (plants) or • non-random mating alter gene flow

5. (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

6. Chapter 24: The Origin of Species • What is a species? • What kinds of barriers keep different species isolated so they cannot mate? • How are new species created? • Allopatric speciation – • when a geographic barrier isolates a population blocks gene flow • ex. mountain range emerging, new river dividing a field, island • Adaptive radiation • evolution of many diversely adapted species from a • common ancestor • Seen on islands • Sympatric speciation • intrinsic factors such as chromosomal changes (plants) or • non-random mating alter gene flow

7. N 1.3 million years Dubautia laxa MOLOKA'I KAUA'I MAUI 5.1 million years Argyroxiphium sandwicense O'AHU LANAI 3.7 million years HAWAI'I 0.4 million years Dubautia waialealae Dubautia scabra Dubautia linearis Figure 24.12 Adaptive radiation

8. Chapter 24: The Origin of Species • What is a species? • What kinds of barriers keep different species isolated so they cannot mate? • How are new species created? • Allopatric speciation – • when a geographic barrier isolates a population blocks gene flow • ex. mountain range emerging, new river dividing a field, island • Adaptive radiation • evolution of many diversely adapted species from a • common ancestor • Seen on islands • Sympatric speciation • intrinsic factors such as chromosomal changes (plants) or • non-random mating alter gene flow • ex. oats, cotton, tobacco, potatoes, wheat • Autopolyploidy • An individual has more than 2 chromosome sets derived from • a single species from an error in meiosis

9. 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

10. Chapter 24: The Origin of Species • What is a species? • What kinds of barriers keep different species isolated so they cannot mate? • How are new species created? • Allopatric speciation – • when a geographic barrier isolates a population blocks gene flow • ex. mountain range emerging, new river dividing a field, island • Adaptive radiation • evolution of many diversely adapted species from a • common ancestor • Seen on islands • Sympatric speciation • intrinsic factors such as chromosomal changes (plants) or • non-random mating alter gene flow • Autopolyploidy • An individual has more than 2 chromosome sets derived from • a single species from an error in meiosis • Allopolyploidy • 2 different species produce the polyploid hybrid

11. 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

12. Monochromatic orange light 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. Normal light EXPERIMENT 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 Sympatric speciation: non-random mating

13. Chapter 24: The Origin of Species • What is a species? • What kinds of barriers keep different species isolated so they cannot mate? • How are new species created? • What is the difference between gradualism & punctuated equlibrium?

14. 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

15. Pigmented cells (photoreceptors) Pigmented cells Epithelium Nerve fibers Nerve fibers (a) Patch of pigmented cells. The limpet Patella has a simple patch of photoreceptors. (b) Eyecup. The slit shell mollusc Pleurotomaria has an eyecup. Cornea Cellular fluid (lens) Fluid-filled cavity Epithelium Optic nerve Pigmented layer (retina) Optic nerve (d) (c) Pinhole camera-type eye. The Nautilus eye functions like a pinhole camera (an early type of camera lacking a lens). Eye with primitive lens. The marine snail Murex has a primitive lens consisting of a mass of crystal-like cells. The cornea is a transparent region of epithelium (outer skin) that protects the eye and helps focus light. Cornea Lens Retina Optic nerve Complex camera-type eye. The squid Loligo has a complex eye whose features (cornea, lens, and retina), though similar to those of vertebrate eyes, evolved independently. (e) Figure 24.14 A–E

16. (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. Chimpanzee fetus Chimpanzee adult Human adult Figure 24.15 B Human fetus Chapter 24: The Origin of Species 5. What other mechanisms can influence evolution/speciation? DEVELOPMENTAL FACTORS… A) Differences in allometric growth (proportional growth of body structures)

17. Ground-dwelling salamander. A longer time peroid for foot growth results in longer digits and less webbing. (a) 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. (b) Figure 24.16 A, B

18. Chicken leg bud Region of Hox gene expression Zebrafish fin bud Figure 24.18 Chapter 24: The Origin of Species 5. What other mechanisms can influence evolution/speciation? DEVELOPMENTAL FACTORS… A) Differences in allometric growth (proportional growth of body structures) B) The expression of homeotic genes (which determine the “body plan” of an organism) may change through mutation.

19. Most invertebrates have one cluster of homeotic genes (the Hox complex), shown here as coloredbands on a chromosome. Hox genes direct development of major body parts. A mutation (duplication) of the single Hox complex occurred about 520 million years ago and may have provided genetic material associated with the origin of the first vertebrates. First Hox duplication 5 1 3 4 2 In an early vertebrate, the duplicate set of genes took on entirely new roles, such as directing the development of a backbone. Hypothetical vertebrate ancestor (invertebrate) with a single Hox cluster A second duplication of the Hox complex, yielding the four clusters found in most present-day vertebrates, occurred later, about 425 million years ago. This duplication, probably the result of a polyploidy event, allowed the development of even greater structuralcomplexity, such as jaws and limbs. Second Hox duplication Hypothetical early vertebrates (jawless) with two Hox clusters The vertebrate Hox complex contains duplicates of many ofthe same genes as the single invertebrate cluster, in virtuallythe same linear order on chromosomes, and they direct the sequential development of the same body regions. Thus, scientists infer that the four clusters of the vertebrate Hoxcomplex are homologous to the single cluster in invertebrates. Vertebrates (with jaws) with four Hox clusters Figure 24.19