Key Concepts in Speciation and the Biological Species Concept

1. Chapter 24 The Origin of Species Key Concepts (1)Biological species concept: reproductive isolation (2) Speciation: geographic separation or in sympatry (3) Macroevolution: occurs through accumulation of many generations of change and speciation events

2. Figure 24.1 The flightless cormorant (Nannopterum harrisi), one of many new species that have originated on the isolated Galápagos Islands Speciation that occurred in isolation Geographic barrier = ?

3. (b) Cladogenesis (a) Anagenesis Figure 24.2 Patterns of Speciation • Two basic patterns of evolutionary change can be distinguished • Anagenesis • Cladogenesis

4. The Biological Species Concept • The biological species concept • Defines a species as a population or group of populations whose members have the potential to interbreed in nature and produce viable, fertile offspring but are unable to produce viable fertile offspring with members of other populations

5. 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 Potential to interbreed = future offspring will have genes that come from different individuals in the same species. (Dog example: Great Dane vs Chihuahua)

6. Figure 24.3 The biological species concept is based on the potential to interbreed rather than on physical similarity Problems with this definition: If two populations are geographically separated, how do we know if the individuals from one population are able to breed with individuals from the other population?

7. Other Limitations of the Biological Species Concept • The biological species concept cannot be applied to • Asexual organisms • Fossils • Organisms about which little is known regarding their reproduction

8. Other Species Definitions (see pg. 476) Morphological: species defined by measurable characteristics Paleontological: fossil species identified by morphology Ecological: species are distinguished by ecological factors (e.g., habitat, food) Phylogenetic: species = organisms with a unique genetic history, as seen in a phylogenetic tree.

9. Reproductive Isolation: characterizes all species. • Reproductive isolation • Is the existence of biological factors that impede members of two species from producing viable, fertile hybrids • Is a combination of various reproductive barriers

10. Prezygotic barriers • Impede mating between species or hinder the fertilization of ova if members of different species attempt to mate • Postzygotic barriers • Often prevent the hybrid zygote from developing into a viable, fertile adult

11. Figure 24.4 Reproductive Barriers Pre-zygotic Isolation (prior to fertilization) Habitat Isolation ………….…... no mating attempt Temporal Isolation …………... no mating attempt Behavioral Isolation …………. no mating attempt Mechanical Isolation ………… mating attempt fails Gametic Isolation ……………. mating attempt fails Post-zygotic Isolation (after fertilization) Reduced hybrid viability …….. offspring likely to die Reduced hybrid fertility ……… offspring less likely to reproduce Hybrid breakdown ……………. first generation is fertile, but subsequent generations not fertile See text pgs. 474-475

12. (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 Allopatric speciation Is more common Change due to natural selection and drift

13. A. harrisi A. leucurus Figure 24.6 Allopatric speciation of antelope squirrels on opposite rims of the Grand Canyon

14. 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. Results: Divergent evolution incipient speciation 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?

15. 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 Failure of cell division after chromosome duplication Tetraploidy Diploid gametes

16. 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? (“sik-lids”)

17. 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 Pattern of Speciation: Adaptive radiation “ silversword alliance” = all descended from tarweed

18. 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. Timing of speciation events

19. Macroevolutionary changes Major transformations resulting from prolonged microevolution. Evolutionary novelties mollusc eye complexity Evolution of genes that control development (Evo-Devo) Heterochrony= Differential timing of development Allometric growth Adult retention of juvenile morphology = paedomorphism Changes in spatial patterns: Hox genes

20. Pigmented cells (photoreceptors) Pigmented cells Epithelium Nerve fibers Nerve fibers (b) Eyecup. The slit shell mollusc Pleurotomaria has an eyecup. (a) Patch of pigmented cells. The limpet Patella has a simple patch of photoreceptors. Cornea Cellular fluid (lens) Fluid-filled cavity Epithelium Optic nerve Pigmented layer (retina) Optic nerve 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. (c) (d) Pinhole camera-type eye. The Nautilus eye functions like a pinhole camera (an early type of camera lacking a lens). Cornea Lens Retina Optic nerve (e) 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. Macroevolutionary changes: eye complexity in molluscs

21. (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)

22. (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 Timing of development differs for certain parts of the body.

23. Paedomorphosis: retention of juvenile characteristics in adult

24. Chicken leg bud Region of Hox gene expression Zebrafish fin bud Figure 24.18 Hox genes and the evolution of tetrapod limbs Hox genes affect position and distance of developing structures.

25. 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. Hypothetical vertebrate ancestor (invertebrate) with a single Hox cluster 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 2 3 4 In an early vertebrate, the duplicate set of genes took on entirely new roles, such as directing the development of a backbone. Hypothetical early vertebrates (jawless) with two Hox clusters 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 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 Hox mutations and the origin of vertebrates

26. 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 Evolution Is Not Goal Oriented Evolution of horses