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CHAPTER 24. THE ORIGIN OF SPECIES. 1. OVERVIEW. A. Speciation Is defined as the origin of new species (process by which one species splits into two or more species) Is at the focal point of evolutionary theory which must explain how new species originate and how populations evolve
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CHAPTER 24 THE ORIGIN OF SPECIES 1
OVERVIEW A. Speciation • Is defined as the origin of new species (process by which one species splits into two or more species) • Is at the focal point of evolutionary theory which must explain how new species originate and how populations evolve • Microevolution • Consists of adaptations that evolve within a population, confined to one gene pool • Mechanisms—mutation, natural selection, genetic drift, and gene flow 2
Macroevolution • Refers to evolutionary change above the species level • Deals with the appearance of evolutionary novelties that can be used to define high taxa 3
II. Concept 24.1: Biological Species Concept • Emphasizes reproductive isolation • Species is a Latin word meaning “kind” or “appearance” • Today in addition to appearance, we use differences in physiology, biochemistry, behavior, and DNA sequences to differentiate species A. Biological Species Concept 1. Proposed by Ernst Mayr in 1942 2. A species is defined as a group of populations whose members have the potential to interbreed in nature and produce viable, fertile offspring but who cannot produce viable, fertile offspring with other such populations 4
Reproductive Isolation 1. Defined as the existence of biological factors (barriers) that impede two species from producing viable, fertile offspring 2. Hybrids are the offspring of crosses between different species 3. Development of reproductive isolation can possibly lead to the formation of new species 4. Reproductive isolation can be classified by whether factors act before (prezygotic) or after fertilization (postzygotic) 7
Prezygotic Barriers • Five ways prezygotic barriers can block fertilization between species 1. Habitat Isolation • Two species encounter each other rarely, or not at all, because they occupy different habitats, even though not isolated by physical barriers 9
2. Behavioral Isolation • Species use unique and elaborate courtship behavior to attract mates Courtship ritual of blue-footed boobies 11
3. Temporal Isolation • Two species that breed during different times of day, different seasons, or different years cannot mix gametes Eastern spotted skunk (Spilogaleputorius) Western spotted skunk (Spilogalegracilis) 12
4. Mechanical Isolation • Closely related species may attempt to mate but fail because they are anatomically incompatible and transfer of sperm is not possible Bradybaena with shells spiraling in opposite directions 13
5. Gametic Isolation • Gametes of one species may not be able to fertilize eggs of another species 14
Postzygotic Barriers • Three postzygotic barriers that prevent a hybrid zygote from developing into a viable, fertile adult • Reduced Hybrid Viability • Genetic incompatibility between the two species may stop the development of the hybrid at some embryonic stage or produce frail offspring 16 Ensatina hybrid
2. Reduced Hybrid Fertility • Even though hybrid offspring survives, it may be sterile 17
3. Hybrid Breakdown • Some first-generation hybrids are viable and fertile, but when they mate with another species or with either parent species, the offspring of the next generation are sterile or feeble 18
E. Limitations of the Biological Species Concept • The biological species concept cannot be applied to fossils or asexual organisms (including all prokaryotes) F. Other Definitions of Species • Other species concepts emphasize the unity within a species rather than the separateness of different species 1. The morphological species concept defines a species by structural features • It applies to sexual and asexual species but relies on subjective criteria • Oldest and still most practical 19
2. The ecological species concept views a species in terms of its ecological niche, the set of environmental resources that a species uses, and its role in a biological community • Applies to sexual and asexual species 3. The phylogenetic species concept defines a species as the smallest group of individuals that shares a common ancestor and forms one branch on the tree of life • It applies to sexual and asexual species, but it can be difficult to determine the degree of difference required for separate species 20
II. Concept 24.2: Speciation A. Two modes of speciation are distinguished by the way gene flow among populations is initially interrupted: 1. Allopatric Speciation • Occurs when geographic separation of populations restricts gene flow • Geological process can fragment a population into two or more isolated populations. • Definition of barrier depends on the ability of a population to disperse • Separate populations may evolve independently through mutation, natural selection, and genetic drift 21
Evidence of allopatric speciation can be seen in regions with many geographic barriers typically having more species than do regions with few barriers • Reproductive isolation between populations generally increases as the distance between them increases 22
2. Sympatric Speciation • New species arise within the range of the parent population • Occurs when reproductive barriers evolve between sympatric populations • May occur rapidly 3. Types of Sympatric Speciation • Polyploidy • Defined as 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 (more common) • Polyploidy is more common in plants than animals 23
b. Habitat Differentiation • Sympatric speciation in animals may result from the appearance of new ecological niches • Sexual Selection B. Adaptive Radiation • Occurs when many new species arise from a single common ancestor • Typically occurs when a few organisms make their way to new, distant areas or when environmental changes cause extinctions thus opening up niches for survivors • Ex: Darwin’s finches 31
Allopatric and Sympatric Speciation: 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 34
IV. Concept 24.4: Tempo of Speciation A. Gradual—proposes that species descended from a common ancestor and gradually diverge more and more in morphology as they acquire unique adapations -difficult to prove with fossils B. Punctuated—spurts of rapid change rather than gradual divergence -based on fossils that suddenly appear and disappear -proposed by Eldredge and Gould 35
You should now be able to: • Define and discuss the limitations of the four species concepts • Describe and provide examples of prezygotic and postzygotic reproductive barriers • Distinguish between and provide examples of allopatric and sympatric speciation • Explain how polyploidy can cause reproductive isolation • Define the term hybrid zone and describe three outcomes for hybrid zones over time
