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Teresa Audesirk • Gerald Audesirk • Bruce E. Byers Biology: Life on Earth Eighth Edition Lecture for Chapter 16 The Origin of Species Copyright © 2008 Pearson Prentice Hall, Inc. Chapter 16 Outline • 16.1 What Is a Species? p. 316 • 16.2 How Is Reproductive Isolation Between Species Maintained? p. 317 • 16.3 How Do New Species Form? p. 320 • 16.4 What Causes Extinction? p. 326 Section 16.1 Outline • 16.1 What Is a Species? – Biologists Need a Clear Definition of Species – Species Are Groups of Interbreeding Populations – Appearance Can Be Misleading Clear Definition of Species Needed • In Pre-Darwinian times, the term “species” referred to different kinds of organisms • Species were classified initially based upon appearance Species • Today a species is defined as a group of actually or potentially interbreeding natural populations, which are reproductively isolated from other such groups (biological species concept) Species • Limitations of species definition – Cannot be used to determine species identity among asexually reproducing organisms or among fossils – Often difficult to observe whether members of two different groups interbreed Appearance Can Be Misleading • Organisms of similar appearance sometimes belong to different species • Organisms that differ in appearance may belong to the same species Appearance Can Be Misleading • The cordilleran flycatcher and Pacific slope flycatcher are so similar that birdwatchers can’t tell them apart • These birds were considered to be a single species, but research revealed that they do not interbreed and are in fact two different species Appearance Can Be Misleading • The myrtle warbler and Audubon’s warbler used to be classified as different species – Discovered that the warblers interbreed where their ranges overlap – Today, considered a single species Section 16.2 Outline • 16.2 How Is Reproductive Isolation Between Species Maintained? – Premating Isolating Mechanisms Prevent Mating Between Species – Postmating Isolating Mechanisms Limit Hybrid Offspring Reproductive Isolation • Reproductive isolation occurs when members of one population are unable to interbreed with members of another Isolating Mechanisms • Isolating mechanisms prevent interbreeding and maintain reproductive isolation – Premating isolating mechanisms prevent mating between species – Postmating isolating mechanisms prevent the formation of viable, fertile offspring after mating has occurred Isolating Mechanisms • For most species, two or more isolating mechanisms interact to prevent the formation of fertile offspring Premating Isolating Mechanisms • Premating isolating mechanisms include: – Geographical isolation – Ecological isolation – Temporal isolation – Behavioral isolation – Mechanical incompatibility Premating Isolating Mechanisms • Geographical isolation occurs when populations cannot mate because of physical barriers – In nature, lions do not mate with tigers • Lions live in Africa • Tigers live in India Premating Isolating Mechanisms • Geographically separated populations aren’t necessarily distinct species • The Kaibab and Abert squirrels living in geographically separate areas of the Grand Canyon are physically separated, but still very similar • It is currently unknown if the two populations have diverged into two species Premating Isolating Mechanisms • Ecological isolation occurs when species can’t mate because they occupy different habitats Premating Isolating Mechanisms • White-crowned sparrows inhabit fields and meadows, while white-throated sparrows inhabit dense thickets • Each species of fig wasp breeds in the fruits of a particular species of fig Premating Isolating Mechanisms • Temporal isolation occurs when species can’t mate because they breed at different times Premating Isolating Mechanisms • In nature, Bishop pines and Monterey pines do not interbreed – Bishop pine pollination occurs in summer – Monterey pine pollination occurs in early spring Premating Isolating Mechanisms • Behavioral isolation occurs when species can’t mate because they have different courtship and mating rituals Premating Isolating Mechanisms • Songs and plumage of male songbirds are species specific – Attract females of the same species – Females of other species are unresponsive Premating Isolating Mechanisms • Male frogs embrace any female regardless of species – Female frogs encountering males of a different species utter the “release call” Premating Isolating Mechanisms • Mechanical incompatibility occurs when species cannot mate because their reproductive structures are incompatible Premating Isolating Mechanisms • In animals with internal fertilization, male and female sexual organs may not fit together – e.g., snails of species whose shells have lefthanded spirals may be unable to successfully copulate with snails whose shells have righthanded spirals Premating Isolating Mechanisms • In plants, differences in flower size or structure may attract different pollinators Postmating Isolating Mechanisms • Postmating isolating mechanisms include: – Gametic incompatibility – Hybrid inviability – Hybrid infertility Postmating Isolating Mechanisms • Gametic incompatibility occurs when sperm from one species cannot fertilize eggs of another Postmating Isolating Mechanisms • In animals, fluids of the female reproductive tract my weaken or kill sperm of another species • In plants, pollen from one species may fail to germinate when it lands on the stigma of another species Postmating Isolating Mechanisms • Hybrid inviability occurs when hybrid offspring fail to survive to maturity Postmating Isolating Mechanisms • Hybrid may abort early in development • Hybrid may be unable to reproduce because it display behaviors that are mixtures of the two parental types – Lovebird hybrids have great difficulty learning to carry nest materials during flight Postmating Isolating Mechanisms • Hybrid infertility occurs when hybrid offspring are sterile or have reduced fertility – Mule hybrids (a cross between a horse and a donkey) are sterile – Liger hybrids (a zoo-based cross between a male lion and a female tiger) are sterile Postmating Isolating Mechanisms • Infertility is caused by the failure of chromosomes to pair properly during meiosis, so eggs and sperm never develop Section 16.3 Outline • 16.3 How Do New Species Form? – Geographic Separation of a Population Can Lead to Allopatric Speciation – Ecological Isolation of a Population Can Lead to Sympatric Speciation – Under Some Conditions, Many New Species May Arise Speciation • Speciation is the process by which new species form How Do New Species Form? • Speciation depends on two factors – Isolation – Genetic divergence How Do New Species Form? • When populations are isolated, gene flow between them is blocked • Isolated populations subsequently evolve genetic differences large enough to prevent interbreeding – Differences arise by chance (genetic drift) or through natural selection Mechanisms of Speciation • Allopatric speciation – Isolating mechanism is a physical barrier • Sympatric speciation – Isolation occurs without geographic separation Allopatric Speciation • Allopatric speciation occurs when two populations of a species become separated by a geographical barrier – Colonization of remote islands by mainland organisms – Geological changes such as volcanism, earthquakes, continental drift, and rivers changing course Allopatric Speciation • Allopatric speciation occurs when isolated populations diverge genetically Allopatric Speciation • Is believed to be the most common type of speciation, especially among animals • Two allopatric populations, the Kaibab squirrel and Abert’s squirrel, may be evolving into two separate species Sympatric Speciation • Sympatric speciation occurs when – Two populations of a species living in the same geographical area become restricted to different habitats – Isolated populations diverge genetically Sympatric Speciation • Two sympatric populations of fruit flies (Rhagoletis pomonella) may be evolving into two separate species Sympatric Speciation • One population lays its eggs in hawthorn fruit, while the other prefers apples • The two populations experience very little interbreeding Sympatric Speciation • Males and females prefer the same type of fruit in which they developed • Apples mature two or three weeks later than hawthorn fruit (flies mature and mate at different times) New Species • The above mechanisms of speciation and reproductive isolation lead to forking branches in the evolutionary tree of life, as one species splits into two Adaptive Radiation • Adaptive radiation is the rise of many new species over a relatively short period of time Adaptive Radiation • Occurs when populations of one species invade a variety of new habitats – Finch colonization of the Galápagos Islands – Cichlid fish colonization of Lake Malawi – Tarweed plant colonization of the Hawaiian Islands Section 16.4 Outline • 16.4 What Causes Extinction? – Localized Distribution and Overspecialization Make Species Vulnerable in Changing Environments – Interactions with Other Organisms May Drive a Species to Extinction – Habitat Change and Destruction Are the Leading Causes of Extinction What Causes Extinction? • Extinction is the death of all members of a species • At least 99.9% of all species that ever existed are now extinct What Causes Extinction? • Factors that may make a species vulnerable to extinction include: – Localized distribution – Overspecialization – Competition among species – Habitat destruction Localized Distribution • Wide-ranging species normally do not succumb to local environmental catastrophes Localized Distribution • Species inhabiting extremely limited ranges may become extinct if the area is disturbed – Devil’s Hole pupfish is found in only one spring-fed water hole in the Nevada desert Overspecialization • Species that develop adaptations that favor survival in a specific environment are at risk of becoming extinct – The Karner blue butterfly feeds only on the blue lupine plant – The habitat of the lupine has been significantly reduced by development – Loss of the lupine will lead to extinction of the Karner blue butterfly Competition Among Species • Species that are unable to exploit resources more efficiently and effectively than their competitors may become extinct Competition Among Species • 2.5 million years ago a land bridge (the isthmus of Panama) formed between North and South America • North American species displaced the vast majority of South American species, many of which became extinct Habitat Destruction • Habitat destruction is the single greatest cause of extinction Habitat Destruction • Extinctions due to prehistoric habitat change will be discussed in Chapter 17 • Human activities are the primary cause of present-day habitat destruction – Clearing of tropical rainforests could lead to loss of up to half of all current species over the next 50 years