The Origin of Species. Chapter 16

Transcription

1 The Origin of Species Chapter 16

2 Species Species is defined as a group of actually or potentially interbreeding natural populations, which are reproductively isolated from other such groups (biological species concept) Speciation is the process by which new species arise

3 Mechanisms of Speciation Reproductive isolation occurs when members of one population are unable to interbreed with members of another Isolating mechanisms prevent interbreeding and maintain reproductive isolation Premating (prezygotic) isolating mechanisms prevent mating between species Postmating (postzygotic) isolating mechanisms prevent the formation of viable, fertile offspring after mating has occurred

4 Premating Isolating Mechanisms Premating isolating mechanisms include: Geographical isolation Ecological isolation Temporal isolation Behavioral isolation Mechanical incompatibility

5 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 Geographically separated populations aren t necessarily distinct species

6 Premating Isolating Mechanisms Ecological isolation occurs when species can t mate because they occupy different habitats 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

7 Premating Isolating Mechanisms Temporal isolation occurs when species can t mate because they breed at different times In nature, Bishop pines and Monterey pines do not interbreed Bishop pine pollination occurs in summer Monterey pine pollination occurs in early spring

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9 Premating Isolating Mechanisms Behavioral isolation occurs when species can t mate because they have different courtship and mating rituals Songs and plumage of male songbirds are species specific Attract females of the same species Females of other species are unresponsive

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11 Premating Isolating Mechanisms Mechanical incompatibility occurs when species cannot mate because their reproductive structures are incompatible 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

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13 Postmating Isolating Mechanisms Postmating isolating mechanisms include: Gametic incompatibility Hybrid inviability Hybrid infertility

14 Postmating Isolating Mechanisms Gametic incompatibility occurs when sperm from one species cannot fertilize eggs of another 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

15 Postmating Isolating Mechanisms Hybrid inviability occurs when hybrid offspring fail to survive to maturity 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

16 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 Different chromosome numbers Liger hybrids (a zoo-based cross between a male lion and a female tiger) are sterile Infertility is caused by the failure of chromosomes to pair properly during meiosis, so eggs and sperm never develop

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19 How Do New Species Form? Speciation depends on two factors Isolation Genetic divergence 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

20 Mechanisms of Speciation Allopatric speciation Isolating mechanism is a physical barrier Sympatric speciation Isolation occurs without geographic separation Parapatric speciation Ranges overlap,variations in mating habits Adaptive Radiation Pop. from 1 species spread out & inhabit new habitats

21 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 occurs when isolated populations diverge genetically

22 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

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24 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 Two sympatric populations of fruit flies (Rhagoletis pomonella) may be evolving into two separate species One population lays its eggs in hawthorn fruit, while the other prefers apples The two populations experience very little interbreeding

25 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)

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27 Parapatric Speciation Extremes of a species over a broad range (cline) differ from each other. Ranges overlap a little or adjacent. Variations in mating habits allows reproductive isolation

28 Adaptive Radiation Adaptive radiation is the rise of many new species over a relatively short period of time 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

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31 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 Factors that may make a species vulnerable to extinction include: Localized distribution Overspecialization Competition among species Habitat destruction

32 Localized Distribution Wide-ranging species normally do not succumb to local environmental catastrophes 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

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34 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

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36 Competition Among Species Species that are unable to exploit resources more efficiently and effectively than their competitors may become extinct Exotic species introduction of non-native species to an area Usually can out compete native species (no predators)

37 Habitat Destruction Habitat destruction is the single greatest cause of extinction 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

38 Patterns of Evolution Gradualism proposes that evolutionary change is slow, gradual, & continuous. Punctuated equilibrium proposes that species have long periods of stability interrupted by geologically brief periods of significant change during which many new species evolved

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40 Patterns of evolution Macroevolution large scale evolutionary change Microevolution gene frequency changes in local populations Divergent evolution one population becoming isolated from the rest of the species & following a different course of evolution Black bear (omnivore) Polar bear (carnivore)

41 Patterns of evolution Convergent evolution populations of different species exposed to similar selective pressures & exhibit similar adaptations Whale/shark/seal/penguin body shape Parallel evolution two ancestral species resemble each other & have evolved along similar lines Australian marsupials & placental mammals