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Chapter 24 The Origin of Species Evolutionary theory must explain how new species originate and how populations evolve • Speciation the origin of new species within a population Microevolution consists of adaptations that evolve ______________________, confined to one gene pool species level Macroevolution refers to evolutionary change above the _________________ •Biologists compare morphology, physiology, biochemistry, and DNA sequences when grouping organisms The biological species concept is based on the potential to interbreed rather than on physical similarity (a) Similarity between different species (b) Diversity within a species Reproductive Isolation • Reproductive isolation is the existence of biological factors (barriers) that viable impede two species from producing ___________, fertile offspring • different Hybrids are the offspring of crosses between _______________ species • Reproductive isolation can be classified by whether factors act before or after fertilization • block fertilization Prezygotic barriers _______________________ from occurring by: – Impeding different species from attempting to mate – Preventing the successful completion of mating – Hindering fertilization if mating is successful Zonkey Liger Wholphin Grolar Bear Zorse Prezygotic barriers • Habitat isolation: Two species encounter each other rarely, or not at all, because they occupy different habitats, even though not isolated by physical barriers • Temporal isolation: Species that breed at different times of the day, different seasons, or different years cannot mix their gametes • Behavioral isolation: Courtship rituals and other behaviors unique to a species are effective barriers • Mechanical isolation: Morphological differences can prevent successful mating • Gametic isolation: Sperm of one species may not be able to fertilize eggs of another species Postzygotic barriers • developing Postzygotic barriers prevent the hybrid zygote from _______________ into a viable, fertile adult: – Reduced hybrid viability – Reduced hybrid fertility – Hybrid breakdown • Reduced hybrid viability: Genes of the different parent species may interact and impair the hybrid’s development • Reduced hybrid fertility: Even if hybrids are vigorous, they may be sterile • Hybrid breakdown: Some first-generation hybrids are fertile, but when they mate with another species or with either parent species, offspring of the next generation are feeble or sterile Prezygotic barriers Habitat Isolation Temporal Isolation Individuals of different species (a) Mating attempt (c) (d) (b) Mechanical Isolation Behavioral Isolation (e) (f) Prezygotic barriers Gametic Isolation Postzygotic barriers Reduced Hybrid Viability Reduced Hybrid Fertility Hybrid Breakdown Viable, fertile offspring Fertilization (g) (h) (i) (j) (k) (l) Speciation can take place with or without geographic separation • Speciation can occur in two ways: Allopatric speciation – _______________________ Sympatric speciation – _______________________ (a) Allopatric speciation (b) Sympatric speciation Allopatric (“Other Country”) Speciation In allopatric speciation, gene flow is interrupted or reduced when a population is divided into geographically isolated subpopulations The definition of barrier depends on the ability of a population to disperse mutation Separate populations may evolve independently through _____________, natural selection genetic drift ___________________, and ___________________ Regions with many geographic barriers typically have more species than do regions with fewer barriers Adaptive radiation - the emergence of numerous - spreads to many new species from a common ancestor environments Allopatric speciation of antelope squirrels on opposite rims of the Grand Canyon A. harrisi A. leucurus Allopatric speciation in frogs India Madagascar 88 mya 65 mya 56 mya Sympatric (“Same Country”) Speciation • In sympatric speciation, speciation takes place in geographically overlapping populations Polyploidy • plants Polyploidy is much more common in __________ than in animals. Many important crops (oats, cotton, potatoes, tobacco, and wheat) are polyploids • sets of chromosomes due to Polyploidy is the presence of extra _______ accidents during cell division • An autopolyploid is an individual with more than two chromosome sets, one species derived from ________________ 2n = 6 4n = 12 Failure of cell division after chromosome duplication gives rise to tetraploid tissue. 2n 4n Gametes Offspring with produced tetraploid are diploid.. karyotypes may be viable and fertile. An allopolyploid is a species with multiple sets of chromosomes derived from different species ___________________ Species B 2n = 4 Unreduced gamete with 4 chromosomes Meiotic error Species A 2n = 6 Normal gamete n=3 Hybrid with 7 chromosomes Unreduced gamete with 7 chromosomes Normal gamete n=3 Viable fertile hybrid (allopolyploid) 2n = 10 Habitat Differentiation • Sympatric speciation can also result from the appearance of new ecological niches ____________ • For example, the North American maggot fly can live on native hawthorn trees as well as more recently introduced apple trees Sexual Selection • sympatric Sexual selection can drive _________________ speciation • Sexual selection for mates of different colors has likely contributed to the speciation in cichlid fish in Lake Victoria The Time Course of Speciation • Broad patterns in speciation can be studied using the fossil record, morphological data, or molecular data Two models for the tempo of speciation - branches diverge gradually - most morphological changes - continuous evolution - long periods of stasis followed by episodes of punctuated speciation ex. polyploid plants