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THE ORIGIN OF SPECIES Chapter 24 Speciation Speciation The origin of a new species Speciation is a focal point of evolutionary theory Must explain how new species originate and how populations evolve Microevolution: Changes in allele frequency in a population over time Macroevolution: Broad patterns of evolutionary change above the species level Biological Species Concept • Biological Species Concept (used by Campbell’s Biology) defines a species as: • A group of populations whose members have the potential to interbreed in nature and produce viable, fertile offspring • Gene flow between populations keeps the populations as one species • Drawbacks: • Can not use biological species concept to analyze fossil species • Can not apply to species that reproduce asexually (all prokaryotes) Other Definitions of Species • Morphological Species Concept: • Defines species by structural and anatomical differences and similarities • Ecological Species Concept: • Defines species by their ecological niche • Phylogenetic Species Concept: • Defines species as the smallest group of individuals on a phylogenetic tree Getting Back to Biological Species Concept • By defining a species as a group of individuals that can successfully reproduce, let’s focus on what keeps organisms from NOT reproducing • Reproductive isolation: Existence of biological barriers that prevent two species from producing viable, fertile offspring • Hybrids • Offspring of crosses between species • Can classify reproductive isolation as before or after fertilization Prezygotic Barriers: Before Fertilization • Prezygotic barriers • Block fertilization from ever happening by: • Impeding different species from attempting to mate • Preventing the successful completion of mating • Hindering fertilization if mating is successful Prezygotic barriers Habitat Isolation Temporal Isolation Individuals of different species (a) MATING ATTEMPT (c) (d) (b) Gametic Isolation Mechanical Isolation Behavioral Isolation (e) (f) FERTILIZATION (g) Postzygotic Barriers: After Fertilization • Postzygotic barriers • Prevent hybrid zygote from developing into a viable, fertile adult by: • Reduced hybrid viability • Reduced hybrid fertility • Hybrid breakdown Postzygotic barriers Reduced Hybrid Viability Reduced Hybrid Fertility Hybrid Breakdown VIABLE, FERTILE OFFSPRING FERTILIZATION (h) (i) (j) (k) (l) How Does Speciation Occur? • Speciation can occur with or without geographic isolation • Allopatric speciation (with isolation) • Sympatric speciation (without isolation) (a) Allopatric speciation. A population forms a new species while geographically isolated from its parent population. (b) Sympatric speciation. A subset of a population forms a new species without geographic separation. Allopatric Speciation • Allopatric (“other country”) • Gene flow is interrupted when population divided into geographically isolated subpopulations • Geographic barrier prevents members of subpopulation from interacting • Ability of population to disperse defines how much of a barrier is needed • Separate populations may evolve independently through mutation, natural selection and genetic drift Sympatric Speciation • Sympatric (“same country”) • Speciation takes place in geographically overlapping populations • Although less common than allopatric, speciation can occur if gene flow is reduced by factors such as: • Polyploidy • Habitat differentiation • Sexual selection Polyploidy • Polyploidy • Mistakes in cell division result in extra sets of chromosomes • Rare in animals but common in plants • Many important crops such as oat and wheat are polyploids Cell division error Polyploidy 2n = 6 Tetraploid cell 4n = 12 2n 2n Gametes produced by tetraploids New species (4n) Habitat Differentiation • Ecological niche • Sum of an organism’s use of biotic and abiotic resources in an environment • If some organisms can exploit a new niche from the parental population, they may eventually evolve into a new species Hybrid Zones • Hybrid zone • Region in which members of different species mate and produce hybrids (incomplete reproductive barriers) • Hybrids often have reduced fitness compared to either parental species Hybrid Zones Over Time • When closely related species meet in a hybrid zone, 3 possible outcomes: Reinforcement 1. • Hybrids less fit than parent species, fewer and fewer hybrids Fusion 2. • Hybrids as fit as parents, forms single species Stability 3. • Continued production of hybrids Outcomes in Hybrid Zones Possible outcomes: Isolated population diverges Hybrid zone Reinforcement OR Fusion OR Gene flow Population Barrier to gene flow Hybrid individual Stability How Long Does Speciation Take? • Hard to say but patterns in fossil records, morphological data or molecular data can give some clues • Fossil record shows some species appearing suddenly, remaining unchanged and then disappear • Punctuated equilibrium • Contrasts with model of gradual change • Speciation by punctuated equilibrium model appears fast in geological time (still a long time!) Speciation Patterns (a) Punctuated pattern Time (b) Gradual pattern How Long Does Speciation Take? • Lots of variation in speciation time • Interval can range from 4,000 years in some species (like cichlids) to 40 million years (some insects) • Average is 6.5 million years