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Big Ideas 1.C.1) Speciation and extinction have occurred throughout Earth’s history. 1.C.2) Speciation may occur when two populations become reproductively isolated from each other. 1.C.3) Populations of organisms continue to evolve. Illustrative Examples: o Five major extinctions (25.4) o Human impact on ecosystems and species extinction rates o Directional phenotypic changes (e.g. Finch beaks) © 2011 Pearson Education, Inc. lecture outline o The biological species concept emphasizes reproductive isolation. o Speciation can take place with or without geographic separation. o Hybrid zones reveal factors that cause reproductive isolation. o Speciation can occur rapidly or slowly and can result from changes in few or many genes. [1] Biological Species Concept A biological species is a group of populations whose individuals have the potential to interbreed and produce viable (living), fertile offspring with each other but not with members of other species. No Reproductive Isolation Canis familiaris Subpopulation #1 Canis familiaris Subpopulation #2 Reproductive Isolation Canis familiaris Subpopulation #1 Canis familiaris Subpopulation #2 So what’s getting in the way? Potentially, a lot of things… Reproductive Isolating Mechanisms Reproductive isolation occurs when populations are divided in some way and can no longer interbreed. Any heritable feature of body, form, functioning, or behavior that prevents breeding between one or more genetically divergent populations can cause reproductive isolation. There are Prezygotic or Postzygotic mechanisms. Please refer to handout Figure 24.3 Exploring Reproductive Barriers. Reproductive Isolating Mechanisms Reproductive isolation occurs when populations are divided in some way and can no longer interbreed. Any heritable feature of body, form, functioning, or behavior that prevents breeding between one or more genetically divergent populations can cause reproductive isolation. There are Prezygotic or Postzygotic mechanisms. Please refer to handout Figure 24.3 Exploring Reproductive Barriers. Reproductive Isolating Mechanisms Reproductive isolation occurs when populations are divided in some way and can no longer interbreed. Any heritable feature of body, form, functioning, or behavior that prevents breeding between one or more genetically divergent populations can cause reproductive isolation. There are Prezygotic or Postzygotic mechanisms. Please refer to handout Figure 24.3 Exploring Reproductive Barriers. A Wrench in the works… Pre-zygotic Barriers Habitat Isolation Temporal Isolation Behavioral Isolation Mechanical Isolation Gametic Isolation Postzygotic Barriers Reduced Hybrid Viability Reduced Hybrid Fertility Hybrid Breakdown A Wrench in the works… Pre-zygotic Barriers Habitat Isolation Temporal Isolation Behavioral Isolation Mechanical Isolation Gametic Isolation In any of these cases two possible mating partners never get to first base. Even if they try to do so, fertilization never happens. Postzygotic Barriers Reduced Hybrid Viability Reduced Hybrid Fertility Hybrid Breakdown Invariably, offspring are feeble, sterile and even if F1 is fertile, F2 isn’t. Not a pretty picture! Pre-zygotic Barriers Great Dane ? Chihuahua Post-zygotic Barriers Ensatina salamanders remain isolated based on their habitats. Some hybridization occurs along adjacent borders. Infertile mule (offspring of horse, donkey) Reproductive Isolation Ursa arctos Ursa maritimus Hybridization: closely related species attempting to produce viable, fertile offspring. Ursa arctos Ursa maritimus [1] Biological Species Concept Although helpful in thinking about how speciation occurs, the biological species concept has limitations: It cannot be applied to organisms known only as fossils or to organisms that reproduce only asexually. Thus scientists use other species concepts like morphology (body structure) in some cases. lecture outline o The biological species concept emphasizes reproductive isolation. o Speciation can take place with or without geographic separation. o Hybrid zones reveal factors that cause reproductive isolation. o Speciation can occur rapidly or slowly and can result from changes in few or many genes. Mr. Anderson says… Allopatric “Other country” Parapatric Sympatric “Same country” Speciation Processes Main Modes of Speciation http://www.all-about-reptiles.com/convergent-evolution.html “The study of evolution has led scientists to discover some very interesting evolutionary trends in nature. Similar species that used to look more alike than they do now underwent divergent evolution; species that used to look different but now share really similar characteristics underwent convergent evolution; totally different species that evolved together underwent co-evolution. Brown Bears and Polar Bears illustrate divergent evolution. Scientists think that a long time ago a group of Brown Bears (bottom) became geographically separated from the rest. This isolated group acquired new characteristics, such as the ability to eat meat, blubber to keep warm in snowy weather, and a white coat for camouflage. They slowly evolved into Polar Bears. http://www.duq.edu/sepa/regmed/evolution/aspects.shtml [2] Speciation & Geographic Isolation In allopatric speciation, gene flow is reduced when two populations of one species becomes geographically separated from each other. One or both populations may undergo evolutionary change during the period of separation, resulting in the establishment of prezygotic or postzygotic barriers to reproduction. Main Modes of Speciation [2] Speciation & Geographic Isolation In parapatric speciation (a special case of allopatric speciation), populations mate only with those nearest to them; kind of opportunistic and lazy! Another example… Main Modes of Speciation [2] Speciation & Geographic Isolation In sympatric speciation, a new species originates while remaining in the same geographic area as the parent species. Plant species (and, more rarely, animal species) have evolved sympatrically through polyploidy. Sympatric speciation can also result from habitat shifts and sexual selection. lecture outline o The biological species concept emphasizes reproductive isolation. o Speciation can take place with or without geographic separation. o Hybrid zones reveal factors that cause reproductive isolation. o Speciation can occur rapidly or slowly and can result from changes in few or many genes. [3] Hybrid zones reproductive isolation Many groups of organisms form hybrid zones in which members of different species meet and mate, producing at least some offspring of mixed ancestry. [3] Hybrid zones reproductive isolation Possible Outcomes: Reinforcement: barriers are strengthened and hybrids cease to be formed Fusion: two species continue “interspecies breeding” and become a single unified species (some call this reverse speciation). Stability: hybrids continue to be produced but “parent species” remain separate lecture outline o The biological species concept emphasizes reproductive isolation. o Speciation can take place with or without geographic separation. o Hybrid zones reveal factors that cause reproductive isolation. o Speciation can occur rapidly or slowly and can result from changes in few or many genes. [4] Pace of Speciation New species can form rapidly once divergence begins- but it can take millions of years for that to happen. Certain catastrophic events are thought to accelerate the rate of change among populations. The time interval between speciation events varies considerably, from a few thousand years to tens of millions of years. http://ai.stanford.edu/~serafim/CS374_2006/notes/lecture24.pdf http://www.google.com/imgres?imgurl=http://www.accessexcellence.org/BF/bf02/klein/slides/PhyloftheHom.gif&imgrefurl=http://www.ecotao.com/ho Selected Practice Exam Questions o3 o 10-11 o 13 o 23 o 30