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Macroevolution and Speciation “Ch-ch-ch-changes…” -David Bowie What is a species? • Species means “kind” or “type” Older way of looking at this question: • Morphological Species Concept – Classified organisms into separate species based on ________________ structural / physical differences. – Used by ____________ Carl Linnaeus (Swedish doctor & naturalist, c.1735) but not adequate today. What is a species? With the help of population genetics, in 1940 Ernst Mayr came up with the: • Biological Species Concept – A species consists of groups of populations interbreeding whose members are capable of ____________ fertile offspring AND in nature to produce ______ who do not interbreed with members of different species. (this last part is called ____________________) reproductive isolation Defining a species Biological Species Concept issues: – Still this definition is not perfect: what about asexually reproducing organisms like bacteria or fungi ?! ________________ • These are classified by structural and biochemical differences Budding Yeast Asexual reproduction in E.coli bacteria Defining a species Biological Species Concept issues: – Additionally, some different species that do not interbreed in nature can be made to do so in an artificial environment (ranch, circus, zoo, aquarium, or laboratory). Ex) Zebroids, interspecies hybrids: a cross between a horse and a zebra Zebroid, Mt. Kenya Game Ranch and Animal Orphanage, Kenya How do you create a new species? Usually through geographic isolation (separation) of some members of a population. The mechanisms of genetic drift, mutation, & natural selection all act upon the newly founded population and the original population. Over time, these now separate populations evolve independently. If the two populations are brought back together and are capable of successfully reproducing in nature, we say they are still the same species. If they are NOT capable of this, then we declare them different species. Species boundaries are maintained through geographic and/or reproductive isolation • What separates two species whose space / range overlaps? Reproductive barriers usually keep species boundaries intact. • Reproductive barriers can be put into two categories: – Prezygotic barriers: prevent fertilization – Postzygotic barriers: prevent hybrid from developing into viable, fertile adult Prezygotic Barriers to Species Interbreeding • Ecological / Habitat isolation – Two species occupy different microenvironments (in same area) and thus do not interbreed Ex) Garter snakes: water v. land • Temporal isolation – Reproduce at different times (of day, year, season, etc.) Ex1) flowers: open/pollinated at different time of day Ex2) frogs: mate at different time of season (temp dependent) • Behavioral isolation – Different courtship rituals / mating behavior Ex1) blue-footed boobies “dance” Ex2) frog “calls” --> different “songs” among different species More Prezygotic Barriers to Species Interbreeding • Mechanical isolation – Differences in anatomy / physical structure do not permit interbreeding Ex) flowers: adapted for specific (insect) pollinators • Larger flowers only pollinated by larger bees, smaller bees cannot reach • Gametic isolation – Egg & sperm of two species biochemically incompatible (different recognition proteins on surface usually) – This especially important to externally fertilizing aquatic organisms or organisms with wind-borne gametes Ex1) flowers: will not accept/recognize “foreign” pollen Ex2) fish that fertilize externally, in open water Postzygotic Barriers to Species Interbreeding • Hybrid inviability – Aborted development, usually at early embryonic stage Ex1) bullfrog eggs & leopard frog sperm Ex2) different species of irises • Hybrid sterility – Hybrid offspring survive (& may be strong) but are sterile (often due to abnormal gametes of hybrid) Ex) female horse (2N = 64) and male donkey (2N = 62) results in sterile mule offspring (2N = 63). • Different diploid number prevents proper matching up of homologous chromosomes in meiosis • Hybrid breakdown – F1 hybrid can successfully reproduce with other F1 hybrids or a Parental individual but F2 hybrid unable to reproduce. Ex1) sunflower species Ex2) cotton species Speciation patterns and mechanisms Speciation and Adaptive Radiation • Adaptive radiation refers to an evolutionary pattern in which one species gives rise to many (also referred to as divergent evolution) • This has happened several times in the history of the evolution of species… Evidence? Homologous structures (similar structure, different function) Adaptive radiation most often follows mass extinctions or when a species moves into a new, unoccupied area. In both these cases, there is usually an abundance of available ecological niches, that can be filled by the new “daughter” species. Speciation in honeycreepers The islands of Hawaii were colonized by a single species of finch-like birds. Eventually populations were isolated from each other on separate islands. The diagrams show how they evolved into the various modern honeycreeper species with beak shapes which adapt them for different feeding methods. Allopatric Speciation The term Allopatric Speciation is used to describe a speciation event in which the new species results due to geographic isolation and subsequent adaptation of the emigrants to a new environment. When the two species are brought together again, they do not interbreed. Sympatric Speciation The term Sympatric Speciation is used to describe a speciation event in which reproductive isolation arises within the boundaries/range of the parent species (without geographic isolation). This is most common among plants, and usually results due to an increase in a plant’s chromosome number. The resulting plant is thus considered to be polyploid. Image Sources • • • • Zebroid: www.ceotraveler.com/adventure/kenya.shtml Budding yeast cells: http://www.sirinet.net/~jgjohnso/fungi.html Bacteria: http://library.thinkquest.org/CR0212089/meso.htm Snail speciation & Honeycreeper Beaks: http://www.bbc.co.uk/scotland/education/bitesize/higher/biology/genetics_adaptation/natural_sel ection2_rev.shtml • • • • • • • • Frog Mating Behavior: http://w3.dwm.ks.edu.tw/bio/activelearner/19/ch19c1.html Horse/Mule/Donkey: http://w3.dwm.ks.edu.tw/bio/activelearner/19/ch19c3.html Adaptive Radiation: http://www.cod.edu/people/faculty/fancher/AdaptiveRadiation.htm Boobie Mating Dance: http://www.nwf.org/internationalwildlife/1998/boobies.html Linnaeus portrait & Cinchona watercolor: http://www.linnean.org/ Ernst Mayr: http://www.pbs.org/wgbh/evolution/library/06/2/l_062_01.html Honeycreepers #2: http://www.micro.utexas.edu/courses/levin/bio304/evolution/speciation.html Picture:
