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D2 species and speciation


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Option D2 IB Biology

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D2 species and speciation

  1. 1. Option D: Evolution D2 Species and Speciation
  2. 2. What you need to be able to do and understand: • D.2.1 Define allele frequency and gene pool. • D.2.2 State that evolution involves a change in allele frequency in a population's gene pool over a number of generations. • D.2.3 Discuss the definition of the term species. • D.2.4 Describe three examples of barriers between gene pools. • D.2.5 Explain how polyploidy can contribute to speciation. • D.2.6 Compare allopatric and sympatric speciation. • D.2.7 Outline the process of adaptive radiation. • D.2.8 Compare convergent and divergent evolution. • D.2.9 Discuss ideas on the pace of evolution, including gradualism and punctuated equilibrium. • D.2.10 Describe one example of transient polymorphism. • D.2.11 Describe sickle-cell anemia as an example of balanced polymorphism.
  3. 3. What is a species? Do we have a good idea of what defines a species? What is the bottom line according to the narrator? What is the biological species concept of a species? From this short introduction answer: A group of actually or potentially interbreeding populations, with a common gene pool, which are reproductively isolated from other such groups Biological species concept:
  4. 4. Why the confusion? A group of actually or potentially interbreeding populations, with a common gene pool, which are reproductively isolated from other populations Some species cannot interbreed but they are so close in appearance that they are difficult to tell apart. These are knows as siblings species. Some species only reproduce asexually -binary fission in many single celled organisms -parthenogenesis (reproduction via self-cloning: aphids, some bees, wasps and hornets, some fish and water fleas) -gynogenesis (via the catalyst of a male nearby which does not actually fertilize the eggs: komodo dragons, some sharks, some snails) Some species occasionally mate to produce hybrids -occasionally happens to migrating waterfowl- read this article from Ducks unlimited: American gray treefrogs, Hyla chrysoscelis and Hyla versicolor. They are indistinguishable from one another morphologically but they do not interbreed. One of them is a diploid, and the other is a tetraploid, with twice the number of chromosomes as the other species. Their hybrids are not viable, so they have evolved slightly different mating calls to help them avoid interbreeding with one another. Hyla versicolor Hyla chrysoscelis
  5. 5. What is a species? A group of actually or potentially interbreeding populations, with a common gene pool, which are reproductively isolated from other populations "I have come to realize that there are different concepts of species afloat in biology that have their own utilities. I am an agnostic pluralist about this: there are different concepts of species for different endeavors. There does not seem to be one concept of species that meets everybody‘s needs very well, and so I am inclined to think we are better just leaving the term quite loose and not making a big deal of it." Daniel Dennett (philosopher of science)
  6. 6. What is a species? A group of actually or potentially interbreeding populations, with a common gene pool, which are reproductively isolated from other populations "Where we have sexual reproduction a species can be objectively defined as a group of organisms which reproduce sexually amongst themselves but don't reproduce with members of other species. Where we don't have sexual reproduction - as in asexual species, or in fossils where we have no idea of how they reproduced - then there is no objective definition of the species, and the species just becomes like the genus, the family or the class. It's subject to arbitrary human decision." Richard Dawkins
  7. 7. What is a species? A group of actually or potentially interbreeding populations, with a common gene pool, which are reproductively isolated from other populations "There has been much discussion over the years as to whether species actually exist in nature outside the minds of taxonomists. My own view is that they do exist outside of our minds. They exist by virtue of the evolutionary process, by virtue of the fact that lineages go along through time and split and evolve independently and then split again, and that is how we get the tree of life." Michael Donoghue
  8. 8. What is a species? A group of actually or potentially interbreeding populations, with a common gene pool, which are reproductively isolated from other populations A good example of how similar species can tell each apart can be found in the American cricket: Within a single habitat in the USA, as many as 30 or 40 different species of crickets may be breeding but the female cricket recognizes the song of males of her own species and will breed only with a male who sings that song. The song, and the female recognition of it, constitutes a mate recognition system: the species has a specific mate recognition system by which it can be identified. Actually or potentially interbreeding: they must not be geographically isolated populations Gene pool: is the complete set of unique alleles in a species- these must be common in the population- by changing the gene pool you change the allele frequency. Reproductively isolated: the inability of a species to breed successfully with related species due to geographical, behavioral, physiological, or genetic barriers or differences
  9. 9. Allele Frequency a change in this means evolution Check this out: It ―provide one central source, freely available to all, for the storage of allele frequencies from different polymorphic areas in the HUMAN genome.‖ Allele frequency is the proportion of all copies of a gene that is made up of a particular gene variant (allele). In other words it is a measure of how common an allele is in a population. Allele frequencies range from 0 to 1.0, or as a percentage. Evolution always involves a change in allele frequency in a population's gene pool, over a number of generations
  10. 10. How can barriers happen between gene pools? Temporal isolationGeographical isolation This can be due to either the timing of breeding cycles (seasonal isolation) of closely related species or the time of day or night that species search for mates. A good example is from the pine trees Pinus radiata and P. muricata, which grow together in California. P. radiata sheds its pollen in early February and P. muricata in April. P. radiata P. muricata A change in climate, or a geological change such as an orogenic event (mountain making) or continental drift may, over time, split an originally single population.
  11. 11. Read this article on new speciation genes and how they relate to hybrid infertility ews.2008.1297.html Hybrid infertility Behavioral isolation How can barriers happen between gene pools? Visual and chemical signals are important in mating. May have frequent encounters but no mating because male may not ―display‖ or female may not be ―receptive‖, sex pheromones might be confused When placed in a wind tunnel, male moths will fly to and attempt to mate with an object impregnated with the pheromones of their own species but will not respond to those of other species. Reduced hybrid viability and fertility. Hybrid zygotes fail to develop or fail to reach maturity. Hybrids can also fail to produce functional gametes
  12. 12. How do species form? Speciation is the evolutionary process by which new biological species arise. Imagine that you are looking at a tip of the tree of life that constitutes a species of fruit fly. Move down the phylogeny to where your fruit fly twig is connected to the rest of the tree. That branching point, and every other branching point on the tree, is a speciation event. At that point genetic changes resulted in two separate fruit fly lineages, where previously there had just been one lineage. But why and how did it happen? The place where you have common ancestors It is the formation of a new species by splitting of an existing species.
  13. 13. A tale of two flies - or the ―how‖ speciation happens The scene: a population of wild fruit flies minding its own business on several bunches of rotting bananas, cheerfully laying their eggs in the mushy fruit...
  14. 14. Disaster strikes: A hurricane washes the bananas and the immature fruit flies they contain out to sea. The banana bunch eventually washes up on an island off the coast of the mainland. The fruit flies mature and emerge from their slimy nursery onto the lonely island. The two portions of the population, mainland and island, are now too far apart for gene flow to unite them. At this point, speciation has not occurred—any fruit flies that got back to the mainland could mate and produce healthy offspring with the mainland flies.
  15. 15. The populations diverge: Ecological conditions are slightly different on the island, and the island population evolves under different selective pressures and experiences different random events than the mainland population does. Morphology, food preferences, and courtship displays change over the course of many generations of natural selection.
  16. 16. So we meet again: When another storm reintroduces the island flies to the mainland, they will not readily mate with the mainland flies since they‘ve evolved different courtship behaviors. The few that do mate with the mainland flies, produce inviable eggs because of other genetic differences between the two populations. The lineage has split now that genes cannot flow between the populations. Story from:
  17. 17. Causes- or the ―why‖ speciation happens Geographic isolation Populations are prevented from interbreeding by geographic isolation. Geographic isolation is a common way for the process of speciation to begin: rivers change course, mountains rise, continents drift, organisms migrate, and what was once a continuous population is divided into two or more smaller populations. Any unfavorable habit will pose a barrier between populations.
  18. 18. Reduction of Gene Flow However, speciation might also happen in a population with no specific extrinsic barrier to gene flow. Imagine a situation in which a population extends over a broad geographic range, and mating throughout the population is not random. Individuals in the far west would have zero chance of mating with individuals in the far eastern end of the range. So we have reduced gene flow, but not total isolation. This may or may not be sufficient to cause speciation. Speciation would probably also require different selective pressures at opposite ends of the range, which would alter gene frequencies in groups at different ends of the range so much that they would not be able to mate if they were reunited.
  19. 19. Go to this tutorial and answer these questions from the introduction, then as a class we will watch the animations: 1- What is allopatric speciation? 2- What is sympatric speciation? 3- What is parapatric speciation? 1-Allopatric speciation is just a fancy name for speciation by geographic isolation (ie different geographical area). 2-Sympatric speciation does not require large-scale geographic distance to reduce gene flow between parts of a population. Merely exploiting a new niche may automatically reduce gene flow with individuals exploiting the other niche. This may occasionally happen when, for example, herbivorous insects try out a new host plant (apple maggot flies now eating a new food source – apples (introduced to America many years ago -instead of hawthorns). It happens in the same geographical area. 3-In parapatric speciation there is no specific extrinsic barrier to gene flow. The population is continuous, but nonetheless, the population does not mate randomly. Individuals are more likely to mate with their geographic neighbors than with individuals in a different part of the population‘s range. In this mode, divergence may happen because of reduced gene flow within the population and varying selection pressures across the population‘s range. (not part of your IB syllabus) nt/chp24/2402001.html
  20. 20. Two other examples of allopatric speciation: .htm and
  21. 21. Data based question Lacewing songs Click on the oscillograph to hear the song Oscillograph of Chrysoperla mediterranea Oscillograph of Chrysoperla lucasina From: Songs are part of the process of mate selection in members of different species within the genus Chrysoperla (lacewings). Males and females of the same species have precisely the same ‗song‘ and during the pre-mating period take turns making the songs. 1- Compare the songs of the two species of lacewings. 2- Explain why differences in mating songs might lead to speciation. 3- The ranges of the two species currently overlap. Suggest how differences in song could have developed: a) by allopatric speciation b) by sympatric speciation C. lucasina ranges across most of Europe and eastward into western Asia, and the northern quarter of Africa. C. mediterranea ranges southern to central Europe and across the north African Mediterranean.
  22. 22. Polyploidy can also lead to speciation Polyploidy is when the number of chromosomes in a cell becomes doubled. This can happen by a mutation that simply makes two copies. It can also happen when the chromosomes from two different species are mixed (a form of sympatric speciation). Many plants are both male and female, so they can simply fertilize themselves. Some earthworms can do this too. An example is the gilia plant from the Mojave desert in California. The species Gilia transmontana turned out to be a hybrid of Gilia minor and Gilia clokeyi. It has as many chromosomes as the other two combined, and its flowers have an intermediate shape. Since chromosomes are not all the same length, we can even say which transmontana chromosomes came from which ancestor. Gilia transmontana Gilia minor Gilia clokeyi
  23. 23. Convergent vs Divergent Evolution Divergent Increases the morphological differences between species, as each species adapts to different ecological niches OR it is the process of two or more related species becoming more and more dissimilar. Convergent Decreases morphological differences between species, as each species adapts to similar ecological niches OR it is the process of unrelated species become more and more similar in appearance as they adapt to the same kind of environment. A- Divergent B- Convergent C- Parallel evolution occurs when two species evolve independently of each other, maintaining the same level of similarity. Parallel evolution usually occurs between unrelated species that do not occupy the same or similar niches in a given habitat.
  24. 24. Examples of Convergent Evolution Old World vultures and New World vultures eat carrion, but Old World vultures are in the eagle and hawk family (Accipitridae) and use mainly eyesight for discovering food; the New World vultures are of obscure ancestry, and some use the sense of smell as well as sight in hunting. Birds of both families are very big, search for food by soaring, circle over sighted carrion, flock in trees, and have unfeathered heads and necks. Nubian Vulture (Torgos tracheliotos) is an Old World vulture Turkey Vulture (Cathartes aura) is a New World vulture. See Wikipedia for more examples Other than wings in birds and bats
  25. 25. Examples of Divergent Evolution The red fox lives in mixed farmlands and forests, where its red color helps it blend in with surrounding trees. The ears of the kit fox are larger than those of the red fox. The kit fox's large ears are an adaptation to its desert environment. The enlarged surface area of its ears helps the fox get rid of excess body heat. Similarities in structure indicate that the red fox and the kit fox had a common ancestor. As they adapted to different environments, the appearance of the two species diverged. Red fox Kit fox Can you think of other examples?
  26. 26. Adaptive Radiation It is a process of divergence where initially similar populations become genetically different from each other. When a population disperses into separate geographic locations, or ecological niches within a geographic range, the dispersed populations are exposed to unique sets of selective pressures and therefore change from the original ancestral species. The most famous example is Darwin‘s finches Each new species adapts to its unique environment radiating away from other species genetically (expressed as ecological, behavioral, physiological and morphological differences).
  27. 27. How fast does all this happen? Gradualism Species slowly change through a series of intermediate forms Punctuated equilibrium Long periods of relative stability in a species are ‗punctuated‘ by periods of rapid evolution This was the dominant framework in paleontology for a long time. But, there were gaps in the fossil record (no intermediate forms) so it is hard to see this pace of evolution. But according to this version gaps in the fossil record might not be gaps at all. Events such as geographic (allopatric) speciation and new niches can lead to rapid speciation.
  28. 28. Is this an example of Gradualism or Punctuated equilibrium? Read this story from an article published in Nature: -warming-not-death-of-dinosaurs-led-to-rise-of- mammals.html
  29. 29. A short video on sickle cell anemia When carriers have advantages that allow a detrimental allele to persist in a population, balanced polymorphism is at work. This form of polymorphism often entails heterozygosity for an inherited illness that protects against an infectious illness. Balanced polymorphism Deflated red cells from a human with sickle-cell anemia Normal human red cells A balanced polymorphism is a polymorphism in which the frequencies of the characteristics remain fairly constant over time. Polymorphism is when two or more forms of a phenotype are represented in high enough frequencies to be readily noticeable.
  30. 30. Transient polymorphismA transient polymorphism is one that is changing in frequency over time. Two examples, if a population with a high incidence of the sickle cell anemia gene moves to a location where malaria is no longer present the gene will begin to disappear because the selective pressure in its favour has disappeared but the pressure against it (the homozygous form) continues. Over time its frequency will fall. A paper looking at flowers ontent/early/2009/06/10/aob.m cp141.full Industrial melanism: Biston betularia, the peppered moth, where allelic frequencies continue to change over time. As forests became soot-covered, previously rare dark morphs became more frequent as they were selected for over light morphs. As forests became less soot-covered with pollution controls, the dark moths once again decreased in frequency to low levels as light morphs were selected for over dark morphs.
  31. 31. “But we can’t see evolution happening, so how do we know it’s true?” But we do see it happening, all the time. Here is a recent example of a new species of salamanders evolving in California, from PBS:
  32. 32. Your assignment.. Or How the ―_______‖ became a ―________‖ Create a story similar to the one that I did ―A tale of two flies‖ to explain how speciation happens. Thinking about how species develop and change over time your job is to develop a story on speciation about an organism of your choosing- fictional or non- fictional. Using the proper terminology and scientific principles on speciation (what is in your syllabus) create a story on how any species might have evolved. Your story can be in any format (comic book, picture book, narrative form, children‘s story, young adult, or in the form of a folklore or fairy tale). The description of the assignment and the criteria can be found on moodle