Assessment Statements • Define allele frequency • State that evolution involves a change in allele frequency in a population’s gene pool over a number of generations. • Discuss the definition of the term species • Describe three examples of barriers between gene pools (allopatric,sympatric and polyploidy). • Explain how polyploidy can contribute to speciation. • Compare allopatric and sympatric speciation • Outline the process of adaptive radiation • Compare convergent and divergent evolution • Discuss ideas on the pace of evolution, including gradualism and punctuated equilibrium. • Describe one example of transient polymorphism. • Describe sickle-cell anemia as an example of balanced polymorphism Why Biology is Awesome! The 2012 Nobel Prize for Physiology or Medicine was awarded Monday to Sir John B. Gurdon and Shinya Yamanaka for work that revolutionized the understanding of how cells and organisms develop. They discovered that mature adult cells can also differentiate just like stem cells. So….what we learned about stem cells and what makes them so unique and special when it comes to future treatments is now no longer true, any cell can do that! Cool Adaptations Tripod fish, Bathypterois grallator Pacific Barreleye Fish Eyes Not eyes, water intake http://www.youtube.com/watch?v=Zoygy-8PTtU Star Nosed Mole Quetzalcoatlus 68–65.5 million years ago Largest flying animal of all time. 35ft wing span! Temporal Isolation Over generations, a population can undergo a great deal of change from its original state. But all members of that population are still belonging to the same species unless some members become REPRODUCTIVELY ISOLATED from the original population. Speciation is the separation of two previously interbreeding populations into two populations that can no longer mate to produce fertile, viable offspring. Bufo americanus and Bufo fowleri . The members of these species can be successfully crossed in the laboratory producing healthy, fertile hybrids. However, mating does not occur in the wild even though the geographical distribution of the two species overlaps. The reason for the absence of interspecies mating is that B. americanus mates in early summer and B. fowleri in late summer. Polyploidy Another barrier between gene pools can occur due to polyploidy. A polyploidy organism has more than two sets of chromosomes. The normal number of sets is 2, known as diploid (2n), polyploidy organism could contain 3 or more sets. 3n triploid, 4n tetraploid. Polyploidy is widespread in plants but rare in animals. This happens when sets of chromosomes are not completely separated during cell division. If chromosomes aren’t separated properly in mitosis and the cell fails to divide after telophase then the cell becomes tetraploid. Each chromosome will have a matching pair and will be able to undergo meiosis to form fertile gametes. A tetraploid can cross with another tetraploid to form fertile offspring. If a tetraploid crosses with a diploid plant they would produce triploid plants that would be sterile. In this case, polyploidy acts as a barrier between the diploid and tetraploid species. The populations may become so different that they develop into new species. Tetraploidy can occur through hybridization of diploid species X T.dubius 2n X T.pratensis 2n T.miscellus 4n T.dubius 2n T. porrifolius 2n T. mirus 4n Hybrid Infertility Barriers between gene pools can also occur as a result of hybrid infertility, although this doesn’t lead to speciation. Most plant and animal hybrids are infertile. Ligers, mules (donkey and horse) they are infertile, so they can not lead to the development of a new species. Comparing Sympatric and Allopatric Speciation Sympatric Speciation Allopatric Speciation A new species arises from existing species that is living in the same area. A new species arises because a physical barrier separates it from other members of an existing species. Temporal or behavioral isolation can produce significant changes in the genetic make-up within a species so that a new species is formed. Physical barriers may include mountain ranges, valleys or bodies of water, or humanmade features such as roads, canals or built up areas. Convergent and Divergent Evolution As the process of speciation takes place, new species form that no longer resemble one another. When this occurs, species are said to have diverged from their common ancestor known as divergent evolution. New species evolve as organisms exploit new habitats and reduce competition by occupying their own specific niche. Over generations, physical and behavioral adaptations develop through natural selection that help organisms survive and reproduce in their particular habitat. Example of this is the pentadactyl limb. Many forms have evolved from a common ancestral form. Adaptive Radiation Adaptive radiation is a form of divergent evolution, and occurs as new species develop from a common ancestor as they adapt to new environments. Darwin’s finches on the Galapagos Islands is a good example. The ancestral finch species that first arrived on the islands had a small but thick beak for eating small seeds. On the different islands many other food sources were available Any birds with beaks that were a slightly different shape, making them better at using a new food source, would be more likely to survive and pass on their genes. The change in beak shape would be reinforced with every new generation, eventually producing different groups of birds that were adapted to feeding on new foods. Convergent Evolution Other situations, organisms that have different ancestors may evolve similar characteristics, because conditions favor these particular features. They make it more likely that an organism will survive and reproduce successfully in the environment. This is known convergent evolution. These wings are a good example, they serve a similar function but are derived from completely different structures. Convergent Evolution Similar characteristics but Different ancestors. Divergent Evolution Different characteristics but same ancestor Nature’s Various Ways to answer the question: How to Survive? Texas Horned Lizard Hippo Sweat Blob Fish Yeti Crab Dunkleosteus 380 mya 33ft long (10 meters) 3.6 tons The Pace of Evolution: Gradualism and Punctuated Equilibrium Darwin viewed evolution as a slow, steady process called gradualism, where changes slowly accumulate over many generations and led to speciation. The evolution of the whale to about 52 million years. Punctuated Equilibrium In some cases, the fossil record does not contain any intermediate stages between one species and the next. One explanation is that fossilization is such a rare event the intermediate fossils simply have not been discovered. In 1972c Stephen J Gould suggested that the fossils had not been found because they don’t exist. He proposed punctuated equilibrium. The driving force for evolution is selection pressure, so if the selection pressure is very mild or nonexistent then species will tend to remain the same (equilibrium) When there is a sudden, dramatic change in the environment, there will be new intense selection pressures which lead to rapid development of new species. Example of Punctuated Equilibrium A good example of intense selection pressure in modern times is the use of antibiotics, which has resulted in the appearance of resistant species of bacteria in a very short period of time. Multile-resistant MRSA (Staphylococcus aureus) It has become adapted to new conditions in which there are many antibiotics in use and has developed new strategies to resist them. Gradualism vs Punctuated Equilibrium Polymorphism Polymorphism means ‘many forms’ and describes a situation where an allele has two different forms, which may be selected for in different environments. If the environments are stable then the relative frequencies of the alleles do not change and is known as stable polymorphism. If the environment changes, the frequencies of the different forms start to change and this is known as transient polymorphism. Transient polymorphism In transient polymorphism, one form is gradually being replaced by another. It represents a temporary situation as a by-product of natural selection. For example, during the course of industrial melanism, it was held that the melanic form of peppered moth gradually predominated the non-melanic form in the trees of Manchester, England due to selective pecking of the latter (lighter form) by the birds. Balanced Polymorphism When natural selection stabilizes two or more alleles in a population so that their frequencies do not change, a balanced polymorphism occurs. Sickle-cell anemia provides an example that occurs in certain parts of the world. Sickle-cell is caused by a mutation in one of the genes for the oxygencarrying protein hemoglobin Two alleles of the gene occur HbA and HbS and produce 3 phenotypes. Most people have the genotype HbAHbA and have normal hemoglobin. These individuals do not develop sick-cell but they a very susceptible to malaria. People who are heterozygous HbAHbS have sick-cell trait. They have some sick-shaped cells and some normal cells, and are resistant to malaria. Homozygous individuals HbSHbS have sickle-shaped blood cells and are resistant to malaria but develop sickle-cell anemia, which can be fatal. A balanced polymorphism exists in parts of Africa where malaria is endemic. Both forms of the allele remain in the population. Heterozygotes are selected for due to their resistance to malaria but both homozygotes are selected against because they are vulnerable to either malaria or sickle-cell anemia. An unusually high proportion of the population are heterozygotes because of the resistance to malaria. Another example of balanced polymorphis is the cichlid fish Perssodus microlepis, which is found in Lake Tanganyika in Africa. These small predatory fish dart out from hiding places to bite off the scales of larger prey fish. These cichlids have developed a mouth that is angled to the side so they can attack from behind without being seen. Some have a mouth angled to the left, so they can attack the right flan of their prey, but this means their prey will be watching its right flank when it gets into Perissodus territory. Others have a a mouth angled to the right, and attack from the other side. If numbers increased of either phenotype the prey would spend more time watching that side giving the other an advantage.