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Natural Selection The Mechanism of Existence Natural Selection • Activity and discussion • Darwin based his theory of natural selection on 3 sets of observations: • The Struggle for Existence • Natural variations within the same species • Environment’s role in evolution 1. The Struggle for Existence • Drawing from Malthus’s ideas, Darwin recognized that all species produce excessive no. of offspring. • But due to limited resources in nature, only a small percentage of offspring survive in each generation 2. Variation • Variation refers to differences among the members of the same species. • Individual variation is widespread in all species. • Much of this is heritable i.e. it passes from one generation to the next 3. The Role of the Environment • Darwin observed that a key factor in the survival of an organism was how well it was suited for the environment. • The environment selected those individuals with variations that were best suited for that environment. • E.g. Darwin would’ve said that in a population, the environment favored the individuals born with longer necks so they survived and reproduced and passed their characteristics to the next generation So… • Natural Selection is the process by which individuals with inherited characteristics well suited for the environment leave more offspring on average than individuals with adaptations less suited for the environment • When this happens over many generations, each new generation has a higher proportion of individuals with advantageous traits • This would cause a population to change over time Ecological Niche • An organism’s ecological niche select whether or not it will survive and reproduce. • An Ecological Niche is the sum total of a species’ use of the biotic and abiotic factors in its environment. • Biotic Factors: Predators or prey • Abiotic Factors: Water availability, weather, etc. Survival of the Fittest? • Fitness of an individual is its ability to survive and reproduce in its specific environment. • Natural selection does not make organisms “better”. It doesn’t have a goal. • If the environment changes in some way, another adaptation might be favored. Evidence for Evolution • Evolution has left its marks on all aspects of life: • The Fossil Record • Geographical distribution • Anatomy • Development of species • Molecular Biology The Fossil Record • Most fossils are sound in sedimentary rocks where younger layers of rock, or strata, are deposited on top of older ones • Fossil Record is the chronological collection of life’s remains in the rock layers recorded over time • Paleontologists are scientists who study fossils • Oldest fossils (Prokaryotes) were found in rocks 3.5 Billion years old The Fossil Record: Whale Evolution • Fossils of aquatic fauna are usually the best preserved • Fossil evidence supports the hypothesis that whales evolved from land-dwelling ancestors with four limbs The Fossil Record: Origins and Extinctions • Fossil evidence also provides evidence of the rise and fall of some species. • Dinosaur-like animals first appeared 230 Million years ago and went extinct some 65 Million years ago [Video] • Drawback: Species without hard tissue (like bones or shells) rarely become fossilized and so its little help in establishing the evolutionary history of those organisms. Geographic Distribution: Closely Related but Different • Darwin’s study of finches showed that they were all descendents of the single ancestral species but today all finch species in the Galapagos has a beak adapted to eating a specific type of food • Same for the tortoise species that live on those islands Geographic Distribution: Distantly Related but Similar • Similar environments will select for similar adaptations • E.g. the flightless birds of grasslands look similar to each other but their body structure indicated that they descended from different types of birds Geographical Distribution: Marsupials vs Placental Mammals • Marsupials: Their fetus leave the uterus early and completes further development in a pouched [Video] • Placental mammals: The fetus is protected by a placenta and develops inside the uterus • The reason Australia has so many marsupials is because they all evolved from marsupial ancestors on an island isolated from placental mammals in the distant past Comparative Anatomy: Homologous Structures • Similar structures in species that share a common ancestor are called homologous structures. • The structures that originally functioned in one way in an ancestral species are now modified for new functions. • E.g. limbs of various mammals Comparative Anatomy: Vestigial Structures • Vestigial Structures are remnants of structures that may have had important functions in an ancestral species but have no clear function in some modern descendents. E.g. • Whales have vestigial hip bones • Humans have appendices Comparative Anatomy: Analogous Structures • Analogous structures are structures, in distantly related species, that are anatomically different but serve the same function. E.g. wings of different kinds • The evolutionary idea of descent is based on structure not function Comparative Development • Comparing the various stages of development of various organisms shows embryos of closely related species with similar stages in development. • E.g. Vertebrate embryos are all show pharyngeal gill pouches, even in animals with no gills. Molecular Biology • DNA provides a record of an organism’s ancestry. • The greater the number of differences in DNA, the less likely it is that they share a close common ancestry. E.g. siblings have very similar DNA • Cytochrome c sequence in the mitochondrial DNA is found in all oxygen dependent organisms. • This sequence is highly conserved and the amount of difference in sequence is used to build phylogenetic trees. Evidence for Natural Selection • Since Darwin, there have been several other evidences for Natural Selection. Some of them are: • Artificial Selection • Changes in Beak Shape • Antibiotic Resistance in Bacteria Artificial Selection • Artificial Selection is the selective breeding of domesticated animals to produce offspring with genetic traits that humans value. • Breeders breeding allow only those plants or animals with useful traits to reproduce. • Artificial selection can produce a great deal of change in a species in a short time. • Problems [Video] Changes in Beak Shape • Peter and Rosemary Grant (Princeton University) studied Daphne Major Island finches for 30 years • They found that average beak and body size of the medium ground finch changes as El Niños come and go • El Niño is a warming of the ocean current that passes along the coasts of Peru and Ecuador and affects global weather patterns Changes in Beak Shape • The medium ground finch prefers smaller seeds and due to their abundance in wet years, eat few large seeds • During dry years due to El Niños, all seeds are in short supply (due to fewer plants) and large seeds are the bulk of the diet • Only birds with large and tough beaks survive these years. • With the return of the wet years, the average beak size diverges again Antibiotic Resistance in Bacteria • Tuberculosis (TB) is severe bacterial chest infection caused by Myobacterium tuberculosis. • In 1940s, Streptomycin was developed to fight it and by 1970s it was almost wiped out • Resurgence in 2006 in South Africa and by 2008 had spread to 49 countries. In 2009 1/3 of world population had TB. 5% new infections were resistant to antibiotics • Why? Antibiotic Resistance in Bacteria • Bacteria have many variations. Some can be destroyed by antibiotics, others are resistant. • TB is treated by a 6-9 month course of 4 different antibiotics. Why? • Patients may stop the treatment prematurely and not take all the antibiotics needed to kill all bacteria. • New problem: Staphylococcus aureus, human skin bacteria, cause pimples on skin, pneumonia if they enter the body • Methicillin-resistant strains are in Ontario hospitals. Why?