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Evolution Unit 12 p. 6-14, 417-428, 499-515 Darwin and Natural Selection Beagle Expedition: found unique species that looked similar to species from other parts of the world. Noticed changes in form among members of the same and closely related species. Voyage of the HMS Beagle The Origin of the Species On The Origin of Species by Means of Natural Selection Nov. 1859 Charles Darwin He avoided the term "evolution” “light will be thrown on the origin of man and his history". "endless forms most beautiful and most wonderful have been, and are being, evolved." Natural Selection Natural selection: mechanism for change in populations that occurs when organisms with favorable variations for a particular environment survive, reproduce, and pass these variations on to the next generation (those with less favorable traits are less likely to survive). Referred to by Darwin as “decent with modification” Evolution of Humans The Descent of Man, and Selection in Relation to Sex Charles Darwin 1871 Darwin outlined sexual selection to explain the evolution of humans Darwin’s 4 Facts Exponential Growth of Populations Factors will prevent Exponential Growth(Steady State) Variability among species Traits are inheritable (NOT AQUIRED) Darwin’s 2 Inferences Competition for resources Individuals most fit for their environment will survive. 4 Tenets of Natural Selection All populations have genetic variation. The environment presents challenges to successful reproduction. Individuals tend to produce more offspring than the environment can support. Individuals that are better fit their environment tend to live longer, reproduce more, and spread their genes throughout the population. Natural Selection Natural Selection is accepted as the driving force for evolution. What is a the difference between a scientific hypothesis, law, and theory? Hypothesis, Theory, Law Hypothesis- explanations that are testable through experimentation or observation. Theory- comprehensive explanation of facts, laws, and reasoning that is supported by many observations and multiple lines of evidence. A working model of a natural phenomena. Theories are accepted by the scientific community. ex. Modern Atomic Theory, Cell Theory, Theory of Evolution Law- short, usually mathematical, explanation of a scientific relationship. ex. Law of Conservation of Matter/Energy, Gas Laws, Law of Gravity, Thermodynamics, Newton’s, Kepler’s Evolution Terms Species- group of organism capable of breeding to produce fertile offspring. Population- a particular species in a given area that is interbreeding. Changes happen to individuals, populations of species evolve. Evolution Terms Reproductive Fitness- Advantage of a particular genotype to survive and reproduce at a greater rate. Fitness is specific to a particular environment. (Consider both the biotic and abiotic environment). As the environment changes, so do the fitness values of the genotypes. Fitness is a property of a genotype, not of an individual or a population. Individuals with the same genotype share the same fitness within the same environment. Fitness is measured over one generation or more. The concepts of fitness and adaptation are relevant ONLY in a particular ecological context. There is no such thing as fitness in an absolute sense. Evolution Terms Adaptation- a genetically determined characteristic that improves an organism’s ability to survive and reproduce in a particular environment. Adapt- the evolutionary process by which organisms become better suited to their environments Evolution Terms Structural Adaptations: Can change the size and/or shape of a body part For example: thorns, wings, mimicry (copy the appearance of another species), camouflage (blend in with environment) Change within a population over time (anywhere from 100 years to millions of years) depending on type of adaptation, rate of reproduction and environmental factors Physiological Adaptations: Help populations overcome chemicals they encounter (antibiotics, pesticides, herbicides, etc) Develop rapidly (example: bacterial resistance to penicillin) Evolution Terms Genetic variation- small differences among individuals within a population Genetic Variation must be present for evolution to occur. Polymorphic- the presence of two or more alleles (variations) of a gene in a gene pool. Sources of Genetic Variation Mutation- change in DNA sequence that can slowly introduce new alleles to a population (can be harmful, beneficial or have no significance) Meiosis Random alignment of chromosomes. Crossing Over- exchange of homologous chromosome segments. Sexual Reproduction (fertilization)- combination of alleles of two individuals Immigration- incoming alleles to a gene pool Mutation = a heritable change in the nucleotide sequence of DNA, resulting in an alteration in the products coded for by the gene. (about 0.5% of DNA bases are variable) Consider how the amount of genetic divergence (change) forms a continuum: Microevolution small changes Macroevolution large changes Microevolution = adaptation Macroevolution = speciation Microevolution vs. Macroevolution Microevolution- changes within a species that can occur over dozens or hundreds of generations. Macroevolution- evolution over long periods of time that involve origin of new species. Microevolution Population genetics- change in the genetic composition of populations. Uses Mendel’s Law of inheritance, Biochemical analysis of genes and proteins, and Mathematical models to understand microevolution. Gene Pool- all the genes of a population of organisms. (refers to a single population, not the entire species) Mechanisms for Evolutionary Change Natural Selection Artificial Selection Gene Flow Genetic Drift Founder Effect Population (Genetic) Bottleneck Microevolution in Large Populations Natural selection is the main factor that changes the gene pool of large populations. Peppered Moth (p.423) Malaria/ Sickle Cell Anemia (p.423-424) • http://www.pbs.org/wgbh/evolution/library/01/2/l_012_02. html Gene Flow Mutation Gene flow = introduction or loss of new alleles into the population through immigration or emigration. Microevolution in Small Populations Genetic Drift Founder Effect Genetic Bottleneck (Population Bottleneck) Inbreeding Inbreeding Depression Genetic drift = random shifts in allele frequencies in small populations Founder’s Effect Small number of individuals of a large populations migrate and start a new population. Gene pools of new population can change greatly from the original population if the new population is small. Causes a loss of genetic variation. Can cause inbreeding- the rise of homozygosity within a population. Genetic Bottleneck The number of organisms in a population is drastically reduced (hunting, disease, loss of habitat, etc.) Large populations can become inbred Inbreeding can lead to the increase of the frequency of harmful recessive alleles in the gene pool. Inbreeding Depression- increased homozygosity of a population leads to reduction os fertility and survival. Speciation = Origin of a new Species Species = group of organisms that look alike and have the ability to interbreed and produce fertile offspring in nature Speciation occurs when Interbreeding is prevented Production of fertile offspring is prevented Speciation Species organisms that can reproduce successfully. Genetic Isolation causes speciationpopulations gene pools become so different, they can no longer interbreed. Prezygotic Isolation mechanisms keep members of different populations from fertilizing each other. Postzygotic isolation mechanisms keep populations from producing viable or fertile offspring. Prezygotic Isolation Geographic Isolation- barrier (mountains, canyons, oceanns, etc.) keep members from meeting each other. Ecological Isolation- populations do not crossbreed because their habitats differ adaptive radiation. Behavioral/ Reproductive isolation- differences in courtship and/or mating season Chemical isolation: sperm may not be attracted to an egg or may not penetrate the egg when they find it. Adaptive Radiation Adaptive radiation is rapid evolutionary change characterized by an increase in the morphological and ecological diversity of a single, rapidly diversifying lineage. Phenotypes adapt in response to the environment, with new and useful traits arising. This is an evolutionary process driven by natural selection. Natural Selection leads to Adaptive Radiation and Speciation Geographic Isolation Postzygotic Isolation Zygotes may not develop properly. Odd chromosome numbers in hybrids (mules, ligers, zonies, etc.) complicate meiosis. Plants can become polyploid, making mating difficult with plants having the parental # of chromosomes. Hardy-Weinberg Idealized mathematical model of allele frequencies in gene pools. Hardy Weinberg uses allele frequency to calculate the genotype frequency. Assumptions Organism is diploid Generations do not overlap Population size is large Mutation is negligible Random Sexual Reproduction Migration is negligible Natural Selection does not operate Under these assumptions, allele frequencies remain stable over time. Hardy-Weinberg p- frequency of one allele q- frequency of another allele p2- frequency of homozygous p genotype 2pq- frequency of heterozygous genotype q2- frequency of homozygous q genotype p +q=1 p2 + 2pq + q2 = 1 Pace of Evolution Gradualism- speciation can occur at a slow but constant accumulation of changes. Punctuated Equilibrium- species experience long periods of no change between short bursts of change. Has been proven by evolution of E coli bacteria in nutrient-poor growth conditions. Explains gaps in the fossil record between related species, short transition leads to little chance for fossils. Types of Natural Selection Stabilizing Selection - favors average traits (average sized spiders fair better than both large and small) Directional Selection - favors one extreme of a trait (the longer the beak the better) Disruptive Selection - favors both extremes of a trait (intermediate at disadvantage - often eliminated) Types of Evolution Divergence (ex: adaptive radiation); Single populations splits to adapt to different environments Convergence Unrelated species become similar as they adapt to a similar environment. Analogous structures • Same use, different form • Not evidence of a common evolutionary ancestor Evidence for Evolution Fossil Record Anatomical relationships Embryological development Genetic comparisons Fossil Record Fossils can help provide a record of the history of life and shows evolutionary change. Shows physical record or extinct organisms. Shows organisms before and after divergence of species. Shows the rate of evolutionary change. Radiometric dating- use of the ratio of radioactive isotopes, can be used to put a relative date on a fossil. The record is incomplete - there are large gaps therefore each and every step of evolution can not be followed. Paleontologists look for intermediate species to provide a step by step sequence of evolution. The Fossil Record The succession of forms in the fossil record clearly suggests that organisms change through time, and have descended from a common ancestor. Different groups appear in the fossil record at different times, with a general trend toward the simplest organisms appearing the earliest..this is at odds with the view that they were all created at the same time. Many forms have gone extinct, another observation that is at odds with the view that each species was specially created for a purpose. In some cases, a direct line of descent, and change through time, can be observed in fossils. Foraminifera, small oceanic protozoans, leave a continuous fossil record in oceanic sediments. It is possible to trace their gradual evolution over millions of years. Since Darwin’s day, our knowledge fossil record has improved tremendously, we can trace the evolution of many different groups through fossils: horses, for instance, have a superb fossil record, showing many instances of speciation and many intervals of evolutionary change. Key Facts from the Fossil Record Animal and Plant forms have changed over time. The timespan of evolution is immense. Extinction is the fate of most species that have ever existed. Environments in every locale have changed, often drastically. Anatomical Comparisons Homologous Same form, different use Indication of common evolutionary ancestor Vestigial structures structures Evolutionary remnants of structures that were important in a past ancestor Homologous Structures Closely related species frequently have homologous structures: structures that are similar in their fundamental layout and construction, although they may serve very different purposes. For example, the forelimbs of mammals are constructed from the same skeletal elements: The wings of a bat, a whale, a human, a dog, etc. all contain the same bones, despite their different uses. This suggests that common ancestry, rather than design, plays a role in the construction of species. Vestigial Structures Many species retain structures that only make sense in light of their ancestry. These structures are typically reduced and nonfunctional, but they are inherited from ancestors, in whom they were important to survival or reproduction Example-Whales have an excellent fossil record-showing transitional forms Comparative Development and Embryology If members of a taxonomic unit share a common ancestry, it is reflected in their development: Two of the many examples: limb bud development in whales extraembryonic membranes of the amniote egg Embryological Development There are noted similarities between the embryological development of all vertebrates (fish, chicken, rabbit, human, etc) Presence of gills and tails in all supports evidence that gill-breathing vertebrates preceded air-breathing, terrestrial species Snake Chicken Possum Cat Bat Human Embryology Closely related species go through similar stages of development, although the adults may not resemble each other very closely. For instance, all vertebrate embryos develop gill pouches at some stage, even though in many species, they are lost later. This is suggestive of a common origin for vertebrates. Embryological development is often suggestive of evolution: birds have many developmental features in common with reptilian ancestors, land vertebrate embryos have many features suggestive of an aquatic existence (gill pouches, a notochord, blocks of segmented muscle). Genetic Comparisons All living things use DNA. All living things use the same genetic code. Can show relationships between species (human and chimpanzee DNA are 99% identical and are closer than chimpanzees are to apes) Can help reveal when species diverged from their ancestral types Modern Evidence Since Darwin’s time, there have been hundreds of studies of evolution. Natural selection has been measured in many organisms in the field, and in laboratory populations. An understanding of evolution has also become important to combating disease. The synthesis of Darwin’s theory with Mendelian genetics led to our modern understanding of Evolution. Several early twentieth century evolutionary biologists are widely credited with developing our modern understanding: R.A. Fisher J.B.S. Haldane Sewall Wright Theodosius Dobzhanski Thomas Hunt Morgan