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Biology 1201A
Aleksandra Zecevic

History of Evolution 20 Recognition of Evolutionary Change Aristotle - believed that both inanimate objects and living species had fixed characteristics. Created ladder-like classification of nature from simplest to most complex: minerals to plants, plants to animals, animals to humans, humans to gods. Ladder called Scala Naturae. 14th century - Europeans merged Biblical Creationism with Aristotle’s classification system. Believed that all different kinds of organisms had been created by God, and that species could not change, new species could not arise and none could go extinct. Natural Theology - biological research that involved the naming and cataloguing of all creation, and then identifying its position and purpose in the Scala Naturae. This approach was led by Carolus Linnaeus 15th-18th Century - Biogeography, Comparative Morphology and Geology promoted a growing awareness of change. George-Louis Leclerc, le Comte de Buffon - noted the existence of body parts with no apparent function. If each species is created anatomically perfect for its particular way of life, why do useless structures exist? Buffon proposed that some animals must have changed since their creation, and that Vestigial Structures must have functioned in ancestral organisms. Species “conceived by Nature, and produced by Time”. Vestigial Structures - useless body parts Catastrophism - Georges Cuvier realized that the layers of fossils represented organisms that lived during successive times in the past. The abrupt changes within geologic strata marked dramatic shifts in ancient environments. Reasoned that each layer of fossil represented the remains of organisms that had died in a local catastrophe (ex. flood). Then different species would recolonize the area, and when another catastrophe occurred, they would form a different set of fossils in the next layer. Jean Baptiste de Lamarck - proposed first comprehensive theory of Biological Evolution. Believed that “perfecting principle” caused organisms to become better suited to their environment. Two mechanisms foster evolutionary change: Principle of Use and Disuse - body parts grow in proportion to how much they are used (ex.working out a muscle at the gym). Unused structures get weaker and shrink. Inheritance of Acquired Characteristics - changes that an animal acquires during its lifetime are inherited by its offspring. - These mechanisms do NOT cause evolutionary change because structural changes acquired during an organism’s lifetime are not inherited by the next generation. Lamarck made 4 contributions to the development of evolution. The first 3 were adopted to guide Darwin’s evolutionary theory: 1. Proposed that all species change through time 2. Recognized that changes are passed from one generation to the next. 3. Suggested that organisms change in response to their environments. 4. Hypothesized the existence of specific mechanisms that caused evolutionary change. Changes in Earth Gradualism - James Hutton - geologist. Suggested that SLOW and continuous physical processes acting over LONG periods of time produced the Earth’s major geologic features. Contrasts sharply with catastrophism. Uniformitarianism - Charles Lyell - geologist. Extended gradualism by suggesting that geologic processes such as volcanic eruptions, earthquakes, erosion, and the formation and movement of glaciers sculpted the Earth’s surface and are exactly the same as the geologic processes we observe today. Undermined any remaining notions of an Unchanging Earth. Has taken millions of years to mould the landscape. Population Genetics 17.1a Evolutionary Biologists Describe and Quantify Phenotypic Variation Quantitative Variation - where individuals differ in small incremental ways (ex. number of hairs on their head, weight, height, length of toes). - Quantitative V is often displayed on a bar graph or with a large enough sample, a curve. The width of the curve shows the variability in the popn. The mean shows the average value/amount of the character being measured. Natural selection often changes the mean value of a character or its variability within a popn. Frequency of blue phenotype in a popn of 123 blue geese and 369 white geese is 123/429 = 0.25. And frequency of the white phenotype is 369/492 = 0.75 Qualitative Variation - when a character exists in two or more discrete forms with no intermediate forms present. Ex. Geese have either white or blue feathers. Not both or a mixture. When there are discrete variants of one character, it is called a polymorphism. The trait is described as polymorphic. Human blood groups are another example. 17.1b Phenotypic Variation Can Have Genetic and Environmental Causes Phenotypic variation within popns may be caused by genetic differences btw individuals, differences in envtal factors they experience, or by an interaction btw genetics with envt. Thus sometimes organisms with different genotypes exhibit the same phenotypes. Genotypic and phenotypic variations are not perfectly correlated - ex. black colouration of mice from Arizona is due to a mutation in the Mc1r gene, but black mice from New Mexico don’t share that mutation however they still exhibit the same phenotype. With that said, organisms with the same genotype sometimes exhibit different phenotype - ex. acidity of soil influences flower colour in some plants. *** Knowing whether phenotypic variation is caused by genetic differences, environmental factors, or an interaction of the two is important because Only genetically based variation is subject to evolutionary change. Traits that vary quantitatively will respond to artificial selection only if the variation has some genetic basis. An organism’s phenotype is frequently the product of an interaction between its genotype and the environment. How can we determine whether phenotypic variation is caused by environmental factors or by genetic differences? - We can test for an environmental cause experimentally by changing one environmental variable and measuring the effects on genetically similar subjects. - Breeding experiments can also determine the genetic basis of phenotypic variation (Ex. Mendel – by crossing plants with different phenotypes). Traits that vary quantitatively will respond to artificial selection only if the variation has some genetic basis. 17.1c Several Processes Generate Genetic Variation 2 potential sources of genetic variation: – the production of new alleles and the rearrangement of existing alleles. Most new alleles arise from small-scale mutations in DNA. The rearrangement of existing alleles into new combinations can result in larger scale changes in chromosomes (ex. Through crossing over, and independent assortment during meiosis and random fertilization of sperm and egg). - the shuffling of existing alleles comprehensively into new combinations can produce an extraordinary number of new genotypes and phenotypes in the next generation. That is why unless you have an identical twin, it is extremely unlikely that another person with your genotype has ever lived or will ever live. There are more than 10^600 combinations of alleles possible, however fewer than 10^10 are alive today. 17.1d Populations often Contain Substantial Genetic Variation In studies of chromosomal and mitochondrial DNA, studies suggest that every locus exhibits some variability in its nucleotide sequence. Sometimes the variability is apparent in individuals. Other times, however, the variations detected in the protein-coding regions of DNA may not affect phenotype because they do not change the amino acid sequence of the proteins for which the genes code. 17.2 Population Genetics In order to predict how certain factors may influence genetic variation, population geneticists first describe the genetic structure of a population. They then create hypotheses formalized in mathematical models to describe how evolutionary processes may change genetic structure under specified conditions. Lastly, they test the predictions of the models to evaluate their ideas about evolution. 17.2a All Populations Have a Genetic Structure All populations have a genetic structure. In diploid organisms, an individual’s genotype includes two alleles at every gene locus. The sum of all alleles at all gene loci in all individuals in called the population’s gene pool. To describe the structure of a gene pool, scientists first identify the different genotypes in a representative sample and calculate genotype frequencies. Genotype frequencies – the percentages of individuals possessing a particular genotype. Knowing that each diploid organism has two alleles at each gene locus (either two copies of the same alleles or two different alleles), the scientist can then calculate allele frequencies. For a locus with 2 alleles, scientists use the symbol “p” to represent the frequency of 1 allele and “q” to represent the frequency of the other allele. Allele frequency – the abundance of one allele relative to others at the same gene locus in individuals of a population. Allele frequencies represent the commonness or rarity of each allele in the gene pool. For a gene locus with two alleles, there are three genotype frequencies (homozygote (2) and heterozygote (1)), but only 2 allele frequencies (p and q). The sum of the three genotype frequencies must equal 1, and the sum of the two allele frequencies must equal 1. 17.2b The Hardy-Weinberg Principle Is a Null Model That Defines How Evolution Does Not Occur Scientists often use control treatements to evaluate the effect of a particular factor. The control tells us what we would see if the experimental treatment had no effect. However, in studies that use observational rather than experimental data, there is often no suitable control. In such cases, scientists use null models. Null models predict what they would see if a particular factor had no effect. They serve as theoretical reference points against which observations can be evaluated. The Hardy-Weinberg principle specifies the conditions under which a population of diploid organisms achieves genetic equilibrium. Genetic Equilibrium – the point at which neither allele frequencies nor genotype frequencies change in succeeding generations. Shows that dominant alleles don’t replace recessive ones and that the shuffling of genes in sexual reproduction alone isn’t what causes the gene pool to change. According to the Hardy-Weinberg model, genetic equilibrium is possible only if all of the following conditions are met: 1.) No mutations are occurring 2.) The population is closed to migration from other populations. 3.) The population is infinite in size. 4.) All genotypes in the population survive and reproduce equally well. 5.) Individuals in the population mate randomly with respect to genotypes. If the conditions of the model are met, the allele frequencies of the population will never change, and the genotype frequencies will stop changing after one generation. Microevolution will NOT occur. The model is a null model and serves as a reference point for evaluating the circumstances under which evolution may occur. Determining which of the model’s conditions are not met is a first step in understanding why the gene pool is changing. Agents of Microevolutionary Change Agent Definition Effect on Genetic Variation Mutation A heritable change in DNA Introduces new genetic variation into a population Gene Flow Change in allele frequencies May introduce genetic as individuals join a variation from another population and reproduce population Genetic Drift Variation in relative genotype Reduces genetic variation, frequencies caused by random, especially in small chance disappearance of populations; can eliminate particular genes and alleles as alleles individuals die or do not reproduce Natural Selection Differential survivorship or One allele can replace another reproduction of individuals or allelic variation can be with different genotypes preserved. Nonrandom Mating Choice of mates based on their Does not directly affect allele phenotypes and genotypes frequencies, but usually prevents genetic equilibrium Selection and Fitness 17.1 Variation in Natural Populations In most species, members of a popn look alike, although are not identical. In some species, members vary dramatically in appearance. This is Phenotypic Variation - differences in appearance or function that are passed from generation to generation. -Microevolutionary studies often begin by assessing phenotypic variation within populations. Most characters exhibit quantitative variation. 17.3d Natural Selection Shapes Genetic Variability by Favouring Some Traits Over Others Natural Selection – the process by which beneficial traits become more common in subsequent generations. ** Although natural selection can change allele frequencies, it is the collective phenotype of an individual organism rather than any particular allele, that is successful or not. When individuals survive and reproduce, their alleles (favourable and unfavourable) are passed to the next generation. An organism with harmful or lethal dominant alleles will probably die before reproducing and all the alleles it carries, even the advantageous alleles will go with it. Relative fitness is used to evaluate reproductive success. Relative fitness – the number of surviving offspring that an individual produces compared with the number left by others in the population. So a particular allele will increase in frequency in the next generation if individuals carrying that allele leave more offspring than individuals carrying other alleles. Natural selection tests fitness differences at nearly every stage of the life cycle, however, it exerts little or no effect on traits that appear during an individual’s post-reproductive life (ex. onset of Huntington’s disease is age 40 allowing carriers to reproduce and pass it on to the next generation before the condition presents itself). Scientists measure the effects of natural selection on phenotypic variation by recording changes in mean and variability of characters over time. There are 3 modes of natural selection: 1.) Directional Selection – traits undergo DS when individuals near one end of the phenotypic spectrum have the highest relative fitness. DS shifts a trait away from the existing mean and toward the favoured extreme. After selection, a trait’s mean value is higher or lower than before. This type of selection is extremely common. 2.) Stabilizing Selection – traits undergo SS when individuals expressing intermediate phenotypes have the highest relative fitness. By eliminating the extremes, SS reduces genetic and phenotypic variation and increases the frequency of intermediate phenotypes. The MOST common mode of natural selection (ex. very small and very large newborns are less likely to survive than those born at intermediate masses). 3.) Disruptive Selection – traits undergo DvS when extreme phenotypes have higher relative fitness than intermediate phenotypes so alleles producing extreme phenotypes become more common promoting polymorphism. Under natural conditions, DvS is less common than DS and SS (Ex. situations that allow animals to specialize on particular food souces). 17.5 Adaptation and Evolutionary Constraints Although natural selection preserves alleles that allow for high relative fitness on the individuals that carry them, researchers are cautious about interpreting the benefits that certain traits provide 17.5a Scientists Construct Hypotheses about the Evolution of Adaptive Traits Adaptive Trait – any product of natural selection that increases the relative fitness of an organism in its environment. Adaptation – the accumulation of adaptive traits over time. It is easy to concoct an adaptive explanation for almost any characteristic we observe in nature, but they must be framed as testable hypotheses. They must compare variations of a trait in closely related species living in different environments. When trying to unravel how and why a particular trait evolved, it is important to remember that a trait observed today may have had a different function in the past. A lot of hypotheses predicting ancestral history of traits cannot be tested because certain ancestors no longer exist. Biologists must use anatomical studies of animals and their ancestors, and theoretical models of mechanics of movement to challenge and refine hypotheses. Although evolution has produced all the characteristics of organisms, not all traits are necessarily adaptive. Some traits are the products of chance events and genetic drift and others of basic physical laws (ex. gravity and the ability of seeds to fall to the ground). 17.5b Several Factors Constrain Adaptive Evolution Adaptive traits of most organisms are compromises produced by competing selection pressures. No organism can be perfectly adapted to its environment because environments change over time. When selection occurs in a population, it preserves the alleles that are successful in the current, prevailing environmental conditions so each generation is adapted to the environmental conditions under which its parents lived. If the environment changes from one generation to the next, adaptation will always lag behind. Natural Selection is not an engineer that designs new organisms from scratch. It works with what it has. It acts on new mutations and existing genetic variation. Because new mutations are rare, it mostly acts on alleles that have been present in the population for generations. It just modifies traits so they are not as efficient or perfect as they could be. The agents of evolution can cause microevolutionary changes in the gene pools of populations that may cause their gene pools to diverge, and sometimes the divergence is enough to cause the populations to evolve into different species. Evolutionary Forces A student asks: Is balanced polymorphism a result of disruptive natural selection? Technically, no. Disruptive selection is a type of selection where both extreme phenotypes have a fitness advantage over intermediate phenotypes. Disruptive selection only applies when we are considering this kind of selection on phenotypes that span a continuous range; that is, when we are considering quantitative traits (e.g. height, weight, shades of hair colours, etc). “Polymorphism” is a term we tend to reserve for describing qualitative (i.e. discrete) traits, like horns versus no horns, or the genotypes of the ABo blood group (where there are discrete phenotypes, rather than a continuum). Since disruptive selection is a type of selection dealing with quantitative traits, and polymorphisms deals with qualitative traits, we can’t state that a balanced polymorphism is a result of disruptive selection. But, I like what brought you to that question. A balanced polymorphism refers to a situation where selection pressures maintain more than one ‘morph’ in the population, or more than one allele at non-zero frequencies. So, the one ‘special’ type of selection we discussed that acts on qualitative/discrete traits was ‘heterozygote advantage’. And, the consequence of heterozygote advantage is that it maintains the two alleles at non-zero frequencies in the population. So, heterozygote advantage will result in a balanced polymorphism. A student asks: I was wondering what the main difference is between gene flow and founder effect? Response: I totally understand whe
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