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Evolution Final Summaries.docx

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University of Guelph
BIOL 2400
T.Ryan Gregory

Unit 1: Class 3: From the beginning to the Origin Lamarck's Theory - First law: Sustained use of any organ will gradually strength and enlarge that organ - Second law: Nature has caused individuals to acquire or lose as a result of the influence of environmental conditions to which their race has been exposed - no extinction, no common ancestry, organisms increase up the scale of complexity - new species formed at bottom of scale by spontaneous generation Unit 1: Class 4: Ever since Darwin Neo-Lamarckism - coordinated additions of developmental stages in a species - new additions caused certain organs to be used or disused, triggering Lamarckian mechanism of change Mutationism - discontinuous variation was much more important than continuous variation - new species formed all at once, no intermediates, no gradual change Orthogenesis - internal force pushes species along evolutionary trajectories Saltationism - natural selection is important only in producing minor within-species change - evolution accompanied by macromutations 'The Modern Synthesis' - species are related by descent, but natural selection was agreed as the dominant mechanism - remaining challenges to Darwinism were mutationism, orthogenesis and saltationism Hardy-Weinberg Equilibrium Principle - explains what happens to allele frequencies under idealized conditions (p + 2pq + q = 1) Unit 1: Class 5: Evolution as fact and theory - Evolution as FACT (species are related by descent), THEORY (mechanisms explain how this change occurs) and PATH (actual historical pathway following during course of evolution on Earth) - fossil record was not a big deal to Darwin because it was incomplete, used them for evidence for extinction and geographical distribution instead - Transitional fossils: fossil species that possess a feature that is intermediate in form between its ancestors and descendants (i.e. a bird with teeth - exhibits traits found in two different groups) - can be used to find out information about fossil gaps - Biogeography -species are distributed in clear patterns geographically and NOT just by habitat - major patterns are now known to correspond to Earth history (Wallace's biogeographic region) - Homology: a particular limb trait is derived from the ancestral limb, and all organisms with that particular structure, originated from that ancestor - Embryology: the closer a species are related, the more similar their homology will be - Atavisms: the reappearance or return of an ancestral feature in modern individuals that was previously lost (extra horse digit) - modern birds do not have teeth, however, they can be induced to grow teeth by the transplantation of mammal or reptile oral tissue, BIRDS MAINTAIN THE GENES TO MAKE TEETH - Vestigial structures: Structures that are greatly reduced from their ancestral forms, however THEY STILL FUNCTION (wisdom teeth, tail bone, muscles to move ears, third eyelid) - Sup-optimality due to history - back pain due to vertical spinal column, retina inverted w/ blind spot and can detach, crossover of trachea and esophagus resulting in choking, difficult childbirth due to large heads Case Study: The fact and path of evolution using whales Homology: They have the same limb structure as us, even though the function is different, meaning they descended from a common terrestrial ancestor Vestigial: Hip bones in the modern whale are smaller, genes for smell are non functional, remnants of muscles to move non-existent ears Pakicetus - oldest cetacean known, possibly amphibious, linked with whales due to skull features Ambulocetus - amphibious, had hind legs better suited for swimming thank walking, depends on fresh water (** was not on its way to being anything, they were well suited for being exactly what they were) Rhodocetus - would not have walked well, skeletal features associated with tail for swimming, nostrils move partway back, inner ear intermediate between terrestrial and aquatic mammals Basilosaurus - strictly aquatic, highly elongated body, tiny vestigial hind legs, similar to whales Dorudon - nostrils moved well back on skull, had hind limbs and lacked 'melon organ' for echolocation Fossil Record - the extinct cetaceans are NOT direct ancestors of modern whales NOR are they ancestor-descendants from each other - they have their own branches in the whale family tree - they still have valuable information regarding the kinds of transitions that occurred Molecular Biology - all genetic analyses AGREE that ALL modern whales are most closely related to TERRESTRIAL mammals, rather than other aquatic vertebrates like fishes Important Concepts - very different types of evidence converge in evolutionary biology - mostly closely related living group DOES NOT EQUAL ANCESTRAL - transitional fossils are no necessarily DIRECT ancestors but they show that transitions occurred - vestigial does not necessarily mean functionless - debates about the scientific PATH or mechanism of evolution does not change the FACT of evolution Unit 1: Class 6: Tree thinking - evolutionary trees are diagrams showing a series of branching and relatedness among species over evolutionary time, known as phylogenies - Anatomy of a phylogeny - Tips (terminal nodes): usually represent current species - Internal nodes: represent common ancestors - Root: beginning of the phylogeny - Topology: pattern of the branching - Polytomy: unresolved or multi-branching node - Soft polytomy: you are not sure how the species evolved - the number of possibilities increases exponentially w/ number of taxa - Hard polytomy: 3 species evolved from a common ancestor at once - you can present phylogenies in an unrooted or rooted fashion, rooted contains more information (ingroup/outgroup) where the TOPOLOGY is the most important - you can have cladograms (topology only) or phylograms (the branch length signifies the time) - any reconstructed phylogeny is a HYPOTHESIS about relationships and patterns of branching Cladistics and 'Natural' Classification - natural classification system is not based on superficial similarity but on EVOLUTIONARY RELATEDNESS - ONLY monophyletic groups (clades) should receive taxonomic names - Monophyletic group: includes the most recent common ancestor of all organisms and all of the descendants of that common ancestor (1 cut) - Paraphyletic group: includes the most recent ancestor of all organisms but not all of the descendants of that common ancestor (2 cuts) - Polyphyletic group: does NOT include the most recent common ancestor of all organisms , usually because the common ancestor lacks characteristics of the group (this group puts distant relatives together by excluding closer relatives = convergent evolution) Important Concepts - just because there is a distinct outgroup DOES NOT mean it is the ancestor of the other species - it is equally closely related to all members of that clade - the pattern of branching or topology is what matters, the order of the tips DOES NOT matter - don't be misled by unbalanced or ladderized trees, they can have the same topology as a balanced one - every internal node can be rotated without changing the topology (DON'T read across the tips) - EARLY BRANCHING DOES NOT EQUAL ANCESTRAL!!! - just because only one representative of a lineage is shown, does NOT mean there has not been a lot of branching within that lineage - BEWARE OF INCOMPLETE SAMPLING - the LINEAGES of all living (extant) species have been evolving for the EXACTLY SAME AMOUNT OF TIME because they all go back to a shared ancestor - every species is a mixture of ancestral and derived traits Unit 2: Class 8: An introduction to microevolution and population genetics Microevolution: small scale processes operating within populations to CHANGE ALLELE FREQUENCIES Macroevolution: large scale patterns of change, including the ORIGIN OF NEW SPECIES Extrapolationists - macroevolution is just microevolution extrapolated over long periods of time - define evolution as nothing more than a change in allele frequencies Macroevolutionists - there is more to macroevolution than just microevolutionary (population level) processes - multi-level or hierarchal selection: natural selection operates at multiple levels, including among organisms in populations, but also within genomes, among groups etc. Mendel's Laws of inheritance First law: Segregation - alternative versions of a gene account for variation in inherited characters, dominant alleles are expressed over recessive alleles, the two alleles for each character segregate during gamete production Second law: Independent assortment - during gamete formation the segregation of the alleles at one locus is independent of the segregation of alleles at another locus - Mendelian genetics allows predictions about what happens to alleles within families or a single cross, when you include natural selection and genetic drift, you can have population genetics, which allows predictions about what happens to alleles in entire populations across many generations Population: for sexual species, a group of interbreeding individuals and their offspring Locus: the physical location of a gene on a chromosome Alleles: alternate forms of a gene Gene pool: set of all copies of all alleles in a population that could be contributed to the next generation Frequency: the proportional representation of a phenotype, genotype, gamete or allele in a population Genotype: the set of genes possessed by an organism Phenotype: the physical expression of the genotype (in combination with the environment) Homozygote: an individual with the same two alleles at a given locus Heterozygote: an individual with two different alleles at a given locus The Hardy-Weinberg Equilibrium Principle - based on one locus with two alleles - if nothing is changing allele frequencies then the allele frequencies will NOT change Example - if you have two allele frequencies 0.6 (A) and 0.42(a) 2 - random mating to get genotype frequencies (0.6 = 0.36), (0.4 =0.16), (0.6X0.4=0.24X2 = 0.48) - these mice grow up and mate with the allele frequencies 0.6 and 0.4 - Dominant DOES NOT CHANGE HOW COMMON ALLELES ARE!!!!!! - ANY starting allele frequencies will stay in 'equilibrium' as long as - p + 2pq + q = 1 Conclusion #1 A. If the idealized conditions are met, then the allele frequencies will NOT change from generation to generation B. Equilibrium will be reached after just ONE generation of random mating under idealized conditions Conclusion #2: A. Under idealized conditions, if the allele frequencies are given by p and q, then the genotype frequencies can 2 2 be calculated by multiplying allele frequencies (p :2pq:q ) Allele frequencies: p + q = 1 2 2 Genotype frequencies: p + 2pq + q = 1 - allows you to calculate allele frequencies if you know the genotype frequencies - finding a deviation from equilibrium means MICROEVOLUTION is happening - ex. gene flow, mutation, genetic drift Unit 2: Class 9: Mutation - the source of genetic variation - Weismann: germline and somatic lines are separate, only changes to copy of genes in germline are passed on - mutation by itself is NOT a powerful force DNA - involved in transformation of bacteria from non-virulent into virulent strains - DNA amount is constant within species and is used by viruses to alter host cells - contains a 1:1 ratio of purine (AG) to pyrimidine (TC) [A=T, C=G] - has a double helical structure - DNA --> RNA --> Protein - there is a feedback from protein to DNA altering the expression of genes - new genes can arise from RNA Mutations - errors in the genetic system - occur because the genetic system and its quality control and correction mechanisms are not perfect - source of new genetic variation, without them evolution would not occur - mutations occur without any regard for any consequences, good or bad - types of mutations - 1. Point mutations (new alleles) - 2. Gene duplications (new genes) - 3. Chromosomal mutations (new gene order) - 4. Polyploidy (genome duplications) - the effects of mutated genes may occur in any part of the body, but it is only relevant in evolution if it occurs in a copy of a gene FOUND IN THE GERMLINE (passed onto offspring) Point Mutations - Substitutions - Transition: purine to purine, pyrimidine to pyrimidine - Transversion: purine to pyrmidine or vice versa - Synonymous (silent): no change in the amino acid specified - Non-synonymous (replacement): changes the amino acid specified - sickle cell anemia - Frame shifts and stops (amino acid is stop codon) Chromosomal mutations - mostly change the ORDER of genes and their RELATIVE LOCATION to the regulatory regions - Inversions: switch order within a chromosome - Translocations: a piece of one chromosome breaks off and joins another chromosome, can be reciprocal or non-reciprocal Important Concepts - inversions, breakages, and fusions change gene order rather than generating new alleles or genes - chromosome number is a very flexible character with lots of breakages, fusions and even duplications - related species can have quite different chromosome numbers, BUT they will have SIMILAR BANDING PATTERNS Gene duplications - can lead to NEW GENES - occur by unequal crossing over (same mechanism can result in deletions) Polyploidy - can occur by hybridization or errors of meiosis/germline mitosis - adds a second copy of the entire genome, and thus of every gene - very common in plants - can create a new species instantly Name Description Cause Significance Point mutations - base pair - chance errors during CREATE NEW ALLELES substitution in DNA synthesis or repair of sequences DNA Chromosome - rearrangement or - breaks in DNA CHANGE PROXIMITY mutations fusing of caused by radiation or OF GENES TO EACH chromosomes other factors OTHER AND TO segments, altering REGULATORY gene order REGIONS Gene duplications - duplication of a - unequal crossing ALLOW DUPLICATE short stretch of DNA, over during meiosis GENES TO DIVERGE IN creating an additional FUNCTION copy Genome duplications - addition of a - hybridization or DUPLICATE ALL (polyploidy) complete set of errors in GENES CAN CREATE chromosomes meiosis/mitosis NEW SPECIES 2N --> 4N Mutations and Hardy-Weinberg - one locus, two alleles A(0.9) and a(0.1) - A is wild type and a is recessive loss of function mutation - A is converted to a by mutation at the very high rate of 1/10 000 and there are no mutations that convert a back to A - Genotype frequencies: 0.81 - 0.00001, 0.18, 0.01 +0.00099 - after 1000 generations, the frequency of A changes from 0.9 to 0.81 - it causes variation but it DOES NOT change allele frequencies very quickly, IT IS A WEAK FORCE Mutation rates - a measure of how commonly new mutations occur per unit time - mutation rate = "mu" - mutation rates vary depending on the species because of sexual vs. asexual reproduction, generation time, exposure to mutagens, differences in repair efficiency - males contribute more to the gene pool than females do, because they have a much higher number of cell divisions (<400 vs. 24) Important Concepts - most mutations are NOT harmful, most are NEUTRAL, however, those that affect fitness are usually deleterious - examples of beneficial mutation have been observed, and even a very low rate of mutation is sufficient for natural selection (source of genetic variation that evolutionary forces act on!!!) - whether a mutation is beneficial or harmful DEPENDS ON THE ENVIRONMENT Unit 2: Class 10: Natural Selection Artificial selection: substantial changes in phenotypes generated using only existing genetic variation plus whatever variation is added by mutation Darwin's Postulates - if all of his postulates are true, natural selection will occur 1. Variability - individuals within populations are variable 2. Heritability - the variability is at least partly heritable - offspring tend to look more like their parents 3. Overproduction - not everyone survives and reproduces AND some are more successful than others 4. Survival/Reproduction due to Traits - the differential survival and reproduction of individuals is associated with heritable variation among individuals - there is a trait determining survival Consequences of Darwin's Postulates - individuals with traits that promote improved survival and/or reproduction relative to the rest of the population will leave more offspring, thus the alleles that enhance the survival and reproduction of these individuals will be passed on more frequently than alleles that do not Natural selection: non-random differences in survival and/or reproduction among individual entities on the basis of differences in heritable characteristics The Concept of Darwinian fitness - individuals who survive and reproduce the most have the highest fitness IT DOES NOT refer to those who lift weights or jog - if the differences that contribute to higher reproductive success are NOT heritable, they are IRRELEVANT to evolution - fitness is NOT an after the fact situation, you can't just decide you want something - fitness can be PREDICTED based on traits that would be expected PRIOR to improving survival and production in a particular environment - ORGANISMS DO NOT EVOLVE POPULATIONS DO!!! Important Concepts - natural selection can result from differences in SURVIVAL or RATES OF REPRODUCTION due to HERITABLE VARIATION among individuals WITHIN A POPULATION - what matters is NET REPRODUCTIVE SUCCESS from one generation to the next - i.e. the relative reproduction of one allele over another one Consequences of Selection - natural selection can remove unfit alleles or INCREASE the frequency OR EVEN FIX new mutations that increase fitness Selection and Hardy-Weinberg - natural selection violates one of the conditions for Hardy-Weinberg equilibrium - natural selection can make it impossible to calculate genotype frequencies just by multiplying allele frequencies BECAUSE THEY WILL CHANGE BETWEEN GENERATIONS -natural selection can drive genotype frequencies away from the values predicted by HW equil. ** one generation of random mating will restore frequencies to HW equil. Mutation-selection balance - unfit alleles that are selected against can still persist in the population if they are generated often enough by mutation - the frequency of a deleterious recessive allele at equilibrium - q =√ Types of Natural Selection 1. Stabilizing - selection AGAINST both extremes (FOR intermediate phenotypes) - individuals with intermediate traits have the highest fitness - does NOT alter the population average (ONLY variance) - the one that is most common will be chosen REDUCING variance 2. Directional - selection AGAINST ONE EXTREME - moves population average in one direction - if new phenotypes arise by mutation or recombination, the population may continue to be pushed in that direction - purifying selection --> removing deleterious allele 3. Diversifying - section FOR BOTH EXTREMES (or AGAINST intermediate phenotypes) - creates a bimodal distribution - may lead to two divergent populations 4. Balancing - favours maintenance of more than one allele in a population (polymorphism) - can occur by HETEROZYGOTE advantage or FREQUENCY-DEPENDENT selection - heterozygote advantage = over dominance - Sickle cell anemia: homozygotes are LETHAL but heterozygotes protected against malaria - Cystic fibrosis: homozygotes LETHAL but heterozygotes protected against cholera Frequency-dependent selection - occurs when the fitness of an allele depends on its abundance - can be POSITIVE (fitness increases with abundance) or NEGATIVE (fitness decreases with abundance) Important Concepts - individual organisms under selection DO NOT CHANGE, they either live or die and reproduce or fail to - natural selection acts on organisms, but the consequences occur in POPULATIONS - natural selection acts on phenotype, but what matters is the resulting CHANGE IN ALLELE FRQUENCIES - natural selection is NOT all or nothing, even a SLIGHT reproductive advantage can have major affect on allele frequencies OVER MANY GENERATIONS - even a 1% advantage can have major effects long term - when a population evolves, it is NOT the case that all organisms in the population change in response to pressures, or that the population as a whole shifts together in the same direction - the effects of natural selection accumulate gradually, from ONE GERNERATION TO THE NEXT because only a non-random sample from each generation contributes offspring to the next gen. - natural selection only sorts among what is ALREADY THERE, it does NOT create new variation, most forms of selection deplete variation - what the organisms needs is IRRELEVANT, those with the highest heritable fitness will produce more offspring - selection pressures can change as the environment changes, what is fit now may not be fit tomorrow - natural selection is a process, it has NO FORESIGHT, GOALS, PLANNING - alleles get passed on more if they have a higher fitness than existing alleles NOW, any future potential benefit is irrelevant - populations adapt to conditions of the PAST not the future - it does not matter what the 'ideal' phenotype might be, only the traits with a higher fitness compared to alternatives of a population are relevant - natural selection DOES NOT produce perfection - some individuals surviving and reproducing are better than others due to heritable characteristics - natural selection does not actively select, it is more a force or an outcome - natural selection does not involve conscious thought about fitness, it just happens - genes do NOT WANT anything, either they are passed on or they are not based on their phenotypic effects - natural selection does NOT create genetic variation (mutation does), it is simply the differential survival and reproduction of existing heritable variation - NO INDIVIDUALS CHANGE WHEN A POPULATION EVOLVES, ONLY THE PROPORTIONS OF DIFFERENT ALLELES OR TRAITS DO - natural selection works generation by generation, not just over one generation Unit 2: Class 11: Genetic Drift Mutations - random with respect to fitness, change allele frequencies VERY SLOWLY - can contribute unfit alleles to the population - causes microevolution Natural Selection - non-random with respect to fitness, and can change allele frequencies very QUICKLY - can cause micro or macro evolution Genetic Drift - random changes in allele frequencies occur by genetic drift - represents SAMPLING ERROR , only a UNREPRESENTATIVE subset of the population passes on alleles - this sampling error is UNRELATED to fitness, and is due to chance NOT natural selection, ITS RANDOM - the direction of drift in each generation is UNPREDICTABLE - any previous change in allele frequencies are irrelevant to predicting future changes (coin toss) 1. Population bottlenecks: drastic reduction in size, which can be due to dumb luck, REDUCES variation 2. Founder effects: a subset of the original population - reduced allelic diversity when you continue to take a subset of the previous subset (birds on islands) 3. Gamete sampling error: unrepresentative subsample of gametes is chosen to form zygotes - 0.6A1, 0.4A2 --> 0.7A1, 0.3A2 (changes allele frequencies due to unrepresentative sample) - FEWER samples means MORE error, a larger population is LESS IMPACTED - drift can occur in more than one way, but always involves a random sub sampling of the alleles in the population, which can CHANGE allele frequencies (cause microevolution) but it does NOT lead to adaptation Fixing or losing alleles by chance - if no other forces are acting, eventually a neutral allele will become fixed or lost - i.e. Drunkards walk - if they walk long enough either they will be safe or die - deleterious alleles can be fixed if drift is strong enough - at any given time the probability of a neutral allele becoming fixed by chance is equal to its current frequency (the more common an allele is, the more likely you will get that allele in a sample) - in a larger population, an allele takes longer to fixate - an the straight line, an allele has been fixed, so the other allele for that gene must have been lost!! - CANNOT BE HETEROZYGOUS Likelihood of fixation by drift - if an allele has just risen by mutation its frequency and thus the likelihood it will reach fixation, will depend on its population size - 1/2N - for an allele that is already present in the population, x is number of copies - X/2N - drift is much more influential in SMALL POPULATIONS where natural selection is important in LARGE POPULATIONS Effective population size - absolute population size is total number of individuals, while effective population size is the number of people that are contributing alleles to the next generation Drift among populations - genetic drift reduces heterozygosity WITHIN a population but INCREASES VARIATION AMONG POPULATIONS - the overall average frequency of the allele AMONG populations does NOT change, but the frequency of heterozygotes goes to zero as different populations become fixed or one or the other Unit 2: Class 12: Gene flow and Inbreeding Gene Flow - gene flow is the movement of alleles from one population to another, causing populations to become more alike - if you have mixing between two populations they become more homogenous to one another, what matters is the MIGRATION of ALLELES - it can occur in several different ways: dispersal of gametes (pollination by bees), dispersal of zygotes (wind dispersed seeds), dispersal of larvae or migration of fertile adults ** a large source population has more of an effect on a smaller population, than a small population has on a large on - the allele frequencies on either island (mainland vs. coast) will become more similar to one another - gene flow is a HOMOGENIZING force and REDUCES VARIATION AMONG POPULATIONS - gene flow can increase genetic variation within a population by adding alleles from another population - gene flow can make it impossible to calculate genotype frequencies b/c no allele frequencies Gene flow in evolution - you have to stop gene flow in order speciation or a NEW SPECIES to occur - without any opposing force migration will equalize the allele frequencies among populations at a rate dependent on how fast they migrate - this process can undermine the process of natural selection unless there are barriers to gene flow - i.e. The Great Wall of China - the wall can cause species to diverge relative to those that can still spread their alleles across the wall - gene flow causes microevolution because it changes allele frequencies within a population, but it PREVENTS divergence between two or more populations Inbreeding - inbreeding is the non-random mating of genetic relatives with each other, to concentrate alleles within a lineage - during inbreeding, there is a reduction in the number of heterozygotes, and you get the two dominant genotypes, however, the allele frequencies do NOT change, violating H-W eqn ** Genotype frequencies change during inbreeding, allele frequencies DO NOT - by increasing homozygosity, it can bring deleterious recessive alleles together, exposing them to more selection - inbreeding depression = reductions in fitness from the combination of unfavourable alleles due to inbreeding - inbreeding INCREASES MORTALITY RATES Important Concepts - inbreeding does not cause microevolution because it does not change allele frequencies, it ONLY changes genotype frequencies, reduced heterozygosity, and exposing deleterious RECESSIVE alleles within a population Unit 2: Class 14: Case Study - Microevolutionary processes in action Micro-evolution in Darwin's finches - you can look at genetic variation to find out the source population b/c of the Founder Effect - Adaptive radiation: divergence of one ancestral species into several descendant species that occupy different niches (variability in all the diets!!) - you can manipulate the expression and get different variation in beaks by changing developmental regulatory genes - if you have gradual variation it will lead to a species being able to eat different sized pinecones or seed - however, the changes are very fast from an evolutionary viewpoint and can be REVERSED - natural selection and other micro-evolutionary processes DO operate significant and observable effects in natural populations - evolution is predictable short term, but long term it CANNOT be predicted due to environmental changes Unit 3: Class 15: Direct and indirect evolution and the origin of complex adaptations - adaptation is a characteristic that enhances the survival and/or reproduction of organisms that bear it, relative to the alternative, that has evolved through natural selection - adaptation is NOT the change undergone by an individual organism within its lifetime in response to external conditions (acclimation!!) Direct Adaptive Evolution (Point A to Point B) - individuals within the population with the fittest heritable traits leave more offspring - the distribution of the descendant population is skewed towards fitter traits - mutation introduces new variation, some of which is beneficial - for direct evolution to work all intermediate forms MUST be functional (you cannot have half an eye) - complex phenotypes arise, intermediate traits may be found in fossil record OR still occur in modern species - driven by directional selection Indirect Evolution - trait evolves through a very complex route where it undergoes many changes - the intermediates must be functional for SOMETHING and they must be BETTER than the alternatives - past function and current function does NOT have to be the same as long as it provides an advantage Mechanisms of Indirect Evolution Exaptation (co-option) - co-opted for a given function, but was originally evolved for a different function (fit because of its existing form) - i.e. duct tape - used to tape ducts, now we use it for everything - it takes on a new function while still serving its previous function Possibility #1: A structure with an existing function switches to a new function, which it may perform poorly at first but can be modified by secondary adaptation (ear bones derived from jaw bones) - i.e. jaws migrate and shrink and become part of the ear, where they have completely shifted function Possibility #2: A structure with an existing function takes on a second function (F(x)1 + F(x)2) - i.e. keratin in scales, claws and skin, form early feathers, then complex ones involved in flight - FUNCTION CAN SHIFT MORE THAN ONCE, meaning exaptation can occur many times during the evolution of one single feature Possibility #3: A structure becomes modified for its existing function in such a way that movement into a new environment becomes possible, leading to further modifications of the structure for a new function - essentially, F(x)1 in Environment 1, becomes modified to F(x)2 which works in Environment 2 (fins developed into limbs, fish to tetrapods) Possibility #4: One organ serving two functions becomes specialized for one of them, leading to an organ with one function in its descendant. F(x)1a + 2a --> F(x)1b OR F(x)2b - i.e. tetrapod lungs (breathing) and teleost swim bladder (buoyancy) are both descended from a joint respiratory-buoyant organ in a distant ancestor Possibility #5: Multiple organs serve the same function, but then specialize and diverge into separate functions - i.e. multiple gill arches all serving support to pharynx but some separate to form jaw Possibility #6: An organ becomes reduced in its original function (vestigial) and then takes on a new function in this reduced form (F(x)1 ---> vestigial --> F(x)2 Possibility #7: A non-functional structure takes on a function (can begin as a structure that is simply a byproduct of other features (spandrel) OR the holes in the skull of birds) Duplication Gene Sharing Tinkering (brico
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