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Western University
Biology 1001A
Tom Haffie

Genetic Variation - Dominance status of an allele describes its effect on the phenotype of heterozygotes, NOT whether the allele is helpful, harmful or neutral - Dominance affects evolution rate and outcome - Most genetic disorders are associated with recessive alleles because selection weeds out harmful dominant alleles while recessive alleles can “hide” in heterozygous individuals - Diploidy reduces the effectiveness of natural selection in eliminating harmful recessive alleles – they can be protected from selection by the phenotypic expression on the dominant allele - When a new, favourable allele is dominant, a small proportion of recessive alleles will be sheltered from selection and carried in heterozygous alleles - A beneficial dominant allele will start with a higher initial rate of frequency - A beneficial dominant allele is constrained by diploidy - Most genetic disorders are caused by recessive alleles for this reason - When a new, favourable allele is recessive, eventually selection will remove every copy of the harmful dominant alleles - Recessive allele is able to reach fixation and completely erases all deleterious dominant alleles – frequency of favourable allele will be 1 - Takes a longer period of time for a beneficial, recessive allele to become established - Other constraints on selection - Genetic correlations among traits – can’t select on one trait without seeing selection on other traits; certain loci are pleiotropic in their effects (ex. Darwin’s finches – beak depth, body size, beak length) - A particular trait may be associated with a fitness advantage (ex. peacocks – long, brightly coloured tail increases mating success, but attracts predators) - Organisms must balance fitness demands of different traits - Time – environment may be changing more rapidly than evolution can occur (ex. insects evolve earlier in the year such that fly-catching birds don’t have enough to eat during their peak season) - Amount of genetic variation – if there is very little genetic variation, there might not be enough to support evolution by selection (ex. cheetahs) - Why is genetic variation important? - Affects evolutionary potential and shapes individual fitness - Decreased fitness in individuals produced by mating of closely related parents (ie. inbreeding depression) - Quantifying genetic variation - Of an individual: proportion of heterozygous loci – the higher the proportion of loci at which that someone is heterozygous, higher their genetic diversity; inbreeding coefficient - Of a population: proportion of polymorphic loci; alleles per locus – more alleles, more genetic diversity 3 Modes of Natural Selection 1. Directional selection - Refers to selection against low fitness at one extreme of a distribution and favouring individuals with the beneficial trait at the other extreme - Individuals at on end of the phenotypic spectrum have the highest relative fitness - Shifts a trait away from the existing mean and toward the favoured extreme - If directional selection occurs over a long period of time, there will be a decrease in genetic variation - Because selection often fluctuates from one year to the next, genetic variation can be maintained by selection rather than removed from selection - Spatial variation in selection - Certain genotypes/alleles may be favoured in a particular environment (ex. yellow snails with stripes in areas with fields, brown snails in dark wooded areas) - Population as a whole maintains a high level of variation 2. Stabilizing selection - Individuals that have intermediate phenotypes of a trait will have highest relative fitness – extremes will be selected against - Stabilizing selection reduces genotypic and phenotypic variation and increases the frequency of intermediate phenotypes - Most common mode of natural selection - Ex. human baby birth weights – mean around 7 pounds; larger and smaller babies have lower chance of survival - Opposing forces of directional selection can sometimes produce an overall pattern of stabilizing selection - Directional selection can be imposed in opposite directions by 2 organisms - 2 different directional selections will result in stabilizing selection - Ex. Gall-making fly larvae mature inside “galls” on goldenrod plants. Some wasps parasitize these galls by laying eggs in their walls which the larvae then feed on – since the wasps are too small to penetrate large galls, their eggs are mostly seen in larvae maturing in small galls. On the other hand, some bird species open the galls to feed on the larvae and they prefer large galls. Galls of intermediate size are therefore favoured. 3. Disruptive selection - Opposite of stabilizing selection – extremes have higher relative fitness and are favoured - Bimodal distribution - Population evolves in direction of being polymorphic - Much less common than directional selection and stabilizing selection - Ex. During 1977 drought, finches with long bills were able to open cactus fruits and feed on the fleshy pulp; ones with deep bills were able to strip bark from trees and locate insects; birds with intermediate bill morphologies were not able to access either type of food, but are generally favoured during nondrought years Frequency-dependent selection - Fitness of a genotype/phenotype depends on how frequent it is in the population - Negative frequency-dependent selection – rare forms have a selective advantage/higher fitness just by virtue of being rare (ex. female drosophila prefer rare males – white eyes) - In many predator-prey systems, predators look for most common phenotype in the prey – rare prey have lower odds of being eaten - Rare forms eventually become more frequent  fitness of the rare form drops  other form will become more frequent - Genetic variation is maintained at a stable frequency – keeps polymorphic genetic variation - Positive frequency-dependent selection – common forms are favoured simply because they are more common (ex. brightly coloured poisonous frogs warn predators to stay away – greater prevalence increases likelihood that predators will stay away) - Does not result in stabilization – rare forms will be lost - Weeds out genetic variation Overdominance AKA Heterozygote Advantage - Heterozygous individuals are more favourable - Natural selection will favour heterozygotes when they have higher relative fitness, when diff alleles are favoured in diff environments, and when rare phenotypes have an advantage - Ex. MHC heterozygotes in immune system can defend against twice as many pathogens - Sweaty T-shirt study – males wore the same shirt for 3 days, females smelled them and rated attractiveness – females preferred males with MHC genotypes that were different than their own – if they mated, offspring would be heterozygous - Maintains genetic variation - Form of balancing selection – leads to a balanced polymorphism in which 2 or more phenotypes are maintained in fairly stable proportions over many generations More Evolutionary Forces  Agents of Microevolutionary Change pg. 379 Table 17.2 - What happens if we violate the assumptions of H-W equilibrium? - Mutation - Heritable change in DNA caused by random, spontaneous errors in DNA synthesis - Can have beneficial, harmful (deleterious mutations, lethal mutations), or neutral effects on fitness - Lethal mutations that cause death before reproductive age are eliminated from a population - If an allele is advantageous, it may be preserved by natural selection but once it has been passed on to a new generation, other agents of microevolution determine its fate - Not directed towards the needs of the organisms - Ultimate source of all new genetic/heritable variation – provides new alleles - New mutations exert little or no immediate effect on allele frequencies, but over evolutionary time scales, they have been accumulating in biological lineages for billions of years - Mutation rate tends to be so low that it is outweighed by other forces - Migration AKA Gene Flow - Movement of organisms or their gametes between populations – if the immigrants reproduce, novel alleles can be introduced into the new population - Gene flow happens all the time - Animals may leave the population in which they were born and reproduce somewhere else - Animals may act as dispersal agents and move plants to new populations (ex. blue jay drops an acorn in a different place) - This kind of migration refers to gene flow, not seasonal migration from north to south - If the gene pools are different, even a little gene flow increases genetic variation within a population by introducing new alleles and then tends to equalize allele frequencies between populations - If the gene pools are already similar, even a lot of gene flow will have little effect - Therefore, the evolutionary importance of gene flow depends on the degree of genetic difference between populations and the rate of gene flow between them - Genetic Drift - Opposite of migration - Tends to reduce genetic variability within populations - Unpredictable change in allele frequencies between generations caused by chance events (similar to chance of flipping heads and tails) - Chance deviations from expected results which cause genetic drift occur whenever organisms engage in sexual reproduction, simply because their population sizes are not infinitely large - Particularly common in small populations because only a few organisms contribute to the gene pool and any given allele is present in very few individuals - One allele becomes fixed at a locus, others lost - Due to random sampling errors, not fitness differences - Tends to increase allele frequency differences between populations – different alleles will reach fixation - Leads to the loss of alleles and reduced genetic variability - Population Bottlenecks – the population crashes in size due to disease, starvation, drought, etc. - Allele frequencies of the survivors are much higher, while other alleles are lost from the population - Ex. low levels of genetic diversity in cheetahs - Founder effect – a few individuals from one population go off and colonize a distant locality and start a new population - By chance events, some alleles may be missing from the new population, whereas formerly rare alleles may occur at relatively high frequencies - Nonrandom mating - Many organisms select mates with particular phenotypes and underlying genotypes - Assortative/disassortative mating vs. inbreeding/outbreeding - What happens when individuals are more likely to mate with others with the same genotype or a different genotype? - Assortative – individuals mate with other that resemble them at a locus (snow geese with white feathers mate with others with white feathers; tall women prefer tall men) - Disassortative – opposites attract (white striped individuals only mate with tan striped individuals) - Increases the frequency of homozygous genotypes – there will be fewer heterozygotes than predicted by the H-W model - Inbreeding – special form of nonrandom mating in which individuals that are genetically related mate with each other - Allele frequencies themselves don’t change, but genotypic frequencies do, increasing the frequency of homozygous individuals - Not a very strong evolutionary force because it doesn’t change allele frequency - Inbred populations (zoo populations, endangered species) often have high rates of some genetic disorders because of inbreeding (esp. when there are harmful recessive alleles) - These types of matings are discouraged in most human societies to reduce inbreeding and the production of recessive homozygotes - Selection, mutation, migration (gene flow), genetic drift, and nonrandom mating all take populations out of H-W equilibrium - Some generate heterozygote deficiency, other heterozygote excess - Not all these forces necessarily result in evolution, if evolution is defined as a change in allele frequency Sex - Sex IS NOT reproduction - Doesn’t always combine genetic information from 2 parents - Bacteria reproduce asexually - Plants can reproduce both sexually and asexually - A few vertebrates reproduce asexually (ex. fish species with only females produces diploid egg which divides) - Some insects can reproduce parthenogenetically or sexually - Asexual reproduction is the dominant form of reproduction - Sexual organisms can be dioecious - One individual is male and the other is female - Or simultaneously monoecious - Some species have both male and female function – can produce male and female gametes at the same time (simultaneous hermaphrodites) - Can form daisy chains where they act as females to the individual in front of them and males to the individual behind them - Or sequentially monoecious - Some species may be born one sex and change sex at some point in their lives - Switch from male to female can be due to reaching a specific relative body size (ex. all clownfish are born male and switch to female) - Mostly seen in fish - Size-advantage model of sex change - Large females may have a huge advantage over small females - Large males are not as successful as large females - Protandry – all individuals are first male and then develop female function - If it is opposite (males have more to gain by being large), there will be Protogyny - Why reproduce sexually? - Many reasons not to: - Finding, attracting, competing for mates - Sexually transmitted diseases and infections - Cost of meiosis – only giving haploid number to offspring - Cost of sons (“twofold cost of sex”)– only females produce eggs, females are the limiting resource  asexual female populations can grow much faster - Asexual females should quickly outcompete sexual females - So why do so many species do it? - Asexual species are rare – relatively new in terms of evolution – species that reproduce asexually still display courtship - Sex forms new combinations of alleles - Mutational explanations for sex - Over time, asexual populations accumulate harmful mutations - Asexual lineages will eventually go extinct - Ruby in the Rubbish hypothesis – sex continually re-creates genotypes with fewer or more harmful mutations - Ecological explanations for sex - Sex in constant vs. changeable environments - Lottery Ticket Principle (buying a lot of lottery tickets but giving them the same number) - In a constant environment, females are well adapted - When environments are changeable, offspring may encounter different conditions than the female – makes sense to produce diverse phenotypes and genotypes - Parasites place negative frequency-dependent selection on host genotypes - Red Queen Hypothesis - In co-evolution between hosts and parasites, parasites are very good at infecting hosts with the most common genotypes; advantageous for hosts to have rare genotypes - Sex (producing diversity of offspring genotypes) favoured in parasite-rich environments - Sex seems too complex to be favoured by natural selection – inefficient, costly, but incredibly successful - Long term advantage – purge harmful mutations - Short term advantage – the ability to respond much more rapidly to environmental change; hedge bets against a quickly changing environment (especially one that it rich in parasites) Sexual Selection - Sex has important consequences - Explains sexually dimorphic traits (traits expressed only by one sex), and traits seemingly incompatible with natural selection (don’t improve survivorship) - Traits can be favoured by sexual selection rather then natural selection – competition between members of one sex for members of the opposite sex - Increases their mating success, not their survivorship (ex. manes of male lions, long tails of male widowbirds) - Intrasexual vs. Intersexual Selection - Intrasexual – based on competition between members of the same sex; direct, aggressive competition between males for females - Males may use their large body size, antlers, or tusks to intimidate, injure, or kill rival males - Intersexual – based on interactions between males and females; possession of traits that are favoured by female choice - Males may produce otherwise useless structures simply as a result of females finding them attractive in the past (ex. long, showy tails of widowbirds) - Distinction between intra/intersexual can be blurry - Ex. Bower birds – intra = build bowers to attract females; inter = males sabotage each other’s bowers - Females are often pickier than males. Why? - Males are less selective and more interested in short-term - Comes down to anisogamy – unequal gamete size - Sperm is cheap to produce relative to an egg - Sex differences in parental investment determine which sex is choosy, and which sex competes - Selective forces in females on males vs. females - Which sex has higher AVERAGE fitness? - Which sex has higher POTENTIAL fitness? - What determines fitness of a male vs. fitness of a female? - Male world record – Moulay Ismael 888 children - Female world record – Mrs. Vassilyev 69 children - Females are limited by access to resources, not access to males - Parental investment and sex-role reversal - In some species, males provide much more care to offspring than females (women may lay eggs and abandon them) - Ex. male seahorses care for offspring in their pouches - If both sexes invest heavily in offspring: - As in humans - Which sex is picky and which sex competes? - In terms of long-term mating, both are choosy and both are competing - In terms of short-term mating, males are competing for access to females and females are more choosy - Some traits are important to both sexes - Intelligence, wit - Facial symmetry - Sex differences in mate preferences - Males are interested in traits that give information about fertility status – estrogen-related traits, waist-to-hip ratio, physical appearance indicating youth, health, and fertility, sexual investment - Females are interested in the degree to which men care for children, kindness to children, willingness to invest in offspring, wealth, emotional investment - Sexual selection happens because sex puts different selection pressures on males than it does on females - Sexual selection is the most probable cause of sexual dimorphism – differences in the size or appearance of males and females - Relative parental investment influences which sex competes, which chooses - Sex that invests more becomes limiting for low-investing sex - Sexual selection acts on sex with lower investment - When both sexes invest heavily in offspring, both sexes experience sexual selection - But still differ in short-term mating strategies, types of traits preferred, and nature of jealousy Species and Speciation - Macroevolution – processes involved in generation of biodiversity and the actual origin of species - Defining species – ecological, morphological, biological, phylogenetic, many others - Speciation – isolation, divergence, secondary contact - Why does it matter whether two populations are one species or two? - For purposes of conservation - Ecological Species Concept (ESC) - Defines species as a group of organisms that share a distinct ecological niche - Morphological Species Concept (MSC) - Identifies species by morphological similarity – Do they resemble each other? - If they look different, they will be classified differently - Distinct phenotypic clusters – anything in blue oval will be member of the blue species - Problems with MSC - Not always a clear line – how similar do 2 populations have to be before they are considered the same species? - Some populations contain a large amount of phenotypic variation - Some populations may be dimorphic, polymorphic - Doesn’t tell us anything about the evolutionary processes involved - Reverse problem – human eyes may not detect distinctions between species - Biological Species Concept (BSC) - Group of actually or potentially interbreeding organisms, reproductively isolated from other such groups - More explanatory power than MSC (shared gene pool) - Provides explanation as to WHY individuals in a species resemble each other - More objectively testable than MSC – when members of population A encounter population B in the wild, what happens? Do they populate/mate with each other? Are their offspring fertile? - If the answer is yes, they will all be considered one biological species whether or not they resemble each other - Ex. Ligers are not fertile, so lions and tigers are considered different species. In addition, if lions and tigers encountered each other in the wild, they would not breed. - Ex. Wild horses and zebras may interbreed in the wild, but their hybrids are sterile, so horses and zebras are considered different species - Reproductive Isolating Mechanisms – biological characteristics that prevent the gene pools of 2 species from mixing even when they are sympatric (occupying the same spaces at the same time) - 2 or more may operate simultaneously - Prezygotic Isolating Mechanisms – exert their effects before the production of a zygote - Temporal Isolation – species breed at different times - Ex. 2 species of California pine release pollen at diff times of the year - Ecological Isolation – species live in different habitats - Ex. lions lived in open grasslands and tigers lived in dense forests so they never encountered each other - Behavioural Isolation – species cannot communicate; many animals rely on specific signals to identify the species of a potential mate - Ex. male songbirds and male fireflies both have distinct courtship displays that the females are able to recognize - Mechanical Isolation – species cannot physically mate, often due to shape of copulatory organs - Ex. because of difference in floral structure, 2 species of monkey flower attract different animal pollinators - Gametic Isolation – species have nonmatching receptors on gametes; incompatibility between the sperm of one species and the eggs of another prevents fertilization - Ex. marine invertebrates, Drosophila, plants - Postzygotic mechanisms – operate after zygote formation; species are reproductively isolated if interspecific hybrids have a lower fitness than those produced by intraspecific matings - Hybrid inviability – zygote may be formed, but genetic incompatibilities prevent the hybrid from growing to maturity; rarely make it past embryonic development - Ex. geep - Hybrid sterility – hybrid is physically healthy, but incapable of producing gametes (evolutionary dead-end); results when the parent species differ in the number or structure of their chromosomes, which cannot pair properly in meiosis - These hybrids have zero fitness - Ex. mule, zebroid - Hybrid breakdown – F hybrids are healthy and able to breed with other 1 hybrids and both parental species but there are problems with the F 2 generation (generation produced by hybrids), such as reduced survival and fertility - Reproductive isolation is maintained because there is little long- term mixing of the gene pools - Ex. matings between Drosophila species – each generation experiences a higher rate of chromosomal abnormalities and harmful types of genetic recombination - Problems with BSC - Difficult to tell if two populations would interbreed with each other if they live in geographically separated environment - Useless when dealing with asexually reproducing organisms - Can’t be applied to extinct organisms – they aren’t around anymore to tell us whether or not they would have interbred with each other - Doesn’t apply to hybridization – when 2 species interbreed and produce fertile offspring - Ring Species – a situation in which a population of species has a range of habitats – adjacent populations can exchange genetic material directly, but gene flow between distant populations occurs only through intermediary populations - At one point in the ring, there are 2 defined species, but at other point species are able to interbreed - Ex. Blue and red are distinct species; the rest are interbreeding - Phylogenetic Species Concept (PSC) - Focuses on evolutionary history – the pattern of ancestry and descent - Species – the smallest possible group of organisms for which a unique set of traits can be defined (shared by all members of the group, but not by any others) - To be a distinct species, a population must have been evolutionarily independent long enough to evolve shared characteristic traits (morphological, behavioural, genetic) - Problems with PSC - Time consuming to generate a phylogeny - May hugely increase the number of species over what we currently recognize (any population with its own characteristic traits is identified as a species) - Defining species depends on what definition is used Speciation - Process by which species are formed - Generates biodiversity – without it, tree of life would be twig of life - Achieving reproductive isolation - Usually occurs in allopatry - Allopatric Speciation - Populations become isolated simply because they occupy non-overlapping regions - Physical isolation – ocean, glacier, highway; different selection processes or random changes such as genetic drift on each side of barrier - Divergence - Secondary contact and reinforcement – populations come back into contact from isolation but may or may not resume interbreeding and gene flow - Islands: Hotbeds of speciation - Islands are a special type of allopatric isolation – separated by oceans - Many species originated on islands - Island conditions are very conducive to speciation – isolation, small population sizes result in genetic drift, founder effects, different selection processes - Secondary Contact - If populations haven’t diverged much while separated, they resume interbreeding (hybrid swarm) - Barrier is transient, short-lived - Process of speciation collapses in this case - Alternatively, postzygotic isolating mechanisms may have evolved - Hybrids are less fit than parental forms - Direct selection favouring prezygotic isolating mechanisms (reinforcement) - Reinforcement is a way of avoiding the negative fitness consequences of secondary contact - Controversial whether postzygotic or prezygotic mechanisms usually evolve first - Speciation without allopatry? - Parapatric speciation – geographic regions are adjacent to one another - Less likely to result in speciation - There is still some possibility of gene flow - Less conducive to speciation than allopatric distribution - In very patchy environment, intermediate forms (hybrids) are selected against - Ex. Copper-tolerant and intolerant grasses on polluted and unpolluted soils – there is no habitat in which an intermediate form would be favoured - Selects for Prezygotic isolation to avoid the production of unfit hybrids (reinforcement) - Sympatric speciation – overlapping ranges - Rare in vertebrates - Polyploidy  instant speciation (occurs in many plants) - A normal diploid plant individual undergoes an error and produces diploid gametes, which can fertilize and combine with other diploid gametes resulting in a tetraploid - Tetraploid can fertilize itself and reproduce with other tetraploid individuals - Host race shift – 2 food sources, mating occurs at feeding grounds (ex. apple/hawthorn maggots) - Speciation is much more common in plants than animals - Speciation is reversible - If isolating mechanisms are no longer effective, we see the collapse of speciation and the resumption of a hybrid swarm - Ex. fish that preferred to mate with fish of the same colour can no longer discriminate colour because of pollution and have resumed random mating Phylogeny - Evolutionary history of a group of organisms - Attempts to reconstruct the pattern of evolutionary relationships among species - How we can classify organisms in a way that reflects how recently they shared a common ancestor - Presented as phylogenetic trees - Taxonomy – identification and naming of a species and their placement in a classification – an arrangement of organisms into hierarchal groups that reflect their relatedness - The Linnaean System of Classification – arranges organisms into a taxonomic hierarchy - Species, Genus, Family (group a genera that closely resemble each other), Order, Class, Phyla, Kingdom, Domain (Bacteria, Archaea, or Eukarya) - Part of traditional evolutionary systematics - Principles of Phylogenetics - Evolutionary relationships reflect how recently groups shared a common ancestor - Phylogenies are reconstructed using similarities because we cannot go back in time – more similar organisms are more likely to have shared a common ancestor - Similarity usually reflects recent shared ancestry (homology) - Often, phenotypic traits must be relied on as indicators of genetic similarity and divergence - Homology vs. Homoplasy - Homology is any similarity between characters due to common ancestry - Emerge from comparable embryonic structures and grow in similar ways during development - Homologous structures can differ considerably among species - Homoplasy occurs when characters are similar, but are not derived from a common ancestor - Phenotypic similarities that evolved independently in different lineages - Can be misleading similarity (convergence) or misleading dissimilarity (divergence) - Provide no info about shared genetic ancestry - Convergence - Unrelated species become more and more similar in appearance as they adapt to similar environments - Placement of the eyes is very similar in alligators and hippos - Body shapes and similar aquatic habitats of sharks and killer whales - Divergence - Two or more related species becoming more and more dissimilar - Caused by distinct and different selection pressures, occupation of different ecological niches of closely related species - Homologous traits support true phylogeny; homoplasious traits are misleading - What if we don’t know whether 2 traits are similar due to homology or homoplasy? - Cladistic analysis – only shared, derived traits (synapomorphies) are informative, only pay attention to certain similarities - Cladistics produces phylogenetic hypotheses and classifications that reflect only the branching pattern of evolution; ignores morphological divergence - Parallels the pattern of branching evolution for classification - Groups monophyletic lineages into “clades” - Cladistic approach is favoured by evolutionary biologists because of its evolutionary focus, clear goals, and precise methods – classifications produced by cladistic analysis often differ radically from those of traditional evolutionary systematics - Every species displays a mixture of ancestral characters (old forms of traits) and derived characters (new form of traits) - Derived characters provide the most evolutionary info because once a derived character becomes established, it can be seen in all of that species’ descendants; if they are not lost or replaced by newer characters, derived characters can serve as markers for entire evolutionary lineages - Synapomorphies - Shared by 2 or more groups - Likely that both groups inherited it from a common ancestor - Derived (of recent evolutionary origin) - Types of Traits - Synapomorphies – shared and derived - Symplesiomorphies – shared, but ancestral (trait was already present in common ancestor) - Autapomorphies – unique, derived within the group (only present in one species) - Which trait is ancestral, which derived? - How can we know what traits the common ancestor of a group had? - Use outgroup comparison (comparison with distantly related taxon) - Outgroup Comparison Analysis - Present in outgroup and ALL of ingroup = ancestral - Present in outgroup and SOME of ingroup = ancestral - Absent in outgroup, present in SOME of ingroup = derived in lineage of the subset of the ingroup that shows it - Present in outgroup by completely absent in ingroup = impossible to tell – could be ancestral or derived - Absent in outgroup, present in ALL of ingroup = impossible to tell - Assumption of Parsimony – simplest explanation is usually the best - Explanation that involves the smallest number of evolutionary steps is preferred - Phylogeny requiring the fewest evolutionary changes (appearances or losses of trait) is taken - Assumes similarity due to convergence (many evolutionary steps) is rarer than similarity due to common ancestry (one step) – it is unlikely that the same change evolved twice in one lineage - How to apply parsimony - Ingroup (group of interest) - Birds (no milk, no fur, wings) - Bats (milk, fur, wings) - Chipmunks (milk, fur, no wings) - Outgroup (more distantly related species) - Fish (no milk, no fur, no wings) - Synapomorphies – presence of milk, fur, wings - First tree is more parsimonious because it requires the fewest evolutionary steps - Phylogenetic Analysis - Only synapomorphies are informative - Not all similarities are homologies - Not all homologies are synapomorphies - Homoplasy is misleading - Creates similarity between distantly related groups (convergence) - Erases similarity between closely related groups (divergence) - Most parsimonious tree (fewest evolutionary steps) is usually the best - Phylogenies help classify life - Many currently recognized groups are not monophyletic - Monophyletic group – includes all the descendants of the group’s most recent common ancestor (blue) - Non-monophyletic (polyphyletic, paraphyletic) group – does NOT include all the descendants of the common ancestor (purple, orange); includes species of separate evolutionary lineages - Trace trees back to common ancestor to analyze - Are “reptiles” monophyletic? - NO, crocodiles have a much more recent common ancestor with birds than other reptiles - Reptiles are a paraphyletic grouping - If definition of reptiles were expanded to include birds, then it would be a monophyletic grouping - Other non-monophyletic groups - Prokaryotes – some Archaea are more closely related to Eukarya than Bacteria - Dicots (plants) - Fish – some more closely related to tetrapod vertebrates - Endotherms (birds and mammals) - Be able to identify monophyletic groups from phylogenies and how closely 2 groups on a phylogeny are related (where the branching points are in evolutionary time) Patterns in Macroevolution - The fossil record - Mode & tempo of evolution - Evolutionary trends: changes in body size - Changes in the Earth - Gradualism (Hutton) – the view that the Earth changed slowly over its evolutionary history - Catastrophism (Cuvier) – opposed gradualism; said Earth was changed by violent, catastrophic processes - Charles Lyell expanded gradualism to uniformitarianism – argued that the geologic processes that sculpted that Earth’s surface took place over long periods of time (volcanic eruptions, earthquakes, erosion, and the formation of glaciers) are exactly the same as the processes we see today - Because geologic processes proceed very slowly, it must have taken millions of year to mould the landscape of Earth’s current configuration - The Fossil Record - Paleontology is our primary source of data about the evolutionary history of many organisms - Most fossils are found in sedimentary rocks, which formed when rain and runoff eroded the land and carried fine particles of rock and soil downstream where they settled as sediments, forming successive layers over millions of years - The weight of newer sediments compressed the older layers into a solid matrix of sandstone or shale and fossils were formed within that layer - Fossils usually preserve the hard structures – bones, teeth, shells - Other fossils are moulds, casts, or impressions - Soft-bodied organisms may be preserved in environments where oxygen is scarce, amber (fossilized tree resin), glacial ice, coal, tar pits, or the highly acidic water of peat bogs - How old is that fossil? - Scientists can assign relative and absolute dates to geological strata (layers) and the fossils they contain - In general, youngest layers are on top - In a particular sedimentary stratum, the fossils represent organisms that lived and died at roughly the same time in the past - Geologists use the sequence of strata and their distinctive fossil assemblages to establish the geological time scale - Law of Superposition (Relative Dating) – whenever you are dealing with a sedimentary rock formation, fossils embedded at the bottom are more ancient in evolutionary time - Read rock strata from bottom to top - Tells us relative order of dates, not absolute dates - We can tell which organisms co-existed with each other - Radiometric dating – parent rock formation decays to daughter isotope at predictable rate; expressed in terms of half-life - Isotopes begin to decay as soon as they form at steady, known rates - Count half-lives to estimate absolute age (number of half-lives that have elapsed multiplied by length of each half-life) - Tells us how old rock is, and thus fossil age - Scientists can also use the ratio of C to C present in a fossil that contains organic matter to determine its age - Fossil record is incomplete and biased - Most species have never formed fossils – the 300 000 described fossil species represent less than 1% of the estimated number of species that have ever lived - Biases in the fossil record - Temporal – more likely to find a more recent fossil - Many fossils are destroyed by pressure from overlying rocks or geologic processes and disturbances - Taxonomic – some groups are more likely to fossilize than others (bones, teeth, shells) – even if 2 organisms were equally abundant at a time, there will be more fossils of the organism with hard parts - Habitat – vast majority of fossils are of marine species; only about 10% of living things actually live in the sea; marine environments are much more conducive to fossilization (bodies are preserved in silt) - Fossils rarely form in areas where sediments do not accumulate or where soils are acidic - New species can “appear” in the fossil record via anagenesis or cladogenesis - Anagenesis – accumulation of changes in a lineage as it adapts to changing environments; if morphological chan
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