BIOL_2400 lecture 16-exam

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Lecture 16 The nature of natural selection · 1. Natural selection acts on phenotypes, but evolution consists of changes in allele frequencies. o Natural selection only works on phenotypes, but the consequence results in a change in allele frequency · 2. Natural selection acts on individuals, but its consequences occur in populations. o The “unit of selection” debate: genes or individuals? § Most evolutionary biologists believe that the unit of selection is individuals (some individuals are better than others) however, the changes that happens occurs in gene frequency in a population over time § Some people believe that selection occurs at a gene level (a particular gene does better to produce more replicates of gene over time) · a gene may try to replicate itself at the cost of the individual o Gene is one level below individuals o Selection may also function at the group level (some trait is better for a group) · 3. Natural selection is backward looking, not forward looking. o Response to what the environment was like in one generation and which genes are passed on to the next generation in regards to environmental conditions at the time (natural selection selects traits for the current environment, not the future environments) · 4. Natural selection can produce new traits, even though it acts on existing traits. o o New trait is high oil content but is produced from acting on the original low oil content trait · 5. Natural selection acts on individuals, not for the good of the groups. o Altruism and the group selection fallacy (next week) · 6. Natural selection does cause adaptation, but does not lead to perfection. o Constraints on evolution o many structures evolved from existing ones · 7. Natural selection is nonrandom, but it is not progressive. o Selection does not always increase the complexity of a species · The red-queen hypothesis: the response to selection does NOT improve the survivorship of a species. o Selection prevents elimination and helps species to stay alive/afloat § Ex: parasites evolve mechanisms to overcome hosts evolution of evasion strategies o Evolution is NOT a ladder and imply no “progress” Adaptation: a definition

· A trait, or integrated suite of traits, that increases the fitness of its possessor is called an adaptation and is said to be adaptive · 1. it has a function · 2. it it results from natural selection and is fitness enhancing · 3. it often implies problem (environmental change) and solution (adaptation) o An adaptation occurs in response to something Important concepts regarding adaptation · Natural Selection is not the only mechanism of evolution, but is the only mechanism of Evolution that causes Adaptation. o Ex: genetic drift is a mechanism of evolution but it is random and does not cause adaptation · Demonstrating that the traits are adaptations has been one of the major activities of evolutionary biology since Darwin’s time. · However, a plausible hypothesis about the adaptive value of a trait is the beginning of a careful study, not the end. o When faced with a ‘trait’ that is being put to good use, one ought not to jump to the conclusion that the particular use is the reason the trait is present o May have adapted due to other reasons that are not as obvious · o “Giraffes have long necks to reach the tops of trees” is the classic explanation o However, in reality, giraffes have adapted long necks for fighting other males § Females also increased in neck length (even though they don’t need them) because they share the same genetic makeup as males o § It’s been shown that giraffes more frequently bend down to eat in trees, rather than using the full extent of their neck to feed at their maximum height o Shows how traits may have evolved for other reasons than what is most obvious · Mutualism between red-billed oxpecker and cattle? o Earl observation showed bird resting on ox and thought to eat ticks off of the ox (mutualism; bird eats ticks, ox provided safe environment for bird) o § Experiment; birds were allowed to rest on one group of cattle, and were not allowed to rest on another group counted ticks on both groups § Results show that in the first experiment the subjects without bird had more ticks, the second experiment showed that subjects without birds had less ticks, and the third experiment showed that cattle without birds had more ticks § Cattle with birds always had more wounds, and cattle without

birds always had more earwax § Turns out that bird is parasite (would drink the blood) not necessarily mutualistic relationship o Always need to test to prove the cause for the adaptation Tests for adaptation · 1. Engineering/physiological test: structure must indeed function in the hypothesized sense o Ex: the giraffe experiment previously described that measured eating heights of giraffes · 2. heritability test o Ex: giraffes inherit long necks from parent · 3. fitness test: results of natural selection o Ex: giraffes with longer necks are able to forage more and produce more offspring which also have longer necks · 4. Cause-effect test: the adaptive state must have evolved in the context of the hypothesized cause. Phylogenetic studies may help. o Ex: need to prove giraffes evolved longer necks when food located lower down was scarce Not-adaptive traits · 1. necessary consequences of physics or chemistry · 2. random genetic drift, instead of natural selection o Fixation of allele due to random genetic drift instead of being selected for · 3. correlated to an adaptive feature · 4. inherited trait, not yet adapted to the recently changed environment o Takes many generations for a trait to be adapted to environment environments change over time · 5. Exaptation Exaptation · A trait that evolved for other functions, or for not function at all, but which have been co-opted for a new use, is an exaptation · Bird feather was initially evolved for thermoregulation, and later was used for flight. Feather is an exaptation for flight · “Pre-adaptation” is not a good term natural selection does not prepare for future, selects traits for the current conditions Phenotypic plasticity · The phenotype of some organisms can vary dramatically depending on the environment. · The ability of an organism with a given genotype to change its phenotype in response to changes in the environment is called phenotypic plasticity. o Natural selection cannot occur from phenotypic plasticity because the genotype remains the same · The changing phenotype is not an adaptation; but phenotypic plasticity is itself a trait that can evolve.

o Allows organism to survive in changing environments which is a trait that would be selected for over time Why are adaptations imperfect? Evolutionary constraints on adaptation · Time lag · Historical constraints · Trade-off (e.g. life history, next week) · Genetic constraints o Limited diversity · Developmental/Physiological/Functional constraints o Development is highly regulated, thus changing the developmental network successfully is difficult Time Lag · It takes time for natural selection to operate. A population may be responding to a new environment and has not yet reached the maximum average fitness for a particular trait o May take several generations to reach peak phenotype for a certain environment · Historical contingency · (Almost) Every adaptative trait evolves from something else. · “Fish-like ancestor setup the foundation and limitation for the evolution of vertebrates” · “Five fingers, particularly the opposable thumbs, serve humans well. We have five fingers because some early tetrapods with five digits survived (probably by chance) and others did not.” Lecture 17 What is sex? · Sex (sexual reproduction) is the combining and mixing of chromosomes during offspring production o Meiosis and recombination o Fertilization two gametes come together to become diploid again · Sex (M/F) is different from gender (nonbinary) · Mechanisms of reproduction are diverse o Origin of sexual reproduction · Sex is common · Life originated without sex so meiosis and sexual reproduction must have evolved. · There are disadvantages to sexual reproduction which makes the evolution of

sex difficult to explain (“the paradox of sex”). o Costly (asexual reproduction is “better” superficially) · Experimental community with two species of lizards · · With sexual reproduction, the sex ratio is close to 50/50 · With clonal reproduction, all offspring are females Why have sex? · The fitness cost of sex o Only half of the sexual population can produce offspring o The production of males leads to ‘twofold cost of sex’ § o Evolution should favour asexual reproduction, however, most organisms use sexual reproduction (at least, partially) Benefits of sex · Sex can remove deleterious mutations efficiently o Majority of mutations are deleterious; being able to effectively remove them is important to the survival of the population · Sex can recombine beneficial mutations · Sex results in more genetic variation - faster evolution · · o Asexual; all offspring inherit mutation o Sexual; recombination allows only some individuals to inherit mutation whereas some may not inherit mutation · · o With sexual reproduction, an individual may inherit both mutations while another offspring inherits none o § Selection removes individual with both mutations § Mutations can be sustained in asexual populations due to lack of recombination which means all mutations are inherited to offspring via asexual reproduction Muller’s Ratchet · The process by which asexual genomes accumulate deleterious mutations · o Amount of mutations increase as they are continuously passed on and new mutations form eventually heavily mutated populations will be selected against, causing population to crash Evidence for Muller’s Ratchet; recurrent loss of sex in evening primrose ·

o Genome sequences from 62 individuals, 29 species · 10 transitions from sexuals to asexuals · More deleterious mutations in asexual populations compared to sexual populations o Older asexual populations have greater proportions of deleterious alleles compared to new populations · Number of deleterious mutations increased with time since transition to asexual Sexual reproduction: advantageous alleles · o In asexual reproduction, an advantageous mutation are often “bogged down” or not as effective because they are often attached to a sea of deleterious mutations o In sexual reproduction, recombination can separate advantageous and deleterious mutations recombination can put advantageous alleles together (very fit genotype that spreads in the population over time) § Sexual reproduction promotes the spread of advantageous alleles Advantage of sex · Sexual reproduction creates more genetic variations · Allows for faster evolution - response to changing environmental · Biological arms race: between hosts and parasites o Need to constantly outrun evolving parasites § In a competitive environment, need to evolve to stay in the same place § Other competitors/predators are evolving need to evolve quickly to keep up o ‘Red Queen Effect’ (VanValen 1973) · Advantage of sex: helps species out-run parasites o o In a highly infected area, we would expect host population to use sexual reproduction Costs/benefits · Benefits of sex o Purging of deleterious mutations o Combining beneficial mutations o Generate new genotype o Faster evolution with environmental change § More genetic variation by recombination = faster evolution (likely creating advantageous forms in a challenging environment) · Costs of sex o Two fold cost of sex o Search costs to find a mate o Reduced relationship with offspring o Risk of sexually transmitted disease

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