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EVOLUTION Evolution • process that transformed life on Earth from earliest beginnings to diversity seen today • unifying force for biology • encompasses small & large scale evolution –changes in gene frequency in a population from one generation to next Evolution • Responsible for pattern of relationships between species • as lineages evolve & split & modifications are inherited, evolutionary paths diverge • produces branching pattern of relationships • by studying inherited characteristics & historical evidence can reconstruct evolutionary relationships & represent them on a family tree • phylogeny Phylogeny of Animalia lineage Understanding Phylogeny • root of tree represents ancestral lineage • tips of branches represent descendents of that ancestor • as move from root to tipsmove forward in time Pre-Darwin Theories • supernatural intervention • naturalistic models – based on logic & philosophy • 5th century BC-Greece – Plato (428-348 BC) & Aristotle (384-322 BC) – naturalistic view of origins – Anaximander • believed life arose in water & simpler forms preceded more complex ones • 1766 Georges Buffon – species represented by fossils could be an ancient version of groups of similar living species • Jean Baptiste Lamarck-1800s – relationship of fossils & current due to evolution – proposed that by using or not using a body part individual develops specific characteristics which get passed to offspring – inheritance of acquired characteristics Charles Darwin • formalized theory of evolution-1859 • The Origin of the Species – species living today descended from ancestral species • Diversity is based on different modifications of common ancestors from which varieties of a species descended • theory of how evolution works is natural selection Theory Development • gathered evidence during around-theworld voyage • HMS Beagle-18311836 • southern islands, South American coast & Australia • collected specimens of fossils & living plants & animals Theory Development • began to notice organism made adaptations to live in particular area • Galapagos Islands • noted presence of unique organisms that were like but different from organisms on mainland Theory Development • while on the Beagle, reading Principles of Geology by Charles Lyell (17971875) – suggested gradual changes in Earth’s surface • proposed slow & subtle processes over vast amounts of time led to enormous changes Theory Development • finches interested Darwin • normally seed-eating birds adapted insect-eating habits • subtle form & habit changes of these birds entrenched ideas of change over time • seemed reasonable that organisms had been transformed over time • new structures & habits had developed over time • began to think species descended from ancestral species Theory Development • 1840’s Darwin wrote essay on evolutionary theory • knew would create social furor • delayed publishing • middle 1850’s Alfred Wallace came up with similar theory • Wallace asked Darwin to take a look at his essay to see if it should be published • 1859 Darwin published On the Origin of Species by Means of Natural Selection • Darwin proposed viable mechanism on how species evolve • natural selection Natural Selection • best theory on how evolution occurs • based on two observations • Observation 1: species produce excessive numbers of offspring with limited natural resources • too many individuals for natural resources to support struggle for existence ensues • Observation 2: variation exists among individuals of a population • no 2 are exactly the same Natural Selection • Darwin inferred: • "As many more individuals of each species are born than can possibly survive, and as, consequently, there is a frequently recurring struggle for existence, it follows that any being, if it vary, however slightly, in any manner profitable to itself under the complex and sometimes varying conditions of life, will have a better chance of surviving and thus be naturally selected” Natural Selection • differential reproductive success • individuals with inherited traits better suited to local environment are more likely than lesser fit individuals to survive & reproduce • individuals who function best tend to leave more offspring Evolution • Earth with liquid water has existed for more than 3.6 billion years • cellular life has been around for at least half that time • organized multicellular life has existed for at least 800 million years • major life forms now on Earth are not all represented in past • major life forms of past-no longer living on Earth • all living forms come from previous living forms that were different Methods & Evidence Used to Date Evolutionary Events Radiometric Dating half-life decay of radioactive elements allows scientists to date rocks & materials directly allows scientists to estimate dates Stratigraphy & Fossil Record • • • • • • fossils-preserved remnants or impressions left by organisms that lived in the past – mostdiscovered in sedimentary rocks – each layer contains fossils that represent organisms that lived during particular part of Earth’s history – position in layers reveals age chronology of appearance in rock layers leaves observable signs of evolution oldest are 3.5 billion years old fossil record fits in with information obtained with other evidences fossil record is incomplete in part because organisms with shells or bony skeletons are more likely to be preserved than those without hard body parts Indicators of Common Ancestry • Comparative Anatomy –Homologous Structures –Analogous Structures –Vestigial Structures • Comparative Embryology • Molecular Biology Comparative Anatomy • anatomical similarities among species – same skeletal elements make up forelimbs of humans, cats, whales & bats • structural similarities-not surprising if all mammals came from common ancestor with prototype forelimb • similarity in characteristics that are the result of common ancestry –homology • Homologous structures ANALOGOUS STRUCTURES • insect & bat wings-analogous – separate evolutionary origins • superficially similar • have evolved to do same job • convergent evolution • though bird & bat wings are analogous as wings, as forelimbs they are homologous • birds & bats did not inherit wings from a common ancestor with wings, but did inherit forelimbs from common ancestor with forelimbs Vestigial Structures • • • • • • • • features that were adaptations for organism's ancestorevolved to be non-functional because organism's environment changed presence of such organs suggests common ancestry fish species that live in completely dark caves have vestigial, nonfunctional eye when sighted ancestors ended up living in caves, there was no longer any natural selection that maintained function of the eyes fish with better sight no longer outcompeted fish with worse sight today, these fish still have eyes not functional by-products of fishes' evolutionary history Comparative Embryology • closely related organisms have similar embryologic stages of development • all vertebrates have structures on sides of throat during embryological development-pharyngeal pouches • at this stage embryos of fish, frogs, snakes, birds & mammals look very similar • in fish-pouches become gills • in humans-part of ears & throat Molecular Biology • study of DNA & proteins • if two species have genes & products of genes-proteins with similar sequences of nucleotides must have been copied from common ancestor Molecular Biology • 98% of our DNA sequences match chimpanzees • 95% of amino acids of rhesus monkey hemoglobin are identical to human hemoglobin • 14% of lamprey hemoglobin is similar to human hemoglobin – conclude-rhesus monkeys are more closely related to humans than lampreys – chimpanzees are more closely related to humans than Rhesus monkeys Molecular Biology • master control genes – regulate groups of other genes during embryological development – very similar nucleotide sequences from fruit flies & mammals • conclusion-genes arose in common ancestor Human Evolution • did not evolve from chimpanzees • share recent common ancestor • neither chimpanzee nor human • humans are not higher or more evolved than other living lineages Natural Selection in Action • in some cases natural selection can be observed • data shows shape of finches' beaks on Galapagos Islands has tracked weather patterns • after droughtsfinch have deeper, stronger beaks • lets them eat tougher seeds Natural Selection in Action • human activity has led to environmental changes that have caused populations to evolve through natural selection • population of dark moths in 19th century England, rose & fell in parallel to industrial pollution Natural Selection at Work • How do antibiotics change the phenotype of bacteria and make them resistant? Population & Evolution • individuals do not evolve • populations do – group of individuals of same species living in same place at same time – smallest biological unit that can evolve • evolutionary impact of natural selection can only be seen in tracking how a population changes over time Population Genetics • tracks variations in populations over time • only traits that can be passed on in genes can affect evolution Inheritable Traits • not all traits are heritable • phenotype is result of genotype & environment • only genetic characteristics can be inherited • when population has two or more forms of one phenotype, they are polymorphic Genetic Variation • basic mechanisms of evolutionary change cannot operate without genetic variation • two main sources • Sexual recombination • Mutation Sexual Recombination • during ovum & sperm formation-meisois • genes are rearranged-prophase 1 • original chromosomes are different from chromosomes of parents Sexual Recombination • Sex can introduce new gene • when organisms reproduce sexually, some genetic "shuffling" occurs • bringing together new combinations of genes • might have bushy eyebrows & big nose since mom had genes associated with bushy eyebrows & dad had genes associated with big nose • combinations can be good, bad, or neutral • shuffling can introduce new combinations of genes in every generation • • • • • • • • • Mutations random changes in DNA beneficial, harmful or neutral do not supply what is needed mutation is unrelated to how useful it is not all matter to evolution somatic mutations occur in nonreproductive cells – will not be passed to offspring golden color on half of this Red Delicious apple was caused by somatic mutation seeds do not carry mutation Reproductive Cell Mutations • occur in reproductive cells-eggs & sperm • No change in phenotype – mutation might occur in stretch of DNA with no function – mutation occurs in proteincoding region, but ends up not affecting amino acid sequence of protein • Small change in phenotype – single mutation caused cat's ears to curl backwards slightly • Big change in phenotype – some phenotypic changes are caused by single mutation – can also have strong negative effects for organism or strong benefits – mutations causing death are lethal Causes of Mutations • DNA fails to copy accurately – most mutations that matter to evolution are naturallyoccurring – when cells divide-make copy of DNA • sometimes copy is not quite perfect • small difference from original sequence is a mutation • External influences – exposure to specific chemicals or radiation – cause DNA to break down – when cell repairs DNA, might not do a perfect jobcell ends up with DNA slightly different than original DNA mutation Sickle Cell Anemia Analyzing Gene Pools • to study evolution at population level focus is on gene pool – all alleles in all individuals making up a population at a given time • when frequencies of alleles in population changes over number of generations • Conclude-evolution is occurring Hardy-Weinberg Equilibrium • gene frequencies & genotype ratios in a randomly-breeding population remain constant from generation to generation • tells us recessive alleles do not tend to disappear in a population • if this were to happen population would soon become homozygous • therefore sexual reproduction alone does not lead to evolution • frequency of each allele in a gene pool will remain constant unless acted on by other agents Gene Frequency • measures frequency of particular gene relative to other genes at a particular locus in a population • if genotype contains two genes • total of 16 genes per locus in population of eight individuals: • Aa AA aa aa AA Aa AA Aa • frequency of A = 9/16 = 0.5625 frequency of a = 7/16 = 0.4375 Gene Frequency • use p to indicate frequency of A (dominant) • use q to indicate frequency of a (recessive) • frequencies for all possible genotypes = 1 •p+q=1 Hardy-Weinberg Equation Uses • (p + q)2 = p2 + 2pq + q2 = 1 • formula allows us to determine frequencies by observing the population • public health professionals use it to calculate how many people carry certain alleles for inherited diseases • Example-used to calculate percentage of population carrying an allele for PKUphenylketonuria When Hardy-Weinberg Law Fails to Apply • works in non evolving populations • genetic equilibrium • law says-gene pools remain constant over time • frequencies remain unchanged • if equation does not equal one-evolutionary change is occurring Circumstances that lead to evolutionary change •gene flow •genetic drift •natural selection • • • • • • • • Gene Flow occurs when individuals migrate & mate outside original population many species make up local populations members tend to breed within their group each local population can develop gene pools distinct from that of other local populations members of one population may breed with occasional immigrant from adjacent population of same species introduces new genes or alters existing gene frequencies in residents gene flow reduces genetic differences between populations gene immigration increases variability of gene pool Genetic Drift • change in gene pool due to chance • more impact in small populations Genetic Drift • two situations can shrink populations down to size where genetic drift can take place • bottle neck effect • founder effect Bottle-Neck Effect • result of disaster – flood, earthquake or fire • drastically reduces population size • certain alleles may be present at higher frequency in surviving population than in original one • effect usually reduces overall genetic variability of a population Founder Effect • few individuals colonize isolated area • smaller colony less genetic makeup will represent gene pool of larger population from which colony migrated Natural Selection • process by which variation, introduced by mutation, gene flow, or genetic drift, is refined • individuals with phenotypes not appropriate for an environment will not pass genes on • selection produces adaptations • if individuals having certain genes are better able to produce mature offspring than those without a gene then frequency of genes will increase • expression of natural selection is seen in alterations in gene pools Fitness • genotype's fitness depends on environment in which organism lives • how good particular genotype is at leaving offspring in next generation • not necessarily strongest, fastest, or biggest • includes ability to survive, find mate, produce offspring • ultimately leaving genes in next generation • if brown beetles consistently leave more offspring than green ones because of their color, you would say brown beetles have higher fitness Fitness • • • • caring for offspring producing thousands of young sporting fancy feathers that attract females these strategies increase fitness because help parents get more of their offspring into next generation Natural Selection • results from differential mortality and/or differential fecundity • differential mortality could be termed mortality selection meaning that certain genotypes are less successful than others in surviving to end of their reproductive period • evolutionary impact of mortality selection can be felt anytime from formation of new zygote to end of organism's period of fertility • another way of describing Darwin's criteria of fitness: survival Natural Selection • Fecundity selection • certain phenotypes (thus genotypes) may make disproportionate contributions to gene pool of next generation by producing disproportionate number of young • certain phenotypes are better able than others to contribute genes to next generation • By Darwin's standards they are more fit Natural Selection • population does not "want" or "try" to evolve • natural selection cannot try to supply what organism needs • selects among whatever variations exist in population • result is evolution • natural selection is sometimes interpreted as random • misconception • genetic variation that occurs in a population because of mutation is random • selection acts on that variation in a non-random way • genetic variants that aid survival & reproduction are much more likely to become common than variants that don't Outcomes of Natural Selection • Directional • Diversifying • Stabilizing Directional Changes • shift phenotypic curve of population by selecting in favor of extreme phenotypes • most common when local environment changes or when organisms migrate to new environment • when challenged species must adapt or become extinct • populations that survive often change enough to be considered a new species • most common-extinction Diversifying or Disruptive Changes • lead to balance between 2 or more contrasting phenotypic forms in a population Stabilizing Changes • maintain variations for a particular trait within a narrow range • selects against extreme phenotypes • stabilizing outcomes persist most of time • lead to well adapted populations Evolution Exercise • Beetles on a diet • Beetles of a different color Evolution Exercise Answer • difference in weight in first situation occurred due to environmental influences-low food supply-not because of a change in frequency of genes • example 1 is not evolution • because small body size was not genetically determined, this generation of small-bodied beetles will produce beetles that will grow to normal size if they have a normal food supply • changing color in example 2 is evolution • these two generations of the same population are genetically different • How did it happen?