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Transcript
Lecture 22 Evolution Evolution • three key observations about life – 1. organisms are suited for life in their environments – 2. many forms of life share characteristics – 3. life is diverse • 150 yrs ago – Charles Darwin developed a scientific explanation for these observation • published his theory as the Origin of Species – from his work categorizing species as a member of the HMS Beagle • evolution = descent with modification Evolution • can also be defined as a change in the genetic composition of a population from generation to generation • pattern of evolutionary change is revealed by data taken from biology, geology, physics and chemistry History of Evolution • greek philosophers suggested that life might have changed gradually over time – but Aristotle – viewed species as fixed and said that life-forms could be arranged on a ladder of increasing complexity = scalae naturae History of Evolution • Carolus Linneaus – developed the binomial system for naming species • developed a nested classification system in contrast to Aristotle – grouped animals according to similar characteristics – groups known as class, family, genus – thought the similarities were due to God’s creation Lamarck • many people proposed that life evolves as the environment changes • Jean Baptiste de Lamarck proposed how these changes happened • published his theory in 1809 – year Darwin was born • he was wrong Lamarck: Use & Disuse • many people proposed that life evolves as proposed two principles at work: use and disuse & inheritance of acquired characteristics – e.g. the giraffe stretching his neck to reach the upper leaves of a tree produces a longer neck in subsequent generations – also thought animals evolve because of a drive to become more complex Evolution: Descent with Modification • Charles Darwin – 1809-1882 • naturalist trained at Cambridge • recommended after graduation to Captain Robert Fitzroy – captain of the HMS Beagle Evolution: Descent with Modification • Beagle was embarking on a multi-year voyage around the world to chart coastlines • Darwin observed plants and animals in temperate regions of South America resembled species in the South American tropics more than they did species in the temperate regions of Europe • also studied fossils and saw similarities with living species Galapagos Islands • group of volcanic islands 900 km west of South America • fascinated by the unusual animals and plants • collected several kinds of birds • many were similar to each other but were different species • some traits were unique to specific islands • saw species on these islands that resembled the SA mainland but were seen no where else • used these specimens to formulate his theories on adaptations and descent Natural Selection • Darwin observed many examples of adaptations – inherited characteristics that enhance their survival and reproduction in the environment • linked adaptation to the environment and the origin of a new species Descent with Modification • Darwin never used the term evolution • used the term descent with modification • proposed that similarities between organisms was due to descent from a common ancestor in the remote past • the descendants lived in various habitats & developed adaptations to fit them to their habitat • Linneaus grouped organisms based on similarities but never recognized these similarities were due to descent with modification Natural Selection • Darwin made two observations: – 1. members of a population often vary in their inherited traits • SO: individuals whose inherited traits give them a higher probability of surviving and reproducing will leave more offspring – 2. all species can produce more offspring than their environment can support – many fail to survive • SO: the ability to survive and reproduce will lead to an accumulation of favorable inheritable traits • if these traits make your offspring more successful at coping with its environment = traits will persist over time = NATURAL SELECTION Natural Selection: a recap • 1. NS is a process in which individuals with certain heritable traits survive and reproduce at a higher rate than other individuals who don’t have those traits • 2. Over time, NS can increase the match between organisms and their environment • 3. if an environment changes (or if an individual moves to a new environment) - NS may result in adaptations – sometimes giving rise to new species. The science of evolution • • • • direct observations homology fossil record biogeography The science of evolution • direct observations – evolution observed using scientific studies – soapberry bug – feed on fruits of various plants using a hollow “beak” – beak length must match the depth at which seeds are found within their fruit – introduction of Golden Rain Tree with flatter pods resulted in the evolution of shorter beaks within the bug population The science of evolution • direct observations – evolution observed using scientific studies – drug-resistant bacteria – e.g Staphylococcus aureus The science of evolution • homology = similarity resulting from common ancestry – several types: • 1. anatomical • 2. molecular The science of evolution • homology: • 1. anatomical – closely related species share similar features even though they may have different functions – e.g. forelimbs of humans, cats, whales and bats – comparing early stages of development can reveal additional anatomical homologies – e.g. pharyngeal pouches gills in fish and parts of the ears and throat in mammals – some “leftover” structures can give us important information about evolution = vestigial structures Humerus Pharyngeal pouches Radius Ulna Carpals Metacarpals Phalanges Post-anal tail Chick embryo (LM) Human embryo Human Cat Whale Bat The science of evolution • homology: • 2. molecular – similarities in DNA, RNA and protein sequenes – many genes have acquired new functions – but other genes – e.g. ribosomal proteins – remain remarkably similar from bacteria to humans • although some organisms that are related share characteristics because of common descent – distantly related organisms can resemble one another due to convergent evolution – the independent evolution of similar features in different lineages • i.e. different common ancestors! – marsupials vs. eutherians – two lineages evolved separately but they experienced similar environments and underwent similar adaptations – these shared features are said to be analogous NOT homologous NORTH AMERICA Sugar glider AUSTRALIA Flying squirrel The science of evolution • fossil record – documents patterns of evolution • fossils also show how evolutionary changes have occurred in various groups of organisms – e.g. ankle bones of the dog is unrelated to those of the pig & deer • can also shed light on the origins of new organisms – e.g. pig and deer are closely related to one another Cetaceans and even-toed ungulates (a) Canis (dog) (c) Sus (pig) (d) Odocoileus (deer) The science of evolution • biogeography – the study of biodiversity over space and time • used to understand species distribution • species distribution – the manner in which a taxon is geographically distributed • is influenced by many factors – – – – Speciation Continental drift Extinction Glaciation Species Distribution • Species distribution can be affected by continental drift – 250 MYA – one land mass = Panagea – 200 MYA – Panagea began to break apart – scientists could predict where fossils might be found Genetic Variation in Evolution • smallest unit of evolution = microevolution – can result in a change in allele frequencies in a population over generations – one of the causes – natural selection – other causes: – 1. genetic drift: change in frequency of an allele in a population • e.g. disease resulting in the death of several individuals of a population – can decrease the frequency of an allele in that population – 2. gene flow: transfer of alleles from one population to another • e.g. migration of individuals into a new population Genetic Variation in Evolution • genetic variation – seen in individual variations – genetic variation is the differences in composition of an individual’s genes or other DNA segments (i.e. junk) – genetic variation produces variations in phenotypes – only the genetic component of a phenotype can have evolutionary consequences • phenotypes are not necessarily passed on • e.g. body builder changes his phenotype but doesn’t pass on the bigger muscles Genetic Variation in Evolution • variation within a population – characters that vary within a population: discrete or quantitative – quantitative characters – most heritable variation uses these characters • may can be measured – e.g. height, weight, IQ • vary along a continuum within a population -e.g. hair color, eye color • usually results from the influence of 2 or more genes on a single phenotypic characteristic = known as Polygenic Traits – discrete characters – classified on an “either-or basis” • e.g. purple color of a flower or a white color • usually determined by a single gene with different allele forms producing distinct phenotypes Genetic Variation in Evolution • variation between populations – species also exhibit geographic variation – differences in the genetic composition of separate populations – geographic variation can be observed as a cline – a graded or continuous change in a character along a geographic axis – e.g. gradual decrease in plant height as altitude increases Sources of Genetic Variation • formation of new alleles – arise through mutation – must occur in a germ cell to be heritable – most mutations occur in somatic cells and are loss when the organism dies • altering gene number or position – chromosomal changes that delete, duplicate or rearrange genes – may not necessarily be bad – e.g. crossing over in meiosis • rapid reproduction – increases the rate of mutations • sexual reproduction – produces genetic variability due to combination of gametes Hardy-Weinberg Principle • Developed independently by Godfrey Hardy and Wilhelm Weinberg – Hardy used to play cricket with Reginald Punnett • used to test whether evolution is occurring in a population • population = group of individuals of the same species that live in the same area and interbreed to produce fertile offspring • the population’s genetic make-up = gene pool – all copies of every type of allele at every gene locus in all members of the population – if only one allele exists for a gene locus = fixed allele within the pool (all individuals are homozygous – e.g. EE or ee) – if there is more than one allele – individuals may be homozygous (EE or ee) or heterozygous (Ee) Hardy-Weinberg Principle • each allele has a frequency or proportion in a population – if a gene has two alleles (i.e. E or e) – p is used for the frequency of one allele (i.e. the dominant), q is used for the other (i.e. the recessive allele) – Hardy-Weinberg principle states that the frequency of these alleles in a population will remain constant from generation to generation – if the gene pool is in equilibrium = p+q = 1 (NO EVOLUTION) – p2 + 2pq + q2 = 1 (i.e. 100%) Hardy-Weinberg Principle • • • • two alleles = E and e p = allele frequency for E q = allele frequency for e p2 = EE genotype frequency - expected frequency of the EE genotype in the population • q2 = ee genotype frequency - expected frequency of the ee genotype • 2pq = Ee frequency - expected frequency of the Ee genotype • p+q - must equal 1 (i.e. 100% of the population) – population is NOT evolving Hardy-Weinberg Principle • conditions for HW equilibrium: – 1. no mutations – 2. random mating – 3. no natural selection – 4. extremely large population size – 5. no gene flow in and out of populations Applying the HW Principle • HW principle is used to measure the frequency of the heterozygote – often hard to measure due to its similarity to the dominant homozygote • can be very helpful in diseases because it can predict the frequency of the carrier (heterozygote) • disease – PKU; 1 in 10,000 births – therefore q2 = 0.0001 – This means that 0.01% of the population are homozygous recessive • frequency of q allele = 0.01 (1% of population - square root of 0.0001) – This means that 1% of the population has a recessive allele in their genotype • frequency of p allele = 1.0-0.01 = 0.99 (99% of population) – This means that 99% of the population has a dominant allele in their genotype • 2pq = 2x0.99x0.01 = 0.0198 or 2% of the population Applying the HW Principle • • • • • • according to the HW principle - p2 + 2pq + q2 must equal 1 does it? p2 = 0.9801 q2 = 0.0001 2pq = .0198 0.9801 + 0.0198 + 0.0001 = 1 • so the frequency of PKU as a disease is stable within the population Applying the HW Principle • The frequency of the autosomal recessive disease cystic fibrosis is 1 in 1700 births • Calculate the allele frequencies of p, q, and the genotype frequencies of p2, q2 and 2pq and determine if the population is in HW equilibrium