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Evolution • • • • • Evolution – the process by which each type of organism is descended from ancestors that were similar but not identical to it All life shares a common ancestry Darwin (and independently, his contemporary Alfred Wallace), proposed a mechanism for evolutionary change Many ideas about evolution pre-date Darwin Jean Baptiste Lamarck (1744 – 1829) was the first to propose a mechanism for evolution – Inheritance of acquired characteristics – living organisms modify their bodies through the use or disuse of parts, and these modifications can be inherited by their offspring Darwin and Wallace propose that evolution occurs by natural selection • • In 1858, Darwin and Wallace independently described a mechanism for evolution Darwin published On the Origin of Species by Means of Natural Selection the following year • • • • Darwin’s theory of natural selection included several important observations and conclusions: Observation #1 - Natural populations ( a population consists of all the individuals of one species in a particular area) of all organisms have the potential to increase rapidly – organisms produce far more offspring than can possibly survive Observation #2 - Nevertheless, the sizes of most natural populations and the resources available to support them remain relatively constant over time Conclusion #1 – Therefore, there is competition for survival and reproduction. Many individuals die young and fail to reproduce. • Observation #3 – Individual members of a population differ from one another in their ability to obtain resources, withstand environmental extremes, escape predators etc. (Variation) • Conclusion #2 – The most well-adapted (the “fittest”) individuals are the ones that leave the most offspring. Natural Selection – process by which the environment selects those individuals whose traits best adapt them to that particular environment. • Observation #4 – At least some of the variation among individuals in traits that affect survival and reproduction is due to genetic differences that can be passed on from parent to offspring. • Conclusion #3 – Over many generations, differential, or unequal, reproduction among individuals with different genetic makeup changes the overall genetic composition of the population. Evolution occurs as a result of natural selection. Summary of Darwin’s Theory of Natural Selection • • • • • Overproduction – each species has the capacity to produce more offspring than will survive to maturity Variation – individuals in a population exhibit variation (each has a unique combination of traits such as size, color, and ability to tolerate harsh environmental conditions) Limits on population growth – there is a limited amount of food, water, light, growing space, and other resources available to a population, therefore, organisms must compete for limited resources Differential reproductive success – individuals that possess the most favorable combinations of characteristics are more likely to survive and reproduce Natural selection leads to adaptation – evolutionary modification that improves the chances of survival and reproductive success of a population Neo-Darwinism (Modern Synthesis) • • During the 1930’s and 40’s biologists combined the principles of genetics with Darwin’s theory of natural selection – Neo-Darwinism (Modern Synthesis) Emphasizes the genetics of populations rather than individuals Evidence for evolution 1. Fossil Record – remains or traces typically left in sedimentary rock (but also in bogs, tar, amber, and ice) by previous organisms – – – shows a progression from the earliest, single-celled organisms to the many single-celled and multicellular organisms living today most fossils are dated by their relative position in sedimentary rock may be dated using radioactive isotopes – each isotope has its own rate of decay and differ in their half-life (carbon-14, potassium-40, uranium-235) 2. Comparative Anatomy – related species demonstrate similarities in their structure – Homologous structures – body parts that have similar structure but may have different functions and appearance – probably were derived from the same structure in a common ancestor (example: pentadactyl limb of vertebrates - human arm, cat forelimb, whale front flipper, and bat wing all have a similar arrangement of bones, muscles, and nerves) – Vestigial structures – structures that have no apparent purpose - often are homologous to structures that are found in and used by other organisms (ex: whales evolved from four-legged ancestral mammals – whales do not have hind legs yet they have small pelvic and leg bones embedded in their sides) • Analogous structures – body parts that have the same function but very different internal anatomy (insect wing and bird wing) – may have developed as a result of convergent evolution – unrelated species share the same environment (similar environmental pressures) and independently evolve similar structures 3.Embryological development – all vertebrates have similar patterns of embryological development indicating a common ancestor 4. Molecular comparisons – similarities and differences in biochemistry and molecular biology of various organisms provides evidence for evolution - genetic code is universal – same bases make up DNA, same amino acids make up proteins, same use of mRNA, codons all code for amino acids the same way in all organisms - closely related species have similarities in DNA sequences Artificial Selection • • • • line of evidence that supports evolution by natural selection the breeding of domestic plants and animals to produce specific desirable traits (ex. different breeds of dogs) people are doing the “selecting” rather than the environment people have bred very different dogs, all descendants of the wolf Evolutionary Change in Populations • Evolution occurs in populations, not individuals • Individuals do not evolve in their lifetime • Evolutionary changes are those that occur from generation to generation Population – consists of all the individuals of the same species that live in a particular place at the same time Population Genetics • • • • branch of genetics dealing with the frequency, distribution, and inheritance of alleles in populations gene pool – sum of all the genes of all the individuals in a population including all the alleles for all the genes present in the population allele frequency – the percentage of a specific allele of a given gene locus in the population evolution of populations is best understood in terms of allele frequencies if the allele frequencies remain constant from generation to generation, then the population is not undergoing any evolutionary change and is in genetic equilibrium evolution can be defined as changes in gene frequencies that occur in a gene pool over time (change in the genetic makeup of populations over time) Hardy-Weinberg Principle • • • • a mathematical model that shows that, under certain conditions, allele frequencies and genotype frequencies in a population will remain constant no matter how many generations pass (in genetic equilibrium) Describes the gene pool of a nonevolving population Hardy-Weinberg principle shows that in large populations, the process of inheritance does not by itself cause changes in allele frequencies represents an ideal situation that probably never occurs in the natural world • Example: • In a wildflower population of 500 plants, 80% (0.8) of the flower color alleles are R and 20% (0.2) are r • What will the allele frequencies be in the next generation as a result of sexual reproduction? • Because each gamete has only one allele for flower color, we expect that a gamete drawn from the gene pool at random has a 0.8 chance of bearing an R allele and a 0.2 chance of bearing an r allele • Using the rule of multiplication, we can determine the frequencies of the three possible genotypes in the next generation. – For the RR genotype, the probability of picking two R alleles is 0.64 (0.8 x 0.8 = 0.64 or 64%). – For the rr genotype, the probability of picking two r alleles is 0.04 (0.2 x 0.2 = 0.04 or 4%). – Heterozygous individuals are either Rr or rR, depending on whether the R allele arrived via sperm or egg. • The probability of ending up with both alleles is 0.32 (0.8 x 0.2 = 0.16 for Rr, 0.2 x 0.8 = 0.16 for rR, and 0.16 + 0.16 = 0.32 or 32% for Rr + rR). • The process of sexual reproduction and creating the next generation have maintained the same allele and genotype frequencies that existed in the previous generation. • Genetic frequencies tend to remain constant generation after generation unless something disturbs genetic equilibrium • evolution does not happen automatically • first recognized by G.H. Hardy and W. Weinberg in 1908 • • Hardy-Weinberg principle can be expressed mathematically: p2 + 2pq + q2 = 1 so … frequencies of various genotypes in a population can be calculated as follows: ex. frequency of R 0.8 frequency of r 0.2 sum of frequencies must equal 1 p = frequency of the dominant allele (R) q = frequency of the recessive allele (r) p = 0.8 and q = 0.2 p2 + 2pq + q2 = 1 (0.8)(0.8) + 2(0.8)(0.2) + (0.2)(0.2) = 1 0.64 + 0.32 + 0.04 = 1 p2 = frequency of RR = 0.64 2pq = frequency of Rr = 0.32 q2 = frequency of rr = 0.04 • 1. 2. 3. 4. 5. Hardy-Weinberg principle states that five conditions must be met for genetic equilibrium to occur: random mating – each individual (each genotype) has an equal chance of mating no mutations large population size – not as likely to be affected by genetic drift as a small population no migration (no gene flow) – no exchange of genes with other populations (no movement of individuals into or out of the population) No natural selection – all phenotypes (and therefore genotypes) have an equal chance of surviving • Under these conditions, allele frequencies within a population will remain the same indefinitely • Used as a basis of comparison (if allele or genotype frequencies deviate from the values predicted by the Hardy-Weinberg principle, then the population is evolving) • Evolution occurs when Hardy-Weinberg equilibrium is disrupted by deviations from any of its five main underlying conditions – there are five major causes of evolutionary change: 1. Non random mating changes genotype frequencies – when individuals select mates based on phenotype (and therefore the corresponding genotype), evolutionary change occurs 2. Population size has an important effect on allele frequencies small populations are more subject to the effects of genetic drift – changes in allele frequency due to random chance – may result in the reduction or elimination of an allele regardless of whether or not it was beneficial or harmful • Effects of genetic drift: tends to reduce genetic variability within a small population genetic drift tends to increase genetic variation between different populations Examples of genetic drift: population bottleneck – population undergoes a drastic reduction in size and genetic drift occurs in small population of survivors (ex. as a result of a natural catastrophe) – only a few individuals are available to contribute genes to the future population – as population size increases again, gene frequencies may be very different from the original population – often results in the reduction of variation (ex. cheetah) founder effect – occurs when isolated colonies are founded by a few individuals from a large population (ex. group of migrating birds that get lost or are blown off course by a storm) – only alleles in the descendents will be those few that the colonizers happened to possess 3. mutations – mutations are inevitable even though they occur rarely – however, mutations are the source of new alleles on which natural selection can work – mutations provide the potential for evolution 4. gene flow occurs between populations and changes allele frequencies – generally increases variation within the population migration of breeding individuals (and therefore movement of alleles) results in gene flow – new alleles added to gene pool increases variation within the one population but reduces variation between the two populations tends to counteract the effects of natural selection and genetic drift (these tend to cause single populations to become more distinct) helps maintain all the organisms over a large area as one species 5. Natural Selection changes allele frequencies as species adapt to their environment – over time the proportion of favorable alleles increases in the population – as a result, the population becomes better adapted to its environment over time • Selection Pressure – anything that acts to disturb the Hardy-Weinberg equilibrium and cause evolution • • • Natural Selection mechanism of evolution in which members of a population that possess more successful adaptations to the environment are more likely to survive and reproduce causes differential reproduction among organisms with different alleles – fitness of an organism is measured by its reproductive success (not just whether or not it survives) acts on the phenotype (although phenotype is an expression of genotype) Three major types of Natural Selection 1. Directional Selection – favors phenotypes at one of the extremes of the normal distribution – – selects against both the average individuals and individuals at the opposite extreme over successive generations, one phenotype gradually replaces the others most common during periods of environmental change or when members of a population migrate to a new habitat with different environmental conditions • Stabilizing Selection – selection against the phenotypic extremes, average is favored – associated with a population that is well adapted to its environment, tends to reduce variation 3. Disruptive Selection (Diversifying) – favors two or more different phenotypes at the expense of the mean – – special type of directional selection – trend is in several directions instead of just one results in divergence of groups of individuals within a population • • Natural selection works on genetic variations genetic variation originates with mutations – sexual reproduction also greatly contributes to genetic variation (crossing over, independent assortment, random union of gametes etc) Coevolution • • • occurs when two species interact extensively and exert strong selection pressures on each other examples include predators and prey and organisms that live in symbiotic relationships (mutualism, commensalism, parasitism) as one evolves a new feature or modifies an old one, the other typically evolves new adaptations in response Natural Selection may lead to Extinction • • • • the death of all the members of a species occurs as a result of environmental change some species may be predisposed to extinction as a result of localized distribution and overspecialization Three major environmental changes may drive a species to extinction: – competition for limited resources with other species – introduction of new predators or parasites – habitat change and destruction • greatest cause of extinctions • humans are rapidly destroying habitats • climate changes have caused many extinctions (as a result of plate tectonics) Mass extinctions • disappearances of many varied species in a relatively short period of time over a wide area • fossil record reveals episodes of extensive worldwide extinctions – may have resulted from enormous meteorites hitting earth and kicking up enough dust to block out most of the sun’s rays – most recent catastrophe occurred about 65 million years ago and coincided with the disappearance of dinosaurs Origin of Species and Speciation • Species – definition for “species” has changed over time – biological species concept – current definition: groups of actually or potentially interbreeding natural populations which are reproductively isolated from other such groups and capable of producing fertile offspring – each species has a gene pool that is isolated from that of other species and each is restricted by reproductive barriers from interbreeding with other species Speciation • process by which new species arise – depends on two factors: – Isolation of populations – a population becomes reproductively isolated from other members of the species – there is little gene flow between the different groups – Genetic Divergence – during the period of isolation, the gene pools of the separated populations begin to diverge in genetic composition – they evolve sufficiently large genetic differences so they can no longer interbreed and produce vigorous, fertile offspring Two possible situations lead to speciation: 1. allopatric speciation – speciation that occurs when one population becomes geographically isolated from the rest of the species – population evolves as a result of natural selection and/or genetic drift – most common method of speciation – often results from physical isolation due to the constant changing of the earth’s surface • river shifting courses, glaciers migrating, mountain ranges forming, land bridges forming, etc. • may also result when a small population migrates and colonizes a new area away from the range of the original species • overall effect is to stop gene flow • speciation is more likely to occur if the isolated population is small – genetic drift (including founder effect) has more effect on smaller populations 2. sympatric speciation – a new species develops within the same geographical region as the parental species – more common in plants than animals – usually results from: ecological isolation chromosomal aberrations 1. ecological isolation – the same geographical area may contain two distinct types of habitats (distinct food sources, nesting places etc) – different members of a single species may begin to specialize in one habitat or the other – natural selection in the different habitats may result in speciation 2. changes in chromosome number may cause instantaneous speciation – – – – – polyploidy – common speciation mechanism in plants – possession of more than two sets of chromosomes may occur when a fertilized egg duplicates its chromosomes but does not divide into two daughter cells – all subsequent divisions may be normal and all cells are now tetraploid most tetraploid plants are healthy and vigorous and can go through meiosis gametes produced can only fuse with other gametes from tetraploid plants – cannot fuse with gametes from original parents occurs in plants because plants can self-fertilize or reproduce asexually • • allopolyploidy – occurs when plants of two different species interbreed to form a hybrid following fertilization, if polyploidy occurs (doubling of chromosomes because fertilized egg does not separate into two cells) then proper synapsis and segregation of chromosomes can occur Reproductive isolating mechanisms • structural and/or behavioral modifications that prevent interbreeding between two different species • prevents gene flow and preserves genetic integrity of each species • Premating isolating mechanisms (prezygotic barriers) – prevent mating between species 1. temporal isolation – genetic exchange is prevented between two groups because they reproduce at different times of the day, season, or year 2. behavioral isolation – elaborate courtship behaviors are recognized only by a member of the same species – common in birds – courtship behaviors are only understood by females of the same species 3. mechanical incompatibility (mechanical isolation) – occurs when structural differences in male and female genital organs prevent successful mating – in animals, male and female sexual organs may not fit together – in plants, flower size or structure may prevent pollen transfer between species 4. gametic isolation – if mating takes place between two species, the gametes may fail to combine – may be due to molecular and chemical differences • Postmating isolation mechanisms (postzygotic barriers) 1. hybrid inviability – if fertilization does occur, resulting hybrid may be weak or unable to survive (most spontaneous abort during embryonic development) 2. hybrid sterility – if the hybrid does survive, it will often be sterile (often because chromosomes fail to pair properly during meiosis) – ex. mules, ligers • Speciation may occur rapidly or gradually – two models have been developed to explain evolution as observed in the fossil record: 1. Punctuated equilibrium – suggests that the fossil record accurately reflects evolution as it actually occurs in the history of a species • evolution occurs with long periods of stasis (no evolutionary change) interrupted (“punctuated”) by short periods of rapid speciation • speciation occurs in “spurts” • few transitional forms exist in the fossil record because few transitional forms occur during speciation 2. Gradualism – more traditional view of evolution – evolution proceeds continuously over long periods of time this model says that the fossil record is missing transitional forms because the fossil record in incomplete Adaptive Radiation • • • • when a species gives rise to many new species in a relatively short period of time typically occurs when populations of a single species invade a variety of new habitats and evolve in response to the differing environmental selection pressures Darwin’s finches are a good example – encountered a wide variety of unoccupied habitats in the Galapagos Islands may occur after mass extinctions when the survivors fill empty habitats (ex – adaptive radiation of mammals after extinction of dinosaurs)