Biology 1406 - Notes for exam 5 - Population genetics Ch 13, 14, 15 Species - group of individuals that are capable of interbreeding and producing fertile offspring; genetically similar 13.7, 14.2 Population - group of individuals of a species that live in the same place at the same time; local species ; high probability of actually interbreeding Gene pool - genotype of population - all of the alleles of all of the genes in the population Population gene pool can change over time - this is called organic evolution (microevolution) Change in gene pool measured as change in allele frequencies 13.9 Allele frequency is proportion of chromosomes with each allele of a gene Fig 13.9b Ex. Aa AA Aa aa aa 5 Diploid individuals, 10 a genes in population A = 4/10 so allele frequency =.40 a = 6/10 so allele frequency =.60 Since the proportion of sex cells produced is the same as the allele frequency, we can predict the next generation Fig. 13.9C This gives reference point to measure change Hardy - Weinberg Equilibrium: 13.9 - sexual reproduction (meiosis and fertilization) will not change allele frequencies - a population that is in equilibrium (allele frequencies stable) must have five conditions: - the population must be large - the population must be isolated, no migration into or out of population - no mutations occur to change allele frequency - mating is random - all individuals reproduce equally Any of these conditions that are not true can cause evolution (change in allele frequency)
Mutations : 13.8 - produce new alleles, genes, chromosomes - only inheritable mutations (in gametes) important - important as ultimate source of variation - too rare ( 1 in 100,000 genes / generation) to change allele frequency within short time Population not isolated: 13.11 - migration into or out of population, called gene flow - gene flow between populations can shift allele frequencies; increases genetic variation because different populations may have different allele frequencies - gene flow between populations maintains genetic similarity of populations - isolation (no gene flow) important in speciation Population small : 13.11 - large population size reduces the effects of chance, small populations effected more by chance events - genetic drift is random change in allele frequencies due to random events (matings, deaths, etc) - allele frequencies can change erratically to higher or lower frequency by chance - alleles can be eliminated (0%) or fixed (100%) - tends to reduce genetic variation within population Ex. a population with random deaths ----- and matings <----> AA AA 1 AA Aa aa AA AA AA Aa Aa aa AA Aa Aa aa Aa First generation Next generation allele frequency A = 11/16 A = 8/16 A 1 = 1/16 A 1 = 0 a = 4/16 a = 8/16
Several conditions can cause very small population size: Founder effect - a few individuals form a new population (colonizers or introduced) AA AA aa AA 2 --------------------------------> AA 2 Aa AA Aa aa population new population Bottleneck effect - a few individuals survive a catastrophe (severe unfavorable condition) - surviving individuals form population Genetic drift, founder effect and bottleneck effect can seriously reduce genetic variation in population and threaten long term survival - can not adapt - endangered species, zoo populations Mating is not random: sexual selection 13.14 - individuals (usually females) select mates not by chance - favors phenotypes that mates choose Ex. large showy tails in peacocks - leads to species specific traits, sexual dimorphism, reduces genetic variation Individuals do not reproduce equally: natural selection 13.12-13.17 - proposed by Charles Darwin as major cause of adaptation and change in living organisms - descent with modification 13. 2 - one allele gives individual a reproductive advantage over other individuals - these leave more offspring, produces adaptation; relative fitness - survival of the fittest - what does fittest mean? - alleles that allow the greatest reproductive success increase in frequency, other alleles decrease
How much better or more fit does an allele have to be to be selected? Ex. with a selection coefficient of.01 (individuals with mutant allele leave 101 offspring : individuals with normal allele leave 100 offspring) mutant allele will be fixed in population within about 2,000 generations What determines if an allele is better than another allele? How do these alleles become more common in population? Natural selection is the outcome of set of conditions: - individual phenotypes are determined by genetic material and are heritable (13.2) - variation exist in genetic material among individuals (different phenotypes) - some variations give reproductive advantage (includes surviving, gaining energy, etc.) - many more offspring produced than can survive Natural selection is powerful shaping force in living organisms: - explains more of observed details in biology than any other idea - unifying concept - acts on phenotypes (individuals) but selects alleles Does this affect recessive alleles and neutral variation? 13.16 - editing process, not creating process(structures are modified, not created) 13.2 Natural selection leads to accumulation of alleles that allow individuals to survive and reproduce the best in their particular environment - adaptive evolution Natural selection does not 13.17 - lead to perfection, only the best of available variation in current environment; often compromise among competing variables - plan ahead; selected in current environment (recessive alleles and neutral variation not selected against) - necessarily benefit the species; works on individual reproductive success, species may indirectly benefit - not goal directed, much variation in evolutionary trends 15.13 - limited by historical constraints: - can only act on existing variation due to random mutations; - chance events can drastically alter genetic structure of life
Natural selection can alter variation in population in three ways 13.13 - stabilizing selection favors intermediate phenotypes; typical in stable environment - directional selection favors one phenotype extreme over others; typical in changing environment ; tracking the environment - disruptive selection favors phenotypes at extremes of variation; balanced polymorphism 13.8 Five things can change allele frequency in gene pool and cause evolution: mutations increase genetic variation random chance, gene flow no consistent result genetic drift decrease genetic variation sexual selection species specific traits natural selection adaptation Can natural selection produce complex organs? - random mutations produce variation = chance -natural selection accumulates best variations through better reproductive success in current environment = not chance - small variations accumulate, build on earlier structure 15.12 ex. the eye Speciation - the origin of species ch 14 - species is a reproductive isolated group 14.1-14.2 - usually results from geographic isolation; allopatric speciation 14.4 - no gene flow between populations - mutations and natural selection make populations different - mutations produce reproductive barriers that prevent reproduction between groups How many mutations are necessary to produce a new species? - homeotic control genes very important, especially during embryonic development 15.11 - natural selection (and speciation by natural selection in different populations) modify existing structures - genetic variations allow different reproductive success What is exaptation? 15.12
Reproductive barriers prevent interbreeding Fig 14.3 A prezygotic barriers (before fertilization): - habitat isolation - live or breed in different places - temporal isolation - mating at different times - behavioral isolation - courtship rituals, odors, colors in sexual recognition and attraction - mechanical isolation - reproductive structures are not compatible - gametic isolation - sex cells do not survive or can not unite postzygotic barriers (after fertilization): - hybrid inviability - zygote does not develop properly - hybrid sterility - hybrids can not produce functional sex cells Speciation can occur without geographical isolation sympatric speciation 14.6 - increased number of chromosome due to errors in meiosis - polyploidy 4N, 6N, 8N - most common in plants which self fertilize 14.7 - new species produced in one generation Speciation is process of isolation and genetic change - if populations come back together before reproductive barriers completely develop, are they species? Ex. Ensatina salamanders, deer mouse 14.10 Adaptation and speciation (genetic change) tract the environment 14.11 - in rapidly changing environment change is rapid - punctuated equilibrium is rapid change (500,000 to 1,000,000 years) followed by long period with little change - in stable environment change is slow, stabilizing selection leads to slow change over long periods of time - most rapid change usually occurs in small isolated populations; smaller gene pool, more easily isolated, more environmental fluctuations New environments lead to rapid speciation - adaptive radiation 14.8 - new islands formed - reduced competition after mass extinction Evidence of change : fossil record 13.4-13.6 biogeography comparative anatomy and embryology evolutionary development 15.11 molecular biology (changes in DNA and proteins)
History of the study of evolutionary change: Carolus Linnaeus - 1760 s, introduced method of classifying living organisms - assumed fixed and unchanging species biologists of late 1700 s - concept of changing species Jean Baptiste Lamarck - 1800, first scientific hypothesis of evolution - based on inheritance of acquired characteristics Charles Darwin and Alfred Wallace - 1850 s, natural selection as most important force causing genetic change - modification through decent geneticists of late 1800 s and early 1900 s - genetics explain how traits are inherited Godfrey Hardy and Wilhelm Weinberg and other population geneticists of early 1900 s - mechanisms of change within populations modern synthesis 1940 s combined ideas of modification through descent, genetics, population studies, paleontology (fossils) into modern view of organic evolution molecular biologists - 1950 s to present compared genetic material (DNA) and proteins to show how species are related embryologists of 1800 s to present described how embryos develop from egg evolutionary development 1990 s to present how genes produce body design, in individuals and in the evolution of animals Ability of life to change and role of natural selection (adaptation) is the central unifying concept in biology
Biology 1406 Exam 5 Review Ch. 13, 14 (15) Define the terms: species, population, gene pool, allele frequency, evolution. Calculate the allele frequency for the dominant and recessive alleles, A and a, if there are 15 homozygous dominant, 10 heterozygous and 5 homozygous recessive individuals. Explain the Hardy-Weinberg equilibrium principle. Why is it important to population genetics and understanding how populations change? How does sexual reproduction (meiosis, fertilization) affect the H. W. equilibrium? List the conditions, or factors, that will upset Hardy-Weinberg equilibrium. Describe each of the following and give an example: (tell how each causes evolution) mutation gene flow genetic drift, founder effect, bottleneck effect nonrandom mating (sexual selection) natural selection Which of these increase and which decrease genetic variation in the population? What is the result or outcome of each of these? What does it mean when we say an allele is fixed? Describe the process of natural selection and give examples. Define the term fitness or fittest allele, and relative fitness, as it relates to natural selection. The selection coefficient is a measure of what? How large does it have to be? How is it calculated? List the four conditions that result in natural selection. What is the important outcome of natural selection? Define the term adaptation. Why do we say that natural selection is based on randomness (chance) but is not random? Explain why we say that natural selection is an editing process, not a creating process. List some cans and cannots for natural selection. Give an example of each of these: stabilizing, directional and diversifying selection. Describe the process of speciation by geographical isolation. Why are homeotic (control) genes important? What is exaptation? What is the difference between prezygotic and postzygotic barriers? List 7 ways, or mechanisms, that isolate populations (the reproductive barriers). In some cases numerous mutations will not cause speciation while in other cases only a few mutations cause speciation. Explain how this happens. What event can happen to disrupt the process of allopatric speciation between populations? Species specific courtship and mate recognition behaviors develop that reinforce other reproductive barriers. Is this stabilizing, directional or diversifying selection? Describe speciation by polyploidy. What is polyploidy? Why does it cause speciation? What is the difference between gradual divergence and punctuated equilibrium? What determines the tempo (rate) of evolution? What is adaptive radiation? Give examples of where and when it occurs. List several methods used to understand how life has changed and how living things are related. Describe the ideas, or contributions, of each of the following to the development of the field of population genetics: Carolus Linnaeus late 1700 biologists Jean Baptiste Lamarck Charles Darwin and Alfred Wallace early 1900 geneticists Godfrey Hardy and Wilhelm Weinberg the modern synthesis 1950 s to present molecular biologist 1800 s to present embryologist and 1990 s to present evolutionary development