Teacher Notes. Biology 30 Unit 4 Population Genetics

Transcription

1 Biology 30 Unit 4 Population Genetics General Outcome D1: Students will describe a community as a composite of populations in which individuals contribute to a gene pool that can change over time. A. Genetic Variation main source of variation in a population lies in the differences in the genes carried by chromosomes (Modern theory of evolution ) all the genes found in a population are called its gene pool a larger population will have a more diverse gene pool (more different alleles) a stable population will have a fairly constant, unchanging gene pool within this gene pool, the frequencies of different alleles remains the same this can be shown mathematically using the Hardy-Weinberg principle B. Hardy-Weinberg Principle developed independently by an Englishman Hardy) and a German (Weinberg) states in the absence of any outside forces, the frequency of each allele in a population will not change as generations pass genetic equilibrium this is useful for predicting allele frequencies in a population that are not evolving over time So Why Use it? no change = no evolution if this is true then the following 5 conditions must hold: 1. no mutations 2. no migration 3. large population size 4. random mating 5. no selection (artificial or natural) Symbols p = frequency of the dominant gene q = frequency of the recessive gene p 2 = frequency of the homozygous dominant trait q 2 = frequency of the recessive trait 2pq = frequency of the heterozygous dominant trait Formulae o p + q = 1 p 2 + 2pq +q 2 = 1 ****when working out the problems, you get q first from the problem Example 1 Four percent of the members of a population of pea plants are short. What is the

2 frequency of the recessive allele and the dominant allele? What are the genotype frequencies in this population? First what is q 2? Then calculate q. q 2 = 4% = 0.04 q = 0.2 therefore p = 0.8 (p + q = 1) homozygous dominant = p 2 =(0.8) 2 = 0.64 heterozygous dominant = 2pq = 2(.8)(.2) = 0.32 homozygous recessive = q 2 = (0.2) 2 = 0.04 total 1.00 Example 2 An island population of monkeys with striped fur has a recessive characteristic of fur with no stripes. Out of a population of monkeys, of them do not have stripes. Calculate the frequencies of each allele and each genotype. First what is q 2? Then find q. q 2 = 1260/14000 = 0.09 q = 0.3 (p + q = 1 or = 0.7) p = 0.7 homozygous dominant p 2 = 0.49 heterozygous dominant 2pq = 0.42 homozygous recessive q 2 = 0.09 total 1.00 C. Evolution evolution may be defined as: changes of allele frequencies within a population over time Hardy-Weinberg has the 5 conditions which must be met it provides a theoretical standard to compare real populations with the process of evolution is constantly taking place in our populations there are three agents of evolution 1. genetic drift 2. founder effect 3. bottleneck effect 4. gene flow 5. mutation 6. non-random mating 1. Genetic Drift A change in genetic makeup of a population resulting from chance. Usually in small populations Decrease or alters the frequency of alleles in a population 2. Founder Effect

3 genetic drift that results when a small number of individuals separate from their original population and find a new population allele frequencies of the new population may deviate as the new population expands. 3. Bottleneck Effect A dramatic population reduction in size Often temporary Significant genetic drift Allele frequency in survivors differ from the original population Caused by severe environmental event 4. Gene Flow movement of large numbers of a population either in (immigration) or out (emigration) many alleles may be lost or gained which affects the frequencies few populations are so isolated that they escape this gene flow 5. Mutations changes one allele into a different allele they are neither good nor bad - it all depends upon the environment chromosome mutations cause many problems (chromosomal number change i.e. Down Syndrome) gene mutations may be passed on via gametes and are the main reason for the process of natural selection (Darwin) 6. Non-Random Mating Sexual selection: favours the selection of any trait that influences the mating success of the individual Sexual dimorphism: traits favoured in sexual selection (differences in the appearance of male and females in a population and behavioral differences) o i.e. female mate choice and from male-versus-male competition D. Speciation another way of altering the gene pool a species is a group of similar organisms that can interbreed and produce fertile offspring caused by either geographic or reproductive isolation a population gets separated from the others and over time, mutations cause each population to evolve in different ways! Selective Pressures will Vary geographic barriers include water, mountains, canyons o Evolve in different directions varying habitat conditions, therefore different adaptations evolve reproductive barriers may mean incompatibility of the chromosomes, of different mating rituals or seasons

4 o Usually a result of geographic isolation o Organism in a population no longer mate and produce viable offspring (even after removal of the barrier) o Differences in mating habits, rituals, seasons etc. General Outcome D2: Students will explain the interaction of individuals in a population with one another and with members of other populations. E. Population Relationships populations affect each other in many complex ways there are symbiotic relationships there are also predator-prey relationships all species are in competition with each other over the available resources these resources are usually: food, water and living space this competition may be interspecific: where different populations are competing or intraspecific: where the competition is within the one population Gause s principle (competitive exclusion principle) states that if two populations are competing for a limited resource then one will be eliminated normally, populations can coexist within the same ecosystem an organisms niche is its role or its place within an ecosystem most organisms competing for a resource have a different enough niche that they may coexist Symbiotic Relationships are close relationships between two different species 1. Parasitism one organism benefits and the other is harmed every parasite requires a host organism usually it is a very specific host different types of parasites exoparasites - fleas, ticks, leeches endoparasites - tapeworms, liver flukes, protozoans social parasites - cuckoos 2. Commensalism one species benefits but the other is neither harmed nor helped examples: fox and caribou! shark and remora! orchids and trees 3. Mutualism both species will benefit from the relationship may be obligatory mutualism - each depends upon the other exclusively or

5 facultative mutualism - neither is wholly dependent upon the other examples plants and pollinators! bacteria and human intestines! ants and aphids Predation one organism feeding on another predators have many mechanisms for feeding, but prey also have many ways of avoiding predators if prey have no way of hiding out, then eventually both populations would die if prey had easy hideouts, then the predator population would die out if prey can hide temporarily, then both populations will cycle two types of hiding 1. camouflage - they blend in with their background 2. mimicry - they look like a different organism F. Succession the way in which populations and communities change over time primary succession is when there was no previous life secondary succession is after partial destruction (eg forest fire, road building) pioneer organisms take root first (moss, lichen, insects) and build up the soil layer transition communities take over and continue the process climax community is the stable end populations that develop Case Study - Glacier Bay Pg 604 Questions Pg 603 #27-31 Applying the Concepts #1-6,8 Critical Thinking #2 General Outcome D3: Students will explain, in quantitative terms, the change in populations over time. G. Chaos Theory way of viewing complex systems states that since randomness is a basic feature of many complex systems, long term predictions may well be difficult to impossible that small uncertainties in short term predictions of individual events may be magnified to such an extent over the long term that expected behaviors in complex systems become unpredictable H. Dynamic Equilibrium or Steady State Theory in mature ecosystems populations tend to remain relatively stable over long periods of time I. Populations

6 Populations are characterized by three criteria: o Population size o Population Density o Rate of Change a) Population size can be determined in two different ways either a total count (okay in small area with small population) or representative sampling (sample a small area and multiply by total area) this assumes the population is distributed randomly factors affecting population size 1. natality 2. mortality 3. immigration 4. emigration open populations - where all four factors are functioning closed populations - no immigration or emigration b) Population Density populations are usually given in numbers per unit area D p = N/A D p = population density N = number of individuals in a population A = area (or V=volume) density will change depending upon the area (eg; Canada vs. Vancouver) population density may change over time: increase density means the population is increasing various populations are distributed around the globe distribution may be: 1. clumped 2. random 3. uniform type of distribution depends upon the resource availability most common is a clumped distribution c) Rate of Change Population Growth change in population size,!n= [natality + immigration] [mortality + emigration] rate of change in a population refers to some factor such as density changing over time also referred to as growth rate(gr)! N gr "! t Growth rate is also referred to as a per capita growth rate (cpr), and represents the change in population size!n, relative to the initial population size, N.! N cgr " N Is effective when comparing populations of different sizes i.e. school population

7 vs. community many factors affect the growth of populations the capacity for reproduction is the biotic potential the biotic potential is regulated by four factors: 1. maximum number of offspring per birth 2. chances that the offspring will reach reproductive age 3. number of times per year the organism reproduces 4. the age at which reproduction begins the ability of an environment to support a population is called its carrying capacity (changes with availability of resources) some factors are density - dependent, some are density - independent density - dependent work by increasing death and limiting reproduction as a population increases: disease, food supply, predation density - independent work regardless of the population number: floods, drought, temperature J. Growth Curves different shaped curves depending upon the type of population growth exponential growth gives a J-shaped curve speed of growth is determined by maximum reproductive rate organisms in nature cannot sustain unlimited growth most populations show an S-shaped curve J-shaped Curve occur in closed systems - aquarium containing paramecium or bacteria, yeast or other microorganisms which have short life span and are easy to handle and manipulate 4 definite phases 1. lag phase - adjustment to new environment 2. growth phase - rapid increase in population 3. stationary phase - lack of space, nutrients or accumulation of wastes o population growth ceases o natality = mortality 4. death phase - mortality # natality o nutrients run out and wastes accumulate S-shaped Curve curve is typical when a limiting factor is introduced or increased or when an organism is placed in a new environment reaches new equilibrium carrying capacity is the maximum number of individuals the environment can support

8 Human Population at the top of the J right now 3 stages: hunting/gathering, agriculture, industrial population is almost due for a crash - 1/5 is malnourished - no housing - no water Population vs Habitat K-selected - slow reproduction o long life-span r-selected - rapid reproduction o short life span Histograms they depict the percentages in age groups shape of the histogram tells if a population is growing, stabilized or declining