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Biology 30 Population Genetics The modern theory of evolution recognizes that the main source of variation in a population lies in the differences in the _________ carried by the chromosomes. Genes determine an organism’s appearance and mutations can cause new variations to arise. These variations can be passed from one generation to another. Certain genotypes may be better equipped for survival than others. They may be better at obtaining food and water, protecting themselves from predators or have a higher reproductive potential. When these organisms reproduce, these “successful” genes will be transmitted to the offspring. The offspring will be better able to survive; therefore, subsequent generations would have an increased frequency of these “successful” variant genes. Consequently, there would be _________________, within the group, of individuals better adapted to prevailing conditions. Genes in Human Populations The principles of genetics, established by studies on plants and fruit flies, can be applied to humans. However, the study of human genetics presents some unique problems: - Unlike garden peas and Drosophilia, humans produce few offspring, which makes it difficult to determine the genotypes of both parents & offspring for any particular trait. Observing successive generations takes time. Drosophilia can reproduce every 14 days. Many human traits, including body size, weight and intelligence, are affected by environment as well as genes. _______________________ is one of the most common ways to study human populations. - A representative group of individuals within the population is selected and the trends or frequencies displayed by the group are used as indicators for the entire population. - i.e. Trait for tongue rolling is sampled this way. Approximately 65% of the population carry this dominant gene. A population consists of all the members of a species that occupy a particular area at the same time. i.e. Perch population of Lake Wabamun, dandelion population of school field. The members of a population are more likely to breed with one another than with other populations of the same species. Therefore, genes tend to stay in the population for generation after generation. The total of all the alleles for all of the genes in all the members of a population at one time is called the population’s ________________. Over time the size of a gene pool changes. The gene pool __________ when a mutation changes a gene and the mutation survives. The gene pool ___________ when an allele dies out. The more variety there is in a gene pool, the better the population can survive in a ______________ environment. _________________ is the change in the frequency of genes in a population’s gene pool from one generation to the next. Hardy-Weinberg Principle 1908 – Godfrey Hardy & Wilhelm Weinberg, independently derived the basic principle of population genetics, the Hardy-Weinberg principle. This law states that the frequencies of alleles in a population’s gene pool will remain constant over generations as long as five factors stay constant (genetic equilibrium): 1) 2) 3) 4) 5) Provides a model of an unchanging gene pool. If all of these conditions are met, the frequencies of two alleles, for example A and a, will remain constant in a population for an indefinite time (until conditions change). The Hardy-Weinberg principle allows us to study one trait at a time. We will consider the simplest case, a trait that is controlled by a dominant allele and a recessive allele. The mathematical expression for the HardyWeinberg equilibrium is as follows: Example #1: Suppose a certain allele A has a frequency of 0.6 in a population. The frequency of allele a must be _______ because A + a must equal 1. We can arrange the alleles and their frequencies in a Punnett square. Unlike the genetic Punnett square used to determine individual traits, the eggs & sperm of this Punnett square represent the genes for the entire population. If p represents the frequency of the dominant allele (A), then p2 represents the frequency of the homozygous dominant offspring ______. If q represents the frequency of the dominant allele (A), then q2 represents the frequency of the homozygous recessive offspring ______. As shown in the Punnett square, there are two possible recombinations that will result in the heterozygous genotype ____. Therefore the frequency of the heterozygous genotype is pq + pq = _______. Notice that the sum of all the genotypes is equal to 1.00 or 100 percent: _________ + __________ + __________ = 1.00 _________ + __________ + __________ = 100 percent _________ + __________ + __________ = 1.00 The Hardy Weinberg equation can be used to calculate the proportion of a population that carries recessive alleles for genetic conditions, i.e. sickle cell anemia or cystic fibrosis. It can also be used to calculate the number of individuals with a specific genotype. Using the example from before, calculate the number of individuals of each genotype if the population was composed of 8000 individuals. Example #2: Suppose a certain allele A has a frequency of 0.7 in a population (found in 70% of the genes). Calculate the expected frequencies of the three possible genotypes. If we were just given the distribution of genotypes, how could we predict the frequency of the A and a alleles? Example #3: The frequency of a recessive trait in a population is 4%, what are the allele frequencies? Hardy-Weinberg purposely ignored the external factors that influence populations. The Hardy-Weinberg principle points out that sexual reproduction reshuffles genes but does not by itself cause ____________. If the population does not demonstrate H-W equilibrium, (i.e. it’s allele frequencies change over time), it is in evolutionary change! Factors that bring about Evolutionary Change A population’s gene pool is very unstable. It is constantly influenced by external factors – factors that were intentionally ignored by Hardy and Weinberg. These factors change a population’s genetic makeup, upset the tendency toward genetic stability & lead to evolutionary change. microevolution – the gradual change in allele frequencies (gene pool), of a population over successive generations. i.e. development of DDT resistance in some Anopheles mosquito species is limiting the effectiveness of DDT over time. 1) Natural Selection The nonrandom survival & reproduction of certain genotypes from one generation to the next. Certain traits may be selected for, while others may be selected against. Individuals with greater fitness breed and pass on their favourable characteristics to the next generation. Natural selection occurs when a _____________ produces a phenotype that gives one individual a survival advantage over another. Examples: In North America, individuals who are homozygous for normal hemoglobin have a selective advantage over those who are heterozygous or homozygous for the sickle cell allele. Peppered moth in England – the frequency for the dark wings in the peppered moth changed as levels of air pollution changed. It is possible that human resistance to HIV, the Ebola virus & the West Nile virus, and others will increase in frequency as these viruses become more widespread in the population. 2) Non-Random Mating Random mating means that there is no way to predict which males will mate with which females. In natural populations, unrestricted random mating is probably uncommon for two main reasons: 1) 2) 3) Mutations A mutation is any inheritable change in the DNA of an organism. Mutations occur in a cell as it undergoes ___________ to form an egg or sperm. Two types: 1) Chromosome Mutation – 2) Gene Mutation – If the mutation gives selective advantage to individuals carrying it, then it will increase in frequency and the population gene pool will change over successive generations. While most mutations are neutral, some are harmful and a few are even beneficial. i.e. some people have a rare mutation in a gene that codes for a protein receptor on the surface of white blood cells. In people without the mutation, HIV can use the protein receptor to enter white blood cells. Those that have the mutation, lack the receptor and are therefore resistant to HIV infection. A mutation considered beneficial in one environment may be detrimental in another environment. i.e. Sickle Cell Anemia & Tay-Sachs Disease - Are conditions expressed as homozygous recessive - Carriers (heterozygotes) are usually symptom free. - This is why disease still exists, or it would be selected out of the population. - Tay Sachs is a fatal genetic disease. - Sickle cell anemia can be treated (not cured), and sometimes can be fatal. - African and south eastern Asian - Eastern Jewish Populations 4) Genetic Drift Evolution can occur simply by chance. Random events may bring death or lack of parenthood to some individuals. As a result, alleles may disappear from a population. I.e. Population of 10 guinea pigs. Only one member displays an allele B, for black coat color. If this black coat individual does not mate, the black allele will disappear from the population. A _________ population is more likely to lose alleles from its gene pool than a large population is. (H-W Principle applies to large populations that do not experience genetic drift since chance events are unlikely to affect overall allele frequencies, i.e. predators are unlikely to kill all members of a population that have a recessive allele. There is, however, the rare chance that factors like _______, ________________, or extensive habitat fragmentation could cause genetic drift to occur in large populations). 5) Gene Flow (Migration) Movement of members into ______________, or out of _____________, a population alters its equilibrium. In immigration, new genes are _________ to the population. In emigration, genes are _________ from the population. Speciation refers to the formation of a new _______________. There is an enormous diversification between species that evolution alone cannot explain. A group of similar organisms that can interbreed and produce fertile offspring in the natural environment. It is important to note that speciation and evolution are NOT necessarily the same. Natural selection does not always cause speciation! (i.e. The evolution of the peppered moth did not lead to a new species). There are two general pathways that can lead to the development of a new species: 1) Gradual Speciation (Transformation) Gradually, and naturally over time as a result of mutation and adaptation to changing environmental conditions. i.e. evolution of mammoths ancestral mammoth steppe mammoth wooly mammoth 2) Divergence One or more species arise from a parent species. i.e. The small hoofed Hyracotherium is thought to have been the common ancestor of modern horses, tapirs & rhinoceroses. Speciation can also be instantaneous – due to events such as nondisjunction. Populations and Communities a population refers to all of the individuals of the same species living in the same place at a certain time a community is made up of the populations of all organisms that occupy an area the study of a community involves only the organisms, whereas the study of an ecosystem involves that abiotic and biotic components of an area a habitat is the physical area where a species lives within a habitat, every population occupies an ecological niche – this is referred to as the populations ecological role in the community, including the biotic and abiotic factors under which a species can successfully survive and reproduce Distribution of Populations population patterns can be divided into three patterns: 1) Clumped distribution - occurs when individuals are grouped in patches or aggregations - organisms are distributed according to certain environmental factors - eg.) in river valleys, trees often grow only on the south slopes and grasses dominate the north slope – plant distribution is found in “clumps” 2) Random distribution - occurs when there is neither attraction nor repulsion among members of the population - arbitrary and not very common 3) Uniform distribution - occurs when there is competition among individuals for factors such as moisture, nutrients, light and space Size and Density of Populations population size: the number of organisms of the same species sharing the same habitat at a certain time these numbers may arise from an exact count or an estimate of the total population size using sampling methods eg.)In 1981, there were 27642 northern pike in Sylvan Lake, Alberta population density: the number of organisms per unit space the density (D) of any population is calculated by dividing the total numbers counted (N) by the area (A) occupied by the population: D=N A i.e. If 200 lemmings were living in a 25ha (hectare) area of tundra near Churchill, Manitoba in 1980, their population density would be: we can compare population densities by determining if there have been changes within the same population over a certain time period (we call this the rate of change) Rate of density change can be expressed as follows: rate of density change = change in density change in time - or R = ∆D T D must be calculated showing the most recent dates minus the density at the earliest date – this will show whether there has been an increase or decrease in the population Four factors determine population size: 1) _________: the number of offspring of a species born in one year 2) _________: the number of individuals of a species that die in one year 3) _____________: the number of individuals of a species moving into an existing population 4) ______________: the number of individuals of a species moving out of an existing population population growth can be determined by the following formula: PG or ΔN = (births+immigration) – (deaths + emigration) = (b+i)-(d+e) (factors that ______ the pop’n) – (factors that ______ the pop’n) i.e. The Alberta greater sage grouse population was about 4375 in 1970. By 2002, the population was estimated to be only about 350 birds. Assuming that the population decreased at a constant rate, calculate the growth rate (gr): a change in the number of individuals in a population over a specific time frame, for the population from 1970 to 2002. Note: the calculation of growth rate does not factor in the initial size of the population. The amount of increase in a large population will always be larger than that of a small population. Why? To compare populations of the same species that are different sizes or that live in different habitats, we use the rate of change per individual. This is called per capita growth rate (cgr). i.e. If a colony of 200 cranes had 40 births & 55 deaths, with no migration, what is the per capita growth rate of the crane population? in mature ecosystems, populations tend to remain relatively stable over the long term – this is called dynamic equilibrium or steady state dynamic equilibrium is similar to homeostasis; populations will adjust to changes in the environment to maintain equilibrium populations can either be classified as “open” or “closed” - in open populations, all four factors (natality, mortality, immigration and emigration) are functioning - in closed populations, immigration and emigration do not occur, so changes in natality and mortality will be the only factors that influence population size Growth Curves Three types of growth curves: 1) J-shaped population curves (ideal environment) If a few relatively active individuals are placed in an ideal environment: unlimited space, food, water, without disease and predation, the population can be expected to reproduce at its maximum physiological rate (this is called its biotic potential which is defined on the next page). The only limiting factors would be the rate of gamete formation, mating and survival of offspring. Defined by a brief lag phase, followed by a steep increase in numbers, known as exponential growth. This gives us the characteristic “J” shaped curve. This rapid growth period is usually followed by a sharp decline in the population. **In real life situations, limiting factors curtail growth and the curves tend to level off! 2) Growth curves for open populations typically form “s-shaped” curves a characteristic growth phase, followed by a stationary phase where the curve levels off, the maximum number of individuals that the environment can support has been reached - this number is now the new carrying capacity (K) The highest possible per capita growth rate for a population is called its _________ ____________ (r). biotic potential determines the carrying capacity of that environment there are six factors that regulate biotic potential: 1. Offspring (fecundity): the number of offspring per reproductive cycle 2. Capacity for survival: the number of offspring that survive to reach reproductive age 3. Maturity: the age of reproductive maturity 4. Procreation: the number of times in a life span the organism reproduces 5. Life span 6. Gender ratio – the more females, the greater the biotic potential. 7. Mate availability Limiting factors are factors that limit a habitat’s carrying capacity & therefore limit population growth. (aka environmental resistance) there are two general categories of limiting factors in an environment: - density independent: - abiotic - affect members of a population regardless of population density - density dependent: - biotic - factors that arise from population density that affect members of that population. These have a bigger impact on large populations. 3) Growth curves for closed populations (limited resources) four definite phases can be identified in this curve I) LAG PHASE - the delay that occurs before the population enters a phase of active reproduction II) GROWTH PHASE - the population increases at its fastest rate during this phase - the rate of natality is greater than the rate of mortality - cell cultures and yeasts can grow exponentially (2, 4, 8, 16…) - the expected population increase in a given time (I) can be calculated from the following formula: I = growth rate (R) x current population (N) - i.e.) If the growth rate of paramecia in a closed population was 7.5% per day, and the initial population was 200, there should be an increase of 15 paramecia on the first day (after that it is compounded) III) STATIONARY PHASE - the point where the population size no longer increases - a lack of space, a shortage of nutrients and an accumulation of toxic metabolic wastes cause a reduction in the rate of increase - the rates of natality are equal to the rates of mortality IV) DEATH PHASE - the mortality rate exceeds the natality rate - nutrients run out and wastes accumulate r and K Population Reproductive Strategies K selected strategies: - These populations are found in stable environments who live close to the carrying capacity (K) of the environment. - populations become crowded, causing intraspecific competition - members are usually large in size and produce young that are slow-growing and require parental care - low reproduction rate - i.e.) elk, bear, humans - Summary of strategy: r selected strategies: - undergo many unpredictable changes - usually populations that are small in size - have short life spans and an early reproductive age - high reproductive rate - the offspring grow rapidly and little parental care is needed - a sudden environmental change can result is a large number of deaths. i.e. Alberta – these organisms experience growth in the summer, but die in large numbers as simmer ends. - i.e.) insects, bacteria, annual plants & algae Life History Patterns population cycles that include growth and decline can occur in many populations the snowshoe hare and lynx have cycles that are about 11 years in length Human Population Growth & Age Pyramids Growth curves show how populations change over time – not the age distribution of the members With age pyramids, we are able to predict whether a population will grow, stabilize, or decline an age pyramid with a wide base is characteristic of a rapidly growing population – it indicates a high number of young offspring, but also shows the number of animals capable of reproduction population histograms with a narrower base are often approaching zero population growth, and those with a more narrow base than middle section are showing declining population growth factors that affect population growth: - industrial revolution/technology - advances in medicine - weather Intraspecific and Interspecific Competition “If two populations of organisms occupy the same ecological niche, one of the populations will be eliminated” – this is known as Gause’s Principle and is due to interspecific competition interspecific competition occurs between similar species for a limited resource (food, water…) intraspecific competition occurs within an ecological niche of members within the same species Predation two main ways for animals to avoid predation are: 1) camouflage – an adaptation in form, shape or behavior that better ables an organism to avoid a predator eg.) toxins produced by alder, birch or poplar trees deter animals and insects eg.) butterflies taste bitter to birds eg.) During the winter, rabbits change their coloring to blend with the environment 2) mimicry – involves developing a similar color pattern, shape or behavior that has provided another organism with some survival advantage eg.) eyespots on butterflies coevolution can occurs between two species – this is when the selection pressure Symbiotic Relationships a symbiosis is a relationship in which two different organisms live in a close association there are three main types of symbiotic relationships: 1) PARASITISM - parasites obtain nourishment from their hosts, but do not usually kill their hosts but often will affect the host in a detrimental way - eg.) Dutch elm disease – a parasitic fungus uses the tree for food 2) COMMENSALISM - commensalism is an association between two organisms in which one benefits and the other is unaffected - eg.) the fox and caribou in the arctic – fox will often follow migrating caribou because the caribou kick the snow out of the way so the foxes will have a path to travel on 3) MUTUALISM - a relationship in which two different organisms live together and both benefit from a relationship - eg.) nitrogen fixing bacteria and legume plants (biology 20!) – the plant feed the bacteria sugar and the bacteria make nitrates for the plant - eg.) pollination Chaos Theory Scientists are interested in studying very complex phenomena which seem to defy long term prediction. Eg. Biological communities & populations, weather. A new way of examining why some features of nature are so unpredictable is known as chaos theory. This theory assumes that randomness is a basic feature of many complex systems, long-term predictions may be extremely difficult. Even though features of nature are so unpredictable, they often share similar characteristics: 1) Outcomes of processes in a complex system are extremely sensitive to small differences in the conditions that were present when the process began. 2) Once a process is underway, the relationships among the interacting parts of the phenomena can change as a result of the interactions themselves. 3) Two systems that appear similar at the start may end up being very different, but how the two will differ is unpredictable. Chaos is a normal feature in biological systems. The inability to predict the precise makeup of a community does not mean that communities are entirely unpredictable: communities tend to undergo predictable changes over time called succession. Succession succession is the slow, orderly progressive replacement of the community by another during an areas development succession ends by reaching a climax community there are two possible types of succession: 1) Primary Succession - occurs in an area which no community previously existed - eg.) invasion of plant life of a newly formed volcanic island 2) Secondary Succession - occurs following the complete or partial destruction of a community - eg.) regrowth after a forest fire - the first plants and animals to appear are called the pioneer community - lichen, mosses and insects are often considered pioneer species - pioneer communities develop into seral communities which have plants and animals with longer life cycles than pioneer species - in the end, a climax community is formed where there is a high rate of survival of all species some generalizations about succession: - species composition changes more rapidly during the earlier stages of succession - the total number of species increases dramatically during the early stages of succession, begins to level off during intermediary stages, and usually declines and the climax community becomes established - food webs become more complex and the relationships more clearly defined as succession proceeds - both the total biomass and nonliving organic matter increase during succession and begin to level off during the establishment of the climax community