Population Studies Introduction of Some terms A population consists of all the members of a species that occupy a particular area at the same time 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 same population for generation after generation INTRODUCTION The total of all the genes in all the members of a population at one time is called the … population's gene pool Evolution is the change in the frequency of genes… in a population's gene pool… from one generation to the next. Hardy-Weinberg Law In order to see how a population evolves, it is helpful to examine the genetics of a population that does not change from generation to generation The Hardy-Weinberg Law provides a model of an unchanging gene pool This law states that the frequencies of alleles in a population's gene pool remain constant over generations if all other factors remain constant Hardy-Weinberg Law For a gene pool to be in the Hardy-Weinberg equilibrium, 5 conditions must be met: 1. The population must be closed. This means that no 2. 3. 4. 5. immigration or emigration can occur. Random mating takes place. There can be no mating preferences with respect to genotype. There can be no selection pressure. A specific gene must not affect the survival of the offspring. No mutation of the particular alleles examined can occur. The population must be very large. This equilibrium is based on statistical probabilities and random sampling. Hardy-Weinberg Law If all these conditions are met, the frequencies of two alleles (A and a) will remain constant in a population forever or until conditions change Recall our definition of Evolution Change of frequency of genes or alleles The Hardy-Weinberg law points out that sexual reproduction reshuffles genes but does not by itself cause evolution Hardy-Weinberg Law The mathematical expression of the Hardy- Weinberg equilibrium is… p+q=1 where p = frequency of the dominant allele & q = frequency of the recessive allele Hardy-Weinberg Law Example: suppose a certain allele A has a frequency of 0.6 in a population since the two alleles must add up to 1… then p + q = 1 (1 - 0.6 = 0.4) the frequency of a is 0.4 Let's see what happens during reproduction Hardy-Weinberg Law First, let’s arrange the two alleles and their frequencies on a Punnett square A (0.6) A (0.6) a (0.4) a (0.4) Hardy-Weinberg Law Then, fill in frequencies for the possible offspring A (0.6) a (0.4) A (0.6) a (0.4) AA (0.36) Aa (0.24) Aa (0.24) aa (0.16) Hardy-Weinberg Law Go ahead and add up your values for the allele frequencies. What do you get? The mathematical relationship governing the gene frequencies is… p2 + 2pq + q2 = 1 AA + 2Aa + aa = 1 (or 100%) Since p = 0.6 and q = 0.4, then (0.6)2 + 2 (0.4 x 0.6) + (0.4)2 must equal 1 (0.36) + 2(0.24) + (0.16) = 1 Practice Mutations & Evolutionary Change Mutations violate the conditions for Hardy- Weinberg equilibrium because one gene changes into another and therefore alters gene frequencies in the population Mutations & Evolutionary Change Review: a mutation is any inheritable change in the DNA of an organism 1. Chromosome mutation results form non-disjunction, chromosome breakage or translocation 2. Gene mutation changes in the nucleotides of a DNA molecule If a population has a stable gene pool and gene frequencies, it is not evolving. Mutations If the population does not demonstrate Hardy-Weinberg equilibrium (i.e. its gene frequencies are not stable) it is in evolutionary change Evolutionary Change Micro-evolution a change in the gene pool of a population over successive generations Potential causes of micro-evolution are mutation genetic drift gene flow non-random mating natural selection Evolutionary Change Mutation A new mutation that is transmitted in gametes immediately changes the gene pool of a population by substituting one allele for another A mutation by itself does not have much effect on a large population in a single generation If, however, 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 Evolutionary Change Genetic drift evolution can occur simply by chance Random events may bring death or parenthood to some individuals regardless of their genetic makeup The resulting change in the gene pool is called genetic drift Genetic drift plays more of a role in small populations than in large ones Evolutionary Change Genetic drift Example Flipping a coin 1000 times compared with flipping a coin 10 times Example a population of plants consists of only 25 individuals, 16 are AA, 8 are Aa and 1 is aa AA plants are destroyed in a rock slide, which alters the relative gene frequencies for subsequent populations. Evolutionary Change Founder Effect Genetic drift that occurs when a small number of individuals separate form their original population and start a new population Allele frequencies of the new population will be different than the original population o depend on gene pool of the founding population Evolutionary Change Bottleneck Effect A dramatic reduction in population size resulting in genetic drift The frequency of alleles in the remaining members of the population is very different from the original population. Evolutionary Change Gene flow The gene pools of most populations of the same species exchange genes. This violates the Hardy-Weinberg condition that populations must be closed to be in equilibrium Animals may leave one area and contribute their genes to the pool of a neighbouring population migration or a high wind may disperse seeds or pollen far beyond the bounds of the local population Gene flow between populations may change gene frequencies and therefore may result in evolution Nonrandom Mating Mates are chosen based on different characteristics (not just love the one you’re with) Sexual Selection Chances of being selected depend on animal’s traits (what makes him more desirable to the female) Includes Physical and Behavioural Differences between sexes Nonrandom Mating Sexual Dimorphism Striking physical differences between males and females Male Male Female Female Nonrandom Mating Natural Selection Environment selects for particular traits that are more favourable for surviving in that environment “Survival of the Fittest” Population Interactions Definitions Population – Any group of individuals of the same species who live in the same area at the same time Eg. Population of humans in the food court Community – The association of interacting populations that live in a defined area Eg. Population of the food court, tables, chairs, trays, and pets and wild animals that wander into the food court. Niche – An organism’s habitat and role within a community Includes all factors needed to survive and the organism’s interactions with other species Population Interactions In any community, individuals of many populations need to live among each other Some possible scenarios: Competing for limited resources One species preying on another One species relying on another for survival Competition occurs whenever two or more organisms attempt to exploit a limited resource Food Living space Population Dynamics The interactions among individuals – either within the same population or from different populations – are the driving force behind population dynamics The changes that occur in a population over time Population Dynamics Individuals are always competing for resources in order to survive Competition for resources can occur: Among individuals of the same species Natural selection Survival of the fittest Between individuals of different species Hence, there are two basic categories of competition Intraspecific Competition Intraspecific competition – Competition for limited resources among members of the same species SURVIVAL OF THE FITTEST among members of the same species aka NATURAL SELECTION Eg. Seeds On the forest floor there are thousands of seeds Each seed requires water, nutrients, sunlight, space to grow and mature Only a few seeds will be able to compete successfully to obtain what they need of the limited available resources Intraspecific Competition Intra-specific competition is very common since the members of a population have the same requirements Intra-specific competition occurs when individuals of a species are competing for resources within their niche Interspecific Competition Interspecific competition – Competition for limited resources between members of different species in the same community Tree competing with a shrub for light and growing space Recall a niche is an organism’s habitat and role in a community Due to interspecific competition, no two organisms can share the exact same ecological niche Interspecific Competition If no two species can share the exact same ecological niche – then why is there interspecific competition? Interspecific competition occurs when individuals of two different species are competing for resources within overlapping niches Competition – Gause’s Principle The Theory of Competitive Exclusion Two species with very similar niches cannot survive together because they compete so intensely that one species eliminates the other Competition – Gause’s Principle Experiment: Gause raised two species of paramecium with similar food requirements in the same culture One species always eliminated the other (the particular conditions in the culture determined which species survived) In nature, species can avoid direct competition by Feeding at different times of the day (e.g. Hawks and owls) Dividing resources in some other way (e.g. Different organisms hunt for insects in different parts of coniferous trees) Producer-Consumer Interactions Not all interspecific interactions in a community are classified as competitive… Predation The most obvious population interaction in a community are those in which a predator eats its prey Predators that specialize in eating only one prey species play an important role in controlling the population size of the prey species Eg. Canada lynx and snowshoe hare The terms predator and prey apply not only to animals that eat other animals, but to any type of producer and consumer relationship Eg. Plants and Herbivores Predation Plant defense mechanisms against herbivores: Thorns Microscopic crystals in their tissues Spines or hooks on leaves Distasteful or harmful chemicals Some well-known poisons and drugs are secondary compounds produced by plants: Strychnine Morphine Nicotine Mescaline Predation Active animal defenses against predation Fighting Hiding Escaping Four types of passive defense Predation – Passive Defense Type I Mechanical or chemical defense mechanisms include porcupine quills, the skunk's offensive odour, the bad taste of monarch butterflies Predation – Passive Defense Type II Camouflage or protective coloration makes it difficult to spot prey Predation – Passive Defense Type III Deceptive coloration, warning coloration Predation – Passive Defense Type IV Mimicry, where one species resembles another Monarch and viceroy butterflies Coral snake and harmless species Wasps and non-biting flies Symbiotic Relationships Symbiosis is a close relationship between members of different species (3 categories) 1. Mutualism - Both species benefit from the association Coliform bacteria in the human gut, nitrogen-fixing bacteria in nodules of legumes, protists in a termite's gut 2. Commensalism - One species benefits while the other neither benefits nor is harmed Remora and the shark 3. Parasitism - One species, the parasite, benefits at the expense of the host The parasite takes nourishment directly from the tissues of its host's body Marc’s Botfly Marc’s Botfly After Central America Regulation of Population Size Factors Affecting Growth of Populations The growth of a population is suppressed by Abiotic factors - Non living things in the environment Sunlight, water, soil, air Biotic factors - Living things in the environment Humans, trees, fish, bacteria The combination of these effects is termed environmental resistance Factors that regulate the growth of populations are described as density-dependent or densityindependent Regulation of Population Size The combination of biotic (living) and abiotic (nonliving) factors create environmental resistance Environmental resistance = the combined effects of various interacting factors that limit population growth There are two categories of factors that regulate the growth of populations 1. Density-dependent 2. Density-independent Density Dependent Factors These factors are intensified as the population increases in size Food availability Living space Emigration Competition Disease Reproductive rate Mortality Accumulation of Parasitism wastes Immigration Biotic Factors Density-Independent The occurrence and severity of these factors are unrelated to population size Weather Climate Abiotic factors Growth and Regulation of Populations Population Density Population density = The number of individual organisms in a given area or volume Population Density Dp = N or A Dp = N V D = Density N = number of organisms A= area V = volume Growth and Regulation of Populations Population Density Example: 44 students/100m2 = 0.44 students/m2 12 gophers/10.0m2 = 1.2 gophers/m2 54 minnows/200 mL = 0.27 minnows/mL Growth and Regulation of Populations Population Density Why calculate population density? If you know your community size you can now estimate the size of your population Examples: School = 1000m2, therefore 4.4 students/m2 x 1000m2 = 4400 students Field = 200m2, therefore 1.2 gophers/m2 x 200m2 = 240 gophers Fish tank = 2L = 2000 mL, therefore 0.27 minnows/mL x 2000mL = 540 minnows Population Density Practice A fish tank 10m long, 5m tall and 2m wide is filled with water. The population density of bacteria in the water is 1.5 x 104bacteria/m3 Approximately how many bacteria are in the fish tank? Population Density Practice–cont’d First we need to calculate the volume of the swimming pool: 10m x 5m x 2m = 100m3 The population density of the bacteria is 1.5 x 104bacteria/m3 Therefore, the population density of bacteria in the water is represented by the formula Dp = N Dp x V = N V Population Density Practice–cont’d Dp x V = N N = 1.5 x 104bacteria/m3 x 100m3 = 1 500 000 = 1.5 x 106 bacteria Growth and Regulation of Populations Population Density Is this always 100% accurate? Note that you need to know how a population is distributed within its habitat before taking samples to determine the population size Some populations tend to clump in certain areas, which can affect the accuracy of your estimation Growth and Regulation of Populations Population Growth A population gains individuals by: Natality = Birth Immigration A population loses individuals by: Mortality = Death Emigration The balance between these four factors will determine whether a population size grows, declines, or remains the same Growth and Regulation of Populations Change in Population Size Although this is the formula given to you, we know that Factors that increase population = births and immigration Factors that decrease population = deaths and emigration Therefore, (D N) = (births + immigration) - (deaths + emigration) Growth and Regulation of Populations Calculate the change in the Sandhill Crane population at the banks island breeding site in 1991 Births = 40, Immigrations = 0 Deaths = 55, Emigration = 0 Initial number = 200 (D N) = (births + immigration) - (deaths + emigration) N = (40 + 0) - (55 + 0) = - 15 individuals Growth and Regulation of Populations Percent Population Growth Formula – not given on your data sheet but is intuitive… Recall that change in population is represented by: (D N) = (births + immigration) - (deaths + emigration) Therefore, percent growth = Change in population x 100% Initial population => Percent growth = __[b + i] - [ d + e]_ Initial population x 100% Growth and Regulation of Populations Percent Population Growth Example: Births = 40 Deaths = 55 Initial population size = 200 =>Percent growth = __[b + i] - [ d + e]_ x 100% Initial population PG% = [40 + 0] - [55 + 0] x 100% = -7.5% 200 Growth and Regulation of Populations Population Growth Rate Population growth rate: The change in the number of organisms in a population per unit time growth rate = _D N_ Dt Growth and Regulation of Populations Per Capita Growth Rate Rate of population growth does not take into account the initial size of the population A large population has more individuals that can reproduce compared to a small population To compare populations of the same species that are different sizes or live in different habitats, the change in population size can be expressed as the rate of change per individual This measurement gives us per capita growth rate Growth and Regulation of Populations Per Capita Growth Rate The per capita growth rate can be calculated by the formula: cgr = ΔN N cgr = Per capita growth rate ΔN = Change in the number of individuals in a population N = The original number in the population Growth and Regulation of Populations Per Capita Growth Rate Why measure per capita growth rate? To examine population size as the rate of change per individual Eg. Suppose that in a town of 1000 people there are 50 births, 30 deaths, and no immigration or emigration in a year. Calculate the per capita growth rate. Growth and Regulation of Populations Per Capita Growth Rate Eg. Suppose that in a town of 1000 people there are 50 births, 30 deaths, and no immigration or emigration in a year. Calculate the per capita growth rate. cgr (Growth rate) = Unknown ΔN = 50 births – 30 deaths = +20 N = 1000 people cgr = ΔN = N 20 = 0.02 people 1000 Could the answer be a negative value? Factors that Affect Population Growth Recall that both biotic and abiotic factors limit the growth of a population Population size can be limited by How fast and how often a species can reproduce The ability of a habitat to support the population Biotic Potential of Populations Biotic potential = r Definition = The maximum number of offspring that can be produced by a species under ideal conditions ie. The capacity of populations for exponential growth There are six factors which affect biotic potential of a population Biotic Potential of Populations Six Factors that affect Biotic Potential 1. Age of onset of sexual maturity • The earlier that sexual maturation occurs, the greater the biotic potential 2. Gender ratio • The more females there are, the greater the biotic potential 3. Estrous cycles • The shorter the time between cycles of sexual receptivity, the greater the biotic potential Biotic Potential of Populations Six Factors that affect Biotic Potential 4. Mate availability The more readily available mates are in a population, the greater the biotic potential 5. Litter or clutch size The larger the litter or clutch size, the greater the biotic potential 6. Fecundity Fecundity = average number of offspring produced per female The greater the fecundity of a species the greater the biotic potential Population Growth Patterns Two Types of Graphs/Curves to know: 1. Exponential Population Growth: J-Curve This model predicts unlimited population increase under ideal conditions (usually a closed pop.) of unlimited resources and then a sharp decline in the population 2. Logistic Growth: S-Curve More representative of population in nature This model incorporates the effects of resource limitation and crowding on the population growth rate Population Growth Patterns 1. Exponential Population Growth: J-Curve This model predicts unlimited population increase under ideal conditions (usually a closed pop.) of unlimited resources and then a sharp decline in the population There four phases in this type of growth pattern: 1. 2. 3. 4. Lag phase Growth phase Stationary phase Death phase ("crash") Examples of organisms that exhibit exponential growth include bacteria, yeast, some insects J-Curve J-Curve J-Curve J-Curve Population Growth Patterns 2. Logistic Growth: S-Curve More representative of population in nature This model incorporates the effects of resource limitation and crowding on the population growth rate Natural populations cannot continue to grow exponentially: There is a limit to the number of individuals that can occupy a habitat The carrying capacity is the maximum stable population size that the environment can support for a long period of time. S- Curve Population Growth Patterns Logistic Growth: S-Curve – cont’d In populations exhibiting logistic growth, an equilibrium is reached near the carrying capacity of the environment example p. 585 figure 25.12 Carrying capacity (symbolized as K) is a property of the environment, and it varies over space and time with the abundance of limiting resources S-Curve Carrying Capacity S-Curve K R-SELECTED AND K-SELECTED POPULATION STRATEGIES r-selected Populations Experience periods of exponential growth Characteristics of r-selected species: Small organisms Short life time Great reproductive potential Recall biotic potential (r) is the capacity of populations to grow exponentially High rate of reproduction = r Insects are examples of r-selected populations K-selected Populations Populations that stabilize near the carrying capacity of their environment (K) Characteristics of K-selected species: Larger size Longer generation time Lower reproductive potential Examples include large mammals such as deer, bears, and humans A COMPARISON OF R-SELECTED (OPPORTUNISTIC) AND K-SELECTED (EQUILIBRIAL) POPULATIONS R-Selection K-Selection Climate Variable and/or unpredictable Fairly constant and/or predictable Mortality Density independent Density dependent Survivorship High juvenile mortality Low juvenile mortality Population Size Variable, below carrying capacity Fairly constant, near carrying capacity Level of competition Low High Life History Rapid development Slow development Reproductive Capacity High reproductive capacity Greater competitive ability Age Sexual Maturity Early reproduction Delayed reproduction Body Size Small body size Large body size Reproductive Frequency Usually reproduce only once Repeated reproduction Offspring Many small offspring Fewer, larger offspring Length of Life Short, less than one year Longer, usually more than one year Change in Communities: Succession Succession - The sequence of invasion and replacement of species in an ecosystem over time The sequence of identifiable ecological stages or communities occurring over time in progress from bare rock to climax community Affected by abiotic and biotic factors – climate and interspecific competition Change in Communities: Succession Communities are defined by the populations in them The stage of succession can be determined by the kinds of species present in a community Change in Communities: Succession Primary succession The initial colonization of a barren habitat by pioneer species Soil is produced during this stage e.g. Lichen and mosses growing on rocks Secondary succession Re-building of an area that once supported many organisms e.g. Mount St. Helen’s Change in Communities: Succession Climax Community The stage in ecological succession that is stable and selfsupporting Usually the final stage in the stages of succession Produce more organic material than they use Type of Community Populations Relationship to Sun Pioneer Weeds, fugitive species Lots of sun required Seral Shrubs Less than above Seral Deciduous trees Even less than above Climax Pine trees Least amount of sun