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Standard Bf. : The student will demonstrate an understanding of the interrelationships among organisms and the biotic and abiotic indicators of their environments. Chapter 15 Populations. Section 1 How Populations Grow. What is a Population?.

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Standard bf

Standard Bf

: The student will demonstrate an understanding of the interrelationships among organisms and the biotic and abiotic indicators of their environments.

Chapter 15 populations

Chapter 15Populations

Section 1

How Populations Grow

What is a population

What is a Population?

  • A population consists of all the individuals of a species that live together in one place at one time.

  • Demography – the statistical study of all populations

  • Study the composition of a population and try to predict how the size of the population will change

Three key features of populations

Three Key Features of Populations

  • Population size – most important feature –can affect the population’s ability to survive

  • Population density – the number of individuals that live in a defined space

  • #of individuals =


    population density

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  • Dispersion – the way the individuals of the population are arranged in an area or a volume

  • Population dispersion patterns influence the rate of gene flow among and between species

Gene flow

Gene Flow

  • Migration-the movement of individuals to or from a population (immigrants add alleles and emigrants subtract alleles

  • Gene flow the movement of alleles into or out of a population

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  • Three main patterns of dispersion are possible within a population

  • Randomly spaced – the location of each individual is self-determined or determined by chance within an area or volume

  • Evenly spaced – they are located at regular intervals – territoriality and intraspecies competition for limited resources lead to individuals living at specific distances from one another

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  • Clumped distribution – individuals are bunched together in clusters – may live close together in groups in order to facilitate mating, gain protection, or access food

  • Each one reflects the interactions between the population & environment

Plant dispersion

Plant dispersion

  • Plant also exhibit the same dispersion pattern as those of animals

  • Most common is clumped dispersion pattern

  • Plants close together would compete – uniform dispersion

  • Random – adapted to a variety of conditions and seeds windblown

Survivorship curves

Survivorship curves

  • Is a generalized diagram showing the number of surviving members over time from a measured set of births

  • Give information about the life history of a species

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  • Type I common among large mammals – behavior common –parental care

  • Type II – roughly equal at all ages birds, small mammals

  • Type III – high birth rate/ high infant mortality rate

  • Ex. Invertebrates, fish, plants

Modeling population growth

Modeling Population Growth

  • Population model- a hypothetical population that attempts to exhibit the key characteristics of a real population

  • Demographers can predict what might occur

  • Three stages of complexity

Tagging monarch

Tagging Monarch

  • Monarch Watch was formed in 1992

  • Essential Question:What can we learn about migrationfrom tagged monarchs?

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  • Population grows when birthrate is greater than death rate – P.G. difference between the birthrate & the death rate

  • Exponential growth curve – a curve in which the rate of population growth stays the same – increase steadily

  • To calculate the # of individuals that will be added –multiply the size of the current pop.(N) by the rate of growth (r)

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  • Carrying capacity (K) –the population size that an environment can sustain

  • Carrying capacity can change when the environment changes

  • Population crash is a dramatic decline in the size of a population over a short period of time

  • Limiting factor – has the greatest effect in keeping down the size of population

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  • Density-dependent factors –the effect of limited resources will affect the population density that uses them

  • Competition, predation, parasitism and disease are limiting factors

  • Logistic model – a population model in which exponential growth is limited by a density-dependent factor

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  • Competition for food, shelter, mates, and limited resources tends to increase as a population approaches its carrying capacity

  • Accumulation of wastes also increases

Http www worldometers info

Growth patterns in real populations

Growth Patterns in Real Populations

  • Density-independent factors – growth limited by environmental conditions

  • Weather & climate are the most important

  • Some organisms grow exponential, while others grow logistic growth model,

  • Some will use both at different times as the environment changes

Rapidly growing populations

Rapidly Growing Populations

  • R-strategists – grow exponentially when environmental conditions allow them to reproduce

  • R-strategists have short-life span, reproduce early, have many offspring which are small and mature rapidly with little or no parental care

Slowly growing populations

Slowly Growing Populations

  • K-strategists- population density is usually near the carrying capacity (K) of their environment

  • Long life span, slow maturing process, reproduction late in life, extensive care of their young, tend to live in stable environments

Population pyramids

Population Pyramids




P. 325 Section 1 review all questions

P. 335 Questions 1-3,6,10,13,16, 18

How populations evolve section 2

How Populations EvolveSection 2

  • Hardy-Weinberg principle –the frequencies of alleles in a population do not change unless evolutionary forces act on the population

  • Holds true if population is big enough that members will not mate with relatives

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  • Five principle evolutionary forces: mutation, gene flow, nonrandom mating, genetic drift, and natural selection

  • Can cause the ratios to differ significantly from predicted by the Hardy-Weinberg principle



  • Mutation rates in nature are very slow

  • Not all mutation result in phenotype changes

  • Several codons can code for the same amino acid

  • Mutation is a source of variation

Gene flow1

Gene Flow

  • Migration-the movement of individuals to or from a population (immigrants add alleles and emigrants subtract alleles

  • Gene flow the movement of alleles into or out of a population

Nonrandom mating

Nonrandom Mating

  • Prefer to mate with others that live nearby or are of their own phenotype

  • Mating with relatives (inbreeding) causes a lower frequency of heterozygotes than predicted by Hardy-Weinberg principle

  • Also when organisms choose based on certain traits

Genetic drift

Genetic Drift

  • Genetic drift – the random change in allele frequency in a population

  • Changed by chance events causing isolation

  • Genetic uniformity can reduce disease resistance

  • Lack of genetic diversity may hasten extinction

Natural selection

Natural Selection

  • Causes deviations from H-W proportions by changing frequencies of alleles

  • Frequency will increase or decrease, depending on allele’s effects on survival & reproduction

  • Sickle cell anemia in US

  • One of the most powerful agents of genetic change

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  • Genetic conditions are not eliminated by natural selection because very few of the individuals bearing the alleles express the recessive phenotype

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  • Natural selection enables individuals who express favorable traits to reproduce & pass those traits on – acts on phenotypes not genotypes

  • Selection cannot operate against rare recessive alleles unless heterozygous individuals are common & produce homozygous offspring

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  • Natural selection shapes populations affected by phenotypes that are controlled by one or by a large number of genes

  • Polygenic trait – a trait that is influenced by several genes

  • Normal distribution-a range of phenotypes clustered around an average value

Directional selection

Directional Selection

  • Eliminates one extreme from a range of phenotypes, alleles for that trait become less common

  • Directional selection – the frequency of a particular trait moves in one direction in a range

  • Has a role in the evolution of a single-gene traits

Stabilizing selection

Stabilizing Selection

  • When selection reduces extremes, the frequencies of the intermediate phenotypes increase

  • Distribution becomes narrower tending to”stabilize” the average by increasing the proportion of similar individuals

  • Very common in nature

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