Ch. 15 : Populations

What is a Population? Populations **Population: All individuals of a species living in a certain place Growth? Birth of offspring Decline? Limited resources

Three Key Features of Population – Determines Future Population size Population Density Dispersion **Population size – the # of individuals in a population, may affect the population’s ability to survive THINK –PAIR - SHARE What do we call species that have a very low population count? Would floods or other natural disasters affect smaller or larger populations more greatly? Inbreeding = little genetic variation! Ex: Cheetah’s

Three Key Features of Population – Determines Future Population size Population Density Dispersion **Population density – the # of individuals that live in a given area When a population of a species is small and species are spaced widely apart, reproduction is rare

Three Key Features of Population – Determines Future Population size Population Density Dispersion **Dispersion – the way the individuals of the population are arranged in space: random, even, or clumped

Each pattern reflects the interactions between the populations and its environment

TRY THIS With your partner, give examples of a clumped, even, and random dispersion of three different species

Population Growth Models Population Model: try to predict how a population will grow A population grows when more individuals are born than die in a given period

Growth Model (1) Growth rate and population size **Exponential growth curve model (J-shaped) Rate of growth is constant but, population increases steadily Ex: Bacteria

Growth Rate & Population Size Growth rate and population size Population size has limiting factors Growth is limited by predators, disease, & availability of resources (Food & Water) Eventually, growth slows, & population may stabilize **Carrying capacity – maximum population size that an environment can sustain

Growth Rate & Population Size Exponential Growth Rate of growth is constant but, population increases steadily **Carrying Capacity maximum population size that an environment can sustain (stabilizes)

Resources & Population Size Resources and population size **Density-dependent factors – (food & water), the rate at which they become depleted depends on the population density of the population that uses them

Density Dependent Factors Resources and population size **Logistic Model- Exponential growth is limited by density dependent factors (resource availability)

Density Independent Factors Growth patterns in real population s **Density-independent factors Many species of plants & insects reproduce rapidly Growth is usually limited by density-independent factors – environmental conditions like weather & climate Ex: Mosquito’s

Growth Rate & Population Size Exponential Growth Plants & Insects usually have this model because of rapid reproduction rate **Carrying Capacity Slower growing organisms like whales, bears have this model

Whiteboards – Quick Questions Which type of organisms are represented in this growth pattern?

Whiteboards – Quick Questions The number of individuals of a population in a given area is known as (1) species number (2) population density (3) carrying capacity

Whiteboards – Quick Questions A colony of bacteria that has limited food supply likely will undergo ______ growth (1) exponential (2) natural (3) random (4) logistic

Whiteboards – Quick Questions A colony of bacteria that has limited food supply likely will undergo ______ growth (1) exponential (2) natural (3) random (4) logistic

Whiteboards – Quick Questions The population size an environment can sustain is called (1) population density (2) population modeling (3) population dispersion (4) carrying capacity

Whiteboards – Quick Questions State one reason for the changes in population size represented by line A between years 5 and 10.

Whiteboards – Quick Questions Which letter indicates the carrying capacity of this population?.

Whiteboards – Quick Questions A scientist was studying a population of fish in a pond over a period of 10 years. He observed that the population increased each year for 3 years, and then remained nearly constant for the rest of the study. The best explanation for this observation is that the population had (1) stopped reproducing (2) reached carrying capacity (3) run out of food and migrated to a different pond

How Populations Evolve How populations evolve **Hardy-Weinberg Principle Frequencies of alleles (Bb, bb) in a population don’t change unless evolutionary forces act on the population Ex: Dominant alleles don’t replace recessives unless a particular allele is favored

How Populations Evolve How populations evolve There are 5 evolutionary forces that affect the frequency of alleles in a population Mutation Gene Flow Nonrandom Mating Genetic Drift Natural Selection

How Populations Evolve How populations evolve How do these 5 forces affect allele frequency in a population? You will be assigned a force Read the description (pages 327 – 329) Explain how the force changes allele frequencies Share with your group

How Populations Evolve How populations evolve 1’s = Mutation p ’s = Gene Flow p.328 3’s = Nonrandom Mating p ’s = Genetic Drift p ’s = Natural Selection p. 329

Whiteboards – Quick Questions Mutation rates in nature (1) Occur quickly (2) Never occur (3) Occur slowly (4) Are not the source of variation

Whiteboards – Quick Questions State one possible cause for the population decrease at X

Whiteboards – Quick Questions Which statement is a possible explanation for the changes shown? (1) Species A is better adapted to this environment. (2) Species A is a predator of species B. (3) Species B is better adapted to this environment. (4) Species B is a parasite that has benefited species A.

Populations Evolutiona ry Forces Causes ratios of genotypes to differ significantly (some favored over others)

Populations Evolutiona ry Forces 1. Mutation- source of variation Slow- acts over long periods of time Small mutations may have no effect

Populations Evolutiona ry Forces 2. Gene Flow Occurs as immigrants (new individuals) add alleles to a population as they migrate in Emigrants (departing individuals) take alleles away as they leave

Populations Evolutiona ry Forces 3. Nonrandom Mating When individuals prefer to mate with others nearby or with their own phenotype Inbreeding causes fewer heterozygotes to be present in population Occurs when females are “choosy” about their mates

Populations Evolutiona ry Forces 4. Genetic Drift Change in allele frequency when small populations are greatly changed by chance events (accidental) Occurs randomly as if genes are drifting away Ex: Cheetahs- lack of genetic variation makes them especially vulnerable to disease

Populations Evolutiona ry Forces 5. Natural Selection Frequency of alleles change depending on their effect on survival and reproduction

Natural Selection individuals with sickle cell anemia in U.S. are declining (these homozygotes rarely have children- and are therefore selected against)

Populations Action of natural selection on phenotype s A. Natural selection acts on phenotypes not genotypes Those with favorable traits persevere Only expressed traits can be targets of natural selection

Action of Natural Selection on Phenotypes Only son of the last Tsar of Russia had hemophilia (2 recessive genes) Natural selection removes this condition from the gene pool, but gene is not eliminated because heterozygotes with it don’t express the trait

Populations Action of natural selection on phenotype s B. Genetic conditions aren’t eliminated by natural selection because so few individuals with the affected alleles express the recessive phenotypes

Ex: Cystic Fibrosis

Populations Action of natural selection on phenotype s C. Polygenic traits (skin color, height) Difficult to keep track of a single gene Measure prevalence of the trait in a population Usually form normal (hill shaped) distribution curves with many phenotypes clustered around an average value

Populations Directional Selection Frequency of a trait moves in one direction in a range Plays role in evolution of single gene traits

Directional Selection Ex: When fruit flies are raised in the dark and exposed to light If only the ones that flew towards light were allowed to reproduce- after several generations, the tendency to fly towards light increases

Populations Stabilizing Selection Extremes in a range of phenotypes are reduced Populations contain more intermediate individuals and less alleles promoting extremes

Assessment Describe the Hardy-Weinberg principle List the five forces that cause genetic change in a population Describe why natural selection against an unfavorable recessive allele is a slow process Explain how directional selection and stabilizing selection differ, and whether they tend to increase or decrease diversity Individuals who are heterozygous for the allele for sickle cell anemia are resistant to malaria. Explain the effects of natural selection on the frequency of the sickle cell allele in an area where malaria occurs. Which evolutionary force decreases the genetic diversity of a population by increasing the proportion of similar individuals

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