CHAPTER 15 HOW A POPULATION GROWS

What is a population? ALL INDIVIDUALS OF A SPECIES THAT LIVE IN ONE PLACE AT ONE TIME.

Three key features of a population POPULATION SIZE POPULATION DENSITY - the number of individuals that live in a given area. DISPERSION - the way a population is arranged in a space. (Random, clumped, even)

Factors That Control Population Growth A population’s biotic potential is the size it would be if nothing stopped it. Why don’t populations grow indefinitely?

MODELING POPULATION GROWTH GROWTH RATE (r) = # OF BIRTHS - # OF DEATHS

GROWTH RATE AND POPULATION SIZE EXPONENTIAL GROWTH CURVE- Curve in which the rate of population growth stays the same as the population size increases steadily. (J shaped) Area of Rapid Growth GROWTH LIMITED BY: Predators, disease, and limited resources.

Population Growth Rate Exponential “J” Curve Some populations start slow and then have a burst of growth. This is called EXPONENTIAL growth. What shape curve does it have? Time Population

RAPIDLY GROWING POPULATIONS GROW EXPONENTIALLY WHEN THE ENVIRONMENT ALLOWS r-strategists Many offspring Little parental care High infant mortality EX. INSECTS, PLANTS

CARRYING CAPACITY- THE POPULATION SIZE THAT AN ENVIRONMENT CAN SUSTAIN.

Density dependent factors A variable affected by the number of organisms present in an area. Ex. food, water, shelter, disease

LOGISTIC MODEL A POPULATION MODEL IN WHICH THE GROWTH IS LIMTED BY DENSITY DEPENDENT FACTORS. Competition for food, shelter, mates, and limited resources increase as a population reaches its carrying capacity (k).

Population Growth Rate Logistic “S” Curve Some populations start slow, have a burst of growth, then level off. This is called LOGISTIC growth. What shape is the curve? Time (in days) Population

SLOWLY GROWING POPULATION A POPULATION WHOSE DENSITY IS USUALLY NEAR THE CARRYING CAPACITY. K-strategists Few offspring High parental care Low infant mortality EXAMPLE: Mammals

DENSITY INDEPENDENT FACTORS A VARIABLE THAT AFFECTS A POPULATION REGARDLESS OF DENSITY. EX. CLIMATE, NATURAL DISASTERS

Human Population What kind of curve is world population growth?

Human Population Where is the lag phase? The exponential phase?

Human Population Where is the carrying capacity?

Human Population What historical event could have caused the dip at the arrow?

Human Population Consider the Black Death Plague as an environmental change. Describe the population after the Black Death (yellow arrow) in terms of variation and survival of the fittest.

Human Population Why would the industrial revolution cause a population increase? (think in terms of FWSS and carrying capacity)

Human Population Where do we go from here? Children per Woman: High 2.5 Medium 2.0 Low 1.6

Human Population What is the Earth’s carrying capacity? When will we reach it? Who will survive? ? ? ? ? ? ? ? ? ? ?

How Populations Evolve

Things We Will Know How to summarize Hardy Weinberg principal Describe the five forces that cause genetic change in a population Identify why selection against unfavorable recessive traits is slow Contrast Directional and Stabilizing Selection

Hardy-Weinberg Principle G.H. Hardy and Wilhelm Weinberg researched to understand trait frequencies Found that dominant traits do not overcome recessive traits within populations. Populations typically do not change traits drastically unless acted on by an evolutionary force within the population.

What are the five forces that can cause populations to evolve? Populations Can evolve through Gene mutation Gene flow Nonrandom mating Natural selection Genetic drift Hardy-Weinberg Principle

Gene Mutations Gene mutations are very slow They mutate 1 to 10 times per 100,000 cell divisions Not all gene mutations are result in phenotypic changes, Still, some do cause change. Example: Sickle Cell Anemia Mutation is a source of variation in populations

Gene Flow Migration (individuals entering and leaving a population) is the major form of gene flow. Gene flow occurs because new individuals (immigrants) add alleles to a population Gene flow also occurs when individuals depart (emigrants) taking important traits away.

Nonrandom Mating Mating with individuals nearby is nonrandom mating. Inbreeding is a way to keep traits in a population from changing; lower frequency of heterozygous individuals The most common example of nonrandom mating is when females choose their mates based on popular or beneficial traits

Genetic Drift Natural disasters can wipe out large numbers within a population Losing large numbers causes loss of strong and beneficial traits. Because Natural Disasters are Random: A random change in traits within a population is called a Genetic Drift

Natural Selection Survival of the FITTEST Traits that are beneficial allow certain individuals to survive. Certain traits are not favorable and the animals containing those traits usually do not get to reproduce For example: Sickle Cell anemia is slowly declining in the U.S. because it is rarely passed on to another generation.

Slowly Removing Unfavorable Traits Tsar of Russia Alexi Nikolayevich had a homozygous recessive trait for hemophilia He died early and never passed one his traits, but all of his sisters were carriers Did the trait die with him? NO, every sister passed on the trait for several generations, but no one knew because it was not a phenotypic trait

Distribution of Traits Polygenic traits are traits that are influenced by dozens of genes. Polygenic traits cause normal distribution or an average curve among populations Usually called a Bell Curve

Contrasting Selection Directional selection is when one an extreme trait is favored and the opposite extreme trait is not. One extreme individual lives to reproduce while the other is lost in history Stabilizing Selection is when neither extreme traits survive, but the average individuals survive to reproduce