Chapter 15: Populations• Section 1: How populations grow.– What is a Population?– Modeling Population Growth.– Growth Patterns in Real Populations.

• Section 2: How populations evolve.– The Change of Population Allele Frequencies.– Action of Natural Selection on Phenotypes.– Natural Selection and the Distribution of Traits.

Section 1: How populations grow• What is a population?– We learned this last chapter.– This chapter we will learn:• What causes populations to grow?• What determines how fast they grow?• What factors can slow their growth?

• A population consists of all the individuals of a species that live together in one place at one time.– Examples• All the E. coli living in your intestine.• All of the same species of fish swimming in a pond.

• Populations grow b/c organisms normally reproduce multiple times.

• Population growth is limited due to limits of natural resources.• Demography: the statistical study of all populations.– Demographers study the composition of a population and try

to predict how the size of the population will change.

Three Key Features of Populations1. Population Size: – it is the number of individuals in a population– one of the most important features of any population– can affect the populations ability to survive.– Studies show that very small populations are among those most

likely to become extinct.• Random events and natural disturbances endanger small

populations more than large populations.• Experience more inbreeding

– This leads to less genetic variability.Example: worldwide Cheetah population

Three Key Features of Populations

2. Population density: the number of individuals that live in a given area.– If the individuals of a population are few and are

spaced widely apart, they may seldom encounter one another,

making reproduction rare.– This would be low density.– Population density in USA as of

1990.

Three Key Features of Populations3. Dispersion: the way the individuals of the

population are arranged in space.– Three patterns of dispersion are possible.

1. Self determined or determined by chance: occurs when individuals are randomly spaced.

2. Regular intervals: occurs if individuals are evenly spaced.3. Clumped distribution: individuals are bunched together in

clusters.

– Each pattern reflects the interactions between the population and its environment.

Modeling Population Growth• The first step to determining how a population will

grow is to make a create a hypothetical population and to attempt to show key characteristics of a real population. This is a population model.

• This is useful, b/c demographers can use the model to predict what might occur to a real population if certain changes were made to the real population.

• There are three stages of population growth that are included in the model.

3 stages of population growth1. Growth rate: a population grows when more

individuals are born than die in a given period.2. Growth rate and population growth: when

population is plotted against time on a graph, the population growth curve resembles a J-shaped curve and is called an exponential growth curve.• A curve in which the rate of population growth stays the same, as a result the population sizes increases steadily.

• Exponential curve: to calculate the number of individuals that will be added to a population as it grows, multiply the size of the current population (N) by the rate of growth (r).

• Normally, as we learned last chapter, populations do not always grow unchecked (ie death). The population size that a given environment can sustain is called the carrying capacity (K).

• When the carrying capacity is considered, the population is modeled with a logistic growth curve or logistic model (example on next slide).

K

K is the carrying capacity

Logistic Growth Curve• Logistic model- a population model in which

exponential growth is limited by a density-dependent factor.

• It is a population model that takes into account the declining resources available to populations– Assumes that birth and death rates vary with population size.– When the population is below carrying capacity the growth

rate is rapid.– As the population approaches the carrying capacity, death

rates begin to rise and birthrates begin to decline because competition is increasing.

What is the difference between this two growth curves?

Which has limited natural resources and therefore increased competition?

Which one has the strongest natural selection pressure?

Growth Patterns in Real Populations

-Things aren’t always as simple as suggested in the growth models.

-Often, growth is not limited by density- dependent factors, but instead growth is limited by density-independent factors. For example, mosquitos are most common in the summer (more rain = more places to lay eggs).

-Density-dependent factors: for example: food & water. Resources that become depleted based on population size.

-Density-independent factors: for example: the climate. Factors that aren’t affected by population size.

• The growth of fast growing plants and organisms is often described by an exponential growth model (r- strategist).

• The growth of slower growing plants and organisms is often described by a logistic growth model (K-strategist).

• Most species are located somewhere in the middle.• Some species switch between the two, as their

environment changes.

R- strategists• R- strategists grow exponentially when the environmental

conditions allow them to.– This results in temporarily large populations (mosquitoes in

summer).– When environmental conditions are not ideal, the population

quickly declines.– Reproduce early in life.– Have many offspring each time the reproduce.– Small offspring that mature rapidly with little or no parental care.

K-strategist• Mature slowly.• Often have small population sizes.• Their population density is usually near the carrying

capacity (K) of their environment.• Long life span• Few young• Reproducelate in life• Care for young• Live in stable Environments.

Section 2: How Populations Evolve• Scientists wondered if dominant alleles would

spontaneously replace recessive alleles within populations.

• 1908- G.H. Hardy & Wilhelm Weinberg demonstrated that this is not the case. Dominant alleles do NOT automatically replace recessive alleles.

• Using algebra, they showed that the frequency of alleles in a population does not change.

• The ratio of heterozygous to homozygous individuals does not change from generation to generation unless the population is acted on by processes that favor particular alleles.

• This discovery is called the Hardy-Weinberg principle.– It states that the frequencies of alleles in a population do

not change unless evolutionary forces act on the population• It is an equation that can be used to predict genotype

frequencies in a population.

– For example, a dominant allele that is lethal will not become more common just because it is dominant. (Actually, it kills people, so if those people haven’t reproduces, the allele could become less common).

– The principle holds true for any population as long as: 1) the population is large enough that its members

are not likely to mate with relatives and as long as 2) evolutionary forces are not acting on the population.

5 principle evolutionary forces: these forces can’t be used with Hardy-Weinberg.

1. Gene flow:2. Mutation3. Nonrandom mating4. Genetic drift5. Natural Selection

- These forces cause the ratios of genotypes in a population to differ significantly from those predicted by the Hardy-Weinberg principle.

Evolutionary Force: 1) Mutation

• This is why HW does not hold true for mutations:

• Mutation from one allele to another can eventually change allele frequencies, but it happens very slowly.

• Not all mutations result in phenotypic changes.• Mutation is however a source of genetic

variation and makes natural selection possible.

Evolutionary Force: 2) Gene Flow

• Movement of individuals from one population to another can cause genetic change.

• This movement is called migration and creates gene flow.

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

• It occurs because new individuals (immigrants) add alleles to the population and departing individuals (emigrants) take alleles away.

Evolutionary Force: 3) Nonrandom mating

• Occurs when individuals prefer to mate with others that live nearby or are of their own phenotype. Therefore, mating is not random.

• Example: mating with relatives- causes lower frequency of heterozygotes than would be predicted with the H-W principle.

• Example: choosing a mate based on size, color, abilities, etc.

Evolutionary Force: 4) Genetic Drift• In small populations, the frequency of an alleles can

be greatly changed by an unexpected event (ie natural disaster, fire, etc).

• Small populations that are isolated from one another can differ greatly as a result of genetic drift. (The same disaster that happens to one population may not also happen to the next population).

• This leads to a decrease in mutations, which could mean that alleles frequencies can’t adapt to disease and an individual will be more likely to die from the disease.

Evolutionary Force: 4) Natural Selection

• Causes deviations from the H-W principle by directly changing the frequencies of alleles.

• The frequency of an allele will increase or decrease, depending on the allele’s effects on survival and reproduction.

• Natural selection is one of the most powerful agents of genetic change.

Action of Natural Selection on Phenotypes

• Natural selection enables individuals who express favorable traits to reproduce and pass those traits on to their offspring. Therefore, natural selection acts on phenotypes, not genotypes.

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

• A trait that is influenced by several genes- polygenic trait.– Ex: human height and hair color

Normal & Directional SelectionThis is called normal distribution. This distribution is often seen by polygenic traits in a population. The range of phenotypes is clustered around an average value.

This is direction selection. It is when the frequency of a particular trait moves in one direction in a range. Plays a role in single-gene traits like pesticide resistance. It eliminates one extreme from a range of phenotypes. So, the alleles promoting this extreme become less common in a population.

Stabilizing Selection

• Occurs when selection reduces extremes in a range of phenotypes, the frequencies of the intermediate phenotypes increase. The population ends up containing fewer individuals that have alleles promoting extreme types. The distribution becomes narrower, stabilizing the average, by increasing the proportion of similar individuals. Very common in nature.