How Populations GrowWhat is a population?

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

• What is demography?Demography is the statistical study of all populations.

3 Key Features of Populations1. Population size is the number of individuals

in a population and can affect the population’s ability to survive.

Small populations are more likely to become extinct b/c of inbreeding, a natural disaster or disease.

2. Population density is the number of individuals that live in a given area.

If few and spread out, they may not encounter one another to breed.

3. Dispersion is the way individuals of a population are arranged in a space.

They can be evenly spaced, clumped together or in a random distribution.

Three Patterns of Population Dispersion (p. 321)

Modeling Population GrowthA population model is a hypothetical population that attempts to exhibit the key characteristics of a real population. Allows demographers to predict what might occur in a real population.

Describes the rate of population growth as the difference between the birth and death rates expressed as the # per 1,000 people.

Growth Rate & Population SizeExponential growth curve

Curve in which the rate of population growth stays the same, as a result the population size increases steadily.

This is called the J-shaped curve. (p. 322)

Limiting Factors of Population Growth

Examples include: Predators Disease Availability of resources

Carrying capacity (K) The population size that an environment

can sustain. Density-dependent factors include

limited resources such as food and water, because the rate at which they become depleted depends upon the population density of the population that uses them.

Resources and Population Size Logistic model

Population model in which exponential growth is limited by a density-dependent factor It’s a population model that takes into

account the declining resources available and assumes that birth & death rates vary.

When a population is below carrying capacity, the growth rate is rapid, however as it approaches the carrying capacity, death rates rise and birth rates decline.

Growth Patterns in Real Populations Density-independent factors

Weather & climate• r-strategists

Grow exponentially when environmental conditions allow them to reproduce.

Ex: bacteria, some plants, many insects such as cockroaches & mosquitos

Growth Patterns in Real Populations• K-strategists

Organisms that grow slowly, such as whales, and have small population sizes

They are called K-strategists because their population density is near the carry-capacity of their environment.

Typically larger, slow maturing, long life span and dependent newborns.

Ex: Many endangered species such as tigers, gorillas, and whales

How Population Evolve• Allele Frequencies

Alleles are different forms of a gene• Hardy-Weinberg Principle

The frequencies of alleles in a population do not change unless evolutionary forces act on the population.

It’s a process that favors particular alleles. For example, a dominant lethal allele will not become common just because it’s dominant.

How Population Evolve• Five principle evolutionary

forces Mutations Gene flow Nonrandom mating Genetic drift Natural selection

Mutations• Mutations are a source of variation

that makes evolution possible. Mutations of alleles can change allele frequencies.

However, most genes mutate only about 1-10 times per 100,000 cell divisions.

Not all give phenotypic changes (outward appearance).

Some will change protein function.

Gene Flow• Gene flow is the movement

of alleles into or out of a population.

• Migration of individuals to or from a population either adds alleles or removes alleles.

Nonrandom Mating• Individuals who mate with others

that live nearby or are of their own phenotype. Inbreeding, self-fertilizing plants, and choosing mates based on certain traits

Increases the number of homozygous individualsFemale widowbirdsprefer to mate with males,such as the one shown, thathave long tails over males thathave short tails. This increasesthe proportion of alleles forlong tails in the population.

Genetic DriftThe random change in allele frequency in a population

In small populations, the frequency of an allele can be greatly changed by a chance event, such as a fire or landslide

When an allele is found in only a few individuals, the loss of even one individual can drastically affect the allele frequency. This can result in genetic uniformity which leads to reduced disease resistance.

Cheetahs are endangered. Cheetahs have gone through at least two drastic declines in population size.

Natural SelectionThe process by which individuals that have

favorable variations and are better adapted to their environment survive and reproduce more successfully than less well adapted individuals do.

The frequency of an allele will increase or decrease, depending on the allele’s effects on survival and reproduction. Ex: the allele for sickle cell is declining in frequency in

the U.S. because individuals who are homozygous rarely have children.

Ex: Hemophilia in a family. The last Tsar of Russia’s only son, Alexei

Nikolayevich (yellow circle), had hemophilia, a blood-clotting disorder that affects males who have a single copy of a recessive gene.

Natural Selection & the Distribution of Traits

• Normal distribution Polygenic traits that exhibit a range of

phenotypes clustered around an average value – forms a bell-shaped curve

• Directional selection The frequency of a particular trait

moves in one direction in a range.• Stabilizing selection

The distribution of a particular trait becomes narrower, tending to stabilize the average by increasing the proportion of similar individuals.

Natural Selection & the Distribution of Traits

• Directional selection The dashed line represents the average height for

this population When selection eliminates one extreme from a

range of phenotypes, the alleles promoting this extreme become less common in the population.

Directional selection has a role in the evolution of single-gene traits, such as pesticide resistance in insects.

Normal distribution

Natural Selection & the Distribution of Traits

• Stabilizing selection When selection reduces extremes in a range of

phenotypes, the frequencies of the intermediate phenotypes increase.

As a result, the population contains fewer individuals that have alleles promoting extreme types.

In stabilizing selection, the distribution becomes narrower, tending to “stabilize” the average by increasing the proportion of similar individuals.