 What 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.

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Population size is the number of individuals in a population and can affect the population’s ability to survive.
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Small populations are more likely to become extinct b/c of inbreeding, a natural disaster or disease.
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.
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.

 A 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.

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Exponential growth curve
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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)

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Examples include:
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Predators
Disease
Availability of resources
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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.

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Logistic model
 Population model in
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 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.

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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

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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

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Allele Frequencies
Alleles are different forms of a gene
Hardy-Weinberg Principle
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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.

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Five principle evolutionary forces
 Mutations
 Gene flow
 Nonrandom mating
 Genetic drift
 Natural selection

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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.

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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.

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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 individuals
Female widowbirds prefer to mate with males, such as the one shown, that have long tails over males that have short tails. This increases the proportion of alleles for long tails in the population.

 The random change in allele frequency in a population
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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.

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The 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.
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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.

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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.

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Directional selection
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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

• Stabilizing selection
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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.