Populations

Chapter 15

Definition

• Population –

• Example – the human population of Grapevine, TX

Characteristics of Populations• Population size

– Number of individuals

• Population density– Number of individuals in an area

• Immigration– Organisms moving into a population

• Dispersion– How individuals are arranged in space

Types of Dispersion

Population Growth

= # births - # deaths

– If more individuals are born than die, the population grows

– If more individuals die than are born, the populations shrinks

Carrying Capacity (K)

• # of individuals the environment can sustain

Carrying capacity

What are some factors that would keep a population from going over their carrying capacity?

Density-Dependent Factors

• Limited Resources– Food– Water– Shelter– Competition– Predation– Parasitism

• When limited resources become depleted, population growth slows

Population cycles in the snowshoe hare and lynx

Why do the numbers of predators and prey cycle up and down together?

Density-Independent Factors

• Environmental conditions that limit population growth

– Weather (drought, floods, grass fires)– Climate – Human Disturbances

r- and K-Strategists

• r-strategists– Small– Reproduce often and large numbers– Unstable environment– Exponential growth– ex: mouse, bacteria, rabbits, fish

• K-strategists– Large– Reproduce less frequently, in numbers that they can

manage – parental care– Stable environment– Logistic growth– ex: elephant, bears, whales, horses

Populations evolve because allele frequencies change

• Things that change allele frequencies

– Mutations– Gene flow– Non-random mating– Genetic drift– Natural selection

T. Trimpe 2008 http://sciencespot.net/

Challenge #3Population Dynamics

1. If a population of 50 cottontail rabbits produce 300 young in a year, what is the birth rate for this population?

2. True or False – If the birth rate < death rate, the population will increase in size.

3. What term refers to the movement of animals into a population?

4. Use the graph to answer these questions.

A. In which year did the population decrease the most?

B. In which year did the population increase the most?

Answer each question.

Rabbit Population StudyData recorded at the end of each year.

1. If a population of 50 cottontail rabbits produce 300 young in a year, what is the birth rate for this population?

2. True or False – If the birth rate < death rate, the population will increase in size.

3. What term refers to the movement of animals into a population?

4. Use the graph to answer these questions.

A. In which year did the population decrease the most?

B. In which year did the population increase the most?

The answers are …

300 rabbits per year

Immigration

Year 4

Rabbit Population StudyData recorded at the end of each year.

False – The population would decrease.

Year 1

Gene Flow

• Migration of fertile individuals or gametes between populations

• Reduces differences between populations

Genetic Drift

Change in allele frequency due to chance. Happens in small populations.

Bottleneck Effect (Genetic Drift)

Preserving Variation

• Why are unfavorable alleles not eliminated from population?

• Diploidy

• Heterozygote advantage– Ex: sickle-cell allele and malaria

Natural Selection

• Darwinian fitness – contribution an individual makes to the gene pool of the next generation relative to the contributions of others

• Modes of selection– Directional– Diversifying– Stabilizing

Modes of Selection

Directional Selection

Diversifying Selection

A limited resource is known as a

A. Polygenic trait

B. Logistic model

C. Density-independent factor

D. Density-dependent factor

An organism that grows exponentially isA. Dispersed

B. Polygenic

C. An r-strategist

D. A K-strategist

_____ tends to happen in a population whose individuals have extreme traits.

A. Random mating

B. Directional selection

C. Stabilizing selection

D. Exponential growth

A colony of bacteria that has limited food will undergo ______ growth.

A. Exponential

B. Logistic

C. Natural

D. random

Which of the following is not a cause of genetic change?

A. Genetic drift

B. Random mating

C. Natural selection

D. mutation