Download Chapter 20 Populations

Chapter 20 Populations
Properties of Populations
- A population consists of all the individuals of a species that live in one place at one
time.
- A population study focuses on a population as a whole – how many individuals
were born, reproduce, die, etc.
Population Size
- A population’s size is the number of individuals it contains.
o Size can be difficult to determine
 Plants versus animals (animals move, plants don’t).
 Most scientists will take a sampling of a population to estimate the
size of the population.
- Every population tends to grow because individuals reproduce over their lifetime.
But eventually, limited resources in an environment limit the growth of a
population.
- The statistical study of all populations is called demography.
3 key features of populations
- Most important feature: size.
o The number of individuals in a population, or population size, can affect
the population’s ability to survive.
 Small populations are more likely to become extinct.
 Random events, like fires or floods, endanger small populations.
 There is generally more interbreeding in small populations.
-
Second feature: Population density
o The number of individuals that live in a given area.
 If the individuals of a population are few and spaced far apart, they
may never encounter each other.
 Big problem for reproduction.
-
Third feature: Dispersion
o The way individuals are arranged in a space.
 There are 3 main patterns of dispersion; random distribution, even
distribution, and clumped distribution.
Scientists use models to explain how populations grow
- A population model is a hypothetical population that attempts to exhibit the key
characteristics of a real population.
o By making changes in this hypothetical model, scientists can predict what
might occur in a real population.
Population Dynamics
- All populations are dynamic – they change in size and composition over time.
o Birth rate – the number of births occurring in a period of time.
o Death rate (mortality rate) – the number of deaths in a period of time.
o Life expectancy – how long on average an individual is expected to live.
Age Structure
- The distribution of individuals among different ages in a population is called age
structure.
o In many species, old individuals do not reproduce. Populations with a high
percentage of young individuals have a great potential for rapid growth.
Patterns of Mortality
- Survivorship curves – show the likelihood of survival at different ages throughout
the lifetime of the organism.
o Example: humans and elephants don’t usually die until later in life. This is
called a Type I curve.
o Type II curve – the probability of dying doesn’t change throughout the life
of the organism.
o Type III – Many organisms die when they are very young but if it the
individual survives this early period, they are more likely to survive into old
age.
Measuring Populations
Growth rate
- Whether a population grows, shrinks, or remains the same depends on four
processes; birth, death, immigration, and emigration.
o Immigration – movement of individuals into the population.
o Emigration – movement of individuals out of the population.
- A simple population model that demonstrates the rate of population growth as the
difference between the birthrate and the death rate (scientists assume that
immigration and emigration are zero).
o This model is called a stage I model.
 Birth rate – death rate = growth rate
 In humans, birth and death rates are usually expressed as the
number of births and deaths per thousand people per year.
 Note: when calculating population, to see how many new
individuals will be added, take the growth rate and multiply it
by the number of individuals in the population.
o If the growth rate is positive – the population is
growing.
o If the growth rate is negative – the population if
shrinking.
Growth rate affects population size
- When population size is plotted against time on a graph, the graph generally shows
a J-shaped growth curve.
o An exponential growth curve is a curve in which the rate of population
growth stays the same; as a result the population size increases steadily
(stage II model).
 To calculate the number of individuals that will be added to the
population as it grows, multiply the size of the current population
(N) by the rate of growth (r).
 Populations generally don’t grow unchecked.
o Their growth is usually checked by predators,
disease, and the availability of resources (limiting
factors). Eventually, growth slows, and the
populations may stabilize. The population size that
an environment can sustain is called the carrying
capacity.
Resources affect population size
- As a population grows, resources eventually become limited, and eventually,
depleted.
o The population model can be adjusted to account for the effect of limited
resources, such as food and water.
 These resources are called density-dependant factors, because the
rate at which they become depleted depends on the size of the
population that uses them.
 The population model that takes into account the declining
resources available to populations is called the logistic model
of population growth (stage III) model.
o Logistic model is limited by a density-dependant
factor.
 It takes into account carrying capacity, the
number of individuals the environment can
support over time.

When a population reaches its carrying
capacity, the birth rate equals the death rate
and growth stops. This type of growth is
known as logistic growth.
How this all looks:
Stage I model: calculating the population growth rate
(Rate of growth) r = birthrate – death rate
Stage II model: exponential growth curve
N (change in population) = rN
Once r has been determined for a population, using the stage I model, the number
of individuals that will be added to a population as it grows is equal to the rate of growth
multiplied by the number of individuals in the current population (N).
Stage III model: logistic model
N = rN (K-N) / K
Population size calculations often need to be adjusted by the amount of resources
available (K).
Population Regulation
Real populations exhibit a range of growth patterns
- Many species of plants and insects are generally not limited by density-dependent
factors, but rather by density-independent factors.
o Examples: weather and climate.
Rapidly growing populations
- Many species, including bacteria, some plants and insects, are found in rapidly
changing environments.
- Such species, called r-strategists, grow exponentially when environmental conditions
allow them to reproduce.
o So, sometimes the population is very large, other times, very small.
o Generally, these are the populations where the offspring mature quickly and
with little or no parental care.
Slowly growing populations
- Populations that grow slowly generally have small population sizes.
- These species are called K-strategists, because their population density is usually
near the carrying capacity (K) for the environment.
o They are generally categorized by a long lifespan, few young, and a slow
maturing process, with reproduction late in life. They also provide
extensive care for their young.
Forces That Change Population Allele Frequencies
- Natural selection alters the proportions of alleles within populations
- Populations change in response to evolutionary forces
Allele Frequencies
- G.H. Hardy (English mathematician) and Wilhelm Weinberg (German
physicist)
o Independently demonstrated that dominant alleles do not automatically
replace recessive alleles
o Use of algebra and theories of probability
o Showed frequency of alleles in a population and ratio of heterozygote
individuals to homozygote individuals do not change from generation to
generation
 Unless the population is acted on by processes that favor a
certain allele
 For example dominant lethal alleles (the individual dies before it
can be passed on)
o Hardy-Weinberg Principle: frequencies of alleles in a population do not
change unless evolutionary forces act on the population
Hardy-Weinberg
- Principle holds true if population is:
o Large enough to not mate with relatives
o Not acted on by the five evolutionary forces:
 Mutation
 Gene flow
 Nonrandom mating
 Genetic drift
 Natural selection
2
2
- p +2p+q = 1
o p2: frequency of individuals that are homozygous for the allele A
o 2pq: frequency of individuals that are heterozygous for the alleles A and
a
o q2: frequency of individuals that are homozygous for the allele a
o Predicts genotype frequency
Mutation
- Mutation rated in nature are very slow
o 1 to 10 times per 100,000 cell divisions
- Not all result in phenotypic changes
o Remember, more than one codon can code for the same amino acid
- Mutation is the source of ALL variation making evolution possible
Gene Flow
- Movement of individuals to (immigrants) or from (emigrants) a population can
cause genetic change
o This movement is called migration which causes gene flow
 Gene flow: movement of alleles into or out of a population
 Immigrants: adds alleles to a population
 Emigrants: remove alleles from a population
Nonrandom Mating
- Preference when determining who to mate with
- Inbreeding
o Mating with relatives (inbreeding).
o Lowers frequency of Hardy-Weinberg predicted heterozygote individuals
 Changes frequency of alleles
 Increases proportions of homozygote individuals in a population
- Choose who to mate with based on traits
Genetic Drift
- In small populations:
o Allele frequency can be greatly altered by chance events (for example a fire)
o Loss of one individual greatly effects allele frequency
- Gene drift: random change in allele frequency in a population
o Causes genetic uniformity
 Reduced disease resistance
Natural Selection
- Causes deviation from Hardy-Weinberg Principle
o Directly changes frequency of alleles (increase or decrease)
 Depends on alleles effect on survival and reproduction
o Most powerful agent of genetic change
Natural Selection Acts Only on Phenotypes
- Does not act directly on genes
o Acts on phenotypes, not on genotypes
- Allows individuals who express favorable traits to reproduce and pass on those
traits to offspring
When Selection Acts
- Only expressed characteristics can be targets
o Selection cannot operate against rare, recessive alleles even when they are
unfavorable
- Allele must be common enough to have heterozygote individuals mate to
produce homozygote offspring
Why Genes Persist
- Genetic conditions do not become eliminated by natural selection
o Due to few individuals expressing the recessive phenotype
Natural Selection Changes Trait Distribution in a Population
- Phenotypes are controlled by one or more number of genes
o Polygenic traits: traits influenced by several genes
 Examples include human height and hair color
 These exhibit a range of phenotypes (extremes and clustered
around the average)
 Normal distribution: a bell-shaped curve that results when
the values of a trait in a population are plotted against their
frequency
- Natural selection most strongly influences genes that make the greatest
contribution to the phenotype
Directional Selection
- Selection can eliminate one extreme from a range of phenotypes
o Alleles promoting the extreme phenotype become less common in the
population
o Directional selection: selection that causes the frequency of a particular trait
to move in one direction
 Characterizes the evolution of single gene traits
 For example antibiotic resistance in disease-causing
bacteria
Stabilizing Selection
- Causes the phenotypic extremes to become eliminated
- Causes the frequency of the intermediate phenotypes to increase
o The population has fewer individuals that have alleles promoting extreme
types
- Stabilizing selection: the average form of the trait is favored and becomes more
common
o Very common in nature
Human Population Growth
- started out as hunter/gatherers.
- Then came the development of agriculture (agricultural revolution).
o Domesticating animals and farming.
o When this happened, it lead to a population explosion.
Population Growth Today
- about 20% of the world’s population live in developed countries.
- Most people live in developing countries.
o The growth rate in developing countries is a lot higher than in developed
countries.