Biology 30 AP - Population & Community Dynamics

G. Hardy and W. Weinberg

1908

frequency of alleles and genotypes in a population will remain constant from generation to generation if population is stable and in genetic equilibrium

Genetic equilibrium

no change

frequencies of certain traits will remain same

Microevolution

change present

frequencies of traits change over time (short period)

5 conditions for Hardy-Weinberg Equilibrium

1. A large breeding population

2. Random mating

3. No change in allelic frequency due to mutations

4. No immigration (coming in) or emigration (leaving)

5. No natural selection

Hardy-Weinberg Principle

p + q = 1.00

p = dominant allele

q = recessive allele

values referred to as allele frequencies

Hardy-Weinberg Equation

p² + 2pq + q² = 1.00

frequencies b/w 0.00-1.00

5 agents of evolutionary change

1. Mutation

2. Gene flow

3. Non-random mating

4. Genetic drift

5. Natural selection

Mutation

change in DNA

created by mutagens or mistakes during replication

can provide genetic diversity in a species and new alleles for a gene

Gene flow

migration of alleles from one population to another

Non-random mating

e.g. fertilization in flowers

random mating is uncommon in natural populations

mates are usually selected based on appearance and health

Genetic drift

only some plants in a population reproduce every generation

when plants do not reproduce → allele quickly lost from gene pool

bottleneck and founder effect

Bottleneck effect

parent population contains roughly equal numbers of yellow blue alleles → catastrophe occurs with only few survivors

most survivors have blue alleles → genetic drift → gene pool of next generation will contain mostly blue alleles

Founder Effect

gene pool change that occurs when a small number of individuals start a new, isolated population

few individuals from large population leave → establish a new population → allele frequencies in new population will differ from original population → further deviation as population expands

Natural selection

favourable variations

acts on mutations by selecting for most successful individual who has the mutation

mutation provides selective advantage → individual lives to reproduce → passes on mutation to offspring → evolution

Census

counting total in small area

Sampling

representatively sample small area and multiply by total area

assumes random distribution

4 factors affecting population size in given area

1. Natality (birth)

2. Mortality (death)

3. Immigration (coming)

4. Emigration (leaving)

Open populations

all 4 factors functioning

Closed populations

no migration (immigration and emigration)

Formula for population density

D_p = N/A or N/V

Dp: population density

N: # of individuals in a population

A: area

V: volume

increased density = increased population

Growth rate

change in population size

delta N = (births + immigrations) - (deaths + emigration)

• factors that increase size - factors that decrease size

formula: gr = delta N / delta t (time)

Per capita growth (cgr)

change in population size (delta N) relative to initial population size (N)

formula: cgr = delta N / N

effective when comparing populations of different sizes (e.g. school vs. community)

Biotic potential ( r )

capacity for reproduction

regulated by four factors

Four factors that regulate biotic potential

1. Max. # of offspring per birth

2. Chance that offspring will reach reproductive age

3. # of times per year organism reproduces

4. Age at which reproduction begins

Carrying capacity (K)

ability for environment to support a population

plateau on a graph (ALWAYS)

Density-dependent factors

usually biotic

affect population because of density of population

increase death & limit reproduction as population decreases

e.g. disease, predation, food supply, water quality

Density-independent factors

usually abiotic

affect the population regardless of density

work regardless of population size

e.g. floods, fires, drought

Environmental resistance

environmental conditions limit a species from growing out of control

influenced by abiotic and biotic factors

environment fighting back

J curves

exponential growth

increase in population size at an increasing growth rate

closed systems

often microorganisms with short life spans

e.g. bacteria, algae, yeast

exception: humans

S curves

most natural populations demonstrate a Sigmoid-shaped curve

population increases until limiting factors cause it to reach carrying capacity → resources maintained → population size stabilized

density dependent/independent factors initiate death phase

K-selected strategies

slow reproduction

long life span

longer offspring rearing (taking care of young)

later reproductive maturity

bigger animals

live at densities close to carrying capacity

e.g. elephants, humans, deer, bears

r-selected strategies

rapid reproduction

short life span

little/no rearing

early reproductive age

smaller animals

exploit less crowded niches

high growth rates

e.g. bacteria, yeast, mice, insects

Type I survivorship curve

large mammals

few offspring

low infant mortality

extended lifespan.

e.g. humans

Type II survivorship curve

chances of survival or death are about the same at any age

e.g. squirrels

Type III survivorship curve

low survivorship

high mortality rates early in life

e.g. oysters

Population histograms

useful for studying human populations to see trends and make predictions

double histograms: includes age and sex of a population

often pyramid shaped

Symbiosis

relationship between two or more different species

3 types: parasitism, commensalism, and mutualism

Predator-prey interaction

one species preys on another

Consumer-producer interaction

one species produces a resource for another

producers must be present in order to sustain the needs of consumers

no producers = no consumers = no community

Competition

individuals must compete for resources

2 types: intraspecific and interspecific competition

Parasitism

positive / negative

one benefits

another harmed

parasites require hosts (usually specific)

Commensalism

positive / neutral (0)

one benefits

other unaffected

Mutualism

positive / positive

both benefit

Predation

both predators and prey have specialized adaptations to assist with survival

2 ways to avoid predation

1. Camouflage: blend in

2. Mimicry: look like something scary

Intraspecific competition

individuals within the same population are competing for resources

Interspecific competition

different populations are competing for resources

Gause Principle

competitive exclusion principle

if 2 populations are competing for a limited resource → one eliminated

Niche

an organism’s role or place within an ecosystem

Succession

ways by which populations and communities change over time

Pioneer species

1st organisms to take root and build soil layer

e.g. lichen, moss, ferns, insects

Climax community

stable end populations that develop

Primary succession

community arising from NO previous living populations (i.e. barren land)

Secondary succesion

community begins to re-establish after partial destruction

e.g. forest fire, road paving, cultivation

Obligatory mutualism

each depends on the other exclusively

Facultative mutualism

neither is wholly dependent on the other