1. POPULATIONS

2. Key Features of Populations
1. Size - number of individuals in a population
- determines ability of population to survive
small populations: most likely to become extinct
- more inbreeding increases bad alleles
and reduces fitness
2. Density - number of individuals in a given area
small populations: individuals rarely meet and reproduce due to low density

3. Dispersion - how individuals are arranged in a given area
regular (uniform): evenly spaced intervals
random: location in not orderly
clumped: bunched in clusters

4. Populations are affected by:
- growth rate
- available resources
- predators and disease

5. Population Density
Number of individuals of a particular species per unit area or volume
ex: number of alligators/km2 of swamp number of bacteria/cm2 of agar plate
number of earthworms/cc of soil
If there are 1000 beech trees in a 50 km2 forest, what is the population density of the beech trees?
population density = individuals
unit area

6. Tracking Population Growth
Population Age Structure Pyramid
X-axis : population
Y axis: age
males on left
females on right
shows the distribution of various age groups in a human population
forms the shape of a pyramid when the region is healthy
indication of the reproductive capabilities and likelihood of the continuation of a species

7. Limits to Population Growth
Carrying capacity
maximum number of organisms in a population that an environment can maintain or “carry” without a net increase or decrease

8. Factors That Affect Population Growth Limiting Factor: a condition that restricts population growth
Density Dependant Factor
factor that limits a population more as population density increases
ex: food, water, space, disease

9. Factors That Affect Population Growth
Density Independent Factor
factors that limit population growth that are unrelated to population density
ex: weather events, fires, floods, major changes in a habitat
A population of aphids typically grows exponentially in the wet springs months The population nearly dies off in the hot, dry summer. Weather is a density-independent factor that limits the aphid populations

10. MODELS IN POPULATION GROWTH
Population Model: hypothetical population which exhibits key characteristics of a real population
- allows demographers to predict outcomes that might occur in a real population

11. MODELS IN POPULATION GROWTH
1. Stage I model
growth rate
- birth rate vs death rate
expressed as:
# births and deaths/ 1000 people/ year

12. Stage II model (exponential)
- population size vs time
- J shaped curve
- rate stays same and population
size increases steadily
- growth affected by predators,
disease, and availability of resources
- calculation to predict the number of individuals added to a population
# = N x r
(size of current pop.) (rate of growth)
- carrying capacity (k) – pop. size an environment can sustain

13. Stage III model (logistical)
- S shaped
- exponential growth limited by a density dependant factor
(food and water)
- refers to ability of population to solve day to day problems of living
Size below K: rapid growth rate
Size near K: death rates rise, births
decline growth of population slows
At K: birth = death
population stops growing
- most realistic model in nature

14. Population Growth Cycles
Boom and Bust Cycle
- direct relationship between predator and prey
- causes fluctuations in populations in stable ecosystems
ex: increases in hare population are followed closely by increases in lynx population

15. Population Growth Patterns
r strategists
- rapidly growing populations
- large population size
- affected by density independent factors (weather, climate)
- in good conditions, grow exponentially
- in poor conditions, population size drops quickly
- short life span, reproduce early, many offspring
Ex: insects, plants, bacteria

16. k strategists
- grow slowly
- affected by density dependant factors (food, water)
- small population size
- population density near K (carrying capacity)
- long life span, few young, slow maturation, repro late in life, take care of young
ex: whales , redwood trees

17. Genetics and Evolution
Populations evolve
- each individual in population has own assortment of alleles
- as they reproduce and die, genetic makeup of population changes from generation to generation

18. POPULATION GENETICS study of changes of genetic makeup of populations
Gene frequencies: each member of population has alleles which are common with others
ex: if all members of population have gene to make certain enzyme
frequency = 100%
If only 1 in 100 individuals have gene
frequency = 1/100, , 1%
- over time the gene frequency for a particular allele changes

19. frequencies in a population.
Modern evolution studies the gradual change in allele
frequencies in a population.

20. Sources of Genetic Variation in Populations
1. Mutation
DeVries (dutch botanist) 1900
introduced concept of mutation
evening primroses
occasionally plant appeared with totally new form
plants bred true in future generations
he considered sudden changes to be mutations
major source in genetic variation
most recessive

21. 2. Gene flow through migration
- new individuals (immigrants) add alleles to population
- emigrants take away alleles from population
3. Genetic drift - random change in allele frequency
4. Natural selection
- most powerful agent of genetic change
- alleles most favorable to allow survival, reproduce and increase in population
- reduces harmful genes in a population

22. HARDY WEINBERG LAW 1908: Hardy (English mathematician)
Weinberg (German physician)
- stated that the proportion of genes in a population doesn’t change much over generations (genetic equilibrium)
- Hardy-Weinberg equilibrium describes the null model evolution. For a population to be in Hardy- Weinberg equilibrium, five conditions must be met:

23. 5 Conditions for H.W.E.
1. No Genetic Drift (very large population size)
2. No Migration (no gene flow)
3. No Mutation
4. No Selection (no differential selection/ assortative selection)
5. Random Mating (no differential reproduction: all individuals should have equal chance of reproducing)
If all five of these conditions are met, the allelic and
genotypic frequencies will remain the same from
generation to generation.

24. There are two formulas that must be memorized:
The Hardy-Weinberg formulas allow us to detect some allele
frequencies that change from generation to generation,
thus allowing a simplified method of determining that
evolution is occurring.
There are two formulas that must be memorized:
p2 + 2pq + q2 = 1
p + q = 1
p = frequency of the dominant allele in the population
q = frequency of the recessive allele in the population
p2 = percentage of homozygous dominant individuals
q2 = percentage of homozygous recessive individuals
2pq = percentage of heterozygous individuals

25. Natural Selection Distribution Curves
NORMAL BELL CURVE
normal distribution of data: most of the examples in a set of data are close to the "average," while relatively few examples tend to one extreme or the other
x-axis: the value in question
y-axis: number of data points for each X value
Ex: number people who eat X calories per day

26. Natural Selection Distribution Curves
1. Directional selection
- eliminates one extreme of the phenotypes so it becomes less common
- causes frequency of particular trait to move in one direction
- characterizes evolution of single gene traits
ex: pesticide resistance
antibiotic resistance
peppered moth
shorter                                                                       longer
Giraffe Neck Length

27. 2. Stabilizing selection
- eliminates extremes at both ends of phenotype
- intermediate phenotypes increase
- results in fewer extreme phenotypes
shorter                                                                        longer
Rabbit Leg Length

28. - eliminates average phenotype
3. Disruptive selection
- individuals with either extreme variation of a trait have greater fitness than individuals with average form of trait
- eliminates average phenotype
- results in two extreme phenotypes (new species)
smaller                                                                                 larger
Size of Clams

29. Other Causes of Population Genotype Changes
Founder effect (Ernst Mayr)
- The establishment of a new population by a few original founders carry only a small fraction of the total genetic variation of the parental population
- reason:
a small number of
individuals may
colonize a place
previously uninhabited
by their species
- result:
the frequencies of the genes
may differ from the parental
population

30. Bottleneck effect:
- an evolutionary event in which a significant percentage of a population or species is killed or prevented fro reproducing
- effect:
reduction of a population’s gene pool and the accompanying changes in gene frequency produced when a few members survive the widespread elimination of a species