1. Unit 4: Principles of Ecology
Section 1 – How Populations Grow
Section 2 – How Populations Evolve

2. What is a Population? Population: Growth is based on:
All individuals of a species that live in the same location at the same time
Growth is based on:
Fecundity – ability to give birth
Resources
Adaptability
Demography: study of all populations in one area of an environment

3. Three Features of a Population
Population Size
Ability to survive depends on the number of individuals in a population
Smaller population are more vulnerable to becoming extinct due to random events
The smaller pops. have more inbreeding that can decrease the fitness of the population
Decreased fitness: more individuals with recessive and/or harmful traits

4. Three Features of a Population
Population Density
The amount of individuals that live in a given area
Smaller populations – more space = less availability to reproduce
Larger population – more choice in mate selection

5. Three Features of a Population
Dispersion
The way the organisms are arranged in their space
Random
Even
Clumped
Arranged by self-determination or chance
Arrangement consists of a regular pattern
The individuals are spaced by clusters or chunks

6. Population Modeling

7. Population Modeling

9. Population Modeling Resources & Population Size Logistic Growth
A graph of population size is restricted by density-dependent factors
Density-Dependent Factors
Factors that account for the amount of individuals in the population that are depleting resources and reduce the capability of population growth
Density Independent – weather and climate

10. Rapid Growing Populations
R-strategists: bacteria, some plants and insects
Environmental conditions allow these species to flourish and reproduce quickly
When environment conditions worsen:
The population depletes quickly
The organisms have a short-life either way
Small and quick maturing offspring allowing for no parental care necessary

11. Slow Growing Populations
K-strategists: whales, mammals, etc.
Population density is usual near carrying capacity
Longer life spans and slower maturing processes
Parental care is required and intense

12. Hardy-Weinberg Principle
The frequencies of alleles in a population do not change unless evolutionary forces act upon the population
Only true if:
The population is large
Members do not mate with relatives
No evolutionary forces ↓
Shifted by:
Mutation, Gene Flow, Genetic Drift, Nonrandom mating & Natural Selection

13. Mutation A source of genetic variation
Change of allele frequencies due to mutation are very slow
Not all mutations affect the phenotype
The mutations appear in the genetic code

14. Gene Flow Movement of alleles to or from a population
Caused by migration
The movement of individuals
Immigrants: add alleles
Emigrants: remove alleles
Make a drawing

15. Nonrandom Mating
Preferred mating with individuals nearby or with similar phenotypes
Inbreeding (mating with relatives) causes a lower frequency of heterozygotes – predicted by H-W
In actuality, inbreeding only increases the amount of homozygotes
Also caused by selective mating choosing by females – they prefer stronger and attractive traits
No change to frequencies
Stronger traits – ability to gather food and handsome individuals

16. Genetic Drift Random changes in allele frequencies due to an event
Isolated and smaller population are affected greater by drift
Cheetahs
Uniform DNA to prevent extinction
Reduced disease resistance

17. Natural Selection
Direct changing of allele frequencies by deviating from the H-W proportions
Selection against = reduces the favor of that trait
Ultimately helps raise the favor of the other trait or individual (homozygote or heterozygote)
Not always the most beneficial trait
Example: a male bird has strong abilities to gather food, but is also a carrier of a fatal disease, the phenotype of gathering food can be passed on with the carrier allele as only alleles expressed are acted upon

18. How Selection Acts
Only traits expressed are acted upon – sometimes causing rare recessive alleles to passed on
Once heterozygous individuals produces homozygous offspring – natural selection can be activated
The traits need to become common enough to be selected against
Hemophilia – recessive disorder

19. Persistence of Genes Homozygous recessive = 1/100
Heterozygous = 18/100
So 18 individuals carry the allele without expression
These individuals maintain that gene without even knowing it
The conditions cannot be eliminated as they are not common enough

20. Distribution of Traits
Populations are shaped by polygenic traits in reference to natural selection
Genes that contribute the most to the distributed are influenced the greatest
These traits and genes are measured by measuring each individual then providing an average
Normal distribution:
The average having the highest amount of individuals – creating a hill-shaped diagram
Ask what polygenic means

21. Directional Selection
The frequency of one trait moves in one direction towards the average to become closer normal distribution
Purpose: trying to eliminate one extreme from the range to balance out the average
Need an example – more spines – more protection

22. Stabilizing Selection
Distribution becomes narrower and selection favors similar individuals opposed to the extremes
In other terms: favoring intermediate phenotypes