1. Evolution of Populations
How can populations evolve to form new species?

2. Populations, not individuals, evolve
Can an organism respond to natural selection by acquiring or losing characteristics?.....

3. Populations, not individuals, evolve
An organism cannot develop a new phenotype by natural selection in response to its environment

4. Genes and Variation – genotype and phenotype in Evolution
Remember, organisms inherit 2 alleles for a particular trait
The combination of those alleles determines its Genotype
The genotype and the environmental conditions produces the organisms Phenotype
Natural Selection acts on the Phenotype
Some individuals have a phenotype that is better suited to their environment

5. Evolution occurs as a population’s genes and their frequencies change over time

6. Populations and Gene Pools
Many individuals make up a population
All of the genes of the individuals make up the populations genes
All of the alleles that make up these genes, (of every single individual) in the population is the
GENE POOL
Allelic frequency -is the percentage of any specific allele in the gene pool.

7. Genetic Equilibrium
If the genes in the population do not change, the population is said to be in Genetic Equilibrium.
Evolution is not occurring
Something must disrupt the populations gene pool in order for evolution to occur

8. Sources of Genetic Variation
Mutations- changes to the DNA
a single mutation could have a large effect, but usually evolutionary change is based on the accumulation of many mutations
Mutations are random
Can be beneficial, harmful, or neutral
Mutations do not “try” to make an organism better
Depending where they occur they may not affect evolution
If in a somatic cell they cannot be passed onto the offspring
Genetic Recombination in Sexual Reproduction
Crossing over and independent assortment
Lateral Gene Transfer
organisms that reproduce asexually, like bacteria can pass genes from one organism to the next.
Gene Flow
Movement of genes from one population to another

9. Natural Selection acting on Variations
Natural Selection acts on phenotypes by producing one of three types of selection:
Directional selection
Stabilizing selection
Disruptive selection

10. Natural Selection on Variations
Directional Selection- When natural selection favors one of the extreme variations of a trait.
Example: the change in color of a moth population as the trees they lived in turned a darker color from pollution.
Population changed from paler color to darker color

11. Natural Selection on Variations
Stabilizing Selection – Natural Selection that favors average individuals in a population.
Example: Population of spiders. Large spiders are easily seen and eaten. Small spiders can’t find food. Average spiders survive

12. Natural Selection on Variations
Disruptive Selection- Individuals with either extreme of a trait’s variation are selected for.
Example: Sea Snail shells range from white, to tan, to dark brown. Snails live on rocks. White snails living on light rocks are harder to see. Same for dark snails living on darker rocks. The tan are easily seen on light and dark and are eaten.

13. Other Ways Allele Frequencies May Change
Genetic Drift –change in allele frequencies due to chance.
Has a great affect on SMALL populations because the gene pool is reduced

14. Example: a bird population nests on the side of a cliff
Example: a bird population nests on the side of a cliff. A landslide may eliminate a portion of the population. If, just by chance, more of the birds in the landslide area have a certain allele, that allele’s frequency will decrease.

15. Other sources of Genetic Variation
Migration -movement of individuals into or out of a population
If the population is small, the effect of even a small number of migrating individuals can be great.
Especially if a rare allele is removed from the population

16. Other sources of Genetic Variation
Founder Effect -results from the formation of a new population from just a few individuals who leave a larger population
The new populations frequencies will reflect those of the founding population rather than the original population.

17. Other sources of Genetic Variation
Population Bottleneck
Occurs when an entire population is reduced to just a few individuals, often as a result of some form of disaster, such as disease

18. The Process of Speciation: How does one species become two?
Species – a population or group of populations whose members can interbreed and produce fertile offspring
In order to get new species populations must become Reproductively Isolated from one another
Reproductive Isolation – occurs when formerly interbreeding organisms can no longer mate and produce fertile offspring

19. How does one species become two? Isolating Mechanisms
Reproductive Isolation-can develop in a variety of ways including
Geographic Isolation
Behavioral Isolation
Temporal Isolation
These are examples of Prezygotic mechanisms
- before egg and sperm come together

20. Isolating Mechanisms- Geographic Isolation
A physical barrier divides a population
Called -Allopatric speciation
Populations are separated by geographic barriers such as rivers, mountains, or bodies of water.

21. Isolating Mechanisms Not a physical boundary
Called – Sympatric speciation
Behavioral Isolation
Populations that develop differences in courtship rituals or other behaviors.
Temporal Isolation
When two or more species reproduce at different times.

22. Postzygotic Mechanism
After sperm and egg come together
Example:
Infertile offspring
Huge fetus – die during child birth
Weak offspring that die quickly
These examples do not allow reproduction to continue and lead to isolation.