1. Mechanisms of Evolution
AP Biology

2. Mechanisms of Change Mutation Migration Genetic drift
Founder effect
Bottleneck
Natural selection
AP Biology

3. Genotype vs. Phenotype Genotype Phenotype
Set of genes an organism carries
Phenotype
Observable characteristics
Influenced by both genotype and environment
In defining evolution we are concerned with genotypes that make up a population from generation to generation
AP Biology
Minzenmayer

4. Genetic Variation
Evolution does not operate without variation being present in population
Three primary sources of genetic variation
Mutations
Changes in DNA
Usually requires accumulation of many mutations
Gene flow
Movement of genes from one population to another
Sexual reproduction
Introducing new genes into a population through genetic shuffling
Mutations are random.
mutations can be beneficial, neutral, or harmful for the organism
mutations do not “try” to supply what the organism “needs.”
In this respect, mutations are random
whether a particular mutation happens or not is unrelated to how useful that mutation would be
Only mutations that matter to large scale evolution are those that can be passed to offspring
Germ line mutations—mutations in egg or sperm
AP Biology

5. Gene Flow Movement of genes from one population to another Migration
Examples
Pollen being blown to new destination
People moving to new cities or countries
Important source of genetic variation
AP Biology

6. Gene Flow Amount depends on organism Example Effects on evolution
Sedentary organisms more isolated than mobile ones
Example
Lower rate of gene flow
Corn—wind pollinated
Unlikely to fertilize individuals more than 50 ft away
Higher rate of gene flow
Fruit flies
Released in death valley and recaptured 15 km away
Effects on evolution
Within a population
Introduce or reintroduce genes to a population
Increases genetic variation
Across populations
Makes distant populations genetically more similar
Reducing chances of speciation
Less gene flow between two populations, speciation more likely to occur
AP Biology

7. Genetic Drift
Chance events changing frequency of traits in a population
not adaptation to environmental conditions
not selection
founder effect
small group splinters off & starts a new colony
bottleneck
some factor (disaster) reduces population to small number & then population recovers & expands again but from a limited gene pool
AP Biology
Minzenmayer

8. Effects of Genetic Drift on Evolution
Reduces genetic variation in populations
potentially reducing population’s ability to evolve in response to new selective pressures.
AP Biology

9. Effects of Genetic Drift on Evolution
Can contribute to speciation
small isolated population may diverge from the larger population through genetic drift.
AP Biology

10. Types of Genetic Drift Bottleneck event Founder effect AP Biology
Minzenmayer

11. Genetic Drift—Bottlenecks
Population bottlenecks
occur when a population’s size is reduced for at least one generation
genetic drift acts more quickly to reduce genetic variation in small populations
AP Biology

12. Example of Bottleneck Effect
Northern elephant seals
Reduced genetic variation
Population bottleneck in 1890s due to overhunting
Population size reduced to around 20 individuals
now over 30,000
Very little variation left in this population
AP Biology

13. Cheetahs All cheetahs share a small number of alleles 2 bottlenecks
less than 1% genetic variation
as if all cheetahs are identical twins
2 bottlenecks
10,000 years ago
Ice Age
last 100 years
poaching & loss of habitat
AP Biology
Minzenmayer

14. Founder effect
When a new population is started by only a small group of individuals
just by chance some rare alleles may be at high frequency; others may be missing
Usually reduced genetic variation
skew the gene pool of new population
human populations that started from small group of colonists
example: colonization of New World
AP Biology

15. All birds had an equal chance of survival to colonize the island.
A group of birds are flying at sea. A storm strikes and only a few birds survive. They land on an island and are able to find enough resources to reproduce. A scientist studies what happened and determines that this is the founder effect. Which of the following assumptions is the scientist basing his conclusion on?
All birds had an equal chance of survival to colonize the island.
The birds underwent natural selection during the colonizing of the island.
The birds are able to adapt to their new environment.
The scientist made no assumptions.
AP Biology

16. Genetic drift as an evolutionary factor is:
greater in a population with small numbers than a population with large numbers.
greater in a population with much genetic variation than in a population with little genetic variation.
responsible for the selection of mutations.
connected to the movements of alleles between populations of a single species.
AP Biology

17. Natural Selection
Some individuals are able to reproduce more than others (FITNESS)
Selection acts on any trait that affects survival or reproduction
predation selection
physiological selection
sexual selection
AP Biology

18. Comparing Forces of evolutionary change
Natural selection
traits that improve survival or reproduction will accumulate in the population
adaptive change
Genetic drift
frequency of traits can change in a population due to chance events
random change
AP Biology

19. Predation Selection Predation selection act on both predator & prey
behaviors
camouflage & mimicry
speed
defenses (physical & chemical)
AP Biology
Minzenmayer

20. Physiological Selection
Acting on body functions
disease resistance
physiology efficiency (using oxygen, food, water)
biochemical versatility
protection from injury
HOT STUFF! Some fish had the variation of producing anti-freeze protein
5.5 mya The Antarctic Ocean freezes over
AP Biology
Minzenmayer

21. Sexual Selection Acting on reproductive success
attractiveness to potential mate
fertility of gametes
successful rearing of offspring
Survival doesn’t matter if you don’t reproduce!
AP Biology
Minzenmayer

22. The lion’s mane…
Females are attracted to males with larger, dark manes
Correlation with higher testosterone levels
better nutrition & health
more muscle & aggression
better sperm count / fertility
longer life
But imposes a cost to male
HOT! Is it worth it??
AP Biology

23. Coevolution
Two or more species reciprocally affect each other’s evolution
predator-prey
disease & host
competitive species
mutualism
pollinators & flowers
AP Biology
Minzenmayer

24. Effects of Selection Changes in the average trait of a population
DIRECTIONAL SELECTION
STABILIZING SELECTION
DISRUPTIVE SELECTION
giraffe neck
horse size
human birth weight
rock pocket mice
AP Biology

25. Fitness
Describes how good a particular genotype is at leaving offspring in the next generation relative to how good other genotypes are at it
Example:
If brown beetles consistently leave more offspring than green beetles because of their color, you’d say that the brown beetles had a higher fitness
AP Biology

26. In a hypothetical population of beetles, there is a wide variety of color, matching the range of coloration of the tree trunks on which the beetles hide from predators. The graphs above illustrate four possible changes to the beetle population as a result of a change in the environment due to pollution that darkened the tree trunks

27. Which of the following includes the most likely change in the coloration of the beetle population after pollution and a correct rationale for the change?
The coloration range shifted toward more light-colored beetles, as in diagram I. The pollution helped the predators find the darkened tree trunks.
The coloration in the population split into two extremes, as in diagram II. Both the lighter-colored and the darker-colored beetles were able to hide on the darker tree trunks.
The coloration range became narrower, as in diagram III. The predators selected beetles at the color extremes.
The coloration in the population shifted toward more darker-colored beetles, as in diagram IV. The lighter-colored beetles were found more easily by the predators than were the darker- colored beetles.

28. Fitness Fitness is relative
A genotype may be very fit in one environment and not fit at all in another
Fitness lumps everything that matters to natural selection into one idea
Survival
Mate finding
Reproduction
Fittest individuals not always strongest, fastest or biggest
Fitness of a genotype depends on
Ability to survive
Find a mate
Produce successful offspring
AP Biology

29. Misconceptions about Natural Selection
Does not produce perfection
No population or organism is perfectly adapted
Natural selection has no goals
Natural selection is the simple result of variation, differential reproduction and heredity
“need”, “try” and “want”
are not very accurate
words when it comes
to explaining evolution
Natural selection selects
whatever variations exit in
the population—results in
evolution
AP Biology

30. Misconceptions about Natural Selection
Sometimes interpreted as a random process
Genetic variation in a population is random
Selection that acts on the variation is
non-random
Genetic variation that aids survival and reproduction are more likely to become common that other variation
Natural selection is not random!
AP Biology

31. Misconceptions about Natural Selection
Natural selection acts on individuals
Differential survival
Survival of the fittest
Differential reproductive success
Bear more offspring
Populations evolve
Populations of organisms change over time
Traits which offer greater fitness become more frequent in the population
AP Biology