1. Evolutionary Forces at work What changes populations?
Natural Selection:
Evolutionary Forces at work
What changes populations?

2. Forces of evolutionary change
1. Natural selection
traits that improve survival or reproduction will accumulate in the population
Referred to as: adaptive change
2. Genetic drift
frequency (how often they occur) of traits can change in a population due to chance events
The changes are random

3. 1. Natural Selection
Selection acts on any trait that affects survival or reproduction
a. predation selection
b. physiological selection
c. sexual selection

4. Predation Selection a. Predation selection
acts on both predator & prey
Behaviors
camouflage & mimicry
speed
defenses (physical & chemical)

5. b. 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

6. Physiological selection
Dogs pee on trees…Why don’t trees pee on dogs?
NH3
plant nutrient
animal waste
One critter’s trash is another critter’s treasure!

7. Survival doesn’t matter if you don’t reproduce!
c. 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!

8. Example of Sexual selection: Lion’s mane
It’s FEMALE CHOICE, baby!

9. 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??
YES! This implies greater ‘fitness’ in the male
Even though it’s very hot to have a large mane the benefit of attracting mates and successfully producing & rearing young since you have that large mane outweighs the costs.
Females who chose these males were more “successful” (more, healthier young) and therefore had a greater opportunity to pass on the trait of being attracted to longer darker manes to their daughters and the trait of having longer, darker manes to their sons.

11. In people…being ‘Sexy’ = fitness markers

12. Fertility figures through the ages!

13. Is there a testable hypothesis in there?
Sexual selection
Acts in all sexually reproducing species
the traits that get you mates
sexual dimorphism (male and female are very different :
( di = two / morph = body)
influences both morphology & behavior
can act in opposition to natural selection
Jacanas
Is there a testable hypothesis in there?

14. 2. 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

15. albino deer Seneca Army Depot
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
skew the gene pool of new population
human populations that started from small group of colonists
example: Amish (1700’s/400 people)
Small founder group, less genetic diversity than Africans
All white people around the world are descended from a small group of ancestors
100,000 years ago
(Chinese are white people!)
albino deer Seneca Army Depot

16. Out of Africa Likely migration paths of humans out of Africa
10-20,000ya
10-20,000ya
Migration Paths
According to the "Out of Africa" theory, modern humans appeared as a single African species nearly 100,000 years ago, then spread throughout the world (K.Wong, Is Out of Africa Going Out the Door?, Scientific American 281(2), August 1999).
50,000ya
Many patterns of human traits reflect this migration

17. Bottleneck effect
When large population is drastically reduced by a disaster
famine, natural disaster, loss of habitat…
loss of variation by chance event
alleles lost from gene pool
not due to fitness
narrows the gene pool

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

19. Can lead to‘fixation’
All individuals in the population are homozygous for an allele
Example in book:
Northern Elephant seal – on brink of extinction with 20 left
Laws change, now 170,000 in pop
Every seal is homozygous for all genes tested (no diversity!)
Why might this be a ‘bad’ thing?

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

21. In Class assignment… Go to page 282-285
In Class assignment… Go to page Read about: directional stabilizing disruptive selection…. Explain the picture on the next slide…begin by defining the term. We will discuss in 15 minutes…!

22. Effects of Selection against or in favor or extreme phenotypes
Changes in the average trait of a population
DIRECTIONAL SELECTION
STABILIZING SELECTION
DISRUPTIVE SELECTION
giraffe neck
horse size
human birth weight
rock pocket mice

23. Any Questions??