Download How Populations Evolve

Evolution
Website: http://evolution.berkeley.edu/
Evolution is the explanation of many facts of life
FACTS:
Life has change through time
• Extinction is common
• Species exist today that did not exist before
Today’s species share many features
• Similar DNA and proteins
• Same genetic code
SCIENTIFIC EXPLANATION:
Descent with modification
(theory of evolution)
Independent lines of evidence to support it:
Misconceptions about Evolution
Biology professor interviewed in Colbert Report
Evolution is NOT in conflict with any religion
Evolution, as science, looks for natural explanations to natural phenomena
Darwin’s Finches
2:30min -4:40
4
Peter and Rosemary Grant
• Scientists Peter and Rosemary Grant
have studied many of these species for
the past thirty years.
– Spend months at a time on the islands
– Often know every finch on an island
• They focused on one species:
medium ground finch 6:00min-7:50
• Let’s look at some of their data.
5
Graph showing the distribution of beak depths
for medium ground finches in Year 1 (1976)
Population size
641 finches
Number of finches with
a specific beak depth
6
1. What is the most common beak depth
of the finches living in Daphne Major in
1976? ______________
Write answers on part 2 of evolution handout
7
2. What is the range of beak depths in
this group of medium ground finches?
________________
8
A Change in the Weather
• In a normal year, 130mm of rain would fall.
• In 1976, 137mm of rain fell.
• In 1977
The total rainfall for the entire year: 24mm.
How did this affect the island? Min 7:50-8:15
9
3. What do you think a graph of population size
would look like for year before the drought to
the year after the drought?
B:
A:
Time
C:
Time
D:
Time
Time
Where all the finches equally affected? Min 8:15-9:20
10
1977 Data
Beak sizes of medium ground finches living in Daphne Major in 1976 (gray
bars)
and of the finches that survived the drought of 1977 (black bars).
What changed?
11
4. Approximately what percentage of birds
with the most common beak depth in 1976
survived the drought?
12
1978 Offspring
13
5. What is the most common beak depth of
the finches living in Daphne Major in 1978?
______________
14
6. Did the finch population evolve
from 1976 to 1978?
A: Yes
B: No
C: Maybe
D: I don’t know
16
Microevolution:
descent with modification within a population
POPULATION is a group of organisms of the same species
that live in the same area
1st generation
What has been modified?
the individual?
2nd generation
Proportion of the phenotypes in the population has changed over time
These phenotypes have a genetic basis
Microevolution can also be defined as the
the change in the proportion (frequency) of alleles over time
What was the cause for the change in the population?
Mechanism: Natural selection
Organisms best suited to the environment
have the most resources for survival and reproduction
And
pass their successful traits to offspring.
What was the cause for the change in this population?
Mechanism: Genetic drift
Changes in the gene pool of a small population due to chance event.
Two Types of Genetic Drift
Bottle neck Effect:
Genetic drift resulting from a drastic reduction in population size
Bottle neck effect
Founder effect
Founder Effect:
Genetic drift due to colonization
by a limited number of individuals from a parent population.
Some colonies show high frequency
of certain inherited disorders
Examples:
British colony on Tristan da Cunha has
higher incidence of retinitis pigmentosa
Dutch colony on South Africa has higher
incidence of Huntington's disease
What type of genetic drift is this?
Species that have been endangered and
rebound show low genetic diversity
Hundreds of thousands of Elephant seals
once inhabited the Pacific Ocean.
The Northern population were slaughtered in the 1800s
for their blubber
By 1892, only 50 to 100 individuals were left.
1900 the Mexican and US government decided to protect them
Today, there are approximately 160,000 northern elephant seals.
The Northern population of elephant seals
is less genetically diverse than the Southern Population.
What type of genetic drift is this?
Why is this not considered natural selection?
What was the cause for the change in this population?
Mechanism: Gene flow
The loss or gain of alleles in a population due to
the migration of fertile individuals or gametes between populations
What was the cause for the change in this population?
Mechanism: Mutation
A rare change in the DNA of a gene introducing a new allele
The Hardy-Weinberg principle or equilibrium
can be used to test whether a population is evolving
Condition that need to be met for a
population to be in H-W equilibirum
The Hardy-Weinberg principle or equilibrium
p = proportion (frequency) of the dominant allele in the population
q = proportion (frequency) of the recessive allele in the population
p2 = proportion (frequency) of homozygous dominant individuals
q2 = proportion (frequency) of homozygous recessive individuals
2pq = proportion (frequency) of heterozygous individuals
Population genetics handout
Work in groups of 4:
one pair does simulation 1: population in equilibrium
simulation 2: population under a negative selection pressure
the other pair does simulation 1 and
simulation 3: population with a heterozygous advantage
exchange data from simulation 2 and 3 between pairs
Fill out handout, except for question 7 at the end of handout
Applications of Hardy-Weinberg principle:
The Hardy-Weinberg equation is often used to
test whether evolution is occurring in a population.
Public health scientists use the Hardy-Weinberg equation to estimate
how many people carry alleles (are heterozygous) for certain inherited
diseases.
Complete the set of H-W problems from your evolution handout (part 3)
Natural Selection: a closer look
This type of trait is said to be an adaptation
Types of Natural Selection depending
on which phenotype is being favored
Directional selection: shifts the frequency curve in one direction.
It happens when the environment changes or
a population migrates to a new habitat
Disruptive selection: conditions favor individuals on both extremes.
Stabilizing selection:
Acts against extreme phenotypes
and favors intermediate ones
7. Which type of selection was observed on
the Darwin finches?
A.
B.
C.
D.
Directional
Disruptive
Stabilizing
I am not sure
Misconceptions about Natural Selection:
“Natural Selection gives a species what it needs”
When the environment changes,
a population can survive or die out,
depending on the variation present before the change.
Natural selection is more of an editing process than a creating process
Misconception:
“Natural Selection is a random process”
A population of organisms experiences random mutations
but selection is non-random (one specific mutation is selected)
Natural Selection
results a population better adapted to the environment
(at least momentary)
What happened to the finches after the drought?
Min 9:20-11:20
Misconception:
“Natural selection is the survival of the strongest and fastest”
More important than surviving is reproducing
FITTNESS: The contribution an individual makes to the next generation,
relative to the contributions of other individuals.
Natural selection does not act only on the traits
that allow for survival of the individual
but also on the traits that
increase ability to find a mate and reproduce
8. Which statement describes the swallow with
the greatest evolutionary fitness?
A. A finch that lives to be 3 years old and has four
offspring, two of which survive to reproduce
themselves
B. A finch that lives to be 5 years old and has five
offspring, one of which survives to reproduce
C. A finch that lives to be 2 years old and has four
offspring, all of which survive to reproduce themselves
D. A finch that lives to be 7 years old and has three
offspring, all of which survive to reproduce
E. All are equally fit
Some traits are selected for
even if they are a burden to health or survival of the individual
Because they increase the fitness of the individual
Sexual selection: natural selection
for mating success
due to mate choice or male competition
Misconception:
“Natural selection will produced perfectly engineered organisms”
Natural selection is THE SURVIVAL OF THE FIT ENOUGH
Reasons why there are not faster cheetah:
Variation not present  not possible
Trade-offs
Misconception:
Natural Selection should eliminate bad genes
• Bad genes might not really reduce fitness
Example: Huntington’s disease
• Bad genes might be maintained by heterozygote advantage
Example: Sickle cell disease
Natural Selection is happening today
Pesticides become less effective
with each spraying
Antibiotics become less effective
with successive uses
Artificial Selection:
Humans (instead of nature) select for which individuals in the
population reproduce (those with the desired phenotype)