Transcript Natural Selection

Natural Selection
The unequal survival and reproduction of
organisms due to environmental forces,
resulting in the preservation of favorable
adaptations.
It is a two step process:
1. The production of variation in a population
2. Non-random aspects of survival and reproduction
Natural Selection is a two step process:
Step One: The Production of Variation. (Chance)
Mutations
Meiosis:
recombination due to crossing-over in 1st division
random movement of chromosomes in 2nd division
Random mate selection & fertilization
Step Two: Non-random aspects of survival and reproduction
•
Superior success of certain phenotypes
•
Nonrandom mate choice
Lemurs of Madagascar
Amazonian Frogs
Humans select traits for dogs, pigeons and other animals when
they breed them.
Bred Pigeons came from a single original species
Who selects the traits for wild plans & animals?
NOBODY!!
There is no agent involved in natural selection.
Natural selection is a process of elimination
INDIVIDUALS THAT HAVE TRAITS THAT ARE BEST
ADAPTED FOR THE CURRENT ENVIRONMENT ARE
THE ONES THAT SURVIVE TO BREED AND PASS ON
THEIR GENES TO THE NEXT GENERATION.
Organisms not possessing the beneficial traits either die
or don’t have as many offspring.
Natural Selection is Survival of the
fittest
Natural Selection is a
mixture of both Chance and
necessity
Natural Selection is not goal
directed. It does not have a
long term goal.
What acts as a selection pressure on a
population?
• Competition for food
• Competition for a mate
• Changes in the environment
• Predators
• Parasites
Example of Natural Selection in Action:
Monarch / milkweed
Card game
Video of Darwin’s Finches
Main Types of Selection Pressures
• Directional Selection
– Natural selection favors one extreme of the population
for that trait
– often happens when environment changes in a
consistent way- e.g.climate gets colder.
• Disruptive Selection
– Natural selection favors both extremes selected
– Causes species to diverge
• Stabilizing Selection
– Natural selection favors the average for population
selected
• Sexual Selection
Directional Selection
•
•
•
•
•
Neck of Giraffe
Antibiotic resistance of bacteria
Moth color (melanin)
Camouflage/Mimics
Many sexually selected traits
Directional Selection: Mimicry
(mimic environment)
Stabilizing Selection
• When the extremes of the
trait aren’t as well suited
Examples
• bird clutch size
• Elk Antlers size
• Giraffe neck length
• Tail length in birds
Disruptive Selection
• Causes divergence within the
species
• Occurs when two different
types of resources in one
area
• Results in specialization for
each branched group
• May lead to formation of
new species
• E.g. Darwin’s Finches
Examples of selection pressures...
• Predators - variants with adaptations allowing them to
escape predators have more offspring
- e.g. speed, defensive weapons, camouflage, mimicry
• Prey/Food - variants with adaptations allowing them to
obtain food have more offspring
- e.g. Speed, senses for finding prey/food, weapons for killing
prey or obtaining food, camouflage for stealth
• Climate - those who can survive new climate best have
more kids
- e.g. ice age, change in climate due to migration.
• Mates - variants with adaptations allowing them to attract a
mate to have offspring
- e.g. strong, attractive, good provider
Example #1: Escaping Predation
Peppered Moth (see video clip)
• Early trees had light-colored bark
• Only the light-colored moths survived.
Selection was for less melanin.
• After industrialization, the tree bark was
darker.
• Only the darker colored moths now
survived. Selection was for more melanin.
New info on the Pepper Moth experiment
Example #2: Obtaining Food
• The neck of the
Giraffe
• Co-evolution with
Acacia Trees
• Selection pressure is
source of food
• The Red Queen
Hypothesis...
Example #3
• The leaf bug
• The selection pressure is
predators
• It’s strategy is to mimic a
leaf
Pray Mantis Camouflage
Natural Selection
Or, how did we get here….
Natural Selection
• The Theory of Natural Selection is so simple that
anyone can misunderstand it…. (Anonymous)
• Charles Darwin (1809-1882) saw three problems
in need of a solution.
– Darwin was not the only one to see these problems
BTW
– Other ‘Naturalists’ were struggling with the same issues
Problem the First
• There is change over time in the flora and
fauna of the Earth
– What we would commonly call ‘evolution’
today
– The fossil record showed this to be pretty clear,
even to people in the mid 1800s
– This was not controversial in Darwin’s time,
and is not now.
The Second Problem
• There is a taxonomic relationship among
living things
– People were big into classifying stuff
– It was pretty obvious that there was a
relationship between different species
• Different birds, different grasses, different cats etc
The Third Problem
• Adaptation
– Different kinds of teeth for different animals,
say carnivore ripping teeth and herbivore
grinding teeth
– Different tissues within species
• Heart vs. eye etc.
The Solution!
• Natural Selection provides a mechanistic
account of how these things occurred and
shows how they are intimately related.
• It is one of those ‘oh man is that ever easy,
why didn’t I think of that?’ type things.
How’s it work?
• There is competition among living things
– More are born or hatched or whatever, than survive and
reproduce
• Reproduction occurs with variation
– This variation is heritable
– Remember, there was NO genetics back then, Chuck
knew, he just knew….
– Realized that is wasn’t ‘blending’
How’s it Work?
• Selection Determines which individuals
enter the adult breeding population
– This selection is done by the environment
– Those which are best suited reproduce
– They pass these well suited characteristics on to
their young
How’s it Work?
• REPRODUCTION is the
key, not merely survival
This lecture keeps evolving…..
• Survival of the Fittest (which Chucky D NEVER
said) means those who have the most offspring
that reproduce
• So, the answer to the trilogy of problems is:
• ‘Descent with modification from a common
ancestor, NOT random modification, but,
modification shaped by natural selection’
Other Evolutionary Theories
• Lamarckism
– Inheritance of acquired characteristics
• E.g., giraffes really wanted leaves, so they stretched their necks
and…..
• Sounds crazy, but a lot of people think this way
• ‘We will all have giant heads and tiny bodies someday’
• ‘Cave swelling fish don’t use their eyes so they disappear’
• ‘We don’t use our appendix so it is disappearing’
Silly incorrect evolutionary theories
and ideas
• Orthogenesis
– There is some plan to evolution.
– NO WRONG INCORRECT, THANKS FOR
PLAYING
– The idea of an ‘evolutionary ladder’ fits in here
– It is wrong too……
Problem 3
• If you observe a population and find that
16% show the recessive trait, you know the
frequency of the aa genotype. This means
you know q2. What is q for this population?
Evolution of populations
• Evolution = change in allele frequencies in a
population
– hypothetical: what conditions would cause allele
frequencies to not change?
– non-evolving population
REMOVE all agents of evolutionary change
1. very large population size (no genetic drift)
2. no migration (no gene flow in or out)
3. no mutation (no genetic change)
4. random mating (no sexual selection)
5. no natural selection (everyone is equally fit)
5 Agents of evolutionary change
Gene Flow
Genetic Drift
Mutation
Non-random mating
Selection
Application of H-W principle
• Sickle cell anemia
– inherit a mutation in gene coding for hemoglobin
• oxygen-carrying blood protein
• recessive allele = HsHs
– normal allele = Hb
– low oxygen levels causes
RBC to sickle
• breakdown of RBC
• clogging small blood vessels
• damage to organs
– often lethal
Sickle cell frequency
• High frequency of heterozygotes
– 1 in 5 in Central Africans = HbHs
– unusual for allele with severe
detrimental effects in homozygotes
• 1 in 100 = HsHs
• usually die before reproductive age
Why is the Hs allele maintained at such high
levels in African populations?
Suggests some selective advantage of
being heterozygous…
Single-celled eukaryote parasite
(Plasmodium) spends part of its life
cycle in red blood cells
Malaria
1
2
3
•
Heterozygote
Advantage
In tropical Africa, where malaria is common:
– homozygous dominant (normal)
• die or reduced reproduction from malaria: HbHb
– homozygous recessive
• die or reduced reproduction from sickle cell anemia: HsHs
– heterozygote carriers are relatively free of both: HbHs
• survive & reproduce more, more common in population
Hypothesis:
In malaria-infected cells,
the O2 level is lowered
enough to cause sickling
which kills the cell &
destroys the parasite.
Frequency of sickle cell allele &
distribution of malaria
Hardy-Weinberg lab pod cast
• http://www.youtube.com/watch?v=KmqgZv
Uoq3k
Lab—Part A
Taster
NonTaster
PTC
Control
Non-Tasters=Homo recessive (aa)
Tasters=Homo Dominant (AA) or Heter (Aa)
Figure out the p2 and 2pq for our class
Results and Discussion
• 5 Hardy Weinberg calculations
• Show work—organzied part A-E
• Discussion: In part A evolution did/did not
happen because…. In part B…. In part C
• Explain WHY, use terms
Part B—Testing an ideal Population
Initial Class Frequencies GG____ Gg____ gg___
My initial genotype ___
F1____
F2_____
F3_____
F4_____
F5______
Final Class Frequencies
GG____ Gg____ gg ___
Part C—Selection (homo recessive
selected against)
Initial Class Frequencies GG____ Gg____ gg___
My initial genotype ___
F1____
F2_____
F3_____
F4_____
F5______
Final Class Frequencies
GG____ Gg____ gg___
Part D—Heterozygous Advantage
(Homo dom—may die of maleria (flip coin; homo
recessive—die of sickle cell)
• Initial Class Frequencies GG____ Gg 24/48
gg___
• My initial genotype ___
• F1____
• F2_____
• F3_____
• F4_____
• F5______
Final Class Frequencies
• GG 16/48 Gg 32/48 gg 0/48
Part E—Genetic Drift (break into 3
smaller populations—make
hypothesis)
• Initial Class Frequencies GG____ Gg____ gg___
• My initial genotype ___
• F1____
• F2_____
• F3_____
• F4_____
• F5______
Final Class Frequencies
Frequencies GG____ Gg____ gg___