Natural SelectionThe 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)

MutationsMeiosis:

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

Bred Pigeons came from a single original species

Humans select traits for dogs, pigeons and other animals when they breed them.

Who selects the traits for wild plans & animals?

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.

NOBODY!!

Organisms not possessing the beneficial traits either die or don’t have as many offspring.

There is no agent involved in natural selection.

Natural selection is a process of elimination

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 PredationPeppered 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 SelectionOr, 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

Mutation

Gene FlowGenetic Drift

Selection

Non-random mating

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?Why is the Hs allele maintained at such high levels in African populations?

Suggests some selective advantage of being heterozygous…Suggests some selective advantage of being heterozygous…

Malaria Single-celled eukaryote parasite (Plasmodium) spends part of its life cycle in red blood cells

Single-celled eukaryote parasite (Plasmodium) spends part of its life cycle in red blood cells

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.

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=KmqgZvUoq3k

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 FrequenciesGG____ 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 FrequenciesFrequencies GG____ Gg____ gg___