5/14

• Have book work out • Review • 1st ½ of block: Lecture• 2nd ½ block: species! What does it REALLY

mean???

REVIEW!• Where are we in Evolution and how did we get

here???• 1. Misconceptions of Evolution• 2. Brief history of theory development.• 3. Natural Selection & how it works.• 4. Evidence for Evolution– Fossils, biochemistry, comparative anatomy, artificial

selection• 5. Population Genetics– Allele frequency, genetic equilibrium & H-W equations

What’s Next?• Factors that Affect a gene pool:–Variation: Mutations, sexual recombination +

crossing over–Population size: bigger is better!–Genetic drift• Bottle neck & Founders effect

–Gene flow– Sexual selection

• MicroEvolution: – Types of Natural Selection– Speciation!!!

I.) Sources of Variation in Populations’ Gene Pools

A.) mutation - any change in organisms’ DNA.• Some can change an organism’s

phenotype and this can affect its fitness (positively OR negatively).

B.) Gene “Shuffling” during Meiosis1.) independent assortment: Chromosomes line

up independently of each other.a.) This can result in different phenotypes as alleles are put in different combinations in different gametes.

2.) Crossing over - exchange of genes between homologous chromosomes.

a.) Again, this results in many different allele combinations and possibly different phenotypes.

C.) Be Aware! Sexual reproduction does not change the frequency of alleles in a gene pool, it just mixes them around and this can produce new phenotypes. (This is just like shuffling a deck of cards!)

• Large population:–A larger population has more variation.– It is better able to cope with lose of

individuals especially on a large scale. • Lose ½ of 10,000 vs lose ½ of 1000.

A.) Genetic Drift - random changes in allele frequency in small populations.

1.) Individuals that carry a particular allele may leave more offspring by chance. Think probability: over time, this can cause an allele to become more common in a population.

2.) This can occur due to:a.) Founders effect: when a small group of

organisms founds a new population, they many have different relative frequencies of alleles than the larger population they came from.

i.) If so, the population they start can be quite different from their original population.

Sample of Original

Population

Founding Population A

Founding Population B

Descendants

Section 16-2

b.) Bottleneck effect: occurs when a small population is left behind after a disaster/disturbance and has different relative frequencies than original population.

C.) Gene flow: the movement of alleles from one population to another – due to immigration/emigration.

B.) Nonrandom mating which can be termed sexual selection.1.) Individuals in

population choose mates based on certain traits.

http://www.pbs.org/wgbh/evolution/library/01/6/l_016_09.html

D.) Review: a change in genes (allele freq) that produces a phenotype that is favorable (increases fitness) in a particular environment leads to natural selection (survive or not in that env) and thus Evolution!

• Before we move onto the next topic take a 5 minute break.

Types of Natural Selection

A.) Most traits are polygenic: controlled by many genes.

1.) These traits therefore have a normal distribution and a characteristic shape when phenotypes are graphed.

Ex: Height in Humans

• WS Inferring information from Data– On your own with JUST YOUR BRAIN (so

everything off your desk & no discussion) complete the 3 scenarios. When you are done flip your paper over.

– Compare answers & discuss

B.) Natural Selection can change this normal distribution in 3 ways:

1.) Directional Selection: individuals at one end of the curve have higher fitness than individuals at the middle or other end.

Directional Selection

Food becomes scarce.

KeyLow mortality, high fitness

High mortality, low fitness

Ex: Beak Size of finches

2.) Stabilizing Selection: individuals at the center of the curve have higher fitness.

KeyPe

r cen

t ag e

of P

opu l

a tion

Birth Weight

Selection against both extremes

keep curve narrow and in

same place.

Ex: Normal weight human babies vs. high or low birth weight.Section 16-2

Low mortality, high fitness

High mortality, low fitness

Stabilizing Selection

3.) Disruptive Selection: individuals at both ends of the curve have higher fitness than individuals in the middle.

Disruptive Selection

Largest and smallest seeds become more common.N

umbe

r of B

irds

in P

opul

ation

Beak Size

Population splits into two subgroups specializing in different seeds.

Beak Size

Ex: Food goes from medium size seeds to large and small seeds. Birds with small & large beaks are now favored.

Num

ber o

f Bird

sin

Pop

ulati

on

KeyLow mortality, high fitness

High mortality, low fitness

• http://wps.prenhall.com/wps/media/objects/1110/1136802/17_3.html

In broader terms, these changes from generation to generation can result in speciation: the formation of new species. a. This can be thought of as descent with

modification.b. Through change from generation to generation,

one species may separate into two.c. This idea implies that ALL life originated from a

common ancestor with speciation occurring over and over again!

• Today you will complete a web quest on Speciation.

• You have class time to complete it & then we’ll review.

• HW: for Thursday–Quiz: evidence for evolution (the packet from

class), wp #3 14.4, wp #4 15.1 & wp #5 book work guided questions & allele frequency