evolution of populations vocabulary 1) allele 2) gene pool 3) genotype 4) mutation 5) phenotype 6)...

Evolution of Populations

Vocabulary

1)Allele2)Gene Pool3)Genotype4)Mutation5)Phenotype6)Polygenic Trait7)Population

1. When Darwin developed his theory of evolution, he did not understand:

• how heredity worked.

This left him unable to explain two things:

a. source of variation

b. how inheritable traits pass from one generation to the next

In the 1940’s, Mendel’s work on genetics was “rediscovered” and scientists began to combine the ideas of many branches of biology to develop a modern theory of evolution. When studying evolution today, biologists often focus on a particular population. This evolution of populations is called microevolution.

2. Vocabulary:

population: group of individuals of the same species living in the same area that breed with each other.

2. gene pool: combined genetic info. for all members of a population

2. allele: one form of a gene

2. relative frequency of an allele: # times an allele occurs in the gene pool compared to other alleles (percent)

Example

Relative Frequency:

70% Allele B

30% Allele b

A population of cows has a codominant gene for fur color. RR is Red, RW is ‘roan’, and WW is white cows. The population has 140 Red cows, 220 roan and 50 all white cows. What is the total number of cows in the population?

What is the total number of cows color alleles in the population?

What is the total number of R alleles? What is the total number of W alleles? What is the allele frequency (percent) of ‘R ’

gene? What is the allele frequency (percent) of ‘W ’

gene? (Yes or No) If after ten generations the R gene

frequency is at 5% and the W gene is 95%: has microevolution occurred?

3. Sources of Variation: a. mutations: any change in DNA sequence

♦ Can occur because of:

♦mistakes in replication

♦ environmental chemicals

♦ May or may not affect an organism’s phenotype

b. Gene Shuffling: recombination of genes that occurs during production of gametes

♦ Cause most inheritable differences between relatives

♦ Occurs during meiosis

♦ As a result, sexual reproduction is a major source of variation in organisms.

♦ Despite gene shuffling, the frequency of alleles does not change in a population, unless evolution is occuring

4.. Single gene trait: controlled by single gene with two alleles

♦ Examples: widow’s peak, hitchhiker’s thumb, tongue rolling

4. Polygenic trait: controlled by 2 or more genes, each with 2 or more alleles

♦ Examples: height, hair color, skin color, eye color

Most human traits are polygenic.

Do the following graphs show the distribution of phenotypes for single-gene or polygenic traits?

type: single gene

why? Only two phenotypes possible

Example: tongue roller or non-tongue roller

type: polygenic

why? Multiple (many) phenotypes possible

Example: height range 4 feet to 9 feet tall

5. Natural selection acts on phenotypes, not genotypes.

Example: in a forest covered in brown leaves, dirt and rocks which mouse will survive better brown or white?

Brown, more hidden.

5. If brown is dominant can the a predator tell the difference between:

Mouse with highest fitness will have the most alleles passed on to the next generation.

BB Bb

?

5. Which mouse will have the lowest fitness?

Will the fitness of BB and Bb differ? Why?No, Both BB and Bb have the same fitness

advantage of being brown

BB Bb

?

6. Three ways in which natural selection affects

polygenic traits.

Directional Selection

Food becomes scarce.

Key

Low mortality, high fitness

High mortality, low fitness

Directional Selection

individuals at one end of the curve have higher fitness so evolution causes increase in

individuals with that trait

Key

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f P

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Birth Weight

Selection against both

extremes keep curve narrow and in same

place.

Low mortality, high fitness

High mortality, low fitness

Stabilizing Selection

Stabilizing Selection

22 lbs 8 oz baby born Indonesia

Smallest baby: 9.2 oz

Disruptive Selection

Disruptive Selection

Largest and smallest seeds become more common.

Nu

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f B

ird

sin

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Beak Size

Population splits into two subgroups

specializing in different seeds.

Beak Size

Nu

mb

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f B

ird

sin

Po

pu

lati

onKey

Low mortality, high fitness

High mortality, low fitness

Name the type of selection Orangutans live in the trees of the

Sumatran and Bornean rainforests. If they are too heavy, the branches can’t support their weight and they can’t get the food high up in the trees, if they are too small, they are easy prey for predators. Size of orangutans is a result of what type of selection?

Name the Type of Selection There were rocks covering an

island with predominantly gray lizards living on them, but there were a few lighter in color and a few darker. There was a volcanic explosion on part of the island. What type of selection on the surviving lizards after the explosion

Name the type of selection

Suppose there is a population of rabbits. The color of the rabbits is polygenic and ranges from white to black. If this population of rabbits occurred in an environment that had areas of black rocks as well as areas of white rocks, what type of selection would be exhibited?

Genetic Drift

In small populations, an allele can become more or less common simple by chance rather than because natural selection is acting on specific variations

Two phenomena that result in small populations and cause genetic drift

1.Founder Effect2.Bottleneck Effect

Founder effect

allele frequencies change due to migration of a small subgroup of a population

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: colonization of New World

albino deer Seneca Army Depot

Polydactyly in Amish

populations

Sample of Original Population

Founding Population A

Founding Population B

Descendants

Founder Effect: : Fruit Fruit Flies on Hawaiian islandsFlies on Hawaiian islands

2. Bottleneck effect

major change in allele frequencies when population decreases dramatically due to catastrophe

Bottleneck effect

Bottleneck Effect: Northern Elephant Seal Population

♦ Hunted to near extinction

♦ Population decreased to 20 individuals in 1800’s, those 20 repopulated so today’s population is ~30,000

♦ No genetic variation in 24 genes

3. Hardy-Weinberg principle:

a.k.a: because allele frequencies do not change and therefore populations in this state do not evolve. Five conditions to maintain genetic equilibrium:

3. Hardy-Weinberg principle: allele frequencies in a population will remain constant unless some factor causes them to change

a.k.a: because allele frequencies do not change and therefore populations in this state do not evolve. Five conditions to maintain genetic equilibrium:

3. Hardy-Weinberg principle: allele frequencies in a population will remain constant unless some factor causes them to change

a.k.a: genetic equilibrium because allele frequencies do not change and therefore populations in this state do not evolve. Five conditions to maintain genetic equilibrium:

3. Hardy-Weinberg principle: allele frequencies in a population will remain constant unless some factor causes them to change

a.k.a: genetic equilibrium because allele frequencies do not change and therefore populations in this state do not evolve. Five conditions to maintain genetic equilibrium:

a. Random mating

b.

c.

d.

e.

3. Hardy-Weinberg principle: allele frequencies in a population will remain constant unless some factor causes them to change

a.k.a: genetic equilibrium because allele frequencies do not change and therefore populations in this state do not evolve. Five conditions to maintain genetic equilibrium:

a. Random mating

b. Large population

c. No migration

d.

e.

3. Hardy-Weinberg principle: allele frequencies in a population will remain constant unless some factor causes them to change

a.k.a: genetic equilibrium because allele frequencies do not change and therefore populations in this state do not evolve. Five conditions to maintain genetic equilibrium:

a. Random mating

b. Large population

c. No migration

d. No mutation

e.

3. Hardy-Weinberg principle: allele frequencies in a population will remain constant unless some factor causes them to change

a.k.a: genetic equilibrium because allele frequencies do not change and therefore populations in this state do not evolve. Five conditions to maintain genetic equilibrium:

a. Random mating

b. Large population

c. No migration

d. No mutation

e. No natural selection

The Process of Speciation

Vocabulary

8)Equilibrium9)Isolation10)Probability11)Species12)Subspecies13)Temporal

The Process of SpeciationThe formation of new biological

species, usually by the division of a single species into two or more genetically distinct one.

Species: Group of organisms that are able to reproduce with one another and produce viable offspring

How does speciation occur? First: Reproductive Isolation

Definition: Separation of populations so that they cannot interbreed and produce fertile offspring

What does this mean? The populations now have separate gene pools and respond to natural selection and genetic drift as separate units

Three Isolating Mechanisms: The gene pools must become separate so they can’t keep spreading similar alleles through population. Species can be isolated, forming subspecies and perhaps causing speciation by:1.Geographic Isolation

2.Behavioral Isolation

3.Temporal Isolation

1. Geographic IsolationTwo populations separated by

a geographic barrier; river, lake, canyon, mountain range.

Example: 10,000 years ago the Colorado River separated two squirrel populations.

Kaibab Squirrel Abert Squirrel

This resulted in a subspecies, but did not result in speciation because the two can still mate if brought together

Kaibab Squirrel Abert Squirrel

2. Behavioral Isolation

Two populations are capable of interbreeding but do not interbreed because they have different ‘courtship rituals’ or other lifestyle habits that differ.

Example: Eastern and Western Meadowlark

Eastern and Western Meadowlark populations overlap in the middle of the US

Example: Eastern and Western Meadowlark Male birds sing a mating song

that females like, East and West have different songs. Females only respond to their subspecies song.

3. Temporal Isolation

Populations reproduce at different times

Example: Northern Leopard Frog & North American Bullfrog

Mates in: Mates in:

April July

Conclusion:Geographic, Behavioral and Temporal Isolation all lead to speciation.

However:No examples ever observed in animals

A couple examples that may demonstrate speciation exist in plants and some insects.