How Populations Evolve and Introduction to Population

Genetics

05-23-2016

Chapter 19 sections 1, 2, and 3

Learning Goals for Today:

• Explain the connection between populations, genes, and evolution

• Predict how populations may change over time

How Are Populations, Genes, and Evolution Related?

• Evolutionary change occurs over time to populations

• A population is a group that includes all members of a species living in a given area

• Individuals live/die, reproduce/not …depending on their traits

Population genetics - concepts

• A gene is a segment of DNA located at a particular place on a chromosome

• Different members of a species may have slightly different nucleotide sequences for the same gene, these are called alleles

– Different alleles code for slightly different versions of the same protein

Population genetics - concepts

• In a population, there are usually two or more alleles of each gene

• If the population is diploid,

– an individual with the same two alleles of a gene is homozygous

– an individual with two different alleles of a gene is heterozygous

• Example: human height

– Genes

– Childhood nutrition

– Growth hormones

Genes and the environment interact to determine traits

Example of Tracking Alleles in a Population

• Hamster coat color

• The dominant allele encodes for an enzyme that catalyzes black pigment formation (B)

• The recessive allele encodes for an enzyme that catalyzes brown pigment (b)

• How many alleles will 1 hamster have?

• What color is a Bb hamster?

B B B b

BB Bb bb

b b

chromosomes

genotype

phenotype

Coat-color allele B is

dominant, so heterozygous

hamsters have black coatsEach chromosome

has one allele of the

coat-color gene

Alleles, Genotype, and Phenotype in Individuals

Gene Pools

• Sum of the genes in a population

– Population genetics deals with the frequency, distribution, and inheritance of alleles in populations

– A gene pool consists of all the alleles of all the genes in all individuals of a population

Gene Pools

• The number of copies of each allele in a gene pool is equal to:

1. The number of heterozygous individuals that carry one copy of the allele

2. Twice the number of homozygous individuals for that allele since they carry two copies of the allele

Gene Pools

• Sum of the genes in a population

– If we counted all of the alleles present for one gene in a population we could determine allele frequency for each different allele

– Allele frequency the proportion of one allele relative to all the alleles of that gene

How Are Populations, Genes, and Evolution Related?

• Determining allele frequency• A population of 25 hamsters contains 50 alleles of

the coat color gene

• If 20 of those 50 alleles code for black coats (are B), then the frequency of the black allele (B) is

• 20/50 = 0.4 = 40%

• What is the frequency of the b allele?

BB BB BB

B B B B B B B B

BB

Bb Bb Bb

B B B B b b b b

Bb

Bb Bb Bb

B B B B b b b b

Bb

bb bb bb

b b b b b b b b

bb

bb bb bb

b b b b b b b b

bb

bb

b b

Bb Bb Bb

B B B B b b b b

Bb

Population: 25 individuals Gene pool: 50 alleles

The gene pool for the

coat-color gene contains

20 copies of allele B and

30 copies for allele b

A Gene Pool

Fig. 15-2

Example: calculating allele frequencies

Population of 50 hamsters (25 BB, 25 bb)

Frequency of B allele? Of b allele?

Population of 50 hamsters (all Bb)

Frequency of B allele? Of b allele?

What do the allele frequencies always add up to?

The Big Picture

• Evolution is the change of allele frequencies within a population

– Frequencies change (over generations) = evolution

– Frequencies stable (over generations) = no evolution

How can we tell if a population is evolving?

• The Hardy-Weinberg principle: a mathematical model for population biology

• Evolution: change in allele frequencies

• Mathematician Godfrey H. Hardy and physician Wilhelm Weinberg came up with a way to track allele frequencies

• No change = no evolution– Said to be in “equilibrium”

Conditions required to have no allele change (no evolution occurring)

Conditions1. No mutations must occur in the population 2. There must be no gene flow between

populations• No movement of alleles in or out of the population

3. The population must be very large 4. Mating must be completely random 5. There can be no selection

• All genotypes have equal reproductive success

Under these conditions, allele frequencies in a population will remain the same indefinitely

• Without evolution, alleles in a population will

quickly reach equilibrium

• Once the frequency of alleles in a population is

known, we can use the Hardy-Weinberg equation

• p = frequency of the dominant allele in the population

• q = frequency of the recessive allele in the population

Hardy-Weinberg Principle

• Assume that 2 heterozygous individuals

produce offspring

Where does it come from?

p

p

q

q

pp

(p2)

qq

(q2)

pq

pq

p

p

q

q

pp

(p2)

qq

(q2)

pq

pq

• We can calculate what proportion of individuals in

the next generation will have a given genotype and

phenotype

• You will do this in Lab 9

What does it mean?

• Two alleles exist for color in a certain type of beetle. Red (R) is dominant to blue (r).

• In a specific population of beetles, 51% are red and 49% are blue.

• Assuming the population is in H-W equilibrium, what are the frequencies of the red and blue alleles in the gene pool?

Example

= RR and Rr = rr only

p2 and pq q2 only

q2 = 0.49

q = 0.7

p + q = 1

1 – 0.7 = p

p = 0.3

Example

• Cystic fibrosis is an autosomal recessive disorder that affects 1/2500 (0.0004) Caucasians.

• Assuming the population is in H-W equilibrium, what percentage of Caucasians are carriers?

Example

= CC and Cc = cc only

p2 and pq q2 only

q2 = 0.0004

q = 0.02

p + q = 1

1 – 0.02 = p

p = 0.98

Example

= CC and Cc = cc only

p2 and pq q2 only

Frequency of heterozygotes = 2pq

2(0.98)(0.02) = 2pq

0.0392 or 3.92%

Example

p2 + 2pq + q2 = 1

p = 0.98 q = 0.02

(0.98)2 + 2(0.98)(0.02) + (0.02)2 = ?

1

Check Your Work!

Practice calculating allele frequencies from phenotypes

Population of 100 hamsters

Year 1: 25 brown, 75 black

Year 2: 36 brown 64 black

Did the allele frequencies change?

Evolutionary forces

• Mutations

• Gene flow

• Genetic drift

• Non-random mating

Mutations

• Changes in DNA sequence– Usually occur during DNA replication

– Rare

• A new mutation will appear in an allele

– 1 of every 100,000 human gametes

– 1 of every 50,000 human babies

– Causes very small changes in the frequency of any allele

• But we have

– 20 to 25,000 genes

– 50,000 alleles

– Most newborns have one or two mutations

– New alleles new variation evolution

Mutations

• Changes in DNA sequence

– Usually occur during DNA replication

– Rare

– Source of new alleles

– Spontaneous

– Can be neutral, harmful, beneficial

– Provides a potential for evolutionary change

Mutations

• Changes in DNA sequence

– Usually occur during DNA replication

– Rare

– Source of new alleles

– Spontaneous

– Can be neutral, harmful, beneficial

– Can be inherited (IF gametes carry the mutation)

Gene Flow

• Movement of alleles from one population to another

– Individuals move around

– Gametes (like pollen) can also move

Fig. 15-4

– Baboons live in troops, juvenile males move from troop to troop to rise in social status

– Changes the alleles of the destination population to be more like the source population

• Examples:

• A migrating bird changes flocks

• Grolar bears (gene flow between species)

Gene Flow

Genetic Drift

• Random change in allele frequencies over time brought about by chance alone

– Only makes a difference in small populations

• Examples of bad luck:

– Seeds can fall into a pond or parking lot and never sprout

– Animals can be killed by floods, fires, volcanic eruptions

Genetic Drift

• Imagine you have a population of 20 hamsters

• B = 0.5 and b = 0.5

• If all animals are allowed to breed randomly and yield 20 new hamsters, the frequencies would not change

• But if you chose two randomly and let only them breed and yield 20 new hamsters, allele frequencies would change dramatically

– Two causes that lead to:

• Population bottleneck

• Founder effect

Alleles are more likely to disappear

due to random chance in small

populations

Genetic Drift by Population Bottlenecks

Fig. 15-7

Natural catastrophes, over-hunting

Some examples:

Elephant sealsCheetahsCalifornia condors

Limited genetic variation leaves populations vulnerable to extinction

• A bottleneck event is random

• A plague that kills off individuals lacking a

particular allele is natural selection

• Natural selection kills individuals due to genetic

make-up

• Bottlenecking kills indiscriminately

IMPORTANT!!

Genetic Drift by the Founder Effect

• Small number of individuals leave a large population and establish a new isolated population

• By chance, not all alleles are present in this new smaller population

http://beacon-center.org/blog/2012/01/03/beacon-researchers-at-work-how-the-cricket-lost-its-song/

Genetic Drift by the Founder Effect

• Example:

• All Amish in Lancaster, PA

descended from ~200

mostly Swiss individuals

that migrated in 1744

• Ellis-Van Creveld

syndrome is common

among old order Amish

• Amish: 1 or 2 in 200

• General population: 1 in

1000

Non-Random Mating

• Mating within a population is almost never random

– Lack of mobility

• Many organisms inbreed (self-fertility is common in plants)

– Can increase frequency of homozygous recessive individuals

• Mates have preferences

Fig. 15-9

• Snow geese mate with partners of the same color

– Assortative mating

• In large population, neither inbreeding nor assortative mating will alter allele frequencies

Evolutionary forces

– Mutation

– Gene flow (between populations)

– Small population size (genetic drift)

– Non-random mating

– Selection occurs

Natural selection

• When an allele provides “some little superiority”, the individuals with that allele are favored by natural selection

• Natural selection favors traits that increase an individual’s survival only to the extent that the individual’s survival leads to improved reproduction

• Reproductive success determines the fate of the alleles

Natural Selection and Antibiotic Resistance

• Penicillin: antibiotic, widely-used starting around WWII

• Killed most bacteria• Some individual bacteria were naturally

resistant (rare resistance allele)

Before penicillin treatment

Penicillintreatment

After penicillin treatment

Natural Selection and Antibiotic Resistance

• Penicillin killed most bacteria• Some individual bacteria were naturally resistant

(rare resistance allele)• Natural selection:

– Did not cause genetic changes in individuals– Acted on individuals (killed them)– Caused the population to evolve/change allele

frequency

• Evolution by natural selection is not progressive; it does not make organisms “better”

Patterns in evolutionary change

• Directional selection

• Stabilizing selection

• Disruptive selection

Directional Stabilizing Disruptive

How selection can alter population structure

Directional selection

• Occurs when natural selection favors one extreme of continuous variation

• Over time, the favored extreme will become more common and the other extreme will be less common or lost

Dark-peppered moths were prevalent in the early 19th century

• Historical case in England: peppered moth

Directional Selection

Pre-Industrial revolution

Post Industrial revolution

Stabilizing selection

• Removes individuals from both ends of a phenotypic distribution, thus maintaining the same distribution mean

• Occurs when natural selection favors the intermediate states of continuous variation– Common when there are opposing forces

acting on a trait

Goldilocks

Human Birth Weights: an Example of Stabilizing Selection

Disruptive or diversifying selection

• Removes individuals from the center of a phenotypic distribution and thus causes the distribution to become bimodal

• Occurs when natural selection favors both extremes of continuous variation

• Disruptive selection can lead to two new species

Balanced polymorphism

• Often occurs when environmental conditions favor

heterozygotes

• Normal and sickle-cell hemoglobin alleles coexist in malaria-prone regions of Africa

Types of selection

• Natural

– Kin selection

– Sexual selection

– Frequency-dependent selection

• Artificial

Kin selection

• A type of selection that involves altruistic behavior, e.g., the protection of offspring

• Occurs when natural selection favors a trait that benefits related members of a group

Kin selection

• Worker bees exhibit altruistic behavior• In terms of simple fitness, the worker bee

does not reproduce • However, all of the bees in the hive are close

relatives, a worker bee's genes will be passed to the next generation indirectly

Young bee-eaters remain with parents when breeding opportunities are low

• Habitat is saturated or no more suitable nesting sites

• More beneficial to assist family with siblings

Sexual Selection• In many species, males and females look very

different

• Sexual dimorphism

Sexual selection

• A type of selection in which the forces determined by mate choice act to cause one genotype to mate more frequently than another genotype

Sexual selection in Long-tailed widowbird

• The evolutionary fitness of an organism not only depends upon its ability to survive but also its ability to reproduce

• To reproduce, an individual must obtain a mate and produce viable offspring

• Natural selection favors traits that maximize the ability of an individual to compete for and attract mates, and/or the ability to produce offspring

Sexual Selection

• Some of these differences give a competitive

advantage when competing with each other for

mates

• Intrasexual selection

Sexual Selection

• Some of these differences give a competitive

advantage when attempting to attract mates

• Intersexual selection

Sexual Selection: one explanation for traits that appear

disadvantageous

Fig. 14-14

Frequency-dependent selection

• Individuals with either the common (positive frequency-dependent selection) or the rare (negative frequency-dependent selection) phenotype are selected for

• Negative frequency-dependent selection serves to increase the population’s genetic variance

• Positive frequency-dependent selection usually decreases genetic variance

Lizards in the PNW show negative frequency-dependent selection

• The common side-blotched lizard

• Alternate male strategies leads to frequency-dependent selection

Artificial Selection

People have bred many varieties of animals and

plants

Selective breeding of organisms by humans is

artificial selection

These organisms have been changed to look very

different from the original parent stock

Types of Selection:

• Natural

• Occur in nature (no human intervention)

– Directional selection

– Stabilizing selection

– Disruptive selection

• Artificial

• Organisms bred by humans for specific traits

– Directional

– Stabilizing

– Disruptive

Crop breeding: artificial selection in action

http://the10000yearexplosion.com/pictures/evolutionary-business/1720176

Teosinte: wild ancestor of corn

What traits were selected for in the selective breeding of corn?

Brassica oleracea: many different forms from one ancestor

A cultivar is a plant or grouping of plants selected for desirable characteristics that can be maintained by sexual or vegetative propagation.

Varieties of Canis familiaris

http://www.blueberrybasket.com/catalog/home/home_KDOG.htm

Ancestor to all dogsCanis lupus

Domestic cat breeds: recent example of artificial selection

http://en.wikipedia.org/wiki/List_of_cat_breeds

Closest relative

What happens when artificially selected organisms return to

nature?

Crop and animal breeding

• Genetic variance is the diversity of alleles and genotypes within a population

• Heritability is the fraction of phenotype variation that can be attributed to genetic differences, or genetic variance, among individuals in a population

• Breeders attempt to increase a population’s genetic variance to preserve as much of the phenotypic diversity as possible and to reduce inbreeding

• Inbreeding depression is the reduced biological fitness in a given population as a result of inbreeding

Upcoming lab:

• Lab 9, population genetics

• Hardy-Weinberg

• Equation for calculating allele frequencies from phenotypes