11.4 hardy-weinberg principle

11.4 Hardy-Weinberg principle

• 1st main conclusion: For a given population, one may calculate all genotypic frequencies from the allelic frequencies.

• 2nd main conclusion: Allelic frequencies remain stable over time.


• Ex. #1: You have a sampled population in which you know that the percentage of the homozygous recessive genotype (aa) is 36%. Using that 36%, calculate:a. The frequency of the aa genotypeb. The frequency of the a allelec. The frequency of the A alleled. The frequency of the AA and Aa genotypes


• Given: aa = 36%a. The frequency of the aa genotype

b. The frequency of the a allele

c. The frequency of the A allele

d. The frequency of the AA and Aa genotypes

11.4 Hardy-Weinberg principle• Ex. #2: In a population of 1000 fish, 640 have

forked tail fins and 360 have smooth tail fins. Tail fin shape is determined by 2 alleles: T for the dominant forked tail and t for the recessive smooth tail.

• Find q2, the freq. of smooth-finned fish (recessive homozygotes)

11.4 Hardy-Weinberg principle• Ex. #2: In a population of 1000 fish, 640 have

forked tail fins and 360 have smooth tail fins. Tail fin shape is determined by 2 alleles: T for the dominant forked tail and t for the recessive smooth tail.

• Find the predicted value of q.


• Ex. #2: In a population of 1000 fish, 640 have forked tail fins and 360 have smooth tail fins. Tail fin shape is determined by 2 alleles: T for the dominant forked tail and t for the recessive smooth tail.

• Rearrange the equation p + q = 1 to find the predicted value of p.


• Ex. #2: In a population of 1000 fish, 640 have forked tail fins and 360 have smooth tail fins. Tail fin shape is determined by 2 alleles: T for the dominant forked tail and t for the recessive smooth tail.

• Calculate the predicted genotype frequencies from the predicted allele frequencies.


• Ex. #3: In an imaginary population of 500 blue-footed boobies, determine the frequency of each genotype and each allele for webbed feet.

Boobies


Allele frequencies

# of alleles in gene pool

(1000)

Genotype frequencies

20160320# of animals (500 total)

wwWwWWGenotypes

WebbingNo webbingNo webbingPhenotypes

Tits• Ex. #4: In an imaginary population of

800 tits, determine the frequency of each genotype and each allele for a regular chest bar.


Allele frequencies

# of alleles in gene pool

(1600)

Genotype frequencies

32256512# of animals (800 total)

bbBbBBGenotypes

IrregularRegularRegularPhenotypes


• In order to preserve this equilibrium five conditions must be met:

1. No mutations occur.2. No migration occurs (i.e. the population must be

isolated).3. Population must be large (no genetic drift).4. No natural selection occurs (i.e. all individuals are

equal in reproductive success). 5. Mating must be random (i.e. no sexual selection).


Factors that can alter the allelic frequency of a population:

1. Mutation 2. Migration3. Genetic drift 4. Artificial or natural selection5. Nonrandom mating/sexual selection


1. Mutation: introduction of a new allele causes an immediate, but small, subtle shift in allelic equilibrium.

– Mutations are much more influential in populations with large reproductive output in shore periods of time, e.g. bacteria.


2. Migration: movement of organisms into, or out of, a population.

– Organisms entering or leaving a population may have a genotype different from the rest of the population. In such cases, a change in allelic frequency occurs.

– Gene flow: the movement of genes into, or out of, a population because of migration.


3. Genetic drift: the random change in allelic frequencies that results in a population with distinct characteristics.

– Usually occurs in small populations.a. Dunker population: higher frequency of

occurrence of a particular blood type than the surrounding population. Religious beliefs against marriage outside the group maintained the unusually high frequency of this allele in their population.


3b. Old Order Amish of Lancaster, PA: dwarfism andpolydactylism was introduced to this small community by one individual. There have been 61 cases since the 1770’s. That number almost equals the total recorded worldwide in the same time period.


3c. Population bottleneck: Other factors, e.g. storm, or attack by predators, could kill a large portion of the population, leaving survivors behind with a different allele frequency than the original group.See section11.3, p. 336


“Population bottleneck in Cheetahs” p. 336


4. Artificial or natural selection: determines which individuals in a population will reproduce and pass on their genes.

– Strong selection pressures rapid changes in allele frequency.

– Weak selection pressures more gradual shifts.


5. Nonrandom mating/sexual selection: If certain individuals in a population show a preference for mating with individuals of a particular phenotype, mating is no longer random.

– Sexual selection: (p. 338) one sex chooses individuals of the other sex for mating based on certain detectable characteristics.

– Can changes in allele frequencies of future generations.

– If these changes reproductive isolation, the population may become a new species.

Bringing Order to Diversity

Taxonomy

1. Life is classified according to similarities in characteristics.2. The groups in a classification system become progressively

more inclusive.3. Binomial nomenclature is a system that provides each

species with a unique name.4. Currently all organisms are grouped in one of six

kingdoms.5. The atmosphere of early earth was made up of water,

methane, ammonia, and carbon dioxide primarily.6. The first life probably arose from simple chemicals.7. The first organisms to exist on earth may have been

heterotrophs.

Taxonomy

Universal system of classification:• Arranges organisms according to ever larger, more general

groupings:• Kingdom: broadest categorization of life, e.g. Animalia =

all animals.• Phylum: (pl. Phyla) similar classes; indicates evolutionary

relationships, e.g. Chordata = primitive spine or notochord.

• Class: e.g. Mammalia = hair, produce milk for young.• Order: similar families, e.g. Carnivora = all carnivores.• Family: similar genera, e.g. Canidae = dog family.• Genus: (pl. Genera); species that share many similar

characteristics, e.g. Canis = dogs.• Species: AKA specific epithet. e.g. lupus = gray wolf

– Gray wolf = Canis lupus

Species Are Grouped into Broader Categories

NOW NOW SIXSIX (6)(6)

Taxonomy

Taxonomy

Encarsia wasp

Taxonomy

Horseshoe Crab

Taxonomy

• Commonly held characteristics become more general at each successive level of classification (from species To kingdom).

• The farther up the list you go (from species to kingdom) the more distant the relatedness.

• Each level of classification deals with three things:1. Characteristics that pertain to each group2. Organisms that belong to each group3. Varying degrees of evolutionary relatedness among

the groups.

Taxonomy

1. Assg.: Study Appendix A “A Catalogue of Living Things,” pp. R25 - 31

2. Assg.: Create a mnemonic for remembering the sequence of categories:

– D, K, P, C, O, F, G, S3. Assg.: Choose any three (3) species, each from a

different kingdom (plant, animal, fungus, etc.) and write out their entire taxonomic classification, including scientific namefrom kingdom to species.– Learn how to pronounce it.– Include a picture of the organism.

Cladistics• Derived characters are represented as hash marks

between the branches of a cladogram.– All spp. above a hash mark share the derived character it

represents.

Cladistics• Nodes: places where a branch splits off.

– Represent the most recent common ancestor shared by a clade.

Cladistics• “Snip Rule”: Whenever you snip a branch under a

node, a clade falls off– Used to identify clades– Each clade is nested within the clade that forms just before it.

How many clades are represented in this cladogram?

Six

Cladistics

Taxonomy

Taxonomic classification is not necessarily permanent.

• As new information is gathered, analyzed and interpreted, we incorporate it into our classification hypotheses so they better fit the new data.

• Biologists don’t all agree on the classification of all organisms, e.g. “lumpers” vs. “splitters.”

• Taxonomists don’t all interpret the data similarly.• Thus, several classification schemes exist.

Taxonomy

• Why are scientific names necessary?– To provide a universal means of communicating with

one another, i.e. they are necessary for scientific exactness.

– see Fig. 10.8, p. 257

Taxonomy

Taxonomy

• 1753: Carolus Linnaeus: Binomial nomenclature; two-word system for identifying each species; i.e. “scientific name,” e.g. carnation (Dianthus caryophyllus).– 1st: Genus (genera = plural); must be

capitalized; e.g. Dianthus– 2nd: Species (AKA specific epithet); must

not be capitalized; e.g. caryophyllus– Both names must be italicized (or

underlined, if handwritten)

Taxonomy

• Two rules:1. Genus name can never be used for any other group2. Species name cannot be used for any other species

within the same genus.

ensures individual identity for any organism.

• Scientific names are frequently derived from Latin or Greek, but may be from any language, or may be invented.

Dichotomous Keys

• Dichotomous key: a tool that allows the user to determine the identity of items in the natural world, e.g. trees, wildflowers, mammals, reptiles, rocks, and fish. – Consist of a series of “either/or” choices (couplets)

that lead the user to the correct name of a given item.– "Dichotomous" means "divided into two parts.“

• Therefore, dichotomous keys always give two choices in each step.

Dichotomous Keys• Identification of plants and animals in biology is

frequently aided by using a dichotomous key, a (usually written) device constructed from a series of highly organized statements arranged into couplets. – A couplet consists of (typically) two descriptions which

should represent mutually exclusive choices (often it is a particular combination of characteristics that determines the difference).

Dichotomous Keys• Both choices are read and compared with the

specimen to be identified. – Having several individuals of the same species to

observe is often helpful.

• Once a decision is made, that selection directs you to another couplet (either the next in order or one further on in the key), and this process is repeated until a conclusion (successful identification) is reached.

Dichotomous Keys• At this point a verification step is important:

compare the specimen with any details in the description and/or any available figures.– Also consider habitat and location where the collection

was made. If the description seems satisfactory, a correct identification probably has been achieved.

– If the description is not satisfactory in one or more important particulars, back up to some earlier couplet and start over, questioning each decision more carefully.

Dichotomous Keys• 1a: Bean round . . . . . . . . . . . . . . . . . . . . . . Garbanzo bean• 1b: Bean elliptical or oblong . . . . . . . . . . . . . Go to 2

– 2a: Bean white . . . . . . . . . . . . . . . . . . . . White northern– 2b: Bean has dark pigments . . . . . . . . . . Go to 3

• 3a: Bean evenly pigmented . . . . . . . . . . . . . Go to 4• 3b: Bean pigmentation mottled . . . . . . . . . . . Pinto bean

– 4a: Bean black . . . . . . . . . . . . . . . . . . . . Black bean– 4b: Bean reddish-brown . . . . . . . . . . . . . Kidney bean

Dichotomous Keys

• Class Activity: ID a student dichotomous key• Group Activity: “Whose shoe is it?” dichotomous key

• Hmwk: Create a dichotomous key to identify 20 items in your room

Determining Evolutionary Relationships• Investigation 18A: Using Cladistics to Construct

Evolutionary Trees, pp. 763 – 64• Investigation 18B: Structural Characteristics of

Animals, pp. 764 - 67

11.4 hardy-weinberg principle

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