MICROEVOLUTION

HARDY WEINBERG EQUATIONS

Ms.KimH. Biology

• Darwin difficulty = what is the mechanism of natural selection?? How does it occur??– Genetic basis

• After Darwin’s paper, Mendel inheritance was published (discrete, heritable characteristics “genes”)

• By 1940’s modern synthesis combined ideas from different fields– Population is the unit of evolution– Natural selection is the main mechanism

• Gradualism explained accumulation of small changes led to large ones over time

The Smallest Unit of Evolution

Population genetics• Population genetics is the study of how

populations change genetically over time• Population: – Group of the same species living in the same area

that can interbred• Species: – a group of populations whose individuals have the

potential to interbreed and produce fertile offspring• Gene pool: – the total combination of genes (alleles) in a

population at any one time

“Individuals are selected, but populations evolve.”

What is an Allele?• A complete gene contains 2 parts called alleles• An allele = a alternative form of a gene• You get 1 allele from “mom” and 1 from “dad”• Alleles are made up of DNA• Genotype = the genetic combination of 2 alleles

• The change in the frequency of ALLELES (or how often a certain allele appears) in a population over time

• The change in a population’s gene pool over time

Microevolution

Allele Frequencies • Each allele has a frequency in a

population’s gene pool a # of times it appears in a population

Aa

A

A

a

a

• In a population of wildflowers, the Red allele is A and white allele is a.

• 500 total plants in the population, 20 are white (aa), 320 are red (AA), 160 are pink (Aa)

• How many total alleles for flower color are there in this population?

• 1000 (500 plants with 2 alleles EACH)• a= 20 + 20 + 160 = 200 alleles• A = 320 + 320 + 160 = 800 alleles

Allele Frequencies- EXAMPLE

Allele Frequencies- EXAMPLE (Con’t)

What is the frequency of “A” allele and “a”

allele? Always use a decimal for frequency!

• 800/1000 A = 0.8 • 200/1000 a = 0.2

How do we KNOW if a population is evolving?

• Use the Hardy-Weinberg theorem (or equilibrium) • It is used to describe a population that is

NOT evolving•Conditions in the population are completely RANDOM

Hardy Weinberg Theorem• Conditions for non-evolving (NOT

CHANGING) population:–Very large population size–No migration–No mutations–Random mating–No natural selection

Since all is RANDOM, the null hypothesis is that the population is

not evolving.

RARELY MET IN NATURE

2 equations are used in the Hardy Weinberg Theorem

p + q = 1 (1 means 100% of all alleles)

• This means that there are only 2 possible alleles p and q

– p = dominant allele frequency– q = recessive allele frequency

• The equation that corresponds to the frequency of individuals regarding these 2 alleles:

p2 + 2pq + q2 = 1

p2 + 2pq + q2 = 1• p2 = frequency of homozygous dominant individuals• 2pq = frequency of heterozygous individuals

(frequency of Aa plus aA genotype)• q2 = frequency of homozygous recessive individuals

EXAMPLE• Round head is dominant to cone heads, 51% of the

individuals in the population have round heads. What portion of this 51% are homozygous?

• 0.49 = q2 (recessive), Therefore q = 0.7, so p = 0.3• p2 is the frequency of homozygous dominant

individuals = 0.09 or 9%

Hardy-Weinberg TheoremStates that… • the frequencies of alleles and genotypes

will stay CONSTANT over generations UNLESS acted upon by agents or• It describes a population where allele

frequencies do NOT change• p and q do NOT change over

generations

Hardy-Weinberg EquationWhen using this

equation, we assume fertilization is RANDOM

and all male/female combinations are likely

Conditions for Hardy-Weinberg Equilibrium

• The Hardy-Weinberg theorem describes a hypothetical population

• In real populations, allele and genotype frequencies DO change over time

• http://www.hippocampus.org/Biology;jsessionid=6E6D9D7721EBDBFE9BD00616517846DD

Causes of MicroevolutionA change in the gene pool of a population

over a succession of generations• We use 5 criteria for non-evolution to

determine causes of microevolution• The Hardy Weinberg equations are

used to determine the degree of microevolution that is occurring for a given allele

Cause #1: Small Populations

• Genetic drift: changes in the gene pool of a SMALL population due to chance (usually reduces genetic variability)

• http://highered.mcgraw-hill.com/sites/0072835125/student_view0/animations.html#

• 2nd to last video

How can this happen? Genetic Drift

Genetic Drift Example #1The Bottleneck

Effect • Results from a

reduction in population (natural disaster) such that the surviving population is no longer genetically representative of the original population

Reduced genetic variation means that the population may not be able to adapt to new selection pressures, such as climatic change or a shift in available resources, because that genetic variation that selection would act on may have already drifted out of the population.

Ex: NORTHERN ELEPHANT SEAL

Northern elephant seals have reduced genetic variation probably because of a population bottleneck humans inflicted on them in the 1890s. Hunting reduced their population size to as few as 20 individuals at the end of the 19th century. Their population has since rebounded to over 30,000—but their genes still carry the marks of this bottleneck: they have much less genetic variation than a population of southern elephant seals that was not so intensely hunted.

Genetic Drift Example #2

Founder Effect: a cause of genetic drift due to the colonization by a limited number of individuals from a parent population

http://bcs.whfreeman.com/thelifewire/content/chp24/2402002.html

Founder Effect in Amish

Causes dwarfism and polydactylyhttp://www.mhhe.com/biosci/esp/2001_gbio/folder_structure/ev/m2/s4/evm2s4_7.htm

Cause #2: Migration of AllelesGene Flow:

genetic exchange due to the migration of fertile individuals or gametes between populations (reduces differences between populations)

Cause #3: MutationsMutations: • a change in an organism’s DNA • original source of genetic variation

(raw material for natural selection)• Mutations can immediately alter p and q in a

population• Individual mutations are rare in a population,

but there may be cumulative mutations that have an effect cause evolution

Cause #4: “Picky” MatingNonrandom

mating • “picky” mating• Unequal chances

of each egg getting fertilized

• Assortative mating - choosing individuals more like self

Sexual Selection• Sexual selection is natural selection for

mating success• It can result in sexual dimorphism,

marked differences between the sexes in secondary sexual characteristics

• Intrasexual selection – competition among individuals of one

sex for mates of opposite sex• Ex: Males competing for female attention

• Intersexual selection – when individuals of one sex (usually

females) are choosy in selecting their mates of the other sex

– Selection may depend on the showiness of the male’s appearance

Sexual Dimorphism • Males are usually

more colorful and larger• Does NOT help cope

with environment• DOES lead to

reproductive success• Can be termed

“Female Selection”

Cause #5: Natural SelectionNatural Selection

• differential reproductive success in reproduction due to variation

• only form of microevolution that adapts a population to its environment–Phenotypes are selected FOR or

AGAINST (not genotypes)• In a population at equilibrium, no

phenotypes (therefore, alleles) are selected over other alleles

• http://www.mhhe.com/biosci/esp/2001_gbio/folder_structure/ev/m2/s1/evm2s1_6.htm

• Directional selection favors individuals at one end of the phenotypic range

• Disruptive selection favors individuals at both extremes of the phenotypic range

• Stabilizing selection favors intermediate variants and acts against extreme phenotypes

• http://wps.pearsoncustom.com/wps/media/objects/3014/3087289/Web_Tutorials/17_A02.swf

3 Modes of Natural selection

Directional Selection• This form of selection increases

the expression of the extreme versions of a trait in a population.

• For example: most of the moths had light-colored wings, but dark moths started to appear. Because moths live against tree barks, years later, most of the moths were dark. Thus, more dark moths survived, adding more genes for dark color to the population.

Disruptive Selection• Disruptive selection: a process that

splits a population into two groups.• it tends to remove individuals with

average traits but retain individuals expressing extreme traits at both ends of a continuum.

• For Example, snakes that live on rocks and are grey will survive. Snakes that live on grass and are green will survive. Snakes that have an intermediate coloring would be disadvantaged because it would be more visible to predators.

Stabilizing Selection• It operates to eliminate

extreme expressions of a trait when the average expression leads to higher fitness.

• For example: human babies born with below-normal and above-normal birth weights have lower chances of survival than babies born with average weights.

• Therefore, birth weight varies little in human populations.

DARK ROCK HABITAT LIGHT/DARK ROCK HABITAT MEDIUM COLOR ROCK HABITAT

Sources of Variations1. Mutations–At first the mutation may not be

beneficial to the organism (resistance to antibiotics)–Once antibiotics are introduced the

mutation is beneficial2. Sexual recombination

increases variation

Evolutionary Fitness• The phrases “struggle for existence”/“survival

of the fittest” are commonly used to describe natural selection but can be misleading

• Reproductive success is more subtle and depends on many factors

» Fitness is determined by REPRODUCTIVE AND SURVIVAL success of individual

• Variations & natural selection affect fitness• Sterile individuals even with a relatively fit allele

will not be considered fit overall (=0) • Fitness is not determined by one trait but by the

totality of traits in the organism