MICROEVOLUTIONMICROEVOLUTION

THE SUBTLE CHANGES IN A GENE POOL

Genetics of PopulationsGenetics of Populations

• Continuous variation can be quantified– Eye color and height– Bell curve – Refer to pages 186 - 187

• Polygenic inheritance– Additive effect of 2 or more genes on a single

phenotype character

Environmental conditions can modify the expression of a gene

• Hydrangea colors: blue or pink– Only true for that individual– Not inherited

Inherited Alleles depend on

• Gene mutation

• Crossover

• Independent assortment

• Fertilization

• Change in chromosome number or structure

Tracking the Rate of Genetic Change

• Calculate the allele frequencyCalculate the allele frequency

• Compare with the ideal population as Compare with the ideal population as outlined in H-W Ruleoutlined in H-W Rule– Involves a population in Involves a population in genetic equilibriumgenetic equilibrium– Frequencies are stable generation after Frequencies are stable generation after

generationgeneration

5 Conditions for an Ideal (life-population)

• Random mating

• No mutations

• Very large

• Isolation

• Equal successful reproduction– No natural selection– If reproductive success were different, it would

alter the frequencies in the gene pool

Calculation “particulars”

• Use p + q = 1 – when finding the frequency of the allele

• Use p2 + 2pq + q2 – When finding the frequency of the genotype

500 flowered plantsPink (A) white (aa)

• 20 white

• 480 pink– 320 AA– 160 Aa

• Find the frequency of the A allele– 640 + 160 = 800– 800 / 1000 = .8

• Allele “a” has frequency of 0.2– Now let’s apply this information to specific

genotypes

• Each allele will occur in the same frequency as the original population– Gamete that will have “A” is 0.8

• Calculate the frequencies of the 3 possible genotypes in the next generation

• Frequency of AA?– 0.8 x 0.8 = .64 or 64%

• Frequency of aa?– 0.2 x 0.2 = .04 or 4%

• Frequency of Aa?– 0.2 x 0.8 = 0.16

• BUT THERE IS ALSO “aA” possible– That’s why the formula is 2pq– Heterozygote frequency is 0.32 or 32 %

• If you only knew the genotype information, you can still find the allele frequency– Use square root to solve for p and/or q

• Imagine 1 in 10,000 births has PKU– Q2 = 0.0001– Q = 0.01– P= 1-q– P= 0.99– Carriers?– 2pq = 2 (0.99)(.01)

= 0.0198 which is nearly 2% of the population

• In a population with 2 alleles B and b, the frequency of the allele B is 0.7. What would be the frequency of heterozygotes?– A. 0.7– B. 0.49– C. 0.21– D. 0.42– E. 0.09

• Answer D

• If 16% of the individuals in a population show the recessive trait, what is the frequency of the dominant allele?– A. 0.84– B. 0.36– C. 0.6– D. 0.4– E. 0.48

• Answer C

Microevolution

• Most common driving forces away from equilibrium:– Natural selection– Gene flow– Genetic drift

• Others– Gene mutations

• Only source of new alleles in a population

• Rare enough not to have an immediate effect on the allele frequency

• What would decrease the frequency of an allele?– Lethal mutation

• A mutation that turns out to be an advantage can be maintained through natural selection

• Mutations create new alleles BUT

• Natural selection, gene flow, and genetic drift change the frequencies of alleles in the gene pool

Genetic Drift

Change in allele frequencies over the generations– Gene pool will change – Especially true if population is 100 or less– Negligible in a very large population

• Due to chance alone– Just like rolling dice or flipping coins

Bottleneck EffectGenetic Drift Example

• Segments of a population are destroyed by disasters or hunting

• Usually reduces genetic variability• Serious threat to the survival of a species

– Cheetahs –ice age victims 10.000 years ago then hunted near extinction 1900’s

– Northern elephant seal had been hunted down to 20; now 30,000 but electrophoresis shows no variability in genes

Founder Effect

• Small sample of a population colonizes a new habitat – Darwin’s finches strayed from S. A.– Inherited disorders among humans

• Retinitis pigmentosa frequency higher among an isolated population due to colonists carrying the gene

Gene Flow

• Alleles enter and leave a population as an outcome of immigration and emigration– Most populations are certainly not closed

systems

• Wind carries pollen– Result is that over time gene flow reduce

differences between populations

• Neighboring populations may have been affected by natural selection

• Gene flow will eventually amalgamate the neighbors

• Human migration reduces the variability

Mutations

• Usual rate is 1 in a million

• Not significant source of variation in a gene pool

• But it is the original source of variation

• Serves as raw material for natural selection

Nonrandom mating

• In reality, we do NOT mate randomly

• Assortative mating—select partners like themselves in certain characteristics

• Inbreeding

• Extreme “selfing” is self-fertilization of plants!

• Frequencies of genotypes shows a decrease in heterozygotes

• Wildflower population– Self-fert will increase the frequency of

homozygotes at the expense of heterozygotes

– AA begets AA– aa begets aa

– “Aa” selfs; only half of their offspring will be heterozygous

– Each generation decreases that number

• Result?

• More hom recessive– Greater than H-W prediction would be

• However the p and q frequency of alleles remains the same– p + q = 1

Natural Selection

• H-W says everyone has to be equal in their ability to produce viable, fertile offspring

• Reality is differential success– Some have more offspring– Maybe red flowers produce more offspring that

white flowers (white visible to predators)– Freq of “A” would increase

• This is the only agent of microevolution that can be adaptive– Accumulates and maintains favorable

genotypes in a population– If the environment changes, selection responds

by favoring genotypes adapted to the new conditions

Causes of Microevolution

• Genetic Drift

• Gene Flow

• Mutation

• Nonrandom mating

• Natural selection

• As a mechanism of microevolution, natural selection can be most closely equated with– A. Assortative mating– B. Genetic drift– C. Differential reproductive success– D. Bottlenecking of a population– E. Gene flow

• Answer next page

• C Differential reproductive success

• Selection acts directly on

• A. Phenotype

• B. Genotype

• C. The entire genome

• D. Each allele

• E. The entire gene pool

• A Phenotype

• Most of the variation we see in coat coloration and pattern in a population of wild mustangs in any generation is probably due to– A. New mutations in the preceding generation

– B. Sexual recombination

– C. Genetic drift

– D. Geographic variation within the population

– E. Environmental effects

• B Sexual recombination

• The most likely effect of assortative mating on the frequencies of alleles and genotypes would be– A. Decrease in p2 compared to q2

– B. Trend toward zero for q2

– C. Convergence of p2 and q2 toward equal values

– D. A change in p and q

– E. A decrease in 2pq below the value expected by H-W

• E decrease in 2pq– Reduce heterozygotes– Does not affect p and q

• A founder event favors microevolution in the founding population mainly because– A. Mutations are more common in a new environment

– B. Small population is subject to sampling error in the composition of its gene pool

– C. The new environment is likely to be patchy, favoring diversifying selection

– D. Gene flow increases

– E. Members of a small population tend to migrate

• B small population leads to sampling error

Adaptive EvolutionModes of Natural Selection

• Response to the environment

• Adaptive mutation– Allele frequency shifts– Phenotype frequency shifts

Directional Selection

• Most common during periods of environmental change

• Or when members migrate to a new habitat with different environmental conditions

• Frequency shifts in one direction– Favors rare individuals that deviate from the

average

Peppered Moth

• Case of industrial melanism

• Bird predation contributed to the selection

Protective Coloration

• Rabbits

• Mice

• Tigers

Antibiotic Resistance

• Antibiotics “killed” susceptible cells

• Also allowed (favored) CELLS THAT ARE RESISTANT

• All this in the last 60 years

Pesticide Resistance

• Kills insects, worms, etc.

• Has allowed resistant forms to increase

• Genetically engineered plants– Pesticide resistant– Will still trigger pests to evolve– Coevolution

• Biological controls– Natural enemies of the pests are raised and

released on a particular area– Allows pests (prey) and “predator” to coevolve

Stabilizing Selection

• Culls the extreme variants from the population

• Reduces phenotypic variation

• Maintains the status quo– Favors the “average” for a trait– Human birth weight around 7 lbs– Mortality increases for higher or lower values

Gall Stories

• Page 262 was about a crown gall caused by bacterium Agrobacterium tumefasciens

• Page 288 is a gall caused by a fly larvae– Fly lays eggs on stem– Egg develops into pupa then larva– Larva bores into stem– Eats plant juice and tissues (yum)

• Plant cells respond by rapid growth of tissue which forms a tumor (gall)

• These galls can vary in range depending on the phenotypes of the fly

• Small galls—not favored because a wasp will puncture it and lay its eggs

• Those eggs develop into larvae and eat. . .– Eat the fly larvae!– Reduces that phenotype of fly larvae – Fly larvae that make small galls are selected

against

• Large galls are not favored because birds will chip into the gall and eat the larvae

• Flies natural enemies (wasp and birds) act as predators

• Cause stabilizing selection in favor of intermediate sized galls

Disruptive Selection

• Phenotypes of two extremes are favored

• Intermediates are selected against

• This category also includes sexual selection.– Females are main agents of selection.– It’s their reproductive success that matters

most!

• Balanced polymorphism includes sickle cell story.

• Heterozygotes favored

• Malaria is the selective force in tropical and subtropical habitats.

• African Finch—balanced polymorphism– Only found small or large billed species– Why?

• Available food (sedge—grasslike plant)

• Wet season: Two species of sedge grow– One has hard seeds, other soft

• Birds mate

• Both species of sedge available

• Birds with both sizes of beaks

• Dry season?– Sedge with soft seeds decrease in population– Birds with large bills are favored– Small-billed may not survive

Beak of the Finch

• Darwin’s finches follow the same natural selection path that the story of African finches do.

• Darwin’s finches were the first “natural” (in the wild) evidence for natural selection.

• Artificial selection was already in place.– Breeding of domesticated animals

• The volcanic Galapagos give rise to a rich diversity of environmental habitats.

• Darwin maintains that the beaks are adaptations to different food sources.– Question of seed-beak compatibility if you will.– Harder the seed, the larger and stronger the bill.

• Wet years, the ground finch prefers small seeds– Easier no matter what the bill size is– Selection favors those finches with smaller bills

and the population evolves– Directional stabilization– Just don’t bother with the large, hard seeds

• Dry years, the small and large seeds are less plentiful.

• Finches with larger bills are favored because they simply have an advantage of being able to eat more food.

• Current research includes work done by Princeton researchers Peter Grant and his wife.

• I have one of their books if you want to see it!

• Island with most species. . .Daphne major– Trivia that someone may want to know

1858

• Alfred Wallace had been researching in the East Indies and was ready to submit his findings.– He asked Darwin (whom he respected) to read

it and forward it to Lyell for publication.– Lyell included excerpts on Darwin’s 1844

unpublished essay

• In 1859, On the Origin of Species, was finally published

• Darwin developed and supported the theory much more extensively than Wallace.

• Darwin’s notebook also collaborated that he had developed the theory 15 years before Wallace.

• Even Wallace felt that Darwin deserved most of the credit.

• Darwin only collected data from a few different species. Later learned of the 13 species that inhabit the island chain.

• He also thought that the changes took place s-l-o-w-l-y through gradual adaptations.

• That’s because of the geologic gradualism that he was reading from Lyell.

• “Evolution” or “change through time” was not the debated issue as many think today.

• It has taken many years of genetic evidence (hence post Mendelian time) for the theory to be accepted.

The Darwinian View of Life

• One facet is that evolution is the basis of unity and diversity of life

• Darwin never used the word evolution in The Origin of Species

• He DID use the terms “descent with modification”

• He perceived that all organisms are related through descent from some unknown prototype that lived in the remote past.

• Descendants spilled into various habitats over millions of years and accumulated diverse modifications (adaptations).

• History of life is like a tree.• At each fork is an ancestor common to all

lines of evolution branching from that fork.• This was called “common descent”.• Reality is that most branches of evolution

are dead ends. • 99% of all species that have ever lived are

extinct.

• Darwin actually devoted very little space to the origin of species.

• He really concentrated on how populations became better adapted to their local environments through natural selection.

• This is the theory that spurred controversy.

• Natural selection is a mechanism Darwin proposed to explain the facts of evolution documented by fossils, biogeography, and other types of historical evidence.