Microevolution: Unique Gene Pools

Charles Darwin• Charles Darwin (1809-1882) is credited

with proposing that the mechanism for the process of evolution is natural selection.

• English naturalist and geologist

• Darwin spent five years on a voyage (HMS Beagle) that took him around the world with the majority of his time spent in South America and its neighboring islands.

• Darwin published his theory (20 years later) with compelling evidence for evolution in his 1859 book On the Origin of Species, overcoming scientific rejection of earlier concepts of transmutation of species.

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Charles Darwin

He established that all species of life have descended over time from common ancestors (descent with modification), and proposed the scientific theory that this branching pattern of evolution resulted from a process that he called natural selection, in which the struggle for existence has a similar effect to the artificial selection involved in selective breeding.

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Charles Darwin• By the 1870s the scientific community and much of

the general public had accepted evolution as a fact (change over time/descent with modification).

• However, many favored competing explanations (e.g. Theists evolution, Neo-Lamarckism, Saltationism). It was not until the emergence of the modern evolutionary synthesis from the 1930s to the 1950s that a broad consensus developed in which natural selection was the basic mechanism of evolution.

• In modified form, Darwin's scientific discovery is the unifying theory of the life sciences, explaining the diversity and unity of life.

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Darwin’s Influences & Observations• Thomas Malthus, Charles Leyell, George

Cuvier

• Populations change over time as evidenced by the fossil record.

• There are always more offspring produced than the preceding generation.

• Populations, if left unchecked, grow at a geometric rate (exponential rate) rather than an arithmetic rate.

• Darwin used an example involving elephants to illustrate the points above. He estimated that if elephants underwent unrestricted reproduction, that in 740-750 years there would be 15 million elephants produced from just one original pair.

Exponential vs. Logistic growth

Darwin’s Elephant Problem Cont’d“There is no exception to the rule that every organic being naturally increases at so high a rate that if not destroyed, the earth would soon be covered by the progeny of a single pair ....

(Darwin, 1859 p.64)

(Note: Darwin’s calculation flaws)

* What prevents/”destroys” this innate ability of living creatures to reproduce so prolifically?

•Darwin’s answer: There is always a struggle for existence. What did he mean?

•What is involved in the struggle and who “wins”?

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Darwin’s Observations Cont’d

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• There is variation within a given species and the majority of this variation is inherited. This litter of kittens vary with respect to coat pattern and color.

• Any variation may, to some degree, affect the ability of an organism to reproduce and contribute genes to the gene pool, thus affecting evolutionary success.

• Species change over time. These changes are related to traits that are inherited or arise from an alteration of the genetic code.

• Some inherited traits are beneficial and contribute to survival.

• Whether a trait is beneficial or not is a function of the environment in which it lives.

Take-away points:

• Overproduction leads to:• A struggle for existence but also creates:• Tremendous variation (inherited)• Natural selection “selects” those variants

that are best suited (have the adaptations that lead to differential reproductive success in local environments)

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A closer look: Adaptations and Fitness

• An adaptation is a genetically controlled trait that is favored by natural selection and gives the organism a reproductive advantage ensuring the trait is passed on to its descendants.

• This trait may also allow the individual to survive longer thus increasing the reproductive rate of that individual.

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Cont’d Adaptations and Fitness• The antelope hare lives in the desert, and the

snowshoe hare lives in the mountains.

• What adaptations do you see in these two hares that might provide them with differential reproductive success in their local environments?

• Evolutionary success or fitness refers to the contribution of genes to the gene pool and NOT necessarily how long an organism lives.

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Classic example of adaptations to a “local environment”

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• Earth’s environment is NOT STATIC, but rather ever changing.

• As a consequence, traits or adaptations that were favorable may become unfavorable.

• The peppered moth, Biston betularia is native to England and exists in two forms (morphs), one is dark and the other light with a “peppered” appearance. Birds are its main predator.

• Prior to the industrial revolution, only 2% of the moths were dark.

• The industrial revolution produced vast amounts of sulfur dioxide and soot from the burning of coal which altered the environment. How so?

• Fifty years later 95% of the moths were dark.

• Propose an explanation.

Industrial Melanism (I.M.)

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England has since regulated the burning of coal and as a result, the trees are returning to their original state (A).

Consequently, the coloring among the population of moths in Britain has shifted back so that the peppered moths are once again favored.

What role does inherent variation play in I.M.?

Evolution Defined• Evolution is defined as a change in the

inherited characteristics of biological populations over successive generations.

• Evolutionary processes give rise to diversity at every level of biological organization, from the molecular to the macroscopic.

• As a result diversity is prevalent among molecules such as DNA as well as individual organisms and species of organisms.

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MicroevolutionMicroevolution is simply a change in gene frequency below the species level (at the level of populations).

•Evolution at this scale can be observed over short periods of time such as from one generation to the next.

•Example: The frequency of a gene for pesticide resistance in a population of crop pests increases.

•Potential causes include:

– natural selection favored the gene (SC gene heterozygous advantage)

– the population received new immigrants carrying the gene (gene flow: humans past and present)

– nonresistant genes mutated into a resistant version of the gene

– coevolution

– of random genetic drift from one generation to the next (sudden environmental changes)

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What are genes?• A gene is a sequence of DNA nucleotides

that specify a particular polypeptide chain.

• Many genes code for proteins (polypeptides).

• An allele is a particular form of a gene. For example: B represents the allele for black coat color and b for white coat color.

• Selection acts on phenotype because differential reproduction and success depends on phenotype not genotype.

• Natural selection “selects”/”favors” individuals, but only populations evolve.

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Macroevolution (in brief)Macroevolution is evolution on a scale of separated gene pools (not individuals).

•Think of it as an accumulation of changes which result in speciation (forming a new species but not necessarily).

•What determines if a single species has become 2 different species?

•Macroevolutionary studies focus on change that occurs at or above the level of species, in contrast with microevolution, which refers to smaller evolutionary changes (typically described as changes in allele frequencies) within a species or population.

•The process of speciation may fall within the purview of either, depending on the forces thought to drive it.

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More Evolution Terms

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• Species-a group of interbreeding organisms that produce viable and fertile offspring in nature

• Gene pool-sum total of all the genes in a given species

• Allelic frequency-is the occurrence for a given allele in a population

N.S. needs Variation to work. Sources?

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How does variation in a population or gene pool arise?

1.Mutations, (gene duplication and chromosome fusion) provide the raw material for evolution.

2.Most variation that N.S. acts on comes from meiosis and sexual reproduction produce new recombinants of phenotypes upon which natural selection operates.

The wisteria pictured on the right has a mutation causing it to produce white flowers instead of purple flowers.

Types of Mutations

• MOST mutations are deleterious as well as recessive. Which means?

• Obviously, mutations occurring in somatic cells do not affect future generations.

• Only mutations occurring in gametes affect future generations.

• Mutations can occur at either the gene or chromosomal level.

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Mutations may cause a sheep to have a 5th leg.

But this is not evolution! 

Point Mutations: Synonymous vs. Nonsynonymous

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Point mutations occur when one nucleotide is substituted for another.

The genetic code contains “synonyms” for the coding of amino acids. For example the DNA codons GGA, GGG, GGT, GGC all code for the amino acid proline.

Therefore, as long as the codon has GG in positions 1 & 2, a mutation in position three has no consequence, proline will be coded for regardless.

This sort of mutation is called a synonymous or silent mutation.

Point Mutations: Synonymous vs. Nonsynonymous

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Point mutations that do result in a different amino acid are called a nonsynonymous or missense mutations (if polypeptide is produced) vs nonsense.

Missense mutations can affect the protein in one of THREE ways: (Remember the new amino acid will have a different R group on the protein)

1.It can result in a protein that does not function as well as the original protein. (This happens most often.)

2.It can result in a protein that functions better than the original protein.

3.It can result in a protein that functions like the original protein. This is usually because the R groups are similar. (both polar or both nonpolar, etc.)

Mutations cont’d Gene Duplication

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Genes can be duplicated and occasionally the duplication moves a gene from one chromosome to another. Each gene will accumulate different mutations altering the protein that is subsequently synthesized.

Globins are proteins. A class of the group binds with oxygen (in muscles, erythrocytes, plant root cells). This gene has been duplicated and modified many times. It has given rise to the hemoglobin genes (alpha, beta, fetal, embryonic) as well as myoglobin, leghemoglobin and psuedogenes.

Evolution of Hemoglobin Gene

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Neutral Mutations

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Naturally evolving proteins gradually accumulate mutations while continuing to fold into stable structures.

Ex. Hb4 and its forms (slightly diff seq = slightly diff polypeptide = no functionality lost)

This process of neutral evolution is an important mode of genetic change and forms the basis for the molecular clock.

•Cytochrome c is a small protein found on the mitochondrial membrane. (a highly conserved protein)

•Between mammals and reptiles there are 15 different amino acids or mutations. (neutral Cyt c mutations have been passed on for millions of years)

Neutral Mutations

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• Mammals and reptiles diverged 265 million years ago.

• That means on average cytochrome c mutated every 17 million years.

• In comparing the evolution of other organisms and their cytochrome c one (NEUTRAL) mutation every 17 million years holds true.

Above is a comparison ancestral cytochrome c and human cytochrome c. This gene has been highly conserved as it is a protein used in the electron transport chain of the mitochondria. Missense mutations occur more frequently in pseudogenes (genes that have been duplicated, then mutated and are no longer functional) than in functional genes. Why?

Changes in Cytochrome C

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Hemoglobin Comparison

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• This is a comparison between the differences in the amino acid sequence of human hemoglobin and different species.

• The last three species do not have a distinction between and chains.

• There is an direct relationship between the amino acid sequence and how closely related the organisms are to humans.

• The chain of hemoglobin has 146 amino acids.

Human beta chain 0

Gorilla 1

Gibbon 2

Rhesus monkey 8

Dog 15

Horse, cow 25

Mouse 27

Gray kangaroo 38

Chicken 45

Frog 67

Lamprey 125

Sea slug (a mollusk) 127

Soybean (leghemoglobin) 124

Hemoglobin Comparison

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Frameshift Mutation

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• A frameshift mutation (nonsense) occurs as a result of either an insertion or deletion of a nucleotide. Polypeptide produced is massively dysfunctional or completely non-functional

• This changes the amino acid sequence of the protein from that point forward.

• THECATATETHEFATFATRATBUTNOTHISHAT

• TECATATETHEFATFATRATBUTNOTHISHAT

• THECATATETHEFATFATRATBUTNOOTHISHAT

• Insertion mutations and deletions commonly produce these frameshifts

• Almost all frame shift mutations are deleterious.

• Tay Sachs & Cystic fibrosis are examples

Chromosomal Rearrangement

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There have also been major changes in chromosome structure that result in changes within populations which can, in turn, result in the emergence of new species.

These include:

a.inversions

b.deletions

c.duplication

d.translocations

e.fusions

Chromosomal Rearrangement

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Compare the karyotype of a human (H) and a chimpanzee (C).

Notice the great apes have 24 pairs of chromosomes compared to 23 pairs of chromosomes in a human.

Why the difference?

Chromosome #2 in the human is the result of a fusion of two chimpanzee chromosomes.

Human Impact on Gene Pools

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It is well documented that humans have had an impact on certain gene pools.

For example, humans have selected for certain desirable traits within the mustard family and cultivated different agricultural products for human consumption.

Artificial Selection

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When humans manipulate a gene pool it is called artificial selection. There are often consequences involved in such manipulations. For example in agriculture, farmers try to increase crop production, which may lead to many farmers growing only one variety of a particular crop such as corn. This leads to a loss of genetic diversity. If a disease attacks that particular variety of corn, the farmers growing that variety lose their entire crop.

Antibiotics and Artificial Selection

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• When antibiotics are applied to a population of microorganisms to treat an infection, some of the microorganisms may be naturally immune to the drug.

• Why? A random mutation occurred in the genetic code of the microorganism conferring its resistance.

• These resistant microorganisms continue to flourish and cause disease.

• The only remaining option a physician has is to treat the infection with a different antibiotic and hope that none of the surviving microorganisms possess a different random mutation that makes them resistant to the second antibiotic as well.

Antibiotics and Artificial Selection

• The increase in antibiotic-resistant bacteria has caused doctors to reduce the number of prescriptions written for antibiotics in general.

• About 70% of pathogenic bacteria are resistant to at least one antibiotic and are called “super bugs” or MDR bacteria. (multidrug resistant)

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MRSA or Methicillin-resistant Staphylococcus aureus

• MDR bacteria do not respond to “first line of defense” antibiotics.

• These types of bacteria are most commonly found in hospitals.

• Skin boils or similar lesions that do not heal often result.

• MDR bacteria can attack internal organs upon gaining entry into the body.

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Reducing or Eliminating Gene Pools

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• Human activities often augment genetic drift and diminish gene flow for many species.

• This reduces genetic variation thereby disrupting adaptive processes both locally and globally within a species.

• This impact is illustrated within populations of collared lizards (Crotaphytus collaris) living in the Missouri Ozarks.

• Forest fire suppression has reduced habitat and disrupted gene flow in this lizard, thereby altering the balance toward drift and away from gene flow. This balance can be restored by managed landscape burns. 

Effect of Sexual Reproduction

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Sexual reproduction recombines genes in new ways. This results in unique offspring that differ from either parent or sibling. Humans make 223 different kinds of gametes. Fertilization means that the uniqueness of an individual is 223 223. Or the probability that two siblings will be genetically identical (excluding identical twins) is 446.

Sexual reproduction is like shuffling a deck of cards and every time getting a new and unique hand dealt. It is the major driving force of evolution.

Created by:

Carol LeiblScience Content DirectorNational Math and Science