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.

• Darwin spent five years on a voyage that

took him around the world with the

majority of his time spent in South America

and its neighboring islands.

• Darwin published his theory 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, 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.

• However, many favored competing explanations

and 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 of life.

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Darwin’s Observations

• 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 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 19 million elephants

produced from just one original pair.

Darwin’s Observations

Darwin’s Elephant Problem

“There is no exception to the rule that every

organic being naturally increase at so high a rate

that if not destroyed, the earth would soon be

covered by the progeny of a single pair ....

The Elephant is reckoned to be the slowest

breeder of all known animals, and I have taken

some pains to estimate its probable minimum

rate of natural increase: it will be under the

mark to assume that it breeds when thirty years

old, and goes on breeding till ninety years old,

bringing forth three pairs of young in this

interval; if this be so, at the end of the fifth

century there would be alive fifteen million

elephants, descended from the first pair.”

(Darwin, 1859 p.64)

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Darwin’s Observations

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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.

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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Adaptations and Fitness

• The antelope hare lives in the desert,

and the snowshoe hare lives in the

mountains.

• Explain how the differences in their

traits enhance their ability to survive in

their respective environments.

• Evolutionary success or fitness refers to

the contribution of genes to the gene

pool and NOT how long an organism

lives.

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The Effect of Environmental Change

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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, 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.

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

• Propose an explanation!

Industrial Melanism

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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.

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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Microevolution

Microevolution is simply a change in gene frequency

within a population.

• 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.

• Such a change might come about because

– natural selection favored the gene

– the population received new immigrants carrying the gene (gene flow)

– nonresistant genes mutated into a resistant version of the gene

– of random genetic drift from one generation to the next

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Microevolution

• A gene is a sequence of DNA nucleotides

that specify a particular polypeptide

chain.

• Genes code for proteins.

• 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

survivorship depend on phenotype not genotype.

• Natural selection acts on individuals, but only populations

evolve.

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Macroevolution

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).

• 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 percent occurrence for a given allele

Sources of Genetic Variation

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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. 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.

• 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.

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.)

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.

Myoglobin is a protein that binds with oxygen in the muscles. This gene has

been duplicated and modified many times. It has given rise to the hemoglobin

gene.

Neutral Mutations

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Naturally evolving proteins gradually accumulate

mutations while continuing to fold into stable structures.

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.

• Between mammals and reptiles there are 15 different

amino acids or mutations.

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 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.

Changes in Cytochrome C

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Cytochrome c Comparison

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• A dash indicates that the amino acid is the same one found at that position in the

human molecule.

• All the vertebrate cytochromes (the first four) start with glycine (Gly).

• The Drosophila, wheat, and yeast cytochromes have several amino acids that

precede the sequence shown here (indicated by <<<).

• In every case, the heme group of the cytochrome is attached to Cys-14 and

Cys-17 (human numbering).

Molecular homology of cytochrome c (see three-letter code of amino acids)

1 6 10 14 17 18 20

Human Gly Asp Val Glu Lys Gly Lys Lys Ile Phe Ile Met Lys Cys Ser Gln Cys His Thr Val Glu Lys

Pig - - - - - - - - - - Val Gln - - Ala - - - - - - -

Chicken - - Ile - - - - - - - Val Gln - - - - - - - - - -

Dogfish - - - - - - - - Val - Val Gln - - Ala - - - - - - Asn

Drosophila <<< - - - - - - - - Leu Val Gln Arg Ala - - - - - - Ala

Wheat <<< - Asn Pro Asp Ala - Ala - - - Lys Thr - - Ala - - - - - Asp Ala

Yeast <<< - Ser Ala Lys - - Ala Thr Leu - Lys Thr Arg - Glu Leu - - - - - -

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 a and b chains.

• There is an inverse relationship

between the difference in the

amino acid sequence and how

closely related the organisms are to

humans.

• The b 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 occurs as a result of either an insertion or

deletion of a nucleotide.

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

forward.

• Almost all frame shift mutations are deleterious.

• Recently, bacteria were found growing in a pool of nylon wastes.

(Flavobacterium)

• These bacteria were actually digesting the nylon waste.

• Upon examining the genome of these bacteria, it was found there

was a frameshift mutation in their DNA that caused the production

of three different enzymes that could digest the nylon.

Evolution of Hemoglobin Gene

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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 Leibl Science Content Director National Math and Science