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EXPLORATIONS: AN OPEN INVITATION TO BIOLOGICAL ANTHROPOLOGY

Editors: Beth Shook, Katie Nelson, Kelsie Aguilera and Lara Braff

American Anthropological Association Arlington, VA

2019

CC BY-NC 4.0 International, except where otherwise noted

ISBN – 978-1-931303-63-7

www.explorations.americananthro.org

Chapter 4: Forces of Evolution

Andrea J. Alveshere, Ph.D., Western Illinois University

Learning Objectives

• Describe the history and contributions of the Modern Synthesis.

• Define populations and population genetics as well as the methods used to study them.

• Identify the forces of evolution and become familiar with examples of each.

• Discuss the evolutionary significance of mutation, genetic drift, gene flow, and natural selection.

• Explain how allele frequencies can be used to study evolution as it happens.

• Contrast micro- and macroevolution.

It’s hard for us, with our typical human life spans of less than 100 years, to imagine all the way back, 3.8 billion years ago,

to the origins of life. Scientists still study and debate how life came into being and whether it originated on Earth or in some other region of the universe including some scientists who believe that studying evolution can reveal the complex

processes that were set in motion by God or a higher power). What we do know is that a living single-celled organism

was present on Earth during the early stages of our planet’s existence. This organism had the potential to reproduce

by making copies of itself, just like bacteria, many amoebae, and our own living cells today. In fact, with today’s genetic

and genomic technologies, we can now trace genetic lineages, or phylogenies, and determine the relationships between all of today’s living organisms—eukaryotes animals, plants, fungi, etc.), archaea, and bacteria—on the branches of the

phylogenetic tree of life Figure 4.1).

Looking at the common sequences in modern genomes, we can even make educated guesses about what the genetic

sequence of the first organism, or universal ancestor of all living things, would likely have been. Through a wondrous series of mechanisms and events, that first single-celled organism gave rise to the rich diversity of species that fill

the lands, seas, and skies of our planet. This chapter explores the mechanisms by which that amazing transformation

occurred and considers some of the crucial scientific experiments that shaped our current understanding of the

evolutionary process.

Forces of Evolution | 109

Figure 4.1 Phylogenetic tree of life.

THE MODERN SYNTHESIS

Historical Framework

When learning about biological sciences today, we always recognize the contributions of Charles Darwin and Gregor

Mendel, so it may be surprising to learn that for a time, before we arrived at today’s understanding of genetics and

110 | Forces of Evolution

inheritance, both Darwin’s and Mendel’s work fell out of favor. Neither Darwin’s theory of natural selection, nor Mendel’s

particulate inheritance were individually sufficient to fully explain all the phenomena being observed in the natural

world. It would take many decades, and many careful scientific experiments to solve the puzzle of evolution.

Rethinking Darwin

Figure 4.2B The Modern Synthesis perspective: The ancestral population had a range of variation in neck length. Those individuals with the longest necks would be the most likely to survive to pass on their longer-neck alleles to future generations.

Figure 4.2A The Lamarckian hypothesis: If a short-necked parent often stretched its neck to reach higher branches, each generation of offspring would be born with somewhat longer necks.

As noted in Chapter 2, Darwin’s 1859 book On the Origin of Species made a big splash;

however, as other researchers began doing

what scientists do—testing whether or not the

concept of natural selection could

consistently account for the variation seen in

organisms—they began to find many

exceptions. One reason for this is that, as we

now know, natural selection is only one of the

forces of evolution. Another challenge was a

general lack of understanding about how

variation is initiated and how inheritance

works. Many scientists of the day subscribed

to the concept known as Lamarckian inheritance, which posited that offspring would inherit characteristics that were acquired during their parents’ lifetimes Figure

4.2).Darwin himself, in 1868, promoted an idea called pangenesis, which combines the Lamarckian idea of inheriting acquired characteristics with the idea that particles

from different parts of the body make their way to the sex cells. Alfred Russell Wallace, evolution. Another researcher,

August Weismann, also rejected the idea that acquired characteristics could be passed on. Weismann 1892) devised an

experiment to directly test whether offspring inherited acquired characteristics: he cut the tails off mice, bred them,

and then waited eagerly to find out if the offspring had tails. All the baby mice were born with tails intact, demonstrating

Lamarckian inheritance of acquired characteristics to be incorrect Figure 4.3).

Figure 4.3 Weismann’s mouse-tail experiment showing that offspring do not inherit traits that the parents acquired during their lifetimes.who had arrived at the concept of natural selection independently of Darwin, rejected Lamarckian

Forces of Evolution | 111

Rediscovering Mendel

Figure 4.4 The continuous range of variation observed in hooded rat coat patterns after five generations.

In 1900, two scientists named Hugo de Vries and Carl

Correns, who were independently studying the

mechanisms of inheritance, rediscovered Gregor

Mendel’s work. Mendel’s pea plant experiments provided

the concepts of dominant and recessive traits, which

explained retention of certain characteristics in a way

that Darwin’s idea, blending inheritance, didn’t. The

debate that unfolded was between the Mutationists, who believed that variation was caused by mutations in

distinct, inherited cells, and Biometricians, who believed that individual mutations of discrete hereditary units

could never account for the continuous spectrum of

variation seen in many traits. One set of experiments that helped resolve this debate was a five-year study carried out

by William Castle and John Phillips on laboratory rats Castle and Phillips 1914). The dominant coat color was the gray

wild type, and the piebald or “hooded” color was recessive. He cross-bred the rats multiple ways for five generations

and proved that he could achieve a continuous range of variation; in fact, he even achieved coat pattern variations that

were more extreme than the original maximums of the parent groups Figure 4.4).

Another scientist, Thomas Hunt Morgan, conducted studies in which he induced genetic mutations in populations of

the fruit fly, Drosophila melanogaster Figure 4.5). His work demonstrated that most mutations merely increased

variation within populations, rather than creating new species Morgan 1911).

Figure 4.5 Examples of mutations producing phenotypic variation in a single species of fruit fly.

112 | Forces of Evolution

Tying It All Together

While the biggest leap forward in understanding how evolution works came with the joining synthesis) of Darwin’s

concept of natural selection with Mendel’s insights about particulate inheritance, there were some other big

contributions that were crucial to making sense of the variation that was being observed. R.A. Fisher 1919) and John

Burdon Sanderson Haldane 1924) developed and tested mathematical models for evolutionary change that provided the

tools to study variation and became the basis for the study of population genetics. Sewall Wright 1932) and Theodosius

Dobzhansky 1937) performed studies that revealed the existence of chromosomes as carriers of collections of genes. Edmund Brisco Ford 1949) conducted studies on wild butterflies that confirmed Fisher’s mathematical predictions and

also led to his definition of the concept of polymorphisms to describe alternative phenotypes, or multiple forms of a trait. Ford 1942) also correctly predicted that human blood type polymorphisms were maintained in the population

because they were involved in disease resistance. Julian Huxley’s 1942 book, Evolution: The Modern Synthesis, provided

an easy-to-read summary of the evolutionary studies that had come before. It was with this book that the term Modern Synthesis was first used to describe the integration of Darwin’s, Mendel’s, and subsequent research into a unified theory of evolution. In appealing to the general public, Huxley’s book also found new success establishing a wide acceptance of

the process of evolution.

POPULATION GENETICS

Defining Species and Populations and the Variations Within Them

One of the major breakthroughs in understanding the mechanisms of evolutionary change came with the realization

that evolution takes place at the level of populations, not within individuals. In the biological sciences, a population is defined as a group of individuals of the same species who are geographically near enough to one another that they can

breed and produce new generations of individuals. Species are organisms whose individuals are capable of breeding because they are biologically and behaviorally compatible to produce viable, fertile offspring. Viable offspring are those offspring who are healthy enough to survive to adulthood. Fertile offspring can reproduce successfully to have offspring of their own. Both conditions must be met for individuals to be considered part of the same species. As you can imagine,

these criteria complicate the identification of distinct species in fossilized remains of extinct populations. In those cases,

we must examine how much phenotypic variation is typically found within a comparable modern-day species, and then

determine whether the fossilized remains fall within the expected range of variation for a single species.

Some species have subpopulations that are regionally distinct. These are classified as separate subspecies because they have their own unique phenotypes and are geographically isolated from one another, but if they do happen to encounter

one another, they are still capable of successful interbreeding.

There are many examples of sterile hybrids that are offspring of parents from two different species. For example, horses

and donkeys can breed and have offspring together. Depending on which species is the mother and which is the father,

the offspring are either called mules, or hennies. Mules and hennies can live full life spans but are not able to have

offspring of their own. Likewise, tigers and lions have been known to mate and have viable offspring. Again, depending

on which species is the mother and which is the father, these offspring are called either ligers or tigons. Like mules and

hennies, ligers and tigons are unable to reproduce. In each of these cases, the mismatched set of chromosomes that the

offspring inherit still produce an adequate set of functioning genes for the hybrid offspring, but, once mixed and divided

in meiosis, the gametes don’t contain the full complement of genes needed for survival in the third generation.

Forces of Evolution | 113

For the purpose of studying evolution, we recognize populations by their even smaller units: genes. Each individual,

for genetic inheritance purposes, carries a collection of genes that can be passed down to future generations. For this

reason, in population genetics, we think of populations as gene pools, which refers to the entire collection of genetic material in a breeding community that can be passed on from one generation to the next.

Remember, a gene is the basic unit of information that encodes the proteins needed to grow and function as a living organism. Each gene can have multiple alleles, or variants, each of which may produce a slightly different protein. For example, there are brown- or blue-pigment alleles for eye color green is a slight variant of the brown type). The set

of alleles that an individual inherits for a given gene is known as the genotype e.g., inheriting both brown and blue eye pigments gives a genotype of Bb); while the observable traits that are produced by a genotype is known as the

phenotype e.g., a Bb individual exhibiting the dominant brown eye trait). For genes carried on our human chromosomes our nuclear DNA), we inherit two copies of each, one from each parent. This means we may carry two of the same

alleles a homozygous genotype; eye pigment genotypes BB or bb) or two different alleles a heterozygous genotype; eye pigment genotype Bb) for each nuclear gene. Only one of each of our alleles will get passed on to each of our

children the other will come from the child’s other parent). This means that children often inherit new genotypes and

likely express unique phenotypes, compared to their parents. A common example is when two brown-eyed parents who

happen to be heterozygous for the pigment alleles) have a blue-eyed baby genotype bb; who has inherited the recessive

b alleles from both parents).

Defining Evolution

In order to understand evolution, it’s crucial to remember that evolution is always studied at the population level. Also,

if a population were to stay exactly the same from one generation to the next, it would not be evolving. So evolution

requires both a population of breeding individuals and some kind of a genetic change occurring within it. Thus, the

simple definition of evolution is a change in the allele frequencies in a population over time. What do we mean by allele frequencies? Allele frequencies refer to the ratio, or percentage, of one allele one variant of a gene) compared to the other alleles for that gene within the study population. By contrast, genotype frequencies are the ratios or percentages of the different homozygous and heterozygous genotypes in the population. Because we carry two alleles

per genotype, the total count of alleles in a population will usually be exactly double the total count of genotypes in the

same population with the exception being rare cases in which an individual carries a different number of chromosomes

than the typical two; e.g., Down syndrome results when a child carries three copies of Chromosome 21).

THE FORCES OF EVOLUTION

Today, we recognize that evolution takes place through a combination of mechanisms: mutation, genetic drift, gene

flow, and natural selection. These mechanisms are called the “forces of evolution” and together they can account for all

the genotypic variation observed in the world today. Keep in mind that each of these forces was first defined and then

tested—and re-tested—through the experimental work of the many scientists who contributed to the Modern Synthesis.

Mutation

The first force of evolution we will discuss is mutation, and for good reason: Mutation is the original source of all the

114 | Forces of Evolution

genetic variation found in every living thing. Let’s try again to imagine all the way back in time to the very first single-

celled organism, floating in Earth’s primordial sea. Based on what we observe in simple, single-celled organisms today,

that organism probably spent its lifetime absorbing nutrients and dividing to produce cloned copies of itself. While the

numbers of individuals in that population would have grown as long as the environment was favorable), nothing would

have changed in that perfectly cloned population. There would not have been variety among the individuals. It was only

through a copying error—the introduction of a mutation, or change, into the genetic code—that each new allele was introduced into the population.

When we think of genetic mutation, we often first think of deleterious mutations—the ones associated with negative effects such as the beginnings of cancers or heritable disorders. The fact is, though, that every genetic adaptation that

has helped our ancestors survive since the dawn of life is directly due to a beneficial mutation—a changes in the DNA that provided some sort of advantage to a given population at a particular moment in time. For example, a beneficial

mutation allowed chihuahuas and other tropical-adapted dog breeds to have much thinner fur coats than their cold-

adapted cousins the northern wolves, malamutes, and huskies.

Every one of us has genetic mutations. Yes, even you. The DNA in some of your cells today differs from the original

DNA that you inherited when you were a tiny, fertilized egg. Mutations occur all the time in the cells of our skin and

other organs, due to chemical changes in the nucleotides. Exposure to the UV radiation in sunlight is one common cause

of skin mutations. Interaction with UV light causes UV crosslinking, in which adjacent thymine bases bind with one another Figure 4.6). Many of these mutations are detected and corrected by DNA repair mechanisms, enzymes that patrol and repair DNA in living cells, while other mutations may cause a new freckle or mole or, perhaps, an unusual hair

to grow. For people with the autosomal recessive disease xeroderma pigmentosum, these repair mechanisms do not function correctly, resulting in a host of problems, especially related to sun exposure, including severe sunburns, dry

skin, heavy freckling, and other pigment changes.

Figure 4.6 A crosslinking mutation in which a UV photon induces a bond between two thymine bases.

Most of our mutations exist in somatic cells, which are the cells of our organs and other body tissues. Those will not be passed on to future generations and so will not affect the population over time. Only mutations that occur in the

gametes, the reproductive cells i.e., the sperm or egg cells), will be passed on to future generations. When a new mutation pops up at random in a family lineage, it is known as a spontaneous mutation. If the individual born with

Forces of Evolution | 115

this spontaneous mutation passes it on to his offspring, those offspring receive an inherited mutation. Geneticists have identified many classes of mutations and the causes and effects of many of these.

Point Mutations

A point mutation is a single-letter single-nucleotide) change in the genetic code resulting in the substitution of one nucleic acid base for a different one. As you learned in Chapter 3, the DNA code in each gene is translated through three-

letter “words” known as codons. So depending on how the point mutation changes the “word,” the effect it will have on the protein may be major or minor, or may make no difference at all. One of the most common causes of point mutations

is a chemical change called cytosine methylation. In cytosine methylation, a methyl group is added to a cytosine base, which further converts to thymine after hydrolytic deamination water-induced removal of an amine group; Figure 4.7).

If this mutation is not detected before replication, half of the daughter cells will inherit a thymine T) in the sequence

where a cytosine C) is usually located. This is one of the most common causes of the autosomal dominant disorder

neurofibromatosis type 1 NF1), discussed in Case Study #1 see below).

If a mutation does not change the resulting protein, then

it is called a synonymous mutation. Synonymous mutations do involve a letter nucleic acid) change, but

that change results in a codon that codes for the same

“instruction” the same amino acid or stop code) as the

original codon. Mutations that do cause a change in the

protein are known as non-synonymous mutations. There are several classes of non-synonymous mutations, which

are defined by their effects on the encoded protein:

missense, nonsense, and splice site mutations Figure 4.8). Figure 4.7 The mechanism by which a cytosine-to-thymine point mutation can occur.

A missense mutation produces a change in a single amino acid. In this case, the protein is assembled correctly, both before and after the point mutation, but one amino acid,

encoded by the codon containing the point mutation, is incorrect. This may impact how the finished protein functions

by, for example, preventing it from folding correctly and/or disrupting an enzyme binding site. Nonsense mutations convert codons that encode amino acids into stop codons, meaning that the protein will be assembled correctly up until

the codon containing the mutation and then assembly will be prematurely terminated. Depending on where in the gene

the nonsense mutation falls, this may have a major or very minor impact. A splice site mutation changes the genetic code so that the process of removing the intron sequences from the mRNA is disrupted. This can result in the erroneous

inclusion of an intron sequence or the exclusion of one of the exons that should have been retained.

116 | Forces of Evolution

Mutation Type Illustration Result

No mutation normal DNA) Normal protein produced

Synonymous silent) mutation

Normal protein produced

Missense mutation Slight difference in amino acid sequence

Nonsense mutation Protein terminates early

Chart continued on next page

Forces of Evolution | 117

Mutation Type Illustration Result

Frameshift insertion Major difference in amino acid sequence

Frameshift deletion Major difference in amino acid sequence

Figure 4.8 Examples and results of point and frameshift mutations.

Insertions and Deletions

In addition to point mutations, another class of mutations are insertions and deletions, or indels, for short. As the name suggests, these involve the addition insertion) or removal deletion) of one or more coding sequence letters nucleic

acids). These typically first occur as an error in DNA replication, wherein one or more nucleotides are either duplicated

or skipped in error.

Frameshift mutations are types of indels that involve the insertion or deletion of any number of nucleotides that is not a multiple of three. Because these indels are not consistent with the codon numbering, they “shift the reading frame,”

causing all the codons beyond the mutation to be misread. These mutations can create extensive changes to the protein

sequence, potentially not only causing it to lose function but also possibly creating new enzyme-binding sites, leading

to new interactions between the protein and other components of the cellular environment. Like point mutations, small

indels can also disrupt splice sites. Entire codons or sets of codons may also be removed or added if the indel is a

multiple of three nucleotides.

Transposable Elements, or transposons, are fragments of DNA that can “jump” around in the genome. There are two types of transposons: Class I transposons, or retrotransposons, which are transcribed from DNA into RNA and then “reverse transcribed,” to insert the copied sequence into a new location in the DNA; and Class II transposons, or DNA

transposons, which do not involve RNA— instead, DNA transposons are clipped out of the DNA sequence itself and inserted elsewhere in the genome. Because transposable elements insert themselves into and, in the case of Class II

transposons, remove themselves from) existing DNA sequences, they are frequent gene disruptors. At certain times, and

in certain species, it appears that transposons became very active, likely accelerating the mutation rate and thus, the

genetic variation) in those populations during the active periods.

118 | Forces of Evolution

Chromosomal Alterations

The final major category of genetic mutations are changes at the chromosome level: crossover events, nondisjunction

events, and translocations. Crossover events occur when DNA is swapped between homologous chromosomes while they are paired up during meiosis I. Crossovers are thought to be so common that some DNA swapping may happen

every time chromosomes go through meiosis I. Crossovers don’t necessarily introduce new alleles into a population, but

they do make it possible for new combinations of alleles to exist on a single chromosome that can be passed to future

generations. This also enables new combinations of alleles to be found within siblings who share the same parents. Also,

if the fragments that cross over don’t break at exactly the same point, they can cause genes to be deleted from one of

the homologous chromosomes and duplicated on the other.

Nondisjunction events occur when the homologous chromosomes in meiosis I) or sister chromatids in meiosis II and mitosis) fail to separate after pairing. The result is that both chromosomes or chromatids end up in the same daughter

cell, leaving the other daughter cell without any copy of that chromosome. Most nondisjunctions at the gamete level

are fatal to the embryo. The most widely known exception is Trisomy 21, or Down syndrome, which results from an

embryo that inherits three copies of Chromosome 21: two from one parent due to a nondisjunction event) and one from

the other. Trisomies triple chromosome conditions) of Chromosomes 18 Edwards syndrome) and 13 Patau syndrome) are also known to result in live births, but the children usually have severe complications and rarely survive beyond

the first year of life. Sex chromosome trisomies XXX, XXY, XYY) and X chromosome monosomies inheritance of an X chromosome from one parent and no sex chromosome from the other) are also survivable and fairly common. The

symptoms vary but often include atypical sexual characteristics, either at birth or at puberty, and often result in sterility.

The X chromosome carries unique genes that are required for survival; therefore, Y chromosome monosomies are

incompatible with life.

Chromosomal translocations involve transfers of DNA between non-homologous chromosomes. This may involve swapping large portions of two or more chromosomes. The exchanges of DNA may be balanced or unbalanced. In

balanced translocations, the genes are swapped, but no genetic information is lost. In unbalanced translocations, there is an unequal exchange of genetic material resulting in duplication or loss of genes. Translocations result in

new chromosomal structures called derivative chromosomes, because they are derived or created from two different chromosomes. Translocations are often found to be linked to cancers and can also cause infertility. Even if the

translocations are balanced in the parent, the embryo often won’t survive unless the baby inherits both of that parent’s

derivative chromosomes to maintain the balance).

Case Study #1: Neurofbromatosis Type 1 (NF1)

Neurofibromatosis Type 1, also known as NF1, is a surprisingly common genetic disorder, affecting more people than cystic fibrosis and muscular dystrophy combined. Even more surprising, given how common it is, is how few people

have heard of it. One in every 3,000 babies is born with NF1, and this holds true for all populations worldwide Riccardi

1992). This means that, for every 3,000 people in your community, there is likely at least one community member living

with this disorder. Approximately half of these cases are due to spontaneous mutations—that is, the person is the first

in their family to have the disorder. The other half of the NF1 cases are inherited from a parent with this disorder. NF1

syndrome is an autosomal dominant condition, which means that everyone born with a mutation in the gene, whether inherited or spontaneous, has a 50:50 chance of passing the NF1 syndrome on to each of their children.

Forces of Evolution | 119

Figure 4.9 Photo of a woman with many cutaneous neurofibromas, a common symptom of Neurofibromatosis Type 1.

The NF1 disorder results from disruption of the NF1 gene on Chromosome

17. Studies of individuals with NF1 have identified over 3,000 different

mutations within the gene including small and large indels, point

mutations, and translocations). The NF1 gene is one of the largest known

genes, containing at least 60 exons protein-encoding sequences) in a span of about 300,000 nucleotides. It encodes a correspondingly large protein

called neurofibromin. Neurofibromin is a fascinating protein, and we are

still learning about all its functions.

Studying the symptoms in people who have mutations in an NF1 gene can

provide important insights. There are two other types of

Neurofibromatosis Type 2 and Schwannomatosis) that involve some of the

same symptoms but are much less common than NF1 and are not due to mutations in the same gene or even the same

chromosome).

We know that neurofibromin plays an important role in preventing tumor growth because, when a mutation occurs

causing the NF1 disorder, one of the most common symptoms is the growth of benign non-cancerous) tumors, called neurofibromas. Neurofibromas sprout from nerve sheaths—the tissues that encase our nerves—throughout the body. There is no way to predict where the tumors will occur, or when or how quickly they will grow, although only about 15%

turn malignant cancerous).

Figure 4.10A Photo of a man with large plexiform neurofibroma, another symptom of Neurofibromatosis Type 1.

Figure 4.10B Childhood photo of the same man, illustrating the progressive nature of the NF1 disorder.

The two types of neurofibromas

that are typically most visible are

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