Heritable Variation and Evolution.
Variation (difference between individuals in their genetic makeup [genotype] and physical appearance
[phenotype] provides the raw material of evolution. If there were no variation there could be no
selection (and evolution would essentially grind to a halt) because there would be no differences to
select for or against.
The Discovery of Genes
Heredity was a big problem for Darwin because he didn’t know how it worked. Darwin knew offspring
resembled their parents, but it was widely believed in the 19th century that heritability was a sort of
blending process akin to the way different paints can be mixed to produce a new shade.
The problem with blending inheritance is that if inheritance worked by blending is that if a new
beneficial trait appeared, it would be diluted in a large population and its effect would be swamped.
Discovery of genes – inheritance is particulate.
Gregor Mendel (1822-1884) proved that inheritance is not a blending process. Instead Mendel showed
that discrete particles (we now call them genes) which remain intact through many generations carry the
hereditary information. As a result an individual allele may sometimes be hidden in a generation (e.g.
as a recessive allele in a heterozygote, which shows no visible sign of its presence i.e. does not have a
visible effect on the phenotype), but only reveals its presence in a later generation when its phenotypic
effect shows itself because two copies of the allele are present (as a homozygote).
Because alleles do not blend away they remain intact through many generations whether they have an
effect on the phenotype or not.
Mendel demonstrated the fact that genes are particles with his famous experiments using pea plants
(see box 5.2 pages 142-143 of the text or any introductory biology text for a description of Mendel’s
work)
Gene-centered thinking
We think of bodies as targets of natural selection, which they are because individual bodies live and die,
reproduce or don’t reproduce. However, bodies do not pass on intact from one generation to the next.
Only the instructions for making bodies do so and those instructions (which we call genes) are the
ultimate targets of natural selection. The different versions of genes, which we call alleles, are the
ultimate target of natural selection because alleles can last for many generations and potentially
forever because identical copies pass from generation to generation, making more copies of themselves
and spreading. Changes in population allele frequencies result in evolution.
It is important to remember alleles use bodies to survive long enough to make copies of themselves as a
result of the body reproducing. Each body (“survival machine” is the term Richard Dawkin’s used in his
famous book, the Selfish Gene”) is built by a temporary collection of alleles working together. Individual
alleles that do their jobs better than competing alleles and that cooperate well with others to build well
adapted bodies will become more common and those that don’t will disappear from the gene pool.
To illustrate the idea of how selection judging individual genes based on the products (the bodies) they
build might work, imagine trying to select the best crew of rowers for an 8-man boat from a large pool
of potential rowers.
If you created crews by randomly assigning rowers to crews and racing boats against each other and
repeating the practice many time you would eventually realize that certain rowers tended to be found
more often in winning boats and other rowers in losing boats. Even though strong rowers would
sometimes be in losing boats, on average, they would win more often than weaker rowers. Using the
information on wins you could then build a very strong crew. If you think of different alleles as
analogous to different rowers then similarly, alleles that tend to build more successful bodies on
average tend to be favored by selection and become more common.
Genes
Mendel did not know what genes were, but we know today that they are made of DNA and that they
work by coding the structure of proteins. A protein is a chain of amino acids that are joined together in
a specific sequence. DNA dictates the identity and order in which amino acids are joined together to
make a particular protein.
Structure of DNA
DNA made up of sequence of nucleotides. Each nucleotide includes a sugar, phosphate and one of four
possible nitrogenous bases (adenine and guanine [both purines], and thymine and cytosine [both
pyrimidines]). Thus there are four different nucleotides.
The opposite strands of the DNA molecule are complementary because the strands are held together by
bonds between the opposing bases and adenine bonds only with thymine and cytosine only with
guanine. Thus, knowing the sequence on one strand enables one to construct the sequence on the
other strand.
The sequence of nucleotides in a gene is able to code for the structure of a protein because each three
nucleotide sequence (called a codon) specifies one and only one amino acid in the protein chain. To
make a protein the DNA must first be transcribed into an RNA copy (called mRNA for messenger RNA)
and that mRNA translated into a protein or polypeptide.
The expression “one gene one protein” is widely used, but most genes actually code for multiple
proteins because they join different “exons” the executable or coding portions of a gene together to
make different proteins. This process is called alternative splicing.
Mutations create variation
A change in the structure of DNA, which may perhaps result in a change in the protein coded for, is
called a mutation. Mutations are the ultimate source of all genetic variation. A change to a gene can
result in a new allele (version of a gene) being produced.
Where do new alleles come from?
When DNA is synthesized, an enzyme called DNA polymerase reads one strand of the DNA molecule and
constructs a complementary strand. If DNA polymerase makes a mistake and it is not repaired, a
mutation has occurred.
Mutations are the raw material of evolution. Without variation there can be no selection and selection
is one of the main drivers of evolutionary change. Mutations are the ultimate source of variation.
Types of mutations
A mutation is any change in the structure or amount of hereditary material (DNA or RNA depending on
the organism) on organism possesses.
A mistake that changes one nucleotide on a DNA molecule is called a point mutation. A single point
mutation in a gene that codes for the structure of one of the protein chains in a hemoglobin molecule is
responsible for the condition sickle cell anemia.
Not all mutations that cause a change in a nucleotide result in a change in the amino acid coded for.
These are called silent mutations because they do not cause a change in the protein produced and so
have effect on the body (the phenotype remains unchanged). Mutations that do cause a change in the
amino acid coded for are called replacement mutations.
Another type of mutation occurs when bases are inserted or deleted from the DNA molecule. This
causes a change in how the whole DNA strand is read (a frame shift mutation) and produces a
completely non-functional protein. Because all the amino acids coded for after the insertion/deletion
are affected.
Mutations that affect larger quantities of DNA
There are multiple other forms of mutations that involve larger quantities of DNA. Genes may be
duplicated as may entire chromosomes or even entire genomes. Genes may also be inverted.
Mutation can produce new alleles, but new copies of genes are sometimes produced and gene
duplication appears to be most important source of new genes. Duplication results from unequal
crossing over when chromosomes align incorrectly during meiosis.
Result is a chromosome with an extra section of DNA that contains duplicated genes. Extra sections of
DNA are duplicates and can accumulate mutations without being selected against because the other
copies of the gene produce normal proteins. A gene may completely change over time so gene
duplication creates new possibilities for gene function.
Globin Genes: an example of gene duplication
The human globin genes are examples of products of gene duplication. Globin gene family contains two
major gene clusters (alpha and beta) that code for the protein subunits of hemoglobin. Hemoglobin (the
oxygen-carrying molecule in red corpuscles) consists of an iron-binding heme group and four
surrounding protein chains (two coded for by genes in the Alpha cluster and two in the Beta cluster).
The ancestral globin gene was duplicated and diverged into alpha and beta ancestral genes about 450500 mya. It was later transposed to different chromosomes and followed by further subsequent
duplications and mutations.
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From Campbell and Reese Biology 7th ed.
The lengths and positions of exons and introns in the globin genes are very similar. It is very unlikely
such similarities could be due to chance. (Exons are the parts of genes that code for proteins, introns are
non-coding.
Different genes in alpha and beta families are expressed at different times in development. For
example, in a very young human fetus, zeta (from the alpha cluster) and epsilon (from the beta cluster)
chains are present initially then replaced. Similarly G-gamma and A-gamma chains present in older
fetuses are replaced by beta chains after birth.
Chromosomal alterations are mutations too
Two major forms of chromosomal change are important in evolution: inversions and polyploidy.
A chromosome inversion occurs when a section of chromosome is broken at both ends, detaches, and
flips around and reattaches. The inversion alters the ordering of genes along the chromosome. An
inversion affects linkage (linkage is the likelihood that genes on a chromosome are inherited together
i.e., not split up during meiosis). Inverted sections cannot align properly with another chromosome
during meiosis and crossing-over within inversion produces non-functional gametes. As a result sets of
genes contained within an inversion are inherited together as a unit [also called a “supergene” ]
Inversions are common in Drosophila (fruit flies). The frequency of inversions shows a clinal pattern
and increases with latitude. Inversions are believed to contain combinations of genes that work well in
particular climatic conditions.
Polyploidy
Polyploidy is the duplication of entire sets of chromosomes. A polyploid organism has more than two
sets of chromosomes.
E.g. A diploid (2n chromosomes) organism can become tetraploid (4n), [where n refers to one set of
chromosomes]. Polyploidy is common in plants, rare in animals. Half of all angiosperms (flowering
plants) and almost all ferns are polyploid.
Polyploidy can occur, for example, if an individual produces diploid gametes and self-fertilizes
generating tetraploid offspring. If an offspring later self fertilizes or crosses with its parent, a population
of tetraploids may develop.
If a sterile plant undergoes polyploidy followed by self-fertilization a new species can develop essentially
immediately. Plants can easily reproduce vegetatively (they don’t need to mate) and produce clones of
themselves. Thus, a clump of plants can rapidly develop and maintain itself almost indefinitely. If
further polyploidy events happen the new species may also develop the ability to reproduce sexually.
Cross-fertilization and polyploidy played a major role in the development of many food crops (e.g. the
evolution of bread wheat).
Cross-fertilization of different species, followed by polyploidy, was responsible for the development of
many crop plants e.g. wheat. Cross fertilization means that parents belong to two different species.
Cross-fertilization in plants is common because pollen is windborne or insect-carried and can easily be
delivered to the stamens of a different species.
Evolution of Wheat
Initial cross-fertilization produces a sterile hybrid. It was sterile because there was only one of each
chromosome so chromosomes could not pair up during meiosis.
Triticum monococcum (AA) X wild Triticum (BB) cross produced a sterile hybrid with 14 chromosmomes
(AB; 1-7A and 1-7B). {capitalized letters symbolize species source of chromosomes, number denotes
individual chromosome e.g. 1A, 3B}
Polyploidy of that first sterile hybrid produced Emmer Wheat T. turgidum (AABB) which has 28
chromosomes. Emmer Wheat isn’t sterile. It has two copies of each chromosome (e.g. two 1A
chromosomes, two 3B chromosomes, etc.).
Further cross between Emmer Wheat and another species T. tauschii which has a total of 14
chromosomes (DD) produced another sterile hybrid this one with 21 chromosomes (ABD).
Further polyploid error in meiosis that duplicated the chromosomes produced T. aestivum Bread Wheat
with 42 chromosomes (AABBDD). Those chromosomes are derived from three ancestral species!
Mutation rates
Most data on mutations comes from analysis of loss-of-function mutations. Loss-of-function mutations
cause gene to produce a non-working protein. Examples of loss-of-function mutations include:
insertions and deletions, mutation to a stop codon and insertion of jumping genes.
Some mutations cause obvious phenotypic changes. E.g. Achrondoplastic dwarfism is a dominant
disorder. An achrondoplastic individual’s condition must be the result of a mutation, if his parents do
not have the condition.
Human estimate is 1.6 loss-of-function mutations/genome/generation. A comparison of the entire
genomes of two human children with their parents resulted in an estimate of 70 mutations per child.
Other sources of genetic variation besides mutation
A very important source of variation in offspring results from sexual reproduction. During sexual
reproduction new chromosomes are produced during the process of meiosis (gamete formation) in
which homologous chromosome exchange segments of DNA. In addition, homologous chromosomes
independently assort into gametes so unique combinations of chromosomes occur in each gamete.
Finally, the merger of sperm and egg brings together new combinations of chromosomes derived from
different individuals.
The link between genotype and phenotype
The genetic makeup of an individual is its genotype.
The physical appearance of an individual is its phenotype.
Simple genetic polymorphisms
The traits Mendel studied (fortunately for him) were simple, discrete traits that were controlled by
single genes. When the link between genotype and phenotype is so simple and direct it is easy to see
how genotype affects phenotype. It is also easy to see how selection acting
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For example, alleles of a single gene controls leaf shape in the ivy-leaf morning glory
Similar simple genetic polymorphisms result in various diseases of humans.
Sickle cell anemia, Tay-Sachs disease and Huntington’s Disease are all homozygous recessive disorders
(someone with two copies of the disease-causing allele develops the disorder, heterozygotes and
homozygotes for the “normal” allele do not.)
Most traits however are not under such simple direct control of one or a few genes. Traits, such as
height, do not exhibit discrete categories. Instead variation is continuous. The continuous variation is
the result of differences in genotypes where there are many genes contribute to the value of a trait.
Quantitative traits influenced by multiple genes; generate a normal distribution
Environmental influences on phenotype
The environment also plays a role in quantitative values of traits. Environmental influences can be
factors such as food, but a genes environment includes the activity of other genes, which may influence
how much or even whether a gene is expressed.
Traits differ in their degree of phenotypic plasticity. Height for example can strongly influenced by diet,
but our number of eyes is not.