History of Evolution
20 Recognition of Evolutionary Change
Aristotle - believed that both inanimate objects and living species had fixed characteristics.
Created ladder-like classification of nature from simplest to most complex: minerals to plants,
plants to animals, animals to humans, humans to gods. Ladder called Scala Naturae.
14th century - Europeans merged Biblical Creationism with Aristotle’s classification system.
Believed that all different kinds of organisms had been created by God, and that species could
not change, new species could not arise and none could go extinct.
Natural Theology - biological research that involved the naming and cataloguing of all creation,
and then identifying its position and purpose in the Scala Naturae. This approach was led by
15th-18th Century - Biogeography, Comparative Morphology and Geology promoted a growing
awareness of change.
George-Louis Leclerc, le Comte de Buffon - noted the existence of body parts with no
apparent function. If each species is created anatomically perfect for its particular way of life,
why do useless structures exist? Buffon proposed that some animals must have changed since
their creation, and that Vestigial Structures must have functioned in ancestral organisms.
Species “conceived by Nature, and produced by Time”.
Vestigial Structures - useless body parts
Catastrophism - Georges Cuvier realized that the layers of fossils represented organisms that
lived during successive times in the past. The abrupt changes within geologic strata marked
dramatic shifts in ancient environments. Reasoned that each layer of fossil represented the
remains of organisms that had died in a local catastrophe (ex. flood). Then different species
would recolonize the area, and when another catastrophe occurred, they would form a different
set of fossils in the next layer.
Jean Baptiste de Lamarck - proposed first comprehensive theory of Biological Evolution.
Believed that “perfecting principle” caused organisms to become better suited to their
environment. Two mechanisms foster evolutionary change:
Principle of Use and Disuse - body parts grow in proportion to how much they are used
(ex.working out a muscle at the gym). Unused structures get weaker and shrink.
Inheritance of Acquired Characteristics - changes that an animal acquires during its
lifetime are inherited by its offspring.
- These mechanisms do NOT cause evolutionary change because structural changes acquired
during an organism’s lifetime are not inherited by the next generation. Lamarck made 4
contributions to the development of evolution. The first 3 were adopted to guide Darwin’s
1. Proposed that all species change through time
2. Recognized that changes are passed from one generation to the next.
3. Suggested that organisms change in response to their environments.
4. Hypothesized the existence of specific mechanisms that caused evolutionary change. Changes in Earth
Gradualism - James Hutton - geologist. Suggested that SLOW and continuous physical
processes acting over LONG periods of time produced the Earth’s major geologic features.
Contrasts sharply with catastrophism.
Uniformitarianism - Charles Lyell - geologist. Extended gradualism by suggesting that geologic
processes such as volcanic eruptions, earthquakes, erosion, and the formation and movement of
glaciers sculpted the Earth’s surface and are exactly the same as the geologic processes we
observe today. Undermined any remaining notions of an Unchanging Earth. Has taken millions
of years to mould the landscape.
17.1a Evolutionary Biologists Describe and Quantify Phenotypic Variation
Quantitative Variation - where individuals differ in small incremental ways (ex. number of
hairs on their head, weight, height, length of toes).
- Quantitative V is often displayed on a bar graph or with a large enough sample, a curve.
The width of the curve shows the variability in the popn. The mean shows the average
value/amount of the character being measured. Natural selection often changes the mean
value of a character or its variability within a popn. Frequency of blue phenotype in a
popn of 123 blue geese and 369 white geese is 123/429 = 0.25. And frequency of the
white phenotype is 369/492 = 0.75
Qualitative Variation - when a character exists in two or more discrete forms with no
intermediate forms present. Ex. Geese have either white or blue feathers. Not both or a mixture.
When there are discrete variants of one character, it is called a polymorphism. The trait is
described as polymorphic. Human blood groups are another example.
17.1b Phenotypic Variation Can Have Genetic and Environmental Causes
Phenotypic variation within popns may be caused by genetic differences btw individuals,
differences in envtal factors they experience, or by an interaction btw genetics with envt. Thus
sometimes organisms with different genotypes exhibit the same phenotypes. Genotypic and
phenotypic variations are not perfectly correlated - ex. black colouration of mice from Arizona is
due to a mutation in the Mc1r gene, but black mice from New Mexico don’t share that mutation
however they still exhibit the same phenotype. With that said, organisms with the same genotype
sometimes exhibit different phenotype - ex. acidity of soil influences flower colour in some
*** Knowing whether phenotypic variation is caused by genetic differences, environmental
factors, or an interaction of the two is important because Only genetically based variation is
subject to evolutionary change. Traits that vary quantitatively will respond to artificial selection only if the variation has some
genetic basis. An organism’s phenotype is frequently the product of an interaction between its
genotype and the environment.
How can we determine whether phenotypic variation is caused by environmental factors or by
- We can test for an environmental cause experimentally by changing one environmental
variable and measuring the effects on genetically similar subjects.
- Breeding experiments can also determine the genetic basis of phenotypic variation (Ex.
Mendel – by crossing plants with different phenotypes). Traits that vary quantitatively will
respond to artificial selection only if the variation has some genetic basis.
17.1c Several Processes Generate Genetic Variation
2 potential sources of genetic variation:
– the production of new alleles and the rearrangement of existing alleles. Most new
alleles arise from small-scale mutations in DNA. The rearrangement of existing alleles into new
combinations can result in larger scale changes in chromosomes (ex. Through crossing over, and
independent assortment during meiosis and random fertilization of sperm and egg).
- the shuffling of existing alleles comprehensively into new combinations can produce an
extraordinary number of new genotypes and phenotypes in the next generation. That is why
unless you have an identical twin, it is extremely unlikely that another person with your genotype
has ever lived or will ever live. There are more than 10^600 combinations of alleles possible,
however fewer than 10^10 are alive today.
17.1d Populations often Contain Substantial Genetic Variation
In studies of chromosomal and mitochondrial DNA, studies suggest that every locus exhibits
some variability in its nucleotide sequence. Sometimes the variability is apparent in individuals.
Other times, however, the variations detected in the protein-coding regions of DNA may not
affect phenotype because they do not change the amino acid sequence of the proteins for which
the genes code.
17.2 Population Genetics
In order to predict how certain factors may influence genetic variation, population geneticists
first describe the genetic structure of a population. They then create hypotheses formalized in
mathematical models to describe how evolutionary processes may change genetic structure under
specified conditions. Lastly, they test the predictions of the models to evaluate their ideas about
17.2a All Populations Have a Genetic Structure
All populations have a genetic structure. In diploid organisms, an individual’s genotype includes
two alleles at every gene locus. The sum of all alleles at all gene loci in all individuals in called
the population’s gene pool. To describe the structure of a gene pool, scientists first identify the different genotypes in a
representative sample and calculate genotype frequencies.
Genotype frequencies – the percentages of individuals possessing a particular genotype.
Knowing that each diploid organism has two alleles at each gene locus (either two copies of the
same alleles or two different alleles), the scientist can then calculate allele frequencies. For a
locus with 2 alleles, scientists use the symbol “p” to represent the frequency of 1 allele and “q”
to represent the frequency of the other allele.
Allele frequency – the abundance of one allele relative to others at the same gene locus in
individuals of a population. Allele frequencies represent the commonness or rarity of each allele
in the gene pool.
For a gene locus with two alleles, there are three genotype frequencies (homozygote (2) and
heterozygote (1)), but only 2 allele frequencies (p and q). The sum of the three genotype
frequencies must equal 1, and the sum of the two allele frequencies must equal 1.
17.2b The Hardy-Weinberg Principle Is a Null Model That Defines How Evolution Does
Scientists often use control treatements to evaluate the effect of a particular factor. The control
tells us what we would see if the experimental treatment had no effect. However, in studies that
use observational rather than experimental data, there is often no suitable control. In such cases,
scientists use null models. Null models predict what they would see if a particular factor had no
effect. They serve as theoretical reference points against which observations can be evaluated.
The Hardy-Weinberg principle specifies the conditions under which a population of diploid
organisms achieves genetic equilibrium.
Genetic Equilibrium – the point at which neither allele frequencies nor genotype frequencies
change in succeeding generations. Shows that dominant alleles don’t replace recessive ones and
that the shuffling of genes in sexual reproduction alone isn’t what causes the gene pool to
change. According to the Hardy-Weinberg model, genetic equilibrium is possible only if all of
the following conditions are met:
1.) No mutations are occurring
2.) The population is closed to migration from other populations.
3.) The population is infinite in size.
4.) All genotypes in the population survive and reproduce equally well.
5.) Individuals in the population mate randomly with respect to genotypes.
If the conditions of the model are met, the allele frequencies of the population will never change,
and the genotype frequencies will stop changing after one generation. Microevolution will NOT
occur. The model is a null model and serves as a reference point for evaluating the circumstances
under which evolution may occur. Determining which of the model’s conditions are not met is a
first step in understanding why the gene pool is changing.
Agents of Microevolutionary Change
Agent Definition Effect on Genetic Variation
Mutation A heritable change in DNA Introduces new genetic
variation into a population Gene Flow Change in allele frequencies May introduce genetic
as individuals join a variation from another
population and reproduce population
Genetic Drift Variation in relative genotype Reduces genetic variation,
frequencies caused by random, especially in small
chance disappearance of populations; can eliminate
particular genes and alleles as alleles
individuals die or do not
Natural Selection Differential survivorship or One allele can replace another
reproduction of individuals or allelic variation can be
with different genotypes preserved.
Nonrandom Mating Choice of mates based on their Does not directly affect allele
phenotypes and genotypes frequencies, but usually
prevents genetic equilibrium
Selection and Fitness
17.1 Variation in Natural Populations
In most species, members of a popn look alike, although are not identical. In some species,
members vary dramatically in appearance. This is Phenotypic Variation - differences in
appearance or function that are passed from generation to generation.
-Microevolutionary studies often begin by assessing phenotypic variation within
populations. Most characters exhibit quantitative variation.
17.3d Natural Selection Shapes Genetic Variability by Favouring Some Traits Over Others
Natural Selection – the process by which beneficial traits become more common in subsequent
generations. ** Although natural selection can change allele frequencies, it is the collective
phenotype of an individual organism rather than any particular allele, that is successful or not.
When individuals survive and reproduce, their alleles (favourable and unfavourable) are passed
to the next generation. An organism with harmful or lethal dominant alleles will probably die
before reproducing and all the alleles it carries, even the advantageous alleles will go with it.
Relative fitness is used to evaluate reproductive success.
Relative fitness – the number of surviving offspring that an individual produces compared with
the number left by others in the population. So a particular allele will increase in frequency in the
next generation if individuals carrying that allele leave more offspring than individuals carrying
Natural selection tests fitness differences at nearly every stage of the life cycle, however, it
exerts little or no effect on traits that appear during an individual’s post-reproductive life (ex.
onset of Huntington’s disease is age 40 allowing carriers to reproduce and pass it on to the next
generation before the condition presents itself). Scientists measure the effects of natural selection on phenotypic variation by recording changes in mean and variability of characters over time.
There are 3 modes of natural selection:
1.) Directional Selection – traits undergo DS when individuals near one end of the
phenotypic spectrum have the highest relative fitness. DS shifts a trait away from the
existing mean and toward the favoured extreme. After selection, a trait’s mean value is
higher or lower than before. This type of selection is extremely common.
2.) Stabilizing Selection – traits undergo SS when individuals expressing intermediate
phenotypes have the highest relative fitness. By eliminating the extremes, SS reduces
genetic and phenotypic variation and increases the frequency of intermediate phenotypes.
The MOST common mode of natural selection (ex. very small and very large newborns
are less likely to survive than those born at intermediate masses).
3.) Disruptive Selection – traits undergo DvS when extreme phenotypes have higher
relative fitness than intermediate phenotypes so alleles producing extreme phenotypes
become more common promoting polymorphism. Under natural conditions, DvS is less
common than DS and SS (Ex. situations that allow animals to specialize on particular
17.5 Adaptation and Evolutionary Constraints
Although natural selection preserves alleles that allow for high relative fitness on the individuals
that carry them, researchers are cautious about interpreting the benefits that certain traits provide
17.5a Scientists Construct Hypotheses about the Evolution of Adaptive Traits
Adaptive Trait – any product of natural selection that increases the relative fitness of an
organism in its environment. Adaptation – the accumulation of adaptive traits over time.
It is easy to concoct an adaptive explanation for almost any characteristic we observe in nature,
but they must be framed as testable hypotheses. They must compare variations of a trait in
closely related species living in different environments. When trying to unravel how and why a
particular trait evolved, it is important to remember that a trait observed today may have had a
different function in the past. A lot of hypotheses predicting ancestral history of traits cannot be
tested because certain ancestors no longer exist. Biologists must use anatomical studies of
animals and their ancestors, and theoretical models of mechanics of movement to challenge and
Although evolution has produced all the characteristics of organisms, not all traits are necessarily
adaptive. Some traits are the products of chance events and genetic drift and others of basic
physical laws (ex. gravity and the ability of seeds to fall to the ground).
17.5b Several Factors Constrain Adaptive Evolution
Adaptive traits of most organisms are compromises produced by competing selection pressures.
No organism can be perfectly adapted to its environment because environments change over
time. When selection occurs in a population, it preserves the alleles that are successful in the
current, prevailing environmental conditions so each generation is adapted to the environmental
conditions under which its parents lived. If the environment changes from one generation to the
next, adaptation will always lag behind.
Natural Selection is not an engineer that designs new organisms from scratch. It works with what
it has. It acts on new mutations and existing genetic variation. Because new mutations are rare, it
mostly acts on alleles that have been present in the population for generations. It just modifies
traits so they are not as efficient or perfect as they could be.
The agents of evolution can cause microevolutionary changes in the gene pools of populations
that may cause their gene pools to diverge, and sometimes the divergence is enough to cause the
populations to evolve into different species.
A student asks: Is balanced polymorphism a result of disruptive natural selection?
Technically, no. Disruptive selection is a type of selection where both extreme phenotypes have a fitness advantage over
intermediate phenotypes. Disruptive selection only applies when we are considering this kind of selection on phenotypes that span a
continuous range; that is, when we are considering quantitative traits (e.g. height, weight, shades of hair colours, etc).
“Polymorphism” is a term we tend to reserve for describing qualitative (i.e. discrete) traits, like horns versus no horns, or the
genotypes of the ABo blood group (where there are discrete phenotypes, rather than a continuum). Since disruptive selection is a
type of selection dealing with quantitative traits, and polymorphisms deals with qualitative traits, we can’t state that a balanced
polymorphism is a result of disruptive selection. But, I like what brought you to that question.
A balanced polymorphism refers to a situation where selection pressures maintain more than one ‘morph’ in the population, or more
than one allele at non-zero frequencies. So, the one ‘special’ type of selection we discussed that acts on qualitative/discrete traits
was ‘heterozygote advantage’. And, the consequence of heterozygote advantage is that it maintains the two alleles at non-zero
frequencies in the population. So, heterozygote advantage will result in a balanced polymorphism.
A student asks: I was wondering what the main difference is between gene flow and founder effect? Response: I totally understand whe