Chapter 1 – Evolutionary Biology
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Lamarck believed individuals evolve (giraffe)
Charles Darwin
o Traveled on a 5-year trip on HMS Beagle near South America
o Mockingbirds and tortoises showed different traits on different islands
o Developed theory with two main components:
 Descent with modification: All species descended from one common ancestor
 Natural Selection: Organisms with strong traits are more likely to survive and pass
those traits to their offspring
Darwin’s theory of evolution is now regarded as having 5 parts:
o Evolution as such is the simple proposition that the characteristics of lineages of organisms
change over time.
o Common descent is a radically different view of evolution than the scheme Lamarck had
proposed. Darwin was the first to argue that species had diverged from common ancestors
and that all of life could be portrayed as one great family tree
o Gradualism is Darwin’s proposition that the differences between even radically different
organisms have evolved incrementally, by small steps through intermediate forms. The
alternative hypothesis is that large differences evolve by leaps, or saltations, without
intermediates
o Populational change is Darwin’s thesis that evolution occurs by changes in the proportions
of individuals within a population that have different inherited characteristics.
o Natural selection: changes in the proportions of different types of individuals are caused by
differences in their ability to survive and reproduce—and that such changes result in the
evolution of adaptations, features that appear “designed” to fit organisms to their
environment.
Other theories were developed after Darwin’s
o Neo-lamarckism inclues several theories based on the old idea of inheritance of
modifications acquired during an organism’s lifetime
o Orthogenesis, or “straight-line evolution,” held that the variation that arises is directed
toward fixed goals, so that a species evolves in a predetermined direction without the aid of
natural selection
o Mutationist theories were advanced by some geneticists who observed that discretely
different new phenotypes can arise by a process of mutation. They supposed that such
mutant forms constituted new species, and thus believed that natural selection was not
necessary to account for the origin of species.
Evolutionary Synthesis: fundamental principles of evolution (see pages 10-11)
Chapter 9 – Variation
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Genotype is the genetic constitution of an individual organism or of a group of organisms that are
alike in this respect, at one or more loci singled out for discussion.
A locus is a site on a chromosome, or more usually, the gene that occupies a site
An allele is a particular form of a gene, usually distinguished from other alleles by its effects on the
phenotype
A haplotype is one of the sequences of a gene or DNA segment that can be distinguished from
homologous sequences by molecular methods such as DNA sequencing
Gene copy is the number of representatives of a gene.
There are three methods to determine whether a characteristic is genetic, environmental or both
o Systematically cross phenotypes and analyze Mendelian ratios
o Compare phenotype of relatives with nonrelatives
o Maintain common garden.
Allele frequency is the relative commonness or rarity of an allele; it is the proportion of all gene
copies in the population
Genotype frequency is the proportion of a population that has a certain genotype.
Hardy-Weinberg principle: After one generation of random mating, the genotype frequencies will be
p2:2pq:q2. Moreover, both these genotype frequencies and the allele frequencies will remain
constant in succeeding generations unless factors not yet considered should change them. There
are five assumptions:
o Random mating
o Population is infinitely large
o No new genes are added to population
o Genes do not mutate
o All individuals are equal
The five assumptions are the major factors that cause evolution to occur within populations.
Inbreeding occurs when individuals are more likely to mate with relatives than with nonrelatives, or,
more generally, when the gene copies in uniting gametes are more likely to be identical by descent
than if they joined at random. An example is self-fertilization, which occurs in plants and few
animals
Gene copies are identical by descent if they have descended, by replication, from a common
ancestor, relative to other gene copies in the population
Polymorphism: multiple variants (alleles) exist in population; usually the rare allele has a frequency
greater than 0.01
Imbreeding increases the number of homozygotes, reducing fitness, survival, and fecundity
o Madsen demonstrated that a population of highly homozygous adders did not grow in
population, but once adders from different populations were added, population increased
o Cause of initial ungrowth: inbreeding
Electrophoresis is a technique that determines genetic variation
A population can be in H-W equilibrium but not in linkae equilibrium
o Example: style and stem in plants
 Usually plants have long style/short stamens or short style/long stamen
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However other combinations exist, indicating that style and stamen are indeed
located on different loci
Quantitative (continuous) variation approximately fits a normal distribution. The genetic
component of such variation is often polygenic: that is, it is due to variation at several or many loci,
each of which contributes to the variation in phenotype. Some alleles are additive.
Norm of reaction is the variety of different phenotypic states that can be produce by a single
genotype under different environmental conditions.
Genotype X environmental interaction: the effect of temperature on phenotype differed among
genotypes
Variance: quantifies the spread of individual values around the mean
Phenotypic character = genetic variance + environmental variance
Heritability: proportion of the phenotypic variance that is genetic variance
o This can be determined by measure correlations (regressions) between poarents and
offspring
Artificial variation: used to detect genetic variation in a character
Geographic variation: differences among populations in different geographic areas
o Sympatric: distinct forms or populations have overlapping geographic distributions
o Parapatric: Populations with adjacent but nonoverlapping geographic ranges that come into
contact
o Allopatric: populations with separated distributions
Hybrid zone: region in which genetically distinct parapatric forms interbreed
Cline: gradual change in character/allele frequencies over geographic distance
Ecotype: habitat-associated phenotype
Confused with Counter gradient variation and character displacement
Genetic distance measures how likely it is that gene copies taken from two populations will be
different alleles
Phenograms portray relative difference (similarity)
Chapter 10 – Genetic Drift: Evolution at Random
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Everyday language and scientific terms often have different meanings. For example, randomness
scientifically means that when physical causes can result in any of several outcomes; we cannot
predict what the outcome will be in any particular case.
Random genetic drift: random processes in evolution, such as mutation or random fluctuations in
frequencies
Genetic drift is caused by three components:
1) Sampling error: the proportions of different kinds of items (alleles) in a sample are likely to
differ, by chance, from the proportions in the set of items from which the sample is drawn
2) Coalescence: If enough time is given, all of the genes in a population will descend from (coalesce
to) a single ancestral gene copy.
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3) Random fluctuations: Due to random gene flow or mutations, the frequency of an allele will
eventually approach 0 (allele is lost) or approach 1 (allele is fixed)
There are several important aspects of evolution by genetic drift (7 is listed on page 231)
Effective size of an actual population is the number of individuals in an ideal population (in which
every adult reproduces) in which the rate of genetic drift (measured by the rate of decline in
heterozygosity) would be the same as it is in the actual population. The effective size of a
population can be smaller than the actual size of the populations for a variety of reasons, some of
which are described on pages 231-232.
Genetic drift is also analyzed by molecular evolution
Chapter 11 – Natural Selection and Adaptation
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An adaptation is a characteristic that enhances the survival or reproduction of organisms that bear
it, relative to alternative character states (especially the ancestral condition in the population in
which the adaptation evolved).
Natural selection is the only mechanism known to cause the evolution of adaptations
Examples of adaptations:
o Some young plants grow toward dark areas, which often allow the plant to grow up a tree
trunk. The light leaves high above the ground are borne on long petioles so they capture
more light.
o A flower looks somewhat like an insect and emits a smell. Attracted male insects “mate”
with the flower while pollen is deposited precisely on the insect’s body, which will contact
the stigma of the next flower visited.
Most adaptations are complex and have the appearance of design in order to accomplish some
function, such as growth, feeding, or pollination, which appears likely to promote survival or
reproduction.
Fitness—often called the reproductive success—of a biological entity is its average per capita rate of
increase in numbers. It usually consists of:
o The probability of survival to the various reproductive ages
o The average number of offspring produced via female function
o The average number of offspring produce via male function
“selfish genes”
Chapter 12 – The Genetical Theory of Natural Selection
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The consequences of natural selection at the level of individual organisms depends on the
relationship between phenotype and fitness, and the relationship between phenotype and genotype
These cause a relationship between fitness and genotype, which, in turn, determines whether or not
evolutionary change occurs
Selection can be directional (toward one extreme), stabilizing (toward an intermediate phenotype),
or diversifying (toward multiple phenotypes)
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Since one genotype can give rise to multiple phenotypes (due to environmental influences), fitness
of a genotype is the mean of the fitnesses of its several phenotypes, weighted by their frequencies
The fitness of a genotype is the average lifetime contribution of individuals of that genotype to the
population after one or more generations, measured at the same stage in the life history.
o Ex: the average number of eggs or offspring one generation hence that are descended from
the average egg or offspring (reproductive success)
The per capita growth rate of each genotype is that genotype’s absolute fitness
The relative fitness of a genotype is its value of R relative to that of some reference genotype
The mean fitness is the average fitness of individuals in a population relative to eh fittest genotype
The coefficient of selection is the amount by which the fitness of one genotype is reduced relative to
the reference genotype
The coefficient of selection measures the selective advantage of the fitter genotype, or the intensity
of selection against the less fit genotype
The replacement of relatively disadvantageous alleles by more advantageous alleles is the
fundamental basis of adaptive evolution. An advantageous allele is likely to be initially very rare if it
is a newly arisen mutation or if it was disadvantageous before an environmental change made it
advantageous. An advantageous allele that increases from a very low frequency is often said to
invade a population.
Selection that reduces the frequency of a deleterious mutation or eliminates it is referred to as
purifying selection, which is simply directional selection in favor of the prevalent, advantageous
homozygous genotype.
An example includes rats that are resistant to Warfarin. The rats that remain alive may reproduce,
and pass along the ability to resist rat poison to the offspring. A similar example includes insects
being susceptible to insecticides.
Chapter 13 – Evolution of Phenotypic Traits
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Truncation selection and selection differential can both be understood by an example. Suppose an
experimenter wants to increase the tail length of rats. The experimenter can do this by only
breeding those with tail lengths longer than a certain length. This selection is called truncation
selection. The mean tail length of the selected parents differs from that of the population from
which they were taken by an amount known as the selection differential. The average tail length
among the offspring differs from that of the parent generation as a whole by the amount known as
the response to selection.
Correlated evolution can have two causes: Correlated selection and genetic correlation
o Correlated selection: there is independent genetic variation in two or more characters, but
selection favors some combination of character states over others, usually because the
characters are functionally related
 Ex: Certain spotted species of snakes have irregular flight patterns or tend to be
sedentary, relying on their cryptic patterns to avoid predation whereas striped
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species of snakes tend to flee rapidly in a single direction, probably because visually
hunting predators find it difficult to judge the speed and position of a moving stripe
o The phenotypic correlation between body size and fecundity is what we measure in a
random sample from a population. Just as the phenotypic variance may have both genetic
and environmental components, so too may the phenotypic correlation. Two features of
individuals with the same genotype may vary together because both are mental correlation.
In genetically variable populations, the correlated variation may also be caused by genetic
differences that affect both characters, causing a genetic correlation
Phenotypic plasticity – a genotype has the capacity to produce different phenotypes, suitable for
different environmental conditions
Canalization – It may be advantageous for an animal to attain a fixed body size at maturity or
metamorphosis, despite variations in nutrition or temperature that affect the rate of growth. The
developmental system underlying the character may then evolve so that it resists environmental
influences on the phenotype
Threshold trains are expressed as discrete alternatives, but are controlled by polygenic variation
rather than by single loci.
Chapter 14 – Conflict and Cooperation
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Traits that appear harmful to the possessor but beneficial to other individuals have been explained
by four major classes of individual selection hypotheses: manipulation, individual advantage,
reciprocation, and kin selection
o Manipulation: A donor may dispense aid to a recipient not because it is adaptive, but simply
because the donor is being manipulated or coerced. An example includes a bird laying its
eggs in another bird’s nest, so other birds can take care of the eggs.
o Individual Advantage: Cooperative behavior often evolves simply because it is advantageous
to the individual. Joining a flock or herd, for example, provides protection from predation
owing to safety in numbers. It is often advantageous to each individual to be as close as
possible to the center of the group, thus using other group members as shields and against
approaching predators.
o Reciprocation: It can be advantageous for an individual to help another if the recipient will
provide reciprocal aid in the future. For example, vampire bats, which feed on mammalian
blood, form roosting groups, in which members that have fed successfully sometimes feed
regurgitated blood to other group members that have been unsuccessful in foraging. The
recipients reciprocate at other times. The more general form is the transaction model of
reproductive skew. Dominant individuals gain from the assistance of subordinate helpers,
and “pay” those helpers by allowing them to reproduce just a little more than they could if
they left the group and reproduced on their own.
o Kin selection: Individuals must dispense benefits more often to kin than to nonkin. This can
happen in two ways. First, many animals can distinguish at least some kin from nonkin and
behave differently toward them. Second, the population may be structured, at least at that
point in the life cycle when the altruistic behavior occurs, so that interacting individuals are
more likely to be related than unrelated.
 Kin recognition and cannibalism: Cannibalistic species are less likely to eat related
individuals
 Cooperative breeding: In many species of birds, young are reared not only by their
parents, but also by other individuals that are physiologically able to reproduce, but
do not.
 Social insects: For eusocial animals, those in which nearly or completely sterile
individuals (workers) rear the offspring of reproductive individuals, usually their
parent (or parents).
Chapter 18 – Coevolution: Evolving Interactions among Species
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The possibility that an evolutionary change in one species may evoke a reciprocal change in another
species distinguishes selection in interspecific interactions from selection stemming from conditions
in the physical environment. Reciprocal genetic change in interacting species, owing to natural
selection imposed by each on the other, is coevolution in the narrow sense.
In its simplest form, coevolution is when two species evolve in response to each other (specific
coevolution). For example, deer and wolves improve each other in an “evolutionary arms race”
between prey and predator.
Guild coevolution (diffuse coevolution) occurs when several species are involved and their effets are
not independent. For example, genetic variation in the resistance of a host to two different species
of parasites might be correlated.
In escape-and-radiate coevolution, a species evolves a defense against enemies, and is thereby
enabled to proliferate into a diverse clade. For example, some species of plants that evolved
effective chemical defenses were freed from predation by most herbivorous insects, and thus
diversified, evolving into a chemically diverse array of food sources to which different insects later
adapted and then diversified in turn.
Two challenges that a parasite faces are moving itself or its progeny from one host to another
(transmission) and overcoming the host’s defenses. Some parasites are transmitted vertically, from
a host parent to her offspring, such as Wolbachia bacteria, which are transmitted in insects’ eggs.
Other parasites are transmitted horizontally among hosts in a population via the external
environment (e.g., human rhinoviruses, the cause of the common cold, are discharged by sneezing),
or via-carriers (vectors, such as the mosquitoes that transmit the malaria-causing protest and the
yellow fever virus).
The effects of parasites on their hosts vary greatly. Those that reduce the survival or reproduction
of their hosts are considered virulent. Many people imagine that parasites generally evolve to be
benign because the parasite’s survival depends on that of the host population. However, a parasite
may evolve to be more benign or more virulent depending on many factors.
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Mutualisms are interactions between species that benefit individuals of both species. In symbiotic
mutualisms, individuals are intimately associated for much of the lifetimes. Some mutualisms have
promoted the evolution of extreme adaptations.
According to Brown and Wilson, character displacement describes a pattern of geographic variation
wherein sympatric populations of two species differ more greatly in a characteristic than allopatric
populations.
Ecological release is another geographic pattern, wherein a species or population exhibits greater
variation in resource use and in associated phenotypic characters if it occurs alone than if it coexists
with competing species. Ecological release is most often characteristic of island populations.
Defensive mimicry, in which one or more species gain protection against predators from their
resemblance to one another, provides model systems for studying many evolutionary phenomena.
Traditionally, two forms of defensive mimicry have been recognized. In Batesian mimicry, a
palatable species (a mimic) resembles an unpalatable species. In Mullerian mimicry, two or more
unpalatable species are co-mimics (or co-models). In both cases, predators learn from unpleasant
experience, to avoid potential prey that look like unpalatable species.