Chapter 6 Evolution and Ecology 4/9/2013 11:24:00 PM
Trophy Hunting and Inadvertent Evolution: A Case Study
Trophy hunting removes the largest and strongest individuals from a population, preventing
populations from recovering.
Harvesting of species may also affect sex determination in some species.
What Is Evolution?
Biological evolution is the change in organisms over time.
Evolution includes the relatively small fluctuations that occur continually within populations, as
when the genetic makeup of a population changes from one year to the next.
Evolution is allele frequency change
Genes are composed of DNA, and they specify how to encode proteins.
A given gene can have two or more forms (known as alleles) that result in the production of different
versions of the protein that the gene encodes.
The genotype of an individual can be designated by the letters that represent the individual’s two
copies of each gene.
Evolution can be determined as change over time in the frequencies of different alleles in a
In scientific studies, researchers often use an approach based on the Hardy-Weinberg equation to test
where a population is evolving at one or more genes.
Evolution is descent with modification
Populations accumulate differences over time, and hence, when a new species forms, it differs from
its ancestors in a relatively small number of ways.
A new species not only differs from its ancestors, but also resembles its ancestors, because it
descended from then and continues to share many characteristics with them.
Darwin proposed that populations accumulate differences over time primarily by natural selection,
the process by which individuals with certain heritable characteristics survive and reproduce more
successfully than other individuals because of those characteristics.
Darwin argued that if two populations experience different environmental conditions, individuals
with one set of characteristics may be favored by natural selection in one population, while
individuals with a different set of characteristics may be favored in the other population.
By favoring individuals with different heritable characteristics in different populations, natural
selection can cause populations to diverge genetically from one another over time; thus
accumulating more and more genetic differences. Populations evolve, individuals do not
Natural selection acts as a sorting process, favoring individuals with some heritable traits over
Natural selection ensures that allele frequencies will change over time, causing the population to
Individuals do not evolve, they either r have traits favored by selection or they don’t.
Mechanisms of Evolution
Mutation generates the raw material for evolution
Individuals in populations differ from one another in their observable characteristics, or phenotype.
Many aspects of an organisms phenotype are influenced by its genotype.
Different alleles arise from mutations, caused by events that damage DNA.
Individuals in a population also differ genetically because of recombination, the production of
offspring that have combinations of alleles that differ from those in either of the parents.
Mutations occur too rarely to be a direct cause of significant allele frequency change over short
periods of time.
Natural selection increases the frequencies of advantageous alleles
Natural selection occurs when individuals with particular heritable traits consistently leave more
offspring than do individuals with other heritable traits.
Natural selection can be categorized into three types:
o Directional selection occurs when individuals with one extreme of a heritable phenotypic
trait are favored over other individuals.
o In stabilizing selection, individuals with an intermediate phenotype are favored.
o In disruptive selection, individuals with a phenotype at either extreme are favored.
Some individuals will always have heritable phenotypes that give then an advantage in survival or
reproduction, causing them to leave more offspring than other individuals.
Genetic drift results from chance events
When chance events determine which alleles are passed from one generation to the next, genetic
drift is said to occur.
Genetic drift has four related effects on small populations:
o Because it acts by chance alone, genetic drift can cause allele frequencies to fluctuate
randomly in small populations over time. When this occurs, some alleles eventually
disappear from the population, while other reach fixation.
o By causing alleles to be lost from a population, genetic drift reduces the genetic variation of
the population, making the individuals within the population more genetically similar to one
another. o Genetic drift can increase the frequency of a harmful allele. If a population size is very small,
and an allele has only slightly deleterious effects, genetic drift can overrule the effects of
natural selection, causing the harmful allele to increase or decrease in frequency by chance
o Genetic drift can increase genetic differences between populations because chance events
may cause an allele to reach fixation in one population, yet be lost from another population.
A loss of genetic variation in a population can reduce the capacity of a population to evolve in
response to changing environmental conditions, potentially placing it at risk of extinction, as well as
an increase in the frequency of harmful alleles of any particular gene from one generation to the
Gene flow is the transfer of alleles between populations
Gene flow occurs when alleles are transferred from one population to another via the movement of
individuals or gametes.
Gene flow has two important effects:
o Gene flow tends to make populations more similar to one another genetically.
o Gene flow can introduce new alleles into a population.
Evolutionary change that results in a closer match between the traits of organisms and the conditions
of their environment is an example of adaptive radiation.
Adaptations result from natural selection
Natural selection causes adaptive evolution, which is a process of change in which traits that confer
survival or reproductive advantage tend to increase in frequency over time.
Although gene flow and genetic drift can improve the effectiveness of an adaptation, they can also
do the reverse.
Natural selection is the only evolutionary mechanism that consistently results in adaptive evolution.
Adaptive evolution can occur rapidly
Hundreds of species have altered the timing of key events in their lives in ways that may be a
response to global warming.
Gene flow can limit local adaptation
Gene flow is one of the factors that can limit the extent to which a population is adapted to its local
Adaptations are not perfect Natural selection does not result in a perfect match between organisms and their environments,
because an organisms environment is not static and because organisms face a a number of
constraints on adaptive evolution:
o Lack of genetic variation. If none of the individuals in a population has a beneficial allele of
a particular gene that influences survival and reproduction, adaptive evolution cannot occur
at that gene.
o Evolutionary history. Natural selection works by modifying the traits already present in an
organism, if the necessary genetic variation is present.
o Ecological trade-offs. To survive and reproduce, organisms must perform many essential
functions, and performing one function reduces the ability to perform another.
Adaptive evolution is a key component of the evolutionary process despite these constraints.
The Evolutionary History of Life
The diversity of life results from speciation
Speciation is the process by which one species splits into two or more species.
A species is a group of organisms whose members have similar characteristics and can interbreed.
Speciation most commonly occurs when a barrier prevents gene flow between two or more
populations of a species.
New species can also form if members of two different species can produce fertile hybrid offspring.
In contrast to selection and drift, gene flow typically acts to slow down pr prevent speciation because
populations that exchange many alleles tend to be genetically similar to one another, making it less
likely that reproductive barriers will evolve.
Mass extinctions and adaptive radiations have shaped long-term patterns of evolution
The fossil record documents five mass extinction events in which large proportions of Earth’s
species were driven to extinction worldwide in a relatively short time.
An event in which a group of organisms gives rise to many new species that expand into new
habitats or new ecological roles in a relatively short time is referred to as an adaptive radiation.
After a mass extinction occurs, it takes millions of years for adaptive radiation to increase the
diversity of life seen prior to the mass extinction.
Joint Effects of Ecology and Evolution
Ecological interactions can cause evolutionary change
It is common for speciation to be caused by ecological factors.
Evolution can alter ecological interactions Processes that drive the patterns of evolution over long time scales can also have a profound effect
on ecological interactions. Chapter 15 The Nature of Communities 4/9/2013 11:24:00 PM
“Killer Algae!”: A Case Study
Caulerpa survived because humans facilitated its dispersal and physiological tolerance in the
Native sea grasses become least productive when the invasive algae is most productive, thus
allowing the invasive algae to overgrow native plants.
In reality, species experience multiple interactions, and thus, mathematical models are not always
What Are Communities?
Communities are groups of interacting species that occur together at the same place and time.
Interactions between multiple species are synergistic, making communities into something more than
the sum of their parts.
The existence of a community is dependent on the individual species that are present and how they
interact with one another and their physical surroundings.
Ecologists often delineate communities by their physical or biological characteristics
In most cases, communities end up being defined somewhat arbitrarily by the ecologists who are
Ecologists typically define communities based on the questions they are posing.
Ecologists interested in knowing which species are present in a community must contend with the
difficult issue of accounting for them.
Due to the amount of species and the difficulty of studying many species at one time, ecologists
usually consider a subset of species when they define and study communities.
Ecologists may use subsets of species to define communities
A common way of defining communities is based on taxonomic affinity.
Another useful subset of a community is a guild, a group of species that use the same resources, even
though they might be taxonomically distant.
A functional group is a subset of community that includes species that function in similar ways, but
do not necessarily use the same resources.
Other subsets of communities may allow ecologists to organize species based on their trophic, or
energetic interactions. Food webs can be further organized into trophic levels, or groups of species hat have similar ways of
o The lowest trophic level contains primary producers, which are autotrophs such as plants.
o Primary producers are fed on by the primary consumers, which are herbivores.
o Secondary consumers are carnivores, and will consume the primary consumers.
Almost all food webs that have been studied have two to five levels.
Some species change their feeding status as they mature, and others still can be both primary and
secondary consumers at the same time.
Food webs do not include non-trophic interactions, also referred to as horizontal interactions.
An interaction web is a more accurate description of both the vertical and horizontal interactions
Species diversity are important descriptors of community structure: the set of characteristics that
shape a community.
Community structure is descriptive in nature, but provide the necessary quantitative basis for
generating hypotheses and experiments directed at understanding how communities work.
Species diversity is an important measure of community structure
Species diversity is a measure that combines the number of species and their relative abundances
compared with one another.
Species richness is the easiest metric to determine, by simply counting all the species in the
community one has delineated.
Species evenness, which tells us about the commonness or rarity of species requires knowing the
abundance of each species relative to those of the other species within the community.
The Shannon index is most commonly used to describe species diversity quantitatively.
Biodiversity is a term used to describe the diversity of important ecological entities that span
multiple spatial scales.
Species within communities differ in their commonness or rarity
Rank abundance curves will plot the proportional abundance of each species relative to the others in
rank order, from most abundant to least abundant.
Species diversity estimates vary with sampling effort and scale
In sampling, a point may be reached where any additional sampling will reveal so few new species,
that ceasing to sample will give a good notion of species richness.
The point of “no significant return” for sampling can be determined using a species accumulation
curve. o A species accumulation curve is calculated by plotting species richness as a function of the
o Each data point in a species accumulation curve represents the total number of individuals
and sampling effort up to that point.
o The more samples taken, the more individuals will be added, and the more species will be
In reality, a threshold never occurs in natural systems, because new species are always being found.
Species composition tells us who is in the community
A final element of a community’s structure is its species composition: the identity of the species
present in the community.
Interactions of Multiple Species
Direct interactions occur between tow species and include trophic and non-trophic interactions.
Indirect interactions occur when the relationship between two species is mediated by a third species.
Indirect species interactions can have large effects
A trophic cascade occurs when the rate of consumption at one trophic level results in a change in
species abundance or composition at lower levels.
Indirect effects can also emerge from direct positive interactions called trophic facilitations, which
occur when a consumer is indirectly facilitated by a positive interaction between its prey and another
Important indirect effects can arise from multiple species interactions at one trophic level.
Competitive networks might be important in manipulating species richness in communities.
A network, as opposed to a hierarchy, is an interaction web that is circular rather than linear.
Networks of interacting species indirectly buffer strong direct competition, thus making competitive
interactions weaker and more diffuse.
Species interactions vary greatly in strength and direction
Interaction strength, the effect of one species on the abundance of another species can be measured
experimentally by removing one species from the community and looking at the effect on the other
If the removal of the interactor species has a large decrease in the target species, we know that the
interaction is strong and positive.
If the abundance of the target species increases significantly, we know that the interactor species has
a strong negative effect on the target species.
Dominant species, also known as foundation species, have a large effects on other species, and thus
on the species diversity of communities, by virtue of their considerable abundance or biomass. Species known as ecosystem engineers are able to create, modify, or maintain physical habitat for
themselves and other species.
Keystone species have large effects not because of their abundance, but because of the roles they
play in communities.
Environmental context can change the outcome of species interactions
Interactions among multiple species can vary in strength and direction, and their outcome is highly
dependent on the influence of each of the species in the community.
Chapter 16 Change in Communities 4/9/2013 11:24:00 PM
A Natural Experiment of Mountainous Proportions: A Case Study
The eruption created disturbances that varied in their effects depending on the distance from the
volcano and habitat type.
The life span of communities on Mount St. Helens greatly exceeds our own, and thus scientists must
be content with limited data.
Humans have become the greatest agents of change in the world, and we usually take these actions
with an imperfect understanding of their consequences.
Agents of Change
Succession is change in the species and composition of communities over time.
Succession is the result of a variety of abiotic and biotic agents of change.
Agents of change can be abiotic or biotic
Abiotic agents of change can be placed into two categories:
o Disturbance is an abiotic event that physically injures or kills some individuals and creates
opportunities for other individuals to grow or reproduce.
o Stress occurs when some abiotic factor reduces the growth or reproduction of individuals and
creates opportunities for other individuals.
Abiotic and biotic factors often interact to produce change in communities.
Agents of change vary in their intensity, frequency and extent
A mosaic of disturbed patches can promote species diversity in communities over time, but may not
lead to much successional change.
Basics of Succession
Succession progresses through various stages that include a climax stage.
o The climax stage is thought to be a stable end point that experiences little change until an
intense disturbance sends the community back to an earlier stage.
There is some argument about whether succession can ever lead to a stable end point.
Primary succession and secondary succession differ in their initial stages Primary succession involves the colonization of habitats that are devoid of life, either as a result of
o Primary succession can be very slow, because the first arrivals typically face extremely
o The first colonizers tend to be species that are capable of withstanding great physiological
stress and transforming the habitat in ways that benefit their further growth.
Secondary succession involves the reestablishment of a community in which most, but not all, of the
organisms of organic constituents have been destroyed.
The early history of ecology is a study of succession
When the study of ecology began, it was dominated by scientists who were fascinated by plant
communities and the changes they undergo over time.
Clements thought climax community was seen as a community composed of species that persist over
many years and provided the typed of stability that could potentially be maintained indefinitely.
Gleason thought every community was thought to be the product of a particular place and time, and
thus unique in its own right.
Multiple models of succession were stimulated by lack of scientific consensus
Attempts to integrate early controversial theories, led ecologists to use more scientifically rigorous
methods to explore succession.
Connell and Slayter proposed three models of succession:
o The facilitation model, describes situations in which he earliest species modify the
environment in ways that ultimately benefit later species, but hinder their own continued
Early species in the facilitation model usually are stress-tolerant and are good
A set of species facilitations will eventually lead to a climax community, which can
no longer facilitate the other species, and are displaced only by disturbances.
o The tolerance model assumes that the earliest species modify the environment, but in neutral
ways that neither benefit nor inhibit later species.
Early successional species gave life history strategies that alter them to grow and
Later species persist merely because they have life history strategies that alter them to
tolerate environmental or biological stresses that would hinder early successional
o The inhibition model assumes that early successional species modify the environment in
ways that hinder later successional species.
Later species persist merely because they have life history strategies that allow them
to tolerate environmental or biological stresses that would hinder early successional
species. Animals are also important as drivers of succession, because they not only act as ecosystem
engineers but change assemblages of plants, leading to transitions between successional stages.
Mechanisms of Succession
Studies of succession show that no one model fits one community
Primary succession in Glacier Bay, Alaska
In the first years after new habitat is exposed, a primary or pioneer stage develops, dominated by a
Approximately 30 years after exposure, a second community develops named the Dryas stage, with
higher species richness.
After about 50 years, a third community dominates, then a century later, the third community
200 years later, species diversity sees to decrease, finalizing the community.
Some species had negative effects on later successional species unless they were able to colonize
early, supporting the inhibition model.
Secondary succession in a New England salt marsh
The mechanisms important to succession were fond to be context dependent.
No single model is sufficient to explain the underlying causes of succession.
Primary succession in rocky intertidal communities
Inhibition was thought to be the main mechanism to succession
Experiments show facilitation to be important in early stages
In most successional sequences, especially those with a pioneer stage exposed to physically stressful
conditions, facilitative interactions are important drivers of early succession.
As succession progresses, slow-growing and long-lived species begin to dominate.
As succession proceeds, species richness typically increases.
Alternative Stable States
Natural communities are capable of achieving alternative stable states, where different communities
develop in the same area under different conditions.
A community is said to be stable when it returns to its original state after some perturbation.
Alternative states are controlled by strong interactors If species go missing from a community, alternative trajectories could lead to communities that will
never lead to the original community type.
Systems may also show hysteresis, which is an inability to shift back to the original community type,
even when the original conditions were restored.
Human actions have caused communities to shift to alternative states
Regime shifts are caused by the removal or addition of strong interactors that maintain one
community type over others. Chapter 17 Biogeography 4/9/2013 11:24:00 PM
The Largest Ecological Experiment on Earth: A Case Study
In the past 25 years, the BDFFP has evolved from the question of what the minimum area of
rainforest is needed to maintain species diversity.
Since tropical rainforests require several decades to a century to regenerate, even 100 ha forest
fragments would be ineffective in maintaining species diversity until forest regeneration could
rescue species within the fragments.
Even clearings that were separated by minor distances had hindered the recolonization of forest
fragments by species.
Habitat fragmentation will also expose species to hazards, known as edge effects.
Edge effects may extend up to a kilometer into a forest, influencing the entire area of a 1000 ha
Local conditions are important regulators of species distributions.
Large-scale geographic processes can isolate species over ecological and evolutionary time by
limiting species dispersal.
Biogeography and Spatial Scale
The study of the variation in species composition and diversity among geographic locations is
known as biogeography.
Species richness and composition tend to vary with latitude: the lower tropical latitudes have many
more, and different, species than the higher temperate and polar latitudes.
Species richness and composition vary from continent to continent, even where longitude or latitude
is roughly similar.
The same community type or biome can vary in species richness and composition depending on its
location on Earth.
Patterns of species diversity at different spatial scales are interconnected
The global scale of species diversity takes into account species diversity over the entire world.
The regional scale encompasses smaller geographic areas in which climate is roughly uniform and to
which species are restricted by dispersal limitation.
All the species contained within a region are known as the regional species pool, or the gamma
diversity of the region.
The landscape scale is critical to within region biogeography, with species composition and diversity
varying within a region depending on the landscape properties. Within region biogeography is considered by ecologists in two ways:
o The local scale, which is equivalent to a community, reflects the suitability of the abiotic and
biotic characteristics of habitats for species from the regional species pool once they reach
those habitats through dispersal.
o Species physiology and interactions with other species both influence species diversity at the
local scale, or alpha scale.
o The connection between local and regional scales of species diversity is expressed as a
measure meant known as beta diversity.
o Beta diversity tells us the change in species composition, or turnover across the landscapes as
one moves from one community to another.
The amount of area that a region compasses is highly dependent on the species and community of
Local and regional processes interact to determine local species diversity
Ecologists are interested in knowing just how much variation in species diversity at the local scale is
dependent on larger spatial scales.
The regional species pool provides the raw material for local species assemblages and sets the
theoretical upper limit on species richness for communities in the region.
To determine if species richness is also determined by local conditions, one can plot local species
richness for a community against the regional species richness for that community.
o Three basic types of relationships can be seen in such plots:
If local species richness and regional species richness are equal (slope = 1), then all
the species within a region will be found in the community of that region.
If local species richness is simply proportional to regional species richness, it can be
assumed that local species richness is largely determined by the regional species pool.
If local species richness levels off despite an increasing regional species pool, then
local processes can be assumed t limit local species richness.
The degree to which local richness levels off cant tell us something about how important species
interactions and physical conditions are in setting a saturation point, or a limit on species richness for
Wallace revealed two important global patterns:
o Earths land masses can be divided into six recognizable biogeographic regions containing
distinct biotas that differ markedly in species composition and diversity.
o There is a gradient of species diversity with latitude: species diversity is greatest in the
tropics and decreases toward the poles.
The latitudinal gradient is superimposed over the biogeographic regions. The biotas of biogeographic regions reflect evolutionary isolation
Earth’s six biogeographic regions are the Nearctic, Neotropical, Palearctic, Ethiopian, Oriental, and
To explain how the continents of Earth drifted over the surface, the theory of continental drift was
There are three major types of boundaries between tectonic plates:
o Mid ocean ridges are where lava flows out of the seams between plates and cools, creating
new crust, and forcing the other crusts away, in a process called seafloor spreading.
o Subduction zones are located where two plates meet, and one plate is forced under another
o Faults occur where two plates meet but slide past each other.
The continents that sit on each plate have changed drastically over time due to a result of the tectonic
The evolutionary separation of species by barriers is known as variance.
Species diversity varies with latitude
The species diversity varies with latitude, because different environments at different latitudes have
predictable levels of productivity.
Latitudinal gradients have multiple, interrelated causes
If he dispersal rate is the same worldwide, we can predict that the number of species at any particular
location will reflect a balance between the rates of speciation and extinction.
Subtracting extinction rate from speciation rate, gives us the rate of species diversification, the net
increase of species over time.
Species diversification rate
It was proposed that terrestrial species diversity is highest in the tropics because the topics have the
greatest land area.
Large land area and relatively stable temperatures would foster a decrease in extinction rates and
increase speciation as hypothesized.
Species diversification time
The second type of hypothesis proposes that latitudinal gradients in species diversity are influenced
by evolutionary history.
Even if rates of speciation and extinction were the same worldwide, the tropics should have
accumulated more species, be cause species should have had more uninterrupted time to evolve
Productivity The final hypothesis posits that species diversity is higher in the tropics because that is where
productivity is highest, at least for terrestrial systems.
Species richness increases wit area and decreases with distance
With each increase in area, species richness increases until it reaches a maximum number bounded
by the largest area considered.
Because islands are isolated, species diversity on islands shows a strong negative relationship to
distance from the main source of species.
Island isolation and size are almost always confounded.
Species richness is a balance between immigration and extinction
The first observation of the theory of island biogeography was that for every tenfold increase in
island area, there was a rough do