Download Chapter 21 Populations Evolve in Ecosystems The theory of

Chapter 21
Populations Evolve in Ecosystems
The theory of evolution
 The theory of evolution is the foundation of all biology
 Charles Darwin is credited with developing the theory of evolution
 Based on two important points
o All living organisms descended from a common ancestor and therefore, share a
common chemistry
o Living organisms have the ability to adapt to their environment; the ability of
species of organisms to adapt to their environment is evolution
Natural Selection
 Darwin based his theory on four general statements
 Organisms show differences that can be inherited
 All organisms produce more offspring than can survive and reproduce in subsequent
generations
 Variations among organisms can increase or decrease each individual’s ability to reproduce
 Variations that increase the likelihood of successful reproduction will be passed onto future
generations
 Those individuals in a population with phenotypes better suited to the environment will
have a selective advantage, better able to survive and produce more offspring
 We now refer to change in allele frequencies in a population over time as evolution
 Although there are different means by which allele frequencies can change, natural selection
is the only one that causes to evolutionary adaption
 Although we often hear the term “survival of the fittest”, it is really “survival of the fit
enough”
Fitness
 Reproductive fitness is the ability of an organism to successfully reproduce
o The key to an organisms’ fitness is leaving more copies of its genes for the next
generation
o It’s all about the number of offspring produced
Adaptation
 Evolutionary adaptation is the result of natural selection
 It is an adjustment or series of adjustments a population or species makes in a given
environment over time
 Given enough time, these evolutionary adaptations may result in a new species
Descent with modification
 Evolution occurs at the population level and above
 At the population level, it refers to changes in allele frequency over time
 At higher levels, over much greater periods of time, these changes in a population’s allele
frequencies can lead to new species
 Over even greater periods of time, it can lead to major evolutionary changes, such as the
transformation of a fish into a terrestrial tetrapod
Hardy-Weinberg equilibrium
 In order to determine if a population is evolving, we need to have a point of reference
o
We need to know what a population that is not evolving looks like
 Requirements for Hardy-Weinberg equilibrium
o
Population must be very large
o
Mating must be random
o
No mutations, immigration, or emigration
o
No selective pressure so no natural selection
o
So, there can’t be a natural population that meets all of those requirements
 Hardy-Weinberg equilibrium refers to an idealized situation - a reference point
 With this reference point, any population that does not fit the Hardy-Weinberg equation
must be evolving
 Consider a single gene with two alleles in the gene pool (A and a)
o
Since there are only the two alleles, the sum of their frequencies must be 1
o
If p = frequency of A and q = frequency of a, then p + q = 1
o
If we square both sides (p + q)2 = 12 we get p2 + 2pq + q2 = 1
o
So p2 = frequency of AA
o
2pq = frequency of Aa
o
q2 = frequency of aa
 Practical applications
 Consider cystic fibrosis
o Incidence rate among North American Caucasians = 1/2000
o q2 = 1/2000 = 0.0005; q = 0.02
o p = 0.98, 2pq = 2(0.02)(0.98) = 0.0392
o Therefore, ~4% (1/25) of Caucasians are carriers
Evolution
 Evolution does not have a direction; it does not march toward a perfect organism
 In any particular environment, there are likely to be many organisms that are well-suited to
fill various niches
 These make up communities
 Over time, change is normal
 Environmental conditions can change as well
 Species that were very well-suited to the old environment can now be ill-suited to the new
one
 The species can either adapt via evolutionary process or become extinct
o Humans can cause very rapid environmental changes
Evolutionary relationships
 The concept of descent with modification would predict that we share common
macromolecules with other life forms
 All living organisms have DNA as their genetic material, use RNA as a messenger, and use
the same genetic code to make their proteins
 But DNA can mutate
 Evolutionary theory predicts that more closely related species will have fewer differences in
their DNA sequences; more distantly related species will have more differences
 Genomic analysis indicates that humans and chimpanzees shared a common ancestor about
7 million years ago
o Since that time, the lineage including humans and the lineage including
chimpanzees have diverged
o Research indicates that one of the biggest differences between chimps and humans
is the composition and functioning of the brain
Natural selection
 Natural selection acts on populations subjected to various pressures from the environment
o Climate pressures, intra- and interspecific competition for limited resources, others
 Those individuals with variations that provide a selective advantage will produce more
offspring and make a larger contribution to the gene pool
 The genetic variation is due to mutation to produce new alleles and the genetic
recombination that occurs with sexual reproduction
Genetic bottlenecks
 Stable populations can be devastated by natural catastrophes
 When large portions of populations are suddenly removed from the gene pool, this
promotes evolution without regard to fitness or genetic makeup
 The frequency of alleles in the surviving population is often very different from the
original population
Bottleneck effect
 Sichuan earthquake in 2008 killed >69,000 villagers
 Earthquake in Haiti has greatly affected allele frequencies
Gene flow
 The movement of individuals from one population to another can also affect allele
frequencies
 Alleles are removed from one population and added to the other
 This can introduce or remove disadvantageous alleles
Ecology
 The field of ecology attempts to identify and explain the interactions between the biotic
(living) and abiotic (nonliving) components of ecosystems
o
It’s all about energy and resources
 Individuals of the same species occur in populations
 Populations, in turn, are organized into communities, which include more than one species
The nature of ecosystems
 Biosphere – the regions of the Earth’s waters, crust, and atmosphere inhabited by living
organisms
 Biomes – large areas where organisms interact between each other and their environment
o
These are determined primarily by climate
o
Terrestrial - several distinct types based on temperature and rainfall
o
Aquatic - freshwater and marine
Succession
 Ecological communities undergo constant change
o
The composition of communities are always shifting with changing conditions dominant populations change in a largely predictable manner
o
We can consider primary and secondary succession
 Primary succession occurs in areas where there is no life
o
This can be any new land, such as areas formed from lava flows, beaches, river
deltas, or areas recently cleared or exposed by the movement of glaciers
o
Initially, pioneer species hold newly formed soil in place and add organic
materials, allowing grasses and then large plants to take over
o
The dominant species change - succession
 Secondary succession occurs when a stable community is disrupted
o
Organisms associated with an earlier stage of succession once again become
dominant
o
The land is not newly formed but the pioneer species begin the process again
o
Secondary succession usually occurs much more rapidly than does primary
succession
 Climax communities are stable, mature, equilibrium communities that have resulted from
succession
If it’s all about energy and resources, what is the path of energy in an ecosystem?
o Energy flows through an ecosystem
What about nutrients, a major resource?
o Nutrients cycle through an ecosystem
Energy flow
o There must be a constant supply of energy into ecosystems
o Most ecosystems rely on sunlight energy
o Producers convert sunlight energy to chemical energy in the form of chemical bonds
o
Photosynthesis and cellular respiration are intimately linked
o
Photosynthesis requires sunlight, CO2, and H2O and produces glucose and O2
o
Cellular respiration requires glucose and O2 and produces CO2 and H2O
o As consumers eat producers or other consumers, energy and chemicals are transferred from
one organism to the next
o
In a food chain, energy is transferred as feeding occurs
o
However, most of the energy is lost as heat
 In fact, only about 10% of the energy at any one level is used for adding
biomass (growth) of consumers at the next level
 The 10% rule has some major implications for numbers of organisms and
biomass at each level
o These are our ecological pyramids
o
Most ecosystems can sustain no more than five trophic levels
o
Even then, most 5th level consumers can also feed at lower levels
o
Understanding the ecological pyramid for a community can provide valuable
information about the stability of that community
o Ecosystems rarely involve a single food chain
o
Food webs more accurately depict the myriad of feeding interactions
Cycling of nutrients
 Although essential nutrients have a tendency to remain within ecosystems, they can be
readily accessible or trapped in an inaccessible form
 The availability of these nutrients determines the amount of productivity within an
ecosystem
 Ecologists study biogeochemical cycles of these essential nutrients - water, phosphorus,
nitrogen, carbon
Population growth
 Population growth is dependent on two major rates*
 Growth rate = birth rate - death rate
o
If growth rate > 0, that population has the potential to grow exponentially over
time
o
It can grow at its biotic potential
o
No natural population can grow at its biotic potential indefinitely
 Population-limiting factors will prevent a population from continuing to expand indefinitely
 Carrying capacity is the number of individuals of each population that the ecosystem can
support indefinitely without permanently reducing the productivity of the ecosystem
o
It is a balance between competition for available resources and population
growth
o
Carrying capacity varies with each species, ecological conditions, and time
o
In natural ecosystems, populations often stabilize around their carrying
capacities
o
With successful species, we have usually seen population growth slow as the
population nears its carrying capacity
What about humans?
 When we compare human survival vs. other organisms, we can develop survivorship curves
 The type of survival is linked to reproductive and parental care strategies
What effects have humans had on the planet?
 Pollutants from factories can lead to acid rain
o
But recognizing the problem can sometimes lead to a solution
o
Regulatory controls on sulfur emissions were enacted in 1990
 Increased amounts of CO2 in the atmosphere from the burning of fossil fuels and
deforestation has led to global warming
 More people means less resources and more waste
 By destroying natural habitats, we have also changed whole ecosystems
 This may well be increasing the risk of extinction of many species
 Humans may well be responsible for the sixth mass extinction in all of earth’s history
 Changing natural habitats rather than minimizing our impact
In nature, there are neither rewards nor punishments - there are consequences.
 Roger G. Intersoll