population studies. introduction of some terms a population consists of all the members of a...
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BIOLOGY 30
Population Studies
Introduction of Some terms A population
consists of all the members of a species that occupy a particular area at the same time
The members of a population are more likely to breed with one another than with other populations of the same species Therefore, genes tend to stay in the
same population for generation after generation
INTRODUCTION
The total of all the genes in all the members of a population at one time is
called the … population's gene pool Evolution
is the change in the frequency of genes…
in a population's gene pool… from one generation to the next.
Hardy-Weinberg Law
In order to see how a population evolves, it is helpful to examine the genetics of a population that does not change from generation to generation
The Hardy-Weinberg Law provides a model of an unchanging gene pool
This law states that the frequencies of alleles in a population's gene pool remain constant over generations if all other factors remain constant
Hardy-Weinberg Law
For a gene pool to be in the Hardy-Weinberg equilibrium, 5 conditions must be met:1. The population must be closed. This means that
no immigration or emigration can occur. 2. Random mating takes place. There can be no
mating preferences with respect to genotype. 3. There can be no selection pressure. A specific
gene must not affect the survival of the offspring.
4. No mutation of the particular alleles examined can occur.
5. The population must be very large. This equilibrium is based on statistical probabilities and random sampling.
Hardy-Weinberg Law
If all these conditions are met, the frequencies of two alleles (A and a) will remain constant in a population forever or until conditions change
Recall our definition of Evolution Change of frequency of genes or alleles
The Hardy-Weinberg law points out that sexual reproduction reshuffles genes but does not by itself cause evolution
Hardy-Weinberg Law
The mathematical expression of the Hardy-Weinberg equilibrium is…
p + q = 1 where p = frequency of the dominant
allele&
q = frequency of the recessive allele
Hardy-Weinberg Law
Example: suppose a certain allele A has a
frequency of 0.6 in a population since the two alleles must add up to 1…
then p + q = 1 (1 - 0.6 = 0.4) the frequency of a is 0.4
Let's see what happens during reproduction
Hardy-Weinberg Law
First, let’s arrange the two alleles and their frequencies on a Punnett square
a
(0.4)
A
(0.6)
a
(0.4)
A
(0.6)
Hardy-Weinberg Law
Then, fill in frequencies for the possible offspring
a
(0.4)
A
(0.6)
a
(0.4)
A
(0.6)
aa(0.16)
Aa(0.24)
Aa(0.24)
AA(0.36)
Hardy-Weinberg Law
Go ahead and add up your values for the allele frequencies. What do you get?
The mathematical relationship governing the gene frequencies is…
p2 + 2pq + q2 = 1AA + 2Aa + aa = 1 (or 100%)
Since p = 0.6 and q = 0.4, then (0.6)2 + 2 (0.4 x 0.6) + (0.4)2 must equal 1
(0.36) + 2(0.24) + (0.16) = 1
Practice
Mutations & Evolutionary Change
Mutations violate the conditions for Hardy-Weinberg equilibrium because one gene changes into another and therefore alters gene frequencies in the population
Mutations & Evolutionary Change
Review: a mutation is any inheritable change in the DNA of an organism1.Chromosome mutation
results form non-disjunction, chromosome breakage or translocation
2.Gene mutation changes in the nucleotides of a DNA
molecule
If a population has a stable gene pool and gene frequencies, it is not evolving.
Mutations
If the population does not demonstrate Hardy-Weinberg equilibrium (i.e. its gene frequencies are not stable) it is in evolutionary change
Evolutionary Change
Micro-evolution a change in the gene pool of a
population over successive generations Potential causes of micro-evolution
are mutation genetic drift gene flow non-random mating natural selection
Evolutionary Change
Mutation A new mutation that is transmitted in
gametes immediately changes the gene pool of a population by substituting one allele for another
A mutation by itself does not have much effect on a large population in a single generation
If, however, the mutation gives selective advantage to individuals carrying it, then it will increase in frequency and the population gene pool will change over successive generations
Evolutionary Change
Genetic drift evolution can occur simply by chance Random events may bring death or
parenthood to some individuals regardless of their genetic makeup
The resulting change in the gene pool is called genetic drift
Genetic drift plays more of a role in small populations than in large ones
Evolutionary Change
Genetic drift Example
Flipping a coin 1000 times compared with flipping a coin 10 times
Example a population of plants consists of only 25
individuals, 16 are AA, 8 are Aa and 1 is aa
AA plants are destroyed in a rock slide, which alters the relative gene frequencies for subsequent populations.
Evolutionary Change
Founder Effect Genetic drift that occurs when a small number
of individuals separate form their original population and start a new population
Allele frequencies of the new population will be different than the original population o depend on gene pool of the founding population
Evolutionary Change
Bottleneck Effect A dramatic reduction in population size
resulting in genetic drift
The frequency of alleles in the remaining
members of the population is very different from the original population.
Evolutionary Change
Gene flow The gene pools of most populations of
the same species exchange genes.· This violates the Hardy-Weinberg condition
that populations must be closed to be in equilibrium
Animals may leave one area and contribute their genes to the pool of a neighbouring population
migrationor a high wind may disperse seeds or pollen far beyond the bounds of the local populationGene flow between populations may change gene frequencies and therefore may result in evolution
Nonrandom Mating
Mates are chosen based on different characteristics (not just love the one you’re with)
Sexual Selection Chances of being selected depend on
animal’s traits (what makes him more desirable to the female)
Includes Physical and Behavioural Differences between sexes
Nonrandom Mating
Sexual Dimorphism Striking physical differences between
males and females
Female
MaleMale
Female
Nonrandom Mating
Natural Selection Environment selects for particular traits
that are more favourable for surviving in that environment
“Survival of the Fittest”
Interactions in
Ecological Communities
Population Interactions Definitions
Population – Any group of individuals of the same species who live in the same area at the same time Eg. Population of humans in the food court
Community – The association of interacting populations that live in a defined area Eg. Population of the food court, tables, chairs,
trays, and pets and wild animals that wander into the food court.
Niche – An organism’s habitat and role within a community Includes all factors needed to survive and the
organism’s interactions with other species
In any community, individuals of many populations need to live among each other Some possible scenarios:
Competing for limited resources One species preying on another One species relying on another for survival
Competition occurs whenever two or more organisms attempt to exploit a limited resource Food Living space
Population Interactions
The interactions among individuals – either within the same population or from different populations – are the driving force behind population dynamics The changes that occur in a population
over time
Population Dynamics
Individuals are always competing for resources in order to survive
Competition for resources can occur: Among individuals of the same species
Natural selection Survival of the fittest
Between individuals of different species Hence, there are two basic categories of
competition
Population Dynamics
Intraspecific competition – Competition for limited resources among members of the same species SURVIVAL OF THE FITTEST among
members of the same species aka NATURAL SELECTION Eg. Seeds
On the forest floor there are thousands of seeds
Each seed requires water, nutrients, sunlight, space to grow and mature
Only a few seeds will be able to compete successfully to obtain what they need of the limited available resources
Intraspecific Competition
Intraspecific Competition
Intra-specific competition is very common since the members of a population have the same requirements Intra-specific competition
occurs when individuals of a species are competing for resources within their niche
Interspecific competition – Competition for limited resources between members of different species in the same community
Tree competing with a shrub for light and growing space
Recall a niche is an organism’s habitat and role in a community
Due to interspecific competition, no two organisms can share the exact same ecological niche
Interspecific Competition
Interspecific Competition
If no two species can share the exact same ecological niche – then why is there interspecific compeition?
Interspecific competition occurs when individuals of two different species are competing for resources within overlapping niches
Competition – Gause’s PrincipleThe Theory of Competitive
Exclusion Two species with very similar
niches cannot survive together because they compete so intensely that one species eliminates the other
Competition – Gause’s Principle
Experiment: Gause raised two species of paramecium with similar food requirements in the same culture One species always eliminated the other (the
particular conditions in the culture determined which species survived)
In nature, species can avoid direct competition by Feeding at different times of the day (e.g.
Hawks and owls) Dividing resources in some other way (e.g.
Different organisms hunt for insects in different parts of coniferous trees)
Not all interspecific interactions in a community are classified as competitive…
Producer-Consumer Interactions
Predation
The most obvious population interaction in a community are those in which a predator eats its prey
Predators that specialize in eating only one prey species play an important role in controlling the population size of the prey species Eg. Canada lynx and snowshoe hare
The terms predator and prey apply not only to animals that eat other animals, but to any type of producer and consumer relationship Eg. Plants and Herbivores
Predation
Plant defense mechanisms against herbivores:
Thorns Microscopic crystals in their tissues Spines or hooks on leaves Distasteful or harmful chemicals
Some well-known poisons and drugs are secondary compounds produced by plants: Strychnine Morphine Nicotine Mescaline
Predation
Active animal defenses against predation
Fighting Hiding Escaping
Four types of passive defense
Predation – Passive Defense
Type I Mechanical or chemical defense mechanisms include porcupine quills, the skunk's offensive odour, the bad taste of monarch butterflies
Predation – Passive Defense
Type IICamouflage or protective coloration makes it difficult to spot prey
Predation – Passive Defense
Type IIIDeceptive coloration, warning coloration
Predation – Passive Defense
Type IVMimicry, where one species resembles another Monarch and viceroy
butterflies Coral snake and
harmless species Wasps and non-biting
flies
Symbiotic Relationships
Symbiosis is a close relationship between members of different species (3 categories)
1. Mutualism - Both species benefit from the association Coliform bacteria in the human gut, nitrogen-
fixing bacteria in nodules of legumes, protists in a termite's gut
2. Commensalism - One species benefits while the other neither benefits nor is harmed Remora and the shark
3. Parasitism - One species, the parasite, benefits at the expense of the host The parasite takes nourishment directly from the
tissues of its host's body
12 3
Marc’s Botfly
Marc’s Botfly
After Central America
Growth and Regulations
of Populations
Regulation of Population Size Factors Affecting Growth of Populations
The growth of a population is suppressed by · Abiotic factors - Non living things in the
environment · Sunlight, water, soil, air
· Biotic factors - Living things in the environment· Humans, trees, fish, bacteria
The combination of these effects is termed environmental resistance
· Factors that regulate the growth of populations are described as density-dependent or density-independent
Regulation of Population Size
The combination of biotic (living) and abiotic (nonliving) factors create environmental resistance Environmental resistance = the combined
effects of various interacting factors that limit population growth
· There are two categories of factors that regulate the growth of populations
1. Density-dependent
2. Density-independent
Density Dependent Factors
These factors are intensified as the population increases in size Food availability Living space Competition Reproductive rate Accumulation of
wastes Immigration
Emigration Disease Mortality Parasitism Biotic Factors
Density-Independent
The occurrence and severity of these factors are unrelated to population size Weather Climate Abiotic factors
Population density = The number of individual organisms in a given area or volume
Population Density
Dp = N or Dp = N
A V D = Density N = number of organisms A= area V = volume
Growth and Regulation of Populations
Population Density
Example: 44 students/100m2 = 0.44 students/m2
12 gophers/10.0m2 = 1.2 gophers/m2
54 minnows/200 mL = 0.27 minnows/mL
Growth and Regulation of Populations
Population Density
18
Why calculate population density?
If you know your community size you can now estimate the size of your population
Examples: School = 1000m2, therefore
4.4 students/m2 x 1000m2 = 4400 students Field = 200m2, therefore
1.2 gophers/m2 x 200m2 = 240 gophers Fish tank = 2L = 2000 mL, therefore
0.27 minnows/mL x 2000mL = 540 minnows
Growth and Regulation of Populations
Population Density
Population Density Practice A fish tank 10m long, 5m tall and 2m wide
is filled with water. The population density of bacteria in the
water is 1.5 x 104bacteria/m3
Approximately how many bacteria are in the fish tank?
Population Density Practice–cont’d
First we need to calculate the volume of the swimming pool: 10m x 5m x 2m = 100m3
The population density of the bacteria is 1.5 x 104bacteria/m3
Therefore, the population density of bacteria in the water is represented by the formula
Dp = N Dp x V = N V
Population Density Practice–cont’d
Dp x V = N N = 1.5 x 104bacteria/m3 x 100m3
= 1 500 000 = 1.5 x 106 bacteria
Is this always 100% accurate? Note that you need to know how a
population is distributed within its habitat before taking samples to determine the population size
Some populations tend to clump in certain areas, which can affect the accuracy of your estimation
Growth and Regulation of Populations
Population Density
Growth and Regulation of Populations Population Growth
A population gains individuals by: Natality = Birth Immigration
A population loses individuals by: Mortality = Death Emigration
The balance between these four factors will determine whether a population size grows, declines, or remains the same
Growth and Regulation of Populations
Change in Population Size
Although this is the formula given to you, we know that Factors that increase population = births and immigration Factors that decrease population = deaths and emigration
Therefore,
(D N) = (births + immigration) - (deaths + emigration)
Calculate the change in the Sandhill Crane population at the banks island breeding site in 1991 Births = 40, Immigrations = 0 Deaths = 55, Emigration = 0 Initial number = 200
(D N) = (births + immigration) - (deaths + emigration)
N = (40 + 0) - (55 + 0) = - 15 individuals
Growth and Regulation of Populations
Formula – not given on your data sheet but is intuitive…
Recall that change in population is represented by:
(D N) = (births + immigration) - (deaths + emigration)
Therefore, percent growth = Change in population x 100%
Initial population
=> Percent growth = __[b + i] - [ d + e]_ x 100%
Initial population
Growth and Regulation of Populations
Percent Population Growth
Example: Births = 40 Deaths = 55 Initial population size = 200
=> Percent growth = __[b + i] - [ d + e]_ x 100%
Initial population
PG% = [40 + 0] - [55 + 0] x 100% = -7.5%
200
Growth and Regulation of Populations
Percent Population Growth
Growth and Regulation of Populations
Population Growth Rate
Population growth rate: The change in the number of
organisms in a population per unit time
growth rate = _D N_ D t
Rate of population growth does not take into account the initial size of the population A large population has more individuals
that can reproduce compared to a small population
To compare populations of the same species that are different sizes or live in different habitats, the change in population size can be expressed as the rate of change per individual This measurement gives us per capita
growth rate
Growth and Regulation of Populations
Per Capita Growth Rate
The per capita growth rate can be calculated by the formula:
cgr = ΔN
N cgr = Per capita growth rate ΔN = Change in the number of individuals in a population
N = The original number in the population
Growth and Regulation of Populations
Per Capita Growth Rate
Why measure per capita growth rate?
To examine population size as the rate of change per individual Eg. Suppose that in a town of 1000
people there are 50 births, 30 deaths, and no immigration or emigration in a year.
Calculate the per capita growth rate.
Growth and Regulation of Populations
Per Capita Growth Rate
Eg. Suppose that in a town of 1000 people there are 50 births, 30 deaths, and no immigration or emigration in a year. Calculate the per capita growth rate.
cgr (Growth rate) = Unknown ΔN = 50 births – 30 deaths = +20
N = 1000 peoplecgr = ΔN = 20 = 0.02 people N 1000
Could the answer be a negative value?
Growth and Regulation of Populations
Per Capita Growth Rate
10.3
Recall that both biotic and abiotic factors limit the growth of a population
Population size can be limited by How fast and how often a species can
reproduce The ability of a habitat to support the
population
Factors that Affect Population Growth
Biotic Potential of Populations
Biotic potential = r Definition = The maximum
number of offspring that can be produced by a species under ideal conditions
ie. The capacity of populations for exponential growth
There are six factors which affect biotic potential of a population
Biotic Potential of Populations
Six Factors that affect Biotic Potential
1. Age of onset of sexual maturity • The earlier that sexual maturation
occurs, the greater the biotic potential 2. Gender ratio
• The more females there are, the greater the biotic potential
3. Estrous cycles • The shorter the time between cycles of
sexual receptivity, the greater the biotic potential
4. Mate availability The more readily available mates are in a
population, the greater the biotic potential
5. Litter or clutch size The larger the litter or clutch size, the
greater the biotic potential6. Fecundity
Fecundity = average number of offspring produced per female
The greater the fecundity of a species the greater the biotic potential
Biotic Potential of PopulationsSix Factors that affect Biotic
Potential
Population Growth PatternsTwo Types of Graphs/Curves to
know:1. Exponential Population
Growth: J-Curve This model predicts unlimited population
increase under ideal conditions (usually a closed pop.) of unlimited resources and then a sharp decline in the population
2. Logistic Growth: S-CurveMore representative of population
in nature This model incorporates the effects of
resource limitation and crowding on the population growth rate
Population Growth Patterns
1. Exponential Population Growth: J-Curve
This model predicts unlimited population increase under ideal conditions (usually a closed pop.) of unlimited resources and then a sharp decline in the population
There four phases in this type of growth pattern:
1. Lag phase2. Growth phase3. Stationary phase4. Death phase ("crash")
Examples of organisms that exhibit exponential growth include bacteria, yeast, some insects
J-CurveN
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J-CurveN
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J-CurveN
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J-CurveN
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2. Logistic Growth: S-CurveMore representative of population
in nature This model incorporates the effects of
resource limitation and crowding on the population growth rate
Natural populations cannot continue to grow exponentially:
There is a limit to the number of individuals that can occupy a habitat
The carrying capacity is the maximum stable population size that the environment can support for a long period of time.
Population Growth Patterns
S- CurveP
op
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tion
Siz
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Time
Logistic Growth: S-Curve – cont’d
In populations exhibiting logistic growth, an equilibrium is reached near the carrying capacity of the environment
example p. 585 figure 25.12 Carrying capacity (symbolized as
K) is a property of the environment, and it varies over space and time with the abundance of limiting resources
Population Growth Patterns
S-Curve
Carrying Capacity
Pop
ula
tion
Siz
e
TimeTime
S-Curve
K
Time
Pop
ula
tion
Siz
e
Time
R-SELECTEDAND
K-SELECTED POPULATION STRATEGIES
r-selected Populations Experience periods of exponential
growth Characteristics of r-selected
species: Small organisms Short life time Great reproductive potential Recall biotic potential (r) is the capacity of
populations to grow exponentially High rate of reproduction = r
Insects are examples of r-selected populations
K-selected Populations
Populations that stabilize near the carrying capacity of their environment (K)
Characteristics of K-selected species:
Larger size Longer generation time Lower reproductive potential
Examples include large mammals such as deer, bears, and humans
Young require parental care
A COMPARISON OFR-SELECTED
(OPPORTUNISTIC) AND
K-SELECTED (EQUILIBRIAL) POPULATIONS
R-Selection K-Selection
Climate Variable and/or unpredictable
Fairly constant and/or predictable
Mortality Density independent Density dependent
Survivorship High juvenile mortality Low juvenile mortality
Population Size Variable, below carrying capacity
Fairly constant, near carrying capacity
Level of competition Low High
Life History Rapid development Slow development
Reproductive Capacity
High reproductive capacity
Greater competitive ability
Age Sexual Maturity Early reproduction Delayed reproduction
Body Size Small body size Large body size
Reproductive Frequency
Usually reproduce only once
Repeated reproduction
Offspring Many small offspring Fewer, larger offspring
Length of Life Short, less than one year
Longer, usually more than one year
2 4 5 6
1 4 6 7
Change in Communities: Succession
Succession - The sequence of invasion and replacement of species in an ecosystem over time The sequence of identifiable ecological stages or communities occurring over time in progress from bare rock to climax community
Affected by abiotic and biotic factors – climate and interspecific competition
Communities are defined by the populations in them
The stage of succession can be determined by the kinds of species present in a community
Change in Communities: Succession
Change in Communities: Succession
Primary succession The initial
colonization of a barren habitat by pioneer species
Soil is produced during this stage e.g. Lichen and mosses
growing on rocks
Secondary succession Re-building of an
area that once supported many organisms e.g. Mount St. Helen’s
Climax Community The stage in ecological
succession that is stable and self-supporting
Usually the final stage in the stages of succession
Produce more organic material than they use
Change in Communities: Succession
Type of Communit
y
Populations Relationship to Sun
Pioneer Weeds, fugitive species
Lots of sun required
Seral Shrubs Less than above
Seral Deciduous trees
Even less than above
Climax Pine trees Least amount of sun
3 1 1