Population Studies. Introduction of Some terms A population consists of all the members of a...

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


Type IICamouflage or protective coloration makes it difficult to spot prey


Type IIIDeceptive coloration, warning coloration


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


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


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


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


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


N = (40 + 0) - (55 + 0) = - 15 individuals


Formula – not given on your data sheet but is intuitive…

Recall that change in population is represented by:


Therefore, percent growth = Change in population x 100%

Initial population

=> Percent growth = __[b + i] - [ d + e]_ x 100%

Initial population


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


Percent Population Growth


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


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



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.



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?



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

um

ber

of

org

an

ism

s

Time

J-CurveN

um

ber

of

org

an

ism

s

Time

J-CurveN

um

ber

of

org

an

ism

s

Time

J-CurveN

um

ber

of

org

an

ism

s

Time

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.


S- CurveP

op

ula

tion

Siz

e

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


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



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


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

Date post:	25-Dec-2015
Category:	Documents
Upload:	marilynn-wheeler
View:	216 times
Download:	2 times

Population Studies. Introduction of Some terms A population consists of all the members of a...

Documents