Transcript Document

Population Genetics
{
Understanding Populations

Population: a group of organisms
that belong to the same species and
live in a particular place at the same
time
Example: all of the frogs of one species
living in a pond
 Properties of a population

Population size
 Population density
 Population distrubution

Target #1- I can define population
Population size: the number of
individuals that the population contains

May increase, decrease, undergo cyclical
change, or remain the same over time
 Change in population size can also indicate the
health of the environment

Target #2- I can describe how population size
relates to the wellbeing of a population

The passenger pigeon illustrates the extremes of
population size.
 Were once the most abundant bird in North
America


Nested and bred in the forests of the Midwest and
southern Canada


In the early 1800’s one flock was described as have 2
billion individuals and took approximately 5 hours to fly
overhead
Once people began cutting down the forests, hunters had
easy access to the birds
By the end of the 1800’s the population size was so
small that the pigeons could not form the large
colonies they needed to breed effectively
The Decline of the Passenger Pigeon
Checking for
Understanding
{ How is a population
size related to its
well being

Population density: measures
how crowded a population is
 Expressed as a number of
individuals per area or
volume
 In general, larger organisms,
like lions and other big cats,
have lower population
densities because they
require more resources, and
thus more room to survive
{
Target #3- I can state how population density relates to
the number of members found within a population

High population
density can…
make it easier for
organisms to group
together and find
mates
 Lead to conflict as
individuals compete for
resources
 Make organisms
vulnerable to predators
 Increase the
transmission of
infection


Low population
density can…
 Provide more
beneficial space for
populations
 Make it more
difficult to locate
mates

The harlequin frog
They lived in specific locations called
“Splash Zones”, which are located
along river and stream banks
 Its habitat experienced a period of
warm, dry weather

Case
Studythe
Harlequin
Frog


Resulted in diminished splash zone
areas



{
Water flow decreased and many streams
dried up
Population density increased
The frogs became vulnerable to disease
transmission, predator attach, and assault
from parasitic flies
Cause the population to die off

Except for a select few outside the original
habitat area
Checking for
Understanding
{
Which population of
flamingos is denser:
15 flamingos in a 5
square meter area,
or 40 flamingos in a
10 square meter
area?

Distribution: the spatial
distribution of individuals
within the population
Clumped individuals
cluster together, like in cities
 Uniform individuals are
separated evenly, like in
suburbs
 Random individuals
location is independent of
others, like in rural areas

{
Target #4- I can summarize how populations
can be distributed in the environment
Checking for
Understanding
What are the three
types of population
distribution?
Which of the
pictures below
illustrates each of
these distributions?
{

Populations are dynamic they
are constantly changing in size
and composition over time

To measure change, scientists
look at three different factors
 Birth rate: the number of births
occurring in a period of time
 Death rate: the number of
deaths in a period of time
 Life expectancy: how long on
average an individual is
expected to live
{
Target #5- I can state the three factors that affect
population size and composition over time

Different populations have
different ages and can vary in
proportion of males an females
Age Structure: describes the relative
numbers of organisms of each age
within a population
 Illustrated using an age structure
diagram

{



A visual tool used to show the age
groupings of a population
Can also be used to predict growth
Includes three categories within a
population
 Pre-reproductive age
 Reproductive age
 Post-reproductive age
Target #6- I
can define
age
structure as
it relates to
populations
Target #7- I can draw and interpret age
structure diagrams

Survivorship curve: a series of
three curves that show the
probability that members of a
population will survive to a
certain age
{

Type 1 curve the probability of
dying is small until later in life
 Organisms have few offspring
and nurture young for extended
periods of time
 Example: humans or elephants
Target #8- I can define survivorship curve and
compare the three types of survivorship curves
Type 2 curve the probability
of dying does not change over
time
 Example: birds
 Type 3 curve the probability
of dying when very young is
high, but if organism survives
through period, they will have a
high chance of living to old age
 Organisms have many
offspring, but spend little to
no time nurturing young
 Example: fish and insects

{
Survivorship curve chart
Population Genetics
{
Measuring Populations

Growth rate: the amount by
which a population’s size
changes in a given time

The size depends on 4
processes





Birth
Death
Emigration the movement of
individuals out of a population
Immigration the movement of
individuals into a population
Can be calculated

Birth rate – death rate = growth
rate
Target #9- I can define growth rate and identify the 4 processes
that affect the growth rate of a population

The Exponential Model
Involves the a pattern of
increase in number due to a
steady growth rate
 Occurs when the birth rate
exceeds the death rate
 Graph characteristic
 Line creates a J-shaped curve



{
Shows slow initial growth, and
then a rapid increase in growth
Least likely to represent real
populations (except for microbial
growth)
Target #10- I can explain the pattern described by the
exponential model of population growth

The Logistical Model



Builds on the exponential model, but
accounts for limiting factors of the
environment
Carrying capacity: the number of
individuals the environment can support
over along period of time
Graph characteristics

{
S-shaped curve


When the population is small, birth rates
are high and death rates are low
As the population size approaches
carrying capacity, the growth rate slows
 Births decrease & deaths increase
 When a population reaches its
carrying capacity, the birth rate equals
the death rate
Target #11- I can explain the pattern described by the logistical model of
population growth
Target #12- I can define carrying capacity

Limiting factors like space and resources limits the
growth of a population

As populations grow, competition among individuals for the
shrinking supply of resources intensifies

Limits the ability to reproduce
Target #13- I can explain how limiting factors affect a population

Two kinds of limiting
factors control population
size
Density-independent factors
 Weather, floods, fires, et
 Reduce the population by
the same proportion
regardless of size
 Density-dependent factors
 Resource limitations, like
food shortages or nesting
sites
 Triggered by increasing
population density

Target #14- I can differentiate between densityindependent and density-dependent limiting factors

Disadvantages of small populations

Species become more vulnerable to extinction


Ex: environmental disturbances like storms or
draughts can kill off an entire population or leave
too few individuals to maintain the population
Interbreeding can occur


The mating with relatives
Leads to decreased genetic variability


Populations with low variability are less likely to
adapt to changing environmental conditions
Examples of small populations

California Condor



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Once a prosperous species in the southwestern
United States
By 1980 only a dozen still existed
Several hundred exist today due to conservation
efforts
White Rhinoceros


Only a dozen exist, half of them are in captivity
Reproductive efforts have not been successful
Target #15- I can explain how interbreeding can
threaten the survival of a small population
Population Genetics
{
Genetic Equilibrium
Background info

Evolution occurs within a population


Genetic changes occur within a population over
time, which leads to changes in phenotype
Remember: microevolution involves the
change in gene frequency within a population

Studied using population genetics investigates
changes in gene frequency

Population genetics: the study of
evolution from a genetic point of
view
Referred to as micro-evolution the
change in gene frequencies in a
population
 When studying a population, usually
one trait is studied at one time
 A graph of the frequency of a trait is
usually a bell curve

{


Example: length in a population of fish
Largest number with a particular trait is
the average and few members express
either extreme
Target #16- I can define population genetics
Target #17- I can describe a bell curve

Variation in phenotypes

Causes



Environmental factors
Heredity
Variation in genotypes
Mutation a random change in a
gene that is passed onto offspring
 Recombination a reshuffling of
genes that occurs during gamete
formation
 Random pairing a combining of a
particular pair of gametes is
random

Target #18- I can state how variation occurs in
the phenotypes and genotypes of a population

Remember

A population is all the members
of a single species that occupy a
particular area at the same time



Interbreed and exchange genes
Ex: a population of frogs in a pond
Each member of a population
is assumed to be free to
reproduce with any other
member

The total number of alleles at all
the gene loci in all the members
of a population make up the
gene pool for the populations
Target #19- I can define gene pool
Quick review


What is homozygous
dominant? How do we
show that?

What is heterozygous?

What is homozygous
recessive?
{

Hardy, a scientist, and
Weinberg, a mathematician,
came up with an equation
used to calculate the
genotype and allele
frequencies of a population


{
P2 + 2pq + q2= 1 and p + q = 1
Once you know the allele
frequencies, you can
calculate the ratio of
genotypes in the next
generation
Target #20I can state
the
equation
for and
purpose of
the HardyWeinberg
Principle
Remember the basic formula:


p2 + 2pq + q2 = 1 and p + q = 1
p = frequency of the dominant allele in the
population
 q = frequency of the recessive allele in the
population
 p2 = percentage of homozygous dominant
individuals
2
 q = percentage of homozygous recessive
individuals
 2pq = percentage of heterozygous individuals

Target #21- I can calculate the frequency of
genes using the Hardy-Weinberg equation
PROBLEM #1.
 You have sampled a population in which you know that the
percentage of the homozygous recessive genotype (aa) is 36%.
Using that 36%, calculate the following:


The frequency of the "aa" genotype.

The frequency of the "a" allele.

The frequency of the "A" allele.

The frequencies of the genotypes "AA" and "Aa.“

The frequencies of the two possible phenotypes if "A" is completely
dominant over "a."
 Problem #2
 Within a population of butterflies, the color brown (B) is
dominant over the color white (b). And, 40% of all
butterflies are white. Given this simple information,
which is something that is very likely to be on an exam,
calculate the following:

The percentage of butterflies in the population that are
heterozygous.

The frequency of homozygous dominant individuals.
Target #22- I can identify and describe the 5 conditions
needed for Hardy-Weinberg equilibrium in a population

The Hardy-Weinberg Principle
 States that allele frequencies in a gene pool will remain at
equilibrium after one generation of random mating in a
large, sexually reproducing population as long as five
conditions are met:
 No mutations
 Genetic mutation alteration in alleles due to change
in DNA
 Under Hardy-Weinberg, mutations don’t happen, or
changes are balanced
 No Genetic Drift
 Genetic drift random changes in allele frequencies
by chance

Changes are insignificant in large populations
Target #22- cont.

No Gene Flow
 Gene flow the sharing of alleles between two
populations through interbreeding

When populations are isolated, and there is no migration,
gene flow does not occur
Random Mating
 Occurs when individuals pair by chance
 No Selection
 The environment selects certain phenotypes to
reproduce and have more offspring than other
phenotypes
 If selection does not occur, no phenotype is favored


In real life, HardyWeinberg conditions are
rarely met


Allele frequencies change
every generation
Significance of HardyWeinberg equilibrium is
that microevolution can be
detected by noting
deviations from
equilibrium
{
Target #23- I
can state the
significance
of HardyWeinberg
equilibrium
Population Genetics
{
Disruption of genetic equilibrium
The conditions that allow for evolution to
occur are mutations, genetic drift, gene
flow, non-random mating, and natural
selection


The opposite of the conditions needed for
Hardy-Weinberg Equilibrium
Background Info

Mutations
Permanent genetic changes
 Allows for heritable genetic
diversity among members of
a population
 Occur randomly


Evolution is not directed

Mutations do not occur
because an organism “needs
it”
{
Target
#24- I can
list how
mutations
result in
evolution
Target #25- I can explain how genetic drift
can cause evolution in a population

Genetic Drift
The changes in the allele frequencies of a gene pool due to
chance
 Has greater affects in smaller populations
 Example
 A death of one individual in a small population could
drastically change the allele frequencies within that
population
 If the member of the population that dies is the only
one with a particular trait, the frequency of the trait
dies with it
 Two types of events can occur that lead to small
populations where genetic drift can occur
 Founder Effect & Bottleneck Effect

Target #26- I can identify how small
populations lead to genetic drift

Founder Effect
Occurs when a few individuals form a new colony
 Only a fraction of the total genetic diversity of the
original gene pool is represented in those individuals


The particular alleles carried by the founders are by chance
alone
Bottleneck Effect

Sometimes a population is subjected to near
extinction because of a natural disaster
 Prevents the majority of genotypes from
breeding to form the next generation

Target #26- cont.
Target #27- I can describe how gene flow
leads to evolution in populations

Gene Flow

The movement of alleles among populations

Occurs when individuals migrate from one population to a
different population and then breed in the new population
 immigration & emigration

Nonrandom mating
Occurs when individuals pair
up according to their
genotypes or phenotypes
 Example: mating between
relatives
 In humans leads to
recessive abnormalities
 In other organisms leads
to a tendency toward
homozygous alleles

{
Target #28- I can describe how nonrandom
mating leads to evolution in a population

Sexual Selection
 A type of non-random mating
 The tendency of females to
choose males that have
certain traits
{
Males have a decoration or
mating call that differs from the
female
 Example
 Lions-> males have manes
 Peacocks males have blue
& green feathers and females
are brown

Target #28- cont.
 Natural selection naturally disrupts equilibrium
 Allows some members of a population to survive
and reproduce more so than others
 Operates on variations of traits within a population
 These traits have a range of phenotypes that
often follow a bell-shaped curve
 Three types
 Stabilizing selection
 Directional selection
 Disruptive selection
Target #29- I can describe how natural selection
leads to evolution of a population

Stabilizing selection

Individuals with the average
form of a trait have the
highest fitness
 Can improve adaptation of
the population to those
aspects of the environment
that remain constant
 Example

Swiss starlings lay 4-5 eggs
 More or less than that, the
young have a lower
survival rate
{
Target #30- I
can
differentiate
between the
3 types of
natural
selection in
populations

Directional Selection
 Occurs when an extreme phenotype
is favored and the distribution curve
shifts in that direction
 Changes the average phenotype
 Can occur when a population is
adapting to a changing
environment
 Example
 The change from a small horselike animal to the modern horse
 Correlated with a change in the
environment form forest
conditions to grassland
conditions
Target #30- cont.
{

Disruptive Selection
Two or more extreme
phenotypes are favored
over any intermediate
phenotype
 Example


British land snails exist in
both white and black
phenotypes
 Each color is designed for
two specific and different
environment
Target #30- cont.
Population Genetics
{
Formation of a Species

Speciation: the process of two new species
arising from one ancestral species

Remember: a species is defined as a populations of
organisms that can interbreed with one another and
produce fertile offspring


Each species might have variations
Speciation begins with isolation
Prevents formerly interbreeding populations from breeding
 Two types
 Geographic & Reproductive isolation

Target #31- I can define speciation

Geographic Isolation
The physical separation of
members of a population
 Example: the formation of a
canyon, a river could change
course, or the climate could
change
 The more mobile a species, the
less isolated the populations are
 Results in new species
 Process known as allopatric
speciation
Target #32 I can explain how the geographic isolation
of populations leads to allopatric speciation

{

Reproductive Isolation

Occurs when organisms within a
population become genetically
isolated and can no longer
interbreed with one another


Two general types



Occurs because of a change in mating
practices, a change in reproductive
organs, or a change in mating season
Premating isolation occurs before
mating takes place
Postmating isolation occurs after
fertilization
Known as sympatric speciation
Target #33- I can explain how reproductive isolation of
populations leads to sympatric speciation

The amount of time needed for
speciation to take place varies
according to the organism and the
environment
 Two hypotheses exist to describe
the rate at which speciation
occurs
Gradualism: speciation that occurs
at a regular, gradual rate
 Punctuated Equilibrium: speciation
that occurs in “bursts that happen at
a rapid pace, followed by periods of
little to no change

Target #34- I can contrast the model of punctuated
equilibrium with the model of gradualism