Copyright Pearson Prentice Hall 5-1 How Populations Grow.

Copyright Pearson Prentice Hall

5-1 How Populations Grow


Characteristics of Populations

• Three important characteristics of a population are its:– geographic distribution– density– growth rate


• Geographic distribution, or range, describes the area inhabited by a population.

• Population density is the number of individuals per unit area.

• Growth rate is the increase or decrease of the number of individuals in a population over time.


Population Growth• Three factors can affect population size:

– the number of births– the number of deaths– the number of individuals that enter or leave the

population


• Immigration, the movement of individuals into an area, is another factor that can cause a population to grow.

• Populations can increase by immigration as animals in search of mates or food arrive from outside.


• Emigration, the movement of individuals out of an area, can cause a population to decrease in size.

• Emigration can occur when animals leave to find mates and establish new territories.

• A shortage of food in one area may also lead to emigration.


Exponential Growth• Under ideal conditions with unlimited resources, a

population will grow exponentially.• Exponential growth occurs when the individuals in a

population reproduce at a constant rate.• The population becomes larger and larger until it

approaches an infinitely large size.


Exponential Growth


Logistic Growth

• In nature, exponential growth does not continue in a population for very long.

• As resources become less available, the growth of a population slows or stops.

• Logistic growth occurs when a population's growth slows or stops following a period of exponential growth.


• Logistic growth is characterized by an S-shaped curve.


• Carrying Capacity– The largest number of individuals of a population that a given environment can support is called its carrying capacity.

– When a population reaches the carrying capacity of its environment, its growth levels off. The average growth rate is zero.


5-2 Limits to Growth


Limiting Factors

• The primary productivity of an ecosystem can be reduced when there is an insufficient supply of a particular nutrient.

• Ecologists call such substances limiting nutrients.• A limiting nutrient is an example of a more

general ecological concept: a limiting factor. • In the context of populations, a limiting factor is a

factor that causes population growth to decrease.


Density-Dependent Factors

• A limiting factor that depends on population size is called a density-dependent limiting factor.


Density-Dependent Factors• competition• predation• parasitism• disease


Density-Dependent Factors

• Density-dependent factors operate only when the population density reaches a certain level. These factors operate most strongly when a population is large and dense.

• They do not affect small, scattered populations as greatly.


Wolf and Moose Populations on Isle Royale

Moose Wolves

http://www.phschool.com/webcodes10/index.cfm?fuseaction=home.gotoWebCode&wcprefix=cbp&wcsuffix=2052


Density-Independent Factors• Density-independent limiting factors affect all populations in similar ways, regardless of the population size.

• Examples of density-independent limiting factors include: – unusual weather– natural disasters– seasonal cycles– certain human activities—such as damming rivers

and clear-cutting forests

EVOLUTION OF POPULATIONSGenes and Variation 16–1

Section 16-1

When Darwin developed histheory of evolution, he didn’t know how ____________ worked.

Mendel’s work on ______________ was published during Darwin’s lifetime, but ________________ as important until __________________.

HEREDITY

inheritance in peas

NOT recognized

decades later

http://www.answers.com/topic/gregor-mendel

GAPS IN DARWIN’S THINKING:

How do heritable traits pass from one generation to the next?

How does variation in the population appear?

TODAY we can understand how evolution works better than Darwin ever could because of our knowledge about____________ and ________genes DNA

A _______________ is a group of individuals of the same species that can interbreed and produce fertile offspring.

POPULATION

http://www.3kitty.org/travelrama/Photos/123-21-4x6.jpg

Because members of a population interbreed, they share a common group of genes called a ___________

A gene pool consists of ____________ present in a population, including all the different _________.

GENE POOL

ALL the genes

alleles

Sample Population

48% heterozygous

black

36% homozygous

brown

16% homozygous

black

Frequency of Alleles

allele for brown fur

allele for black fur

The ____________________ is the number of times that an allele occurs in the gene pool compared to the occurrence of other alleles in the gene pool.

relative frequency

Image from BIOLOGY by Miller and Levine; Prentice Hall Publishing ©2006

RELATIVE FREQUENCY is often expressed as a __________________.

EX: In this populationDominant B allele (black) = 40%Recessive b allele (brown) = 60%

percentage


RELATIVE FREQUENCY has _________ to do with whether an allele is______________ or _____________

In this population, the recessive allele is more frequent.

DOMINANT RECESSIVE


NOTHING

IN GENETIC TERMS, __________________ is any change in the relative frequency of alleles in a population.

If the relative frequency of the B allele in this mouse population changed over time to 30%, the population is evolving.

EVOLUTION


BIOLOGISTS can now explain how the _____________ that DARWIN saw is produced!

A __________ is any change in a sequence of _______

SOURCES OF MUTATIONS:__________________________

_____________________

__________________

mutationDNA

Mistakes in DNA replication

variation

Chemicals

http://sickle.bwh.harvard.edu/scd_background.html

Radiation

Many mutations are ___________ and result in death.

Some mutations are ____________ and DON’T affect and organism’s ______________at all.

silent

PHENOTYPE

LETHAL

Image from: BIOLOGY by Miller and Levine; Prentice Hall Publishers©2006

EX: Changing the codefrom GGA to GGUdoesn’t change theamino acid used

GENE SHUFFLING during ______________________

_______________

___________________

MEIOSISCrossing over

REMEMBER !

Independent Assortment

RESULT IS GENETIC RECOMBINATION

Segregation

http://www.emc.maricopa.edu/faculty/farabee/BIOBK/Crossover.gifhttp://waynesword.palomar.edu/lmexer2a.htm

_______________________IS THEMAJOR SOURCE OF VARIATION IN POPULATIONS, but it does _______change the __________________ ofalleles in population!

SEXUAL REPRODUCTION

NOT

relative frequency

http://www.magicbob2000.com/resources/Svengali%20cards.jpg

Shuffling a deck of cards can shuffle to produce many different hands, but doesn’t change the number of kings or queens in the deck.

Single gene traits result in only _______phenotypes.

The number of _______________ produced for a given trait depends on the number of ______ that control that trait.

PHENOTYPES

GENES

TWO

EX: Widow’s peak No widow’s peak

In real populations, phenotypic ratios are determined not only by which allele is _____________, but by _______________ of the allele in the population

FREQUENCYDOMINANT

Presence of widow’s peak in population can be less common even if it is DOMINANT!

_______________ traits are controlled by two or more genes.

POLYGENIC

A bell shaped curve is typical of polygenic traits

Graph from BIOLOGY by Miller and Levine; Prentice Hall Publshing©2006

EVOLUTION OF POPULATIONSEvolution as Genetic Change 16–2

Each time an organism reproduces, it passes its genes on to the next generation.

So __________ = success in passing on genesFITNESS

http://www.cleanfunny.com/pics/animal-giraffe-mother-baby-kiss-kissing.jpg

EX: A population of normally brown lizards. Mutations produce new color choices.

If red lizards are more visible to predators, they

might be less likely to survive.

____________________ on single-gene frequencies can lead to changes in ____________________ and thus to EVOLUTON

NATURAL SELECTION

Bird image from: http://www.germanlis.com/creatures/TN_bird_eating_fish.JPGChart from BIOLOGY by Miller and Levine; Prentice Hall Publishing ©2006

ALLELE FREQUENCIES

Black lizards absorb more heat to warm up faster on cold days so they can move faster to get food and avoid predators. The allele for black may increase in frequency.

Chart from BIOLOGY by Miller and Levine; Prentice Hall Publishing ©2006

When traits are controlled by _______ than one gene, the effects are more complex.

Remember ______________ traits show a bell-curve distribution

more

POLYGENIC


The ___________ of individuals near each other will not be very different, but fitness may vary from one end of curve to the other.

FITNESS


Where fitness varies, ________________ can act!

NATURAL SELECTION

Natural selection can affect thedistribution of phenotypes in 3 ways:

______________________

______________________

______________________

DIRECTIONAL selection

STABILIZING selection

DISRUPTIVE selection

•DIRECTIONAL SELECTION


Individuals at _____________ of the curve have higher fitness than individuals in middle or at other end.

Graph shifts as some individuals fail to survive at one end and succeed and reproduce at other

ONE END

Low mortality, high fitness

High mortality, low fitness

KEY

Food becomes scarce.

Beak size varies in a population

Birds with bigger beaks can feed more easily on harder, thicker shelled seeds.

Suppose a food shortage causessmall and medium size seeds to run low.

Birds with bigger beaks would be selected for and increase in numbers in population.

EXAMPLE OF DIRECTIONAL SELECTION

http://www.animalbehavior.org/ABS/Stars/ONI/Podos_-_finch_graphic.jpg

•STABILIZING SELECTIONGraph from BIOLOGY by Miller and Levine; Prentice Hall Publshing©2006

Individuals in _____________ of the curve have higher fitness than individuals at either end

Graph stays in same place but narrows as more organisms in middle are produced.

CENTER

Key

Per

cen

tag

e o

f P

op

ula

tio

n

Brightness of

Feather Color

Selection against both

extremes keep curve narrow and in same

place.

Section 16-2

Low mortality, high fitness

High mortality, low fitness

Stabilizing SelectionMale birds use their plumage to attract mates. Male birds in the population with less brilliant and showy plumage are less likely to attract a mate, while male birds with showy plumage are more likely to attract a mate.

Male birds withshowier, brightly-colored plumagealso attractpredators, and areless likely to livelong enough tofind a mate.The mostfit, then, is the malebird in the middle--showy, but not tooshowy.

STABILIZING SELECTION


Human babies born with low birth weight are less likely to survive.

Babies born too large have difficultybeing born.

Average size babies are selected for.

EXAMPLE OF STABILIZING SELECTION


•DISRUPTIVE SELECTIONGraph from BIOLOGY by Miller and Levine; Prentice Hall Publshing©2006

Individuals at _____________ of the curve have higher fitness than individuals in middle.

Can cause graph to split into two. Selection creates __________________PHENOTYPES

EXTREMES

TWO DISTINCT

Suppose bird population lives in area where climate change causes medium size seeds become scarce while large and small seeds are still plentiful.

Birds with bigger or smaller beaks would have greater fitness and thepopulation may split into TWO GROUPS. One that eats small seeds and one that eats large seeds.

EXAMPLE OF DISRUPTIVE SELECTION


In a small population this random change in allele frequency based on chance is called _________________GENETIC DRIFT

Genetic drift can occur when a _______ group of individuals colonizes a _____habitat.

small

new

http://www.ucmp.berkeley.edu/fosrec/Filson.html

Individuals may carry alleles in ______________ relative frequencies than in the larger population.

The population they “found” will be different from the parent population

. . . not throughnatural selectionbut by _________

different

chancehttp://www.ucmp.berkeley.edu/fosrec/Filson.html

A situation in which allele frequencies change as a result of themigration of a small subgroup of thepopulation = _________________FOUNDER EFFECT


ARE THERE ANY CONDITIONS IN WHICH EVOLUTION WILL NOT OCCUR?

IS THERE A WAY TO TELL IF THIS IS HAPPENING?

__________________________HARDY-WEINBERG PRINCIPLE


Hardy & WeinbergWho?

Godfrey Hardy1877-1947

Wilhelm Weinberg1862-1937

They developed an equation that predicted the relative frequency of alleles in a population based on the frequency

of the phenotypes in a population.

p2 + 2pq + q2 = 1The Hardy-Weinberg Equation

p2 = the frequency of homozygous dominant genotype

2pq = the frequency of heterozygous genotype

q2 = the frequency of homozygous recessive genotype

HARDY-WEINBERG PRINCIPLE states that allele frequency in a population will remain __________ unless one or more ________ cause the frequency to __________.

In a situation in which allele frequencies remain constant

( = _________________ ) populations will NOT EVOLVE!

constant

Genetic equilibrium

factorschange

5 CONDITIONS REQUIRED TO MAINTAIN GENETIC EQUILIBRIUM FROM GENERATION TO GENERATION 1. _________________________ 2. _________________________ 3. _________________________ 4. _________________________

5. _________________________

Must be random mating

Population must be large

No movement in or out

No mutations

No natural selection

In order for ______ ______ to occur, all members of the population musthave equal opportunity to produceoffspring.

In natural populations, like____, ______, ___, or _____________,members compete or even fight for theopportunity to mateso mating is_______________

Random mating

lions wolves elk mountain sheep

http://www.wasatchcomputers.net/gallery/elk_fight.jpg

NOT RANDOM

Mating in populations is rarely ________

Many species select mates based on certain _______ such as size or strength.

randomhttp://ideiasemdesalinho.blogs.sapo.pt/arquivo/Peacock%20with%20its%20tail%20fanned%20out_Tony%20Ruta.jpg

traits

For _______ __________ to occur, apopulation must be ________, so ____________ doesn’t cause changes in allele frequency by random chance.

genetic equilibrium

genetic driftlarge

http://www.sturgisrallydaily.com/gallery/full/crowd.jpg

________________ can occur, since movement in and out of the population __________ the frequency of ______. changes alleles

http://whiteafrican.com/wp-content/Wildebeast%20Migration-East%20Africa.jpg

NO MIGRATION

For _______ __________ to occur,there must be ____________, whichintroduce new ______, and________________ can take placewhich gives any one _________ asurvival advantage over another.

genetic equilibriumNO mutationsalleles

NO natural selectionphenotype

Evolution vs. Genetic EquilibriumIn some populations, these conditions

may be met or nearly met over longperiods of time, and little or no________ occurs.

BUT in most populations it is _______for ____ conditions of Hardy-Weinbergto be met.

difficult

evolution

ALL

In MOST populations . . .

EVOLUTION happens !


16-3 The Process of Speciation16-3 The Process of Speciation


16-3 The Process of Speciation• Natural selection and chance events can

change the relative frequencies of alleles in a population and lead to speciation.

• Speciation is the formation of new species.• A species is a group of organisms that breed with one another and produce fertile offspring IN NATURE.


Isolating Mechanisms

– The gene pools of two populations must become separated for them to become new species.


• As new species evolve, populations become reproductively isolated from each other.– When the members of two populations cannot interbreed and produce fertile offspring, reproductive isolation has occurred.

• Reproductive isolation can develop in a variety of ways, including:

• behavioral isolation• geographic isolation• temporal / seasonal isolation


Behavioral Isolation• Behavioral isolation occurs when two populations are capable of interbreeding but have differences in courtship rituals or other reproductive strategies that involve behavior.


Geographic Isolation• Geographic isolation occurs when two populations are separated by geographic barriers such as rivers or mountains.

• Geographic barriers do not guarantee the formation of new species.


Temporal Isolation• Temporal isolation or seasonal isolation occurs when two or more species reproduce at different times of the year.


Speciation in Darwin's FinchesSpeciation in the Galápagos finches occurred by:

– founding of a new population– geographic isolation– changes in new population's gene pool– reproductive isolation– ecological competition


Founders Arrive

A few finches—species A—travel from South America to one of the Galápagos Islands.

There, they survive and reproduce.


Geographic Isolation

Some birds from species A cross to a second island.

The two populations no longer share a gene pool.


Changes in the Gene PoolSeed sizes on the second island favor birds with large beaks.

The population on the second island evolves into population B, with larger beaks.


Reproductive Isolation• If population B birds cross back to the first island, they will not mate with birds from population A.

• Populations A and B have become separate species.


17-4 Patterns of Evolution


Macroevolution

• Macroevolution refers to large-scale evolutionary patterns and processes that occur over long periods of time.

• Five important topics in macroevolution are:– extinction– adaptive radiation– convergent evolution– coevolution– gradualism & punctuated equilibrium


1) Extinction: when there are no longer any living members of a species. More than 99% of all species that have ever lived are now extinct!


• Mass Extinction = when large numbers of species disappear– Most scientists agree that most mass extinctions were

caused by several factors:• Volcanoes• Continental drift• Changing sea levels

• The result is the remaining species now have new niches (jobs) to fill, and may then thrive causing bursts of evolution that produces many new species.


• 2) Adaptive Radiation = the process by which a single species or a small group of species evolves into several different forms that live in different ways. – Adaptive radiation is an example of divergent evolution. Adaptive radiation is the kind of divergent evolution when one species diverges into multiple, instead of just 2, species.


3) Convergent Evolution: the process by which unrelated organisms come to resemble one another, due to living in similar environments.

– They develop structures that look and function similarly, but are made up of different parts = analogous structures.

– EX: A dolphin’s fluke and a fish’s tail fin are analogous structures.


• 4) Coevolution = when organisms that are closely connected to one another by ecological interactions evolve together. The two species evolve in response to changes in each other over time.– Bees and the flowers that they pollinate


• 5) Gradualism and Punctuated Equilibrium• Evolution has often proceeded at different rates for

different organisms at different times during the history of life on Earth.

• 2 patterns of evolution:– * gradualism (Darwin’s theory)– * punctuated equilibrium

• Newer theory, still debated


• 5) Gradualism:• Darwin felt that biological change was slow and steady, an idea known as gradualism.


5) Punctuated equilibrium is a pattern of evolution in which long stable periods are interrupted by brief periods of rapid change.


Punctuated equilibrium gradualism

Copyright Pearson Prentice Hall 5-1 How Populations Grow.

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