how populations evolve - hcc learning web

© 2013 Pearson Education, Inc.Lectures by Edward J. Zalisko

PowerPoint® Lectures forCampbell Essential Biology, Fifth Edition, and

Campbell Essential Biology with Physiology,

Fourth Edition

– Eric J. Simon, Jean L. Dickey, and Jane B. Reece

Chapter 13How Populations Evolve

Figure 13.4

Darwin in 1840

NorthAmerica

Great Britain Europe Asia

Africa

SouthAmerica

Cape of Good Hope

Cape Horn

Tierra del Fuego

Australia

Tasmania

New Zealand

HMS Beagle

ATLANTICOCEAN

PACIFICOCEAN

EquatorEquator

PACIFICOCEAN

Fernandina

Isabela

Pinta

Marchena

Santiago

PinzónDaphne Islands

Genovesa

FlorenzaEspañola

SantaCruz

SantaFe San

Cristobal

40 km

40 miles

0

0

GalápagosIslands

• Natural selection is a process in which organisms

with certain inherited characteristics are more likely

to survive and reproduce than are individuals with

other characteristics.

• As a result of natural selection, a population, a

group of individuals of the same species living in

the same place at the same time, changes over

generations.

CHARLES DARWIN AND THE ORIGIN OF SPECIES

© 2013 Pearson Education, Inc.

• Unequal reproductive success

(natural selection)

– Those individuals with traits best suited to the local

environment generally leave a larger share of

surviving, fertile offspring.

Darwin’s Theory of Natural Selection


Figure 13.15-1

Chromosome with geneconferring resistanceto pesticide

Insecticide application

Figure 13.15-2



Figure 13.15-3


Reproduction

Survivors


5 EVIDENCE OF EVOLUTION

1. the fossil record,

2. biogeography,

3. comparative anatomy,

4. comparative embryology, and

5. molecular biology.


Figure 13.6

Figure 13.7-3

Transitional fossils

Biogeography

• Biogeography, the study of the geographic

distribution of species, first suggested to Darwin

that today’s organisms evolved from ancestral

forms.

• Darwin noted that Galápagos animals resembled

species of the South American mainland more than

they resembled animals on similar but distant

islands.


Figure 13.8

Commonringtailpossum

Red kangaroo

Common wombat

Australia

Koala

distribution of marsupial

mammals in Australia.

Comparative Anatomy

• Comparative anatomy

– is the comparison of body structure between

different species and

– attests that evolution is a remodeling process in

which ancestral structures become modified as

they take on new functions.


Bat wingHuman hand

Figure 13.9

Human

Cat Whale

Bat

Homology is the similarity in structures

due to common ancestry

remodeling of the pattern of

bones forming the forelimbs of

mammals

Comparative Embryology

• Early stages of development in different animal

species reveal additional homologous

relationships.

– For example, pharyngeal pouches appear on the

side of the embryo’s throat, which

– develop into gill structures in fish and

– form parts of the ear and throat in humans.

– Comparative embryology of vertebrates supports

evolutionary theory.


Figure 13.10

Post-anal

tail

Chicken embryo

Pharyngeal

pouches

Human embryo

Molecular Biology

• The hereditary background of an organism is

documented in

– its DNA and

– the proteins encoded by the DNA.

• Evolutionary relationships among species can be

determined by comparing

– genes and

– proteins of different organisms.


Figure 13.11

Percent of selected DNA sequences that match a chimpanzee’s DNA

Chimpanzee

100%96%92%

Human

Gibbon

Orangutan

Gorilla

Primate

Old World

monkey

Evolutionary Trees

• Darwin saw the history of life as analogous to a

tree.

– The first forms of life on Earth form the common

trunk.

– At each fork is the last common ancestor to all the

branches extending from that fork.

– The tips of millions of twigs represent the species

living today.


Figure 13.17

Tetrapodlimbs

Amnion

Feathers

Lungfishes

Mammals

Amphibians

Lizardsand snakes

Crocodiles

Hawks and other birds

Ostriches

Am

nio

tes

Tetra

po

ds

Bird

s

Homologous traitshared by all groupsto the right

2

1

3

4

6

5

Figure 13.17

Tetrapodlimbs

Amnion

Feathers

Lungfishes

Mammals

Amphibians

Lizardsand snakes

Crocodiles

Hawks and other birds

Ostriches

Am

nio

tes

Tetra

po

ds

Bird

s

Common ancestor oflineages to the right

Homologous traitshared by all groupsto the right

2

1

3

4

6

5

Figure 13.UN09

Individual

variation

Overproduction

of offspring

Observations

Conclusion

Natural selection:

unequal reproductive success

Figure 13.14

Variation exists among individuals in a population.

Much of this variation is heritable.

Figure 13.19

• Individual variation

– Variation exists among individuals in a population.

– Much of this variation is heritable.

Darwin’s Theory of Natural Selection


Genetic Variation in Populations

• Individual variation abounds in all species.

– Not all variation in a population is heritable.

– Only the genetic component of variation is relevant

to natural selection.


Figure 13.29

(a) Directional selection (b) Disruptive selection (c) Stabilizing selection

Original

population

Evolved

population

Original

population

Phenotypes (fur color)F

req

uen

cy o

f

ind

ivid

uals

Black allele

Pressure of

natural selection

White allele

THE MODERN SYNTHESIS: DARWINISM MEETS GENETICS

• The modern synthesis is the fusion of

– genetics with

– evolutionary biology.


Populations as the Units of Evolution

• A population is

– a group of individuals of the same species, living in

the same place at the same time and

– the smallest biological unit that can evolve.


Figure 13.18

(a) Two dense populations oftrees separated by a lake

(b) A nighttime satellite view of North America

• The total collection of alleles in a population at any

one time is the gene pool.

• When the relative frequency of alleles changes

over a number of generations, evolution is

occurring on its smallest scale.

Populations as the Units of Evolution


Analyzing Gene Pools

• A gene pool

– consists of all the alleles in a population at any one

time and

– is a reservoir from which the next generation draws

its alleles.

• Alleles in a gene pool occur in certain frequencies.


• Alleles can be symbolized by

– p for the relative frequency of the dominant allele

in the population,

– q for the frequency of the recessive allele in the

population, and

– p + q = 1.

• Note that if we know the frequency of either allele

in the gene pool, we can subtract it from 1 to

calculate the frequency of the other allele.



• Genotype frequencies can be calculated from

allele frequencies (if the gene pool is stable = not

evolving).

• The Hardy-Weinberg formula

– p2 + 2pq + q2 = 1

– can be used to calculate the frequencies of

genotypes in a gene pool from the frequencies of

alleles.



Figure 13.UN10

Frequency of

homozygotes

for one allele

Frequency of

heterozygotes

Frequency of

homozygotes

for alternate allele

Frequency of

one allele

Frequency of

alternate allele

Figure 13.20

Figure 13.21

Allele frequencies

Genotype frequencies

Sperm

Eggs

p 0.8

(R)

q 0.2

(r)

p 0.8

R

q 0.2

r

RR

p 0.8

R

q 0.2

r

p2 0.64

rR

pq 0.16 q2 0.04 rr

pq 0.16 Rr

(RR)

p2 0.64 q2 0.04

(rr)

2pq 0.32

(Rr)

Population Genetics and Health Science

• The Hardy-Weinberg formula can be used to

calculate the percentage of a human population

that carries the allele for a particular inherited

disease.


Microevolution as Change in a Gene Pool

• How can we tell if a population is evolving?

• A non-evolving population is in genetic equilibrium,

also known as Hardy-Weinberg equilibrium,

meaning the population’s gene pool is constant

over time.

• From a genetic perspective, evolution can be

defined as a generation-to-generation change in a

population’s frequencies of alleles, sometimes

called microevolution.


Evolutionary Fitness

• Relative fitness is

– the contribution an individual makes to the gene

pool of the next generation

– relative to the contributions of other individuals.


MECHANISMS OF EVOLUTION

• The main causes of evolutionary change are

– genetic drift,

– gene flow, and

– natural selection.

• Natural selection is the most important, because it

is the only process that promotes adaptation.


Genetic Drift

• Genetic drift is a change in the gene pool of a

small population due to chance.


Figure 13.23-1

RR

rr

Rr

RR

RR

RR Rr

RR

Rr

Rr

Generation 1p 0.7

q 0.3

Figure 13.23-2

Only 5 of 10

plants leave

offspring

RR

rr

Rr

RR

RR

RR

RR Rr

RR

Rr

Rr

rr RR

Rr

rr

RR

Rr

Rr Rr

rr

Generation 1p 0.7

q 0.3

Generation 2p 0.5

q 0.5

Figure 13.23-3

Only 5 of 10

plants leave

offspring

RR

rr

Rr

RR

RR

RR

RR Rr

RR

Rr

Rr

Only 2 of 10

plants leave

offspring

RR

rr RR

Rr

rr

RR

Rr

Rr Rr

rr

RR

RR

RR

RR

RR

RR

RR

RR RR

Generation 1p 0.7

q 0.3

Generation 2p 0.5

q 0.5

Generation 3p 1.0

q 0.0

The Bottleneck Effect

• The bottleneck effect

– is an example of genetic drift and

– results from a drastic reduction in population size.

• Passing through a “bottleneck,” a severe reduction

in population size,

– decreases the overall genetic variability in a

population because at least some alleles are lost

from the gene pool, and

– results in a loss of individual variation and hence

adaptability.


Figure 13.24-1

Original

population

Figure 13.24-2

Original

population

Bottleneckevent

Figure 13.24-3

Original

population

Bottleneckevent

Survivingpopulation

Figure 13.25

• Cheetahs appear to have experienced at least two

genetic bottlenecks:

1. during the last ice age, about 10,000 years ago,

and

2. during the 1800s, when farmers hunted the

animals to near extinction.

• With so little variability, cheetahs today have a

reduced capacity to adapt to environmental

challenges.

The Bottleneck Effect


The Founder Effect

• The founder effect is likely when a few individuals

colonize an isolated habitat.

• This represents genetic drift in a new colony.

• The founder effect explains the relatively high

frequency of certain inherited disorders in some

small human populations.


Figure 13.26

South

America

Tristan da Cunha

Africa

Gene Flow

• Gene flow

– is another source of evolutionary change,

– is separate from genetic drift,

– is genetic exchange with another population,

– may result in the gain or loss of alleles, and

– tends to reduce genetic differences between

populations.


Figure 13.27

Three General Outcomes of Natural Selection

• If we graph the coat color of a population of mice,

we get a bell-shaped curve.

• If natural selection favors certain fur-color

phenotypes,

– the populations of mice will change over the

generations and

– three general outcomes are possible.


1. Directional selection shifts the overall makeup

of a population by selecting in favor of one

extreme phenotype.

2. Disruptive selection can lead to a balance

between two or more contrasting phenotypic

forms in a population.

3. Stabilizing selection favors intermediate

phenotypes, occurs in relatively stable

environments, and is the most common.

Three General Outcomes of Natural Selection


Figure 13.29

(a) Directional selection (b) Disruptive selection (c) Stabilizing selection

Original

population

Evolved

population

Original

population

Phenotypes (fur color)F

req

uen

cy o

f

ind

ivid

uals

Figure 13.UN11

Original

population

Evolved

population

Pressure of

natural selection

Directional selection Disruptive selection Stabilizing selection

Sexual Selection

• Sexual selection is a form of natural selection in

which individuals with certain traits are more likely

than other individuals to obtain mates.

• Sexual dimorphism is a distinction in appearance

between males and females not directly associated

with reproduction or survival.


Figure 13.30

(a) Sexual dimorphism in a finch species (b) Competing for mates

Evolution Connection: An Evolutionary Response to Malaria

• We can see the results of past natural selection in

present-day humans.

• Malaria first emerged as a serious threat to people

in Africa just 10,000 years ago,

– long after humans had established populations

around the globe,

– therefore only producing evolutionary responses in

malarial regions.


• Sickle hemoglobin

– is a mutation that denies the malarial parasite

essential access to human hemoglobin and

– distorts the shape of red blood cells.

• Individuals with one copy of this sickle allele

(heterozygotes) are relatively resistant to malaria.

• Individuals with two copies (homozygotes) are

usually fatally ill.



• In the African tropics,

– malaria is most common and

– the frequency of the sickle-cell allele is highest.



Figure 13.31

Areas with highincidence ofmalaria

Frequencies of thesickle-cell allele

0–2.5%

10.0–12.5%

2.5–5.0%

5.0–7.5%

7.5–10.0%

12.5%

Co

lori

ze

d S

EM

Africa

Asia

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