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Lecture 5/23

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About myself

• 林雨德

• Dept. of Life Sciences

• kirklin@ntu.edu.tw

• Office hour: by appointment

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About the course

• I will give the next 5 lectures for this course.• You will turn in an exercise sheet every

lecture.• There will be a quiz every week on the

material from the previous week. The last quiz will be given in the final exam.

• All quizzes will be in English, and take about 15 minutes.

• There is no final exam per se for my lectures.

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About discipline• You need to come to lectures to earn credits

for quizzes and exercises.• I will not give make up quizzes, unless you

can provide official documents that justify your absences.

• Coming in late, leaving early, or having a cell phone ringing are annoying behaviors. Be respectful.

• All lectures will be posted on website after the lecture.

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Get a piece of paper, print your name and ID#,

and be ready to write down the answers to the exercises during the lecture.

Do not look up answers in the textbook.I want your own thoughts.

You are free to discuss with your neighbors.

Turn in the paper at the end of class.

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Genes Within Populations

Chapter 21

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Exercise: Why are there so many different species?

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God must be very fond of beetles!

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Charles Darwin

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Falkland IslandsMarch 1833March 1834

April 1832

Jan. 1836

Sept. 1835GalapagosIslands

July 1834

Dec. 1831

Oct. 1836

Darwin’s 5-year journey on HMS Beagle

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Darwin’s historical visit to the Galapago islands

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Darwin saw thateach island had its own kind of giant tortoises.

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Darwin saw thatsimilar finches had different types of beaks.

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Falkland IslandsMarch 1833March 1834

April 1832

Jan. 1836

Sept. 1835GalapagosIslands

July 1834

Dec. 1831

Oct. 1836

Darwin’s other observations & collections over the 5-year journey

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Species must have changed with time in respond to the environments.

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Evolution: change through time(decent with modification)

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How do species changed through time.

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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Lamarck's hypothesis – variation is aquired

stretching stretching

reproduction

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Jean Baptiste Lamarck

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• Acquired traits (e.g., bigger biceps) do not change the genes transmitted by gametes to offspring.

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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

reproduction

reproduction

reproduction

Darwin's hypothesis – variation is inherited

growth

to adult

growth

to adult

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• Individuals within a population differ among each other. (individual variations)

• Populations grow rapidly, yet natural causes (food, predation, etc.) set limits.

• Individuals with advantageous traits are selectedover less ones. (differential reproduction & survivorship)

• Individuals with advantageous traits produce offspring with advantageous traits. (variation is inherited)

• Thus, the population change through time.

The process of natural selection

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Not only could populations change with time, but over long period of time new species could come in place.

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Exercise: How do finches evolve into different species?

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1858--Alfred Wallace had a similar thought as Darwin’s.

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1859--Darwin published ‘On the Origin of Species by Means of Natural Selection’.

The controversy started.

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Populations of living things change with time (microevolution).

Gradual accumulation of changes leads to speciation: the occurrence of new species (macroevolution).

Evolution: Change through time

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Individuals within a population differ from each other. HOW?

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– Population = members of the same species that occupy the same area.

– Each new generation, genetic mechanisms (mutations, independent assortment, crossing–over, random recombination) create new variations of individuals within a population.

Genetic variation in nature

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– The proportion of different variations may change over time (i.e. microevolution), due to various reasons.

– One of the reason is that different variations do well under different environment. That is,the environment (nature) is a selection agent-- natural selection.

Microevolution

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darkercoloration

lightercoloration

Natural selection –Different phenotypes/genotypes do well under different environments

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What are phenotypes & genotypes?

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• Individuals within a population differ in their phenotypes and genotypes. (individual variations)

• Populations grow rapidly, yet natural causes (food, predation, etc.) set limits.

• Advantageous phenotypes are selected over less advantageous ones. (differential reproduction & survivorship)

• Alleles conferring advantageous phenotypes increase in frequencies. (frequencies of different genotypes changed over time).

• Individuals with advantageous phenotypes and genotypes. produce offspring with advantageous phenotypes and genotypes. (variation is inherited)

The new synthesis of natural selection

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Survival of the Fittest

• Fitness is generally defined as the number of surviving offspring left in the next generation. It is a relative measure. Natural selection favors phenotypes with the greatest fitness.

• Differential reproduction & survivorshipis the key feature of natural selection.

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50

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200

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0

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12 13 14 15 16

12 13 14 15 16

12 13 14 15 16Length of adult female water strider (mm)

Num

ber o

f egg

s la

id p

er d

ayLi

fe s

pan

of a

dult

fem

ale

(day

s)N

umbe

r of e

ggs

laid

dur

ing

lifet

ime

Several fitness measurement

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Individuals do not evolve, populations do.

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Natural section is only one of the mechanism that may cause the evolution

of populations.

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Allele frequencies may change through time due to following mechanisms…

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The 5 mechanisms of evolutionary change-- Mutation

• Mutation creates new alleles, thus change allele frequencies.

• It’s the ultimate source of genetic variations.

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T

AG

G

G

GC

C

UV light DNAAllele A

Allele A no more

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• Movement (emigration & immigration) of alleles from one population to another change allele frequencies.

The 5 mechanisms of evolutionary change-- Gene flow

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Allele A

Allele B

CarryAllele A

+ Allele A

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• Assortative mating - phenotypically similar individuals preferentially mate with each other, and causes frequencies of particular alleles to increase disproportionately.

The 5 mechanisms of evolutionary change-- Nonrandom mating

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Self-fertilization

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• Frequencies of particular alleles may change by chance alone.

– bottleneck effect - drastic reduction in population size

– founder effect - few individuals found a new population

The 5 mechanisms of evolutionary change-- Genetic drift

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Genetic Drift - Bottleneck Effect

Original Reduction Rebound New

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Genetic Drift - Founder Effect

Bye!

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Consequences of natural selection-- some examples

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Coat color matching to avoid predators

Pocket mice in the Tularosa Basin of New Mexico

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1.0

0.8

0.6

0.4

0.2

44 42 40 38 36 34 32 30Latitude (degrees North)

Enzyme types change to optimize metabolism

Mummichog along the coast of North America

Freq

uenc

y of

col

d-ad

apte

d al

lele

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Pesticidemolecule

Insect cellmembrane

Target site

Target site

Insect cells with resistance allele at pen gene:decreased uptake of the pesticide

Insect cells with resistance allele at kdr gene:decreased number of target sites for thepesticide

Resistanttarget site

Cell membrane target site change to increase

resistance to insecticide.

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Exercise: Reconsider this question.How do finches evolve into different species?

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Exercise: Do they increase, maintain, or decrease genetic variation?

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You got to have variation to have evolution

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Variation is the spice of life !

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Natural selection can maintain variation, sometimes

• Negative frequency-dependent selection– Phenotype fitness depends on its frequency

within the population. If selection favors rare phenotypes, variation will be maintained.

• Oscillating selection– Selection favors different phenotypes at

different times.

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20 40 60 80 100

Color form frequency

Dark brownMedium brownLight brown

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Per

cent

of c

olor

form

take

n by

fish

pre

dato

rs

Negative frequency-dependent selection

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Three Modes of Natural Selection

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Modes of Natural Selection

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DIRECTIONALSELECTION

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Selection on beak size of finches

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beak depth

1976

1978Averagebeak depth,1978

Averagebeak depth,

1976

Beak depth (mm)

Shift of average beak depth during drought

5 6 7 8 9 10 11 12 13 140

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Num

ber

of in

divi

dual

s

Selection on beak size of finches

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711977 1980 1982 1984

Dry year

Dry year

Dry year

Wet year

Bea

k de

pth

Selection on beak size of finches

72Mean beak depth of parents (mm)

Medium ground finch8

8 9 10 11

9

10

11

Bea

k de

pth

ofof

fspr

ing

(mm

)

Beak size variations are heritable

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Peppered Moths and Industrial Melanism

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Industrialization favored melanism

• Until the 19th century, peppered moths had predominately light-colored wings. Subsequently, dark individuals became predominant because industrial smog helped turn lichens on tree trunks dark.

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Pollution control reversed the trend

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STABLIZINGSELECTION

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1020305070100

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2 3 4 5 6Birth weight in pounds

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Perc

ent o

f birt

hs in

pop

ulat

ion

Percent infant mortality

Selection on Birth Weight

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DISRUPTIVESELECTION

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Selection on Color in Guppies

• Guppies are found in small streams in S. America and nearby Trinidad.

– Due to dispersal barriers, guppies can be found in pools below waterfalls with high predation risk, or pools above waterfalls with low predation risk.

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Selection on Color in Guppies

Predator

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Evolutionary Change in Spot Number

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Pocket mice in the Tularosa Basin of New Mexico

Another disruptive selection

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Limits to Natural Selection

• Selection requires genetic variation– Intense selection may remove variation

from a population at a rate greater than mutation can replenish.

• Gene interactions affect allelic fitness (epistatic interactions)

– Selection to increase a good gene may increase a bad gene in the same time.

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Limits to Selection

• Genes have multiple effects (pleiotropy)– Selection to increase a desired effect may

increase an undesired bad effect in the same time.• Selection requires genetic variation

– Intense selection may remove variation from a population at a rate greater than mutation can replenish.

• Gene interactions (epistasis)– Selection to increase an allele, which have good

effect in individual A, but bad effect in individual B.

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Left eyeof insect

Ommatidia

Right eyeof insect

Natural selection can’t work on variations without genetic basis

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1900110

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1920 1940 1960

Year

1980 2000

Ken

tuck

y D

erby

win

ning

tim

e(s

econ

ds)

Natural selection can’t work on traits that run out of genetic variation

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Evolution: Change with time

Do populations of living things change with time?

(Does microevolution occur?)

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Evolution: Change with time

If you don’t believe it, you can go out and measure a population for yourself. There are countless studies that support microevolution.

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Evolution: Change with time

Do populations of living things change with time (microevolution)?

Does gradual accumulation of changes lead to speciation (macroevolution)?

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The problem is

NO ONE HAS WITNESSEDTHE OCCURANCE OF

SPECIATION

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The problem is

ALL WE HAVE ARE CIRCUMSTANTIAL EVIDENCE

THAT SPECIATION OCCURRED

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Who killed this guy?

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Is he the murderer?

No one witnessed the occurrence of the murder.

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But we have circumstantial evidence.

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You are the jury.

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Jury!

Here are the evidence for macroevolution

(speciation).

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The Evidence for Evolution

Chapter 22

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Morphology and function

The 14 finch species in the Galapagos Islands are closely related. Despite the relatedness, beak shapes are morphologically dissimilar. The close correspondence between beak shapes and food types suggested that they were shaped by evolution. Thus, different finch species are the product of evolution.

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Geospizafuliginosa

Geospizaconirostris

Geospizadifficilis

Groundand

cactusfinches

Geospizafortis

Geospizamagnirostris

Geospizascandens

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CamarhynchusparvulusCamarhynchuspsittacula

CamarhynchuspauperCactospizaheliobatesCactospizapallida(woodpecker finch)Platyspizacrassirostris

Treefinches

Vegetariantree finch

Warblerfinches

CerthideafuscaCerthideaolivacea

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Homologous structuresStructures with different appearances and functions that all derived from a common ancestor. Thus, different life forms are the product of evolution.

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Artificial selection

Artificial selection can create drastically different morphology from the original ones. Morphology could be so different that they are hardly the same species. Natural selection, as a similar process, should be able to do the same.

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Highpopulation

Bristle number in Drosophila0 10 20 30 40 50 60 70 80 90 100 110

Lowpopulation

Initialpopulation

Mea

n

Mea

n

Mea

n

Num

ber o

f ind

ivid

uals

After 35 yrs of artificial selection

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107Teosinte Intermediates Modern corn

Artificial selection of agriculture plants

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Greyhound Mastiff

DachshundChihuahua

Domestication of wolves

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Simpler organisms are found in older rocks; newer rocks contain more complex organisms. It suggests that life forms are not created randomly.

Complexity of life forms

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Complexity of life forms

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Transition of life forms, such as a fossil snake with legs and a fossil aquatic mammal with four legs, provide evidence that life forms transit from one to the next.

Filling in gaps in the fossil record

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Modern toothed whales

Ambulocetus natansprobably walked on land (as do modern sea lions) and swam by flexing its backbone and paddling with its hind limbs (as do modern otters)

Pakicetus attockilived on land, but its skull had already evolved whale characteristics

Rodhocetus kasrani'sreduced hind limbs could not have aided it in walking or swimming. Rodhocetusswam with an up-and-down motion, as do modern whales

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Development (Embryology)

Different organisms exhibit similar embryological forms. It suggests common ancestor.

Early embryo development in all animals goes through similar stages, producing structures not seen in adults of the more complex organisms, e.g., pharyngeal slits in humans.

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Pharyngeal slits in distantly related organisms

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Some structure in the higher organisms do not appear to be better “designed” compared to lower organisms.

Imperfect structures

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Imperfect vertebrate eyes

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Organs with no apparent modern function, but resemble ancestral structures. It provides evidence that life forms transit from one to the next.

Vestigial structures

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Vestigial structures

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The Molecular Similarity

• For a given molecular, distantly related organisms have a greater number of differences than closely related organisms.

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Convergent Evolution

• Convergent evolution - selection that favors changes making two or more groups more similar

– similar results occur under similar selection pressure, for example, marsupial-placental convergence

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Convergent Evolution

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What’s your verdict?

Does gradual accumulation of changes lead to speciation (macroevolution)?

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Exercise:Are you a follower of me now?

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Darwin’s theory on speciation“by means of natural selection” is controversial.

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Is religion itself a product of evolution?

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Is religion itself a product of evolution?

• The God Gene: How Faith Is Hardwired into Our Genes by molecular biologist Dean Hamer.

• Hamer not only claims that human spirituality is an adaptive trait, but he also says he has located one of the genes responsible, a gene that just happens to also code for production of the neurotransmitters that regulate our moods.

• Our most profound feelings of spirituality may be due to little more than an occasional shot of intoxicating brain chemicals governed by our DNA.