a7 Wk1 Notes

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ANTH 7:How Humans EvolvedWEEK 1

Jeff SnyderUCLA Center for Behavior, Evolution, and

CultureDepartment of Anthropology

OUTLINE: Chapter 1

• Darwin in historical context

• Darwin’s enduring idea: Darwin’s Postulates

• How can a natural selection produce l i ?complexity?

• Evolution can be quite rapid

• Darwin’s difficulties

OUTLINE: Chapter 1






Science in Darwin’s Time

• Earth sciences and biology were derived from biblical (Old Testament) accounts and Greek/Roman philosophy– Predictions from a literal translation of the Bible

• The Earth is relatively young (6 days of creation)

• The Earth should be marked by one cataclysm/catastrophe – Noah’s flood

• Each species is one fixed type (Adam named them, Noah preserved them on the ark)

– Biology as understood from Greek/Roman philosophy• Species are fixed types permanently distinguished from one another

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Direct observation of the world did not match these traditional views

• Geology revealed– Many layers of extinct life – the Earth looked like a series of cataclysms– A sequence of fossil types fromq yplower layers to higher layers

• Mammals• Reptiles and birds• Fish• Invertebrates

Direct observation of the world did not match these traditional views

• Biology together with Geology (paleontology) revealed– Related anatomical forms– Fossils of intermediate formsMany species have the remnants of anatomical structures– Many species have the remnants of anatomical structures that had no apparent use (vestigial organs)

Related anatomy of living animals and intermediate fossil forms

Organisms are similar in ways that are unrelated to function

• But, all have the same basic skeletal plan!

Bats flyDugongs swim Moles dig

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Science in Darwin’s time

• Darwin was not the first to suggest evolution –he was the first to suggest evolution through natural selection

• Previous versions of evolution also attempted• Previous versions of evolution also attempted to explain related anatomical forms and intermediate fossil forms but were not logically tenable ideas and would not be supported by empirical evidence.

Lamarckism (Jean‐Baptiste Lamarck) was the most enduring incorrect version of

evolution• Ladder of progression from simple life‐forms to higher (better) life forms with humans at the top of the Great Chain of Life

• Acquired characteristics/traits are passed on to offspring – Eg: Cheetahs that run fast in their lifetime acquire strong leg muscles so offspring of these cheetahs acquire strong leg muscles from their parents

– Hertability and genetics does not work this way

Charles Darwin challenged conventional view

• Son of upper class English doctor

• Medical school dropout at Edinborough

• Mediocre divinity student at Cambridge

• But,…avid naturalist & collector

Darwin was recommended for position of ship’s naturalist

• Sailed on the HMS Beagle• Under command of Capt Fitzroy

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Darwin’s journey was a transforming experience

• Ship’s mission was to chart coast of S. America

• Voyage lasted 5 years (1832‐36)

• Made extensive collection of plants, animals

• Kept a detailed journal of his research and ideas

• Darwin was deeply affected by the diversity of life he encountered

Darwin was fascinated by his duties

• Made extensive collection of plants, animals

K t d t il d j l f• Kept a detailed journal of his research and ideas

• Darwin was deeply affected by the diversity of life he encountered

Darwin returned to England in 1836

• Settled in London

• Courted Emma Wedgwood

• Cataloged his collections

• Established network of scientific colleagues

• Sought an explanation of the origin of species

OUTLINE: Chapter 1






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By 1838, Darwin formulated outline of theory

1. The ability of a population to expand is infinite, but the ability of the environment to support populations is always finitepopulations is always finite.

2. Organisms within populations vary, and this variation affects ability of organisms to survive and reproduce.

3. Variations are transmitted from parents to offspring

Thomas Malthus

Birds of the Galapagos show us how natural selection works

Daphne Major

Medium Ground Finch

Peter Grant Rosemary Grant

Postulate 1: Environment constrains population growth

• Severe drought occurred 1976‐78

• Drought affectedDrought affected seed availability

• Many birds died of starvation

Postulate 2: Individuals vary in ability to survive and reproduce

• Beak size varies

• Small beaked birds have trouble with large seeds

• During drought, larger beaked birds were at advantage

Before Selection

Frequencyof each

Beak Depth

Beak Depth

After Selection

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Postulate 3: Variation is transmitted from parents to offspring

• Beak size is inherited

Characteristics of population changed over time

• Large beaked adults survived better

• Large beaked birds had large beaked offspring

• Mean beak size increased in population

Natural selection can produce change or maintain status quo

• Small beaked birds can’t find enough food

• Large beaked birds have higher juvenile mortality

• Selection favors intermediate beak size

• At equilibrium, selection will maintain stasis (no change)

Selection is always active

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Important points from biological evolution

• Selection occurs on individuals that are competing with other individuals within populations

• Not only competing for survival, but also reproductive success

• Fitness is context specifici i t th i– …in comparison to others in group

– …at a specific place and time (natural selection does not have a final goal/optimal in mind – fitness is more offspring, now; or survive now until reproduction is possible)

– …sometimes at the expense of others regardless of the moral/ethical implications

• Hence, selection maximizes individual fitness – not group fitness

Individual vs. Group Selection

• Group selection used to be widely believed and cited as the cause of many adaptations

• Wynne‐Edwards argued in favor of group selection in the early 1960’s based on evidence of reproductive restraint in birds– Many birds do not lay as many eggs as they are able to

– Wynne‐Edwards argued that this behavior was for the good of the species or local group.

– Too many birds could equal starvation for the group

Individual vs. Group Selection

• Lack’s response: Optimal clutch size• He asked if these birds would actually increase their fitness by laying “more eggs” when able.– Provisioning for nestlings requires time and energy– When there are too many nestlings, all chicks in the brood y g ,risk starvation

– Hence selection favors the optimal number of offspring• Too few individuals outcompeted• Too many yields lower fitness as well

– The optimal clutch size can vary according to yearly conditions and food availability

OUTLINE: Chapter 1






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Natural selection explains the spread of favorable types

• Text book example: the salt and pepper moth in England

• Originally mottle brown like oak bark

• Coal burning caused bark to be black

• Black morph spread because it was camouflaged

• But note: adaptation arose by chance, spread by natural selection

How can natural selection generate complex adaptations?

• Random processes seem unlikely to produce complex adaptations– If a hurricane blows through a junk yard…

Random changes can’t give rise to complex structure

• Can proverbial monkeys at a typewriter produce Hamlet?• The answer is NOT BLOODY LIKELY. • To see why lets give them a simpler problem. The single sentence

methinks it is like a weasel

• 27 characters (A...Z and space key)• Chance of typing first letter correct = 1/27• Chance of typing 1st and 2nd letters correct = 1/27 x 1/27• Chance of typing all 28 letters correctly = (1/27)28 ≈ 10‐40

• Lifetime of the universe = 1017 seconds

Natural selection allows for cumulative retention of small changes

methinks it is like a weasel

First try: wdlmnlt dtjbkwirzrezlmqco p

10th try: mdldmnls itjiswhrzrhez mecs p

20th try meldinls it iswprke z wecsle20 try meldinls it iswprke z wecsle

30th try methings it iswlike b wecsel

40th try methinks it is like I weasel

43rd try methinks it is like a weasel

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Adaptations evolve in many small steps each favored by natural selection

• Small changes are much more likely to be adaptive.

• Complex adaptations arise through many small steps.

Limpet:Directional information

Beyrich Split Shell:

Better directional i f i

g y p

• Each step favored by selection

Abalone:

Crude image

Spiny Murex:

Better image

information

OUTLINE: Chapter 1






Evolution can produce rapid change

• All dogs descended from wolves

• Dogs were domesticated ≈ 150,000 yrs ago in Asia

• Most breeds domesticated in last few thousand years

Evolution can produce rapid change

• Examples in natureRadiation of Darwin’s finches

(≈ 500,000 yrs)Radiation of African cichlids

( 12 000 )(≈ 12,000 yrs)

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OUTLINE: Chapter 1






Darwin generated a storm of controversy

• Origin of Species a best seller• Only one line about humans, but…

• Many vocal critics make connection explicitconnection explicit

• Reasons for resistance complex– Religious objections– Threatened view of universe as morally structured

– Scientific criticisms of the theory

Bishop Wilberforce

T. H. Huxley

Darwin could not explain variation

• Fleeming Jenkin – Brilliant engineer helped designed first

transatlantic cable

– Fundamental contributions to economics

– Collaborated with Wm Thompson basic

Fleeming Jenkin

physics

• Jenkin raised two objections– How is variation maintained?

– Where do novel variants come from?

Blending of maternal and paternal contributions to offspring destroys

variation• Most Victorians believed in “blending inheritance” – Mother and father both contribute factors to

the offspring– Offspring are averageOffspring are average– Explains why offspring are intermediate

between parents

• Blending inheritance destroys variation– Offspring of extreme parents are intermediate

– Offspring of intermediate parents are intermediate

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Selection can only cull existing variants. Where do novelties come from?

• The Chihuahua problem– All dogs come are descended from wolves

– No wolf is as small as Chihuahua

– How do we get Chihuahuas– How do we get Chihuahuas by subtracting from wolves?

• A big problem because Darwin’s account of complex adaptations depends on the existence of novelties (traits not seen before)

+

Darwin’s critics worried him

• Darwin was ignorant of real mechanics of inheritance– Believed in inheritance of acquired variation

– Knew environment causedKnew environment caused organisms to vary

– Thought might introduce new inherited variation

• Darwin wasn’t able to resolve these problems

OUTLINE: Chapter 2

• Mendel’s experiments

• Understanding Mendel’s observations with modern genetics

Th i li i f l i D i ’• The implications for solving Darwin’s difficulties

• Molecular genetics

OUTLINE: Chapter 2





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The real nature of inheritance had already been discovered by Gregor Mendel

• Born in Silesia, part of Austro‐Hungarian empire, to peasant parents

• Brilliant student, but collapsed during exams

• Joined monastery in Brno• Conducted systematic experiments on y p

inheritance in garden peas, 1856‐63

Mendel’s experiments

• Focused on traits that have 2 forms– Green/yellow seeds

– Smooth/wrinkled seeds

• Cultivated true breeding linesCultivated true breeding lines and then studied results of crosses

• Kept careful records

Mendel crossed two true‐breeding lines

• Cross green × yellow parents

• All the offspring were yellow

• Second generation, 3:1 ratio

• NOTICE – not blending

• Then he crossed the offspring

To explain the pattern of results, Mendel induced two rules

• Characteristics of organisms are determined jointly by “particles” inherited from each

tparent

• Particles from each parent are equally likely to be transmitted to offspring

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Mendel’s work was largely unknown for 50 years

• Mendel published his results• Set aside experiments to become abbot• Mendel’s paper was found in Darwin’s library?

OUTLINE: Chapter 2





Terms• Gene – a segment of chromosomal material that produces a recognizable effect on phenotype and segregates as a unit during gamete formation

• Allele – one of two alternative forms of a gene

• Dominant – describing an allele that results in the same phenotype in the homozygous and heterozygous form– Convention is that alleles for dominant traits are in upper case

• Recessive – describing an allele that that is expressed in the phenotype only in the homozygous state – Convention is that alleles for recessive traits are in lower case

• Single locus genes: AA, Aa, aa

• Examples of alleles: “A” (dominant) or “a” (recessive)

More terms

• Homozygous – same allele at the same locus

– Homozygous dominant – having two dominant alleles of the same gene (example = “AA”)

– Homozygous recessive – having two recessive alleles of the same gene (example = “aa”)

• Heterozygous –two different alleles at the same locus (example =• Heterozygous –two different alleles at the same locus (example = “Aa”)

• Genotype – the combination of alleles that characterizes an individual at some set of genetic loci

• Phenotype – the observable characteristics (traits) of an individual. NOTICE: two individuals may have the same phenotype but differ in genotype

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Terms

• Somatic cell = a cell which forms the body; any cell which is not a gamete (sex cell)

• Diploid = describes that a cell has pairs of each chromosome; pairs chromosomes are calledchromosome; pairs chromosomes are called homologous pairs; somatic cells are diploid

• Haploid = describes that a cell has only one copy of a chromosome(s); sex cells are haploid

Terms

• Mitosis = the process of division of somatic cells, producing new diploid cells

• Meiosis = the process of cell division which produces haploid gametesproduces haploid gametes

• Gametes = sex cells; eggs and sperm

• Zygote = a fertilized ovum (egg)

Modern genetics explains Mendel’s laws

• Cell contains nucleus

• Nucleus contains chromosomes

• Chromosomes come in pairs

• When somatic cells divide, chromosomes are doubled

• Each daughter cell has same complement as original cell

mitosis

Gametes are different than somatic cells

• Gametes (eggs and sperm) have only 1 copy of each chromosome

• When gametes fuse, they create zygote with full set

meiosis

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Discoveries in genetics fit with Mendel’s laws

• Characteristics of organisms are determined jointly by particles inherited from each parent– Particles = genes

– Genes are carried on chromosomes

– Gametes carry single copies of parental chromosomes

• When gametes are formed, variants of genes equally likely to be transmitted

Walter Sutton

With genes, we can retrace the path through Mendel’s garden

• Variants of genes at single locus are called alleles

• Follow distribution of alleles across generationsacross generations

• Use laws of probability to compute how alleles will be distributed

The convention is to label each generation

• F0 = original parents (start point)

• F1 = offspring of original parents

• F2 = next generation, etc.

F0: cross true breeding lines

• Yellow parents produce gametes with yellow alleles (A)

• Green parents produce gametes with green alleles (a)

• Offspring inherit one gameteOffspring inherit one gamete from each parent (all Aa)

• Genetic composition of offspring is not same as either parent (genotype)

• Offspring look same as yellow parent (phenotype)

• Yellow allele is “dominant”

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Phenotype and genotype may differ

Genotype Phenotype

AA Yellow

Aa Yellow

aa Green

F1: Cross offspring

• Each F1 parent produces equal numbers of each allele

⇒F2 have 50% chance of drawing each allele gfrom each parent

⇒Ratio of genotypes1 AA : 2 Aa : 1 aa

⇒Ratio of phenotypes 3 yellow: 1 green

Why sampling gametes yields a 1:2:1 ratio of genotypes

.5 x .5 = .25

2(.5 x .5) =.50

.5 x .5 = 0.25

SETTING UP A PUNNETT SQUARE

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Insert genotypes of breeding individuals into top row and left column

“Parents”

One allele per row/column. (In this case, both parents are heterozygous; that is, Aa)

A a

A

a

A a

A AA

a

A a

A AA Aa

a

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A a

A AA Aa

a Aa

A a

A AA Aa

a Aa aa

OUTLINE: Chapter 2





Why does this matter?

• It’s part of the explanation of how variation is maintained

• Discrete (individual) particles of inheritance (genes/alleles) do NOT blend the way Victorians thoughtthought

• Variation is hidden in as recessive alleles in heterozygous individuals– Selection acts on the expressed trait – the individual’s phenotype

– But heterozygous individuals are carrying the allele that codes for an alternate phenotype

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Mendel also studied other traits

• Experimented with 2‐traits– Color: Yellow or Green

– Texture: Smooth or Wrinkled

• Four combinations of traits– Yellow, smooth

– Yellow, wrinkled

– Green, smooth

– Green wrinkled

Mendel’s experiments led him slightly astray

• Seed texture and seed color transmitted (segregate) independently

• Mendel thought this was true of all traits

• Only traits on different chromosomes segregate independently

Genes on same chromosome are linked

• Genes for particular trait occur at particular site on chromosome (locus)

• Loci are lined up like

Chromosome I Chromosome II

Seed texture

Loci are lined up like beads on string

• Genes on same chromosome tend to stay together

• This is called linkage

loci

Seed color

Genes on same chromosome canrecombine

• Sometimes chromosomes tangle and break during meiosis

• Bits of one chromosome can be swapped between members of homologous pair – crossing over/recombination

• The closer together two loci are, the more likely they are to stay together

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Why does this matter?

• It’s also part of the explanation of how variation is maintained

• Recombination/crossing over produces chromosomes (in gametes) with combinations of genes not present in the parent

• Therefore, offspring may have a different combination of traits (a novel combination) than either parent

• Sexually reproducing species can evolve at a faster rate than asexually reproducing organisms

OUTLINE: Chapter 2





Genes are DNA• Watson, Crick, Wilkins and Franklin

discover structure of DNA• Two sugar‐phosphate backbones

joined by sequence of bases– A = Adenine– T = ThymineT = Thymine– G = Guanine– C = Cytosine

• Vast number of sequences with same stability

⇒DNA can encode vast number of different messages

• Weak bonds make replication easy

What does DNA do?

• Some genes code for proteins– Proteins are manufactured in cells

– Some genes are “instructions” on how to build a specific proteinspecific protein

• Some genes have regulatory functions– “instructions” for where, when, and which proteins will be manufactured

– “instructions” for development

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