CHAPTERS 17 & 18

Evolution and Natural Selection

ORIGINS Aristotle, others observed & studied

patterns of living things Alfred Wallace—(Mid 1800s) Studied

patterns of where species lived & how they might be related

Biogeography—study of patterns in the geographic distributions of species and communities

Comparative Morphology—study of similarities in structures of different species

BIOGEOGRAPHY Rhea—South America Emu—Australia Ostrich—Africa

ORIGINS—QUESTIONS Biogeography—why were some animals

and plants similar in different parts of the world?

Comparative morphology—Why were anatomical structures of different animals similar?

Fossils—Why were deeper layers unlike modern species and higher layers more like modern species?

ORIGINS—THEORIES Georges Cuvier—Noted that dramatic changes

happened in fossil species. Theorized that survivors repopulated the earth after great catastrophes.

Jean Lamarck—Offspring inherit traits that a parent acquires in its lifetime.

CHARLES DARWIN Darwin attended

Cambridge University to study theology, had an interest in natural history

1831—Darwin voyaged on HMS Beagle as a naturalist

Read theories challenging the geographical age of the earth

Noticed differences in species

CHARLES DARWIN When returned to England, studied

notes and specimens Noticed that there are similarities

between species, and between living species and fossils

Deduced that any population can produce more individuals than the environment can support. Who lives and who dies?

Noticed that some modifications of traits allow individuals within a species to have a greater chance of survival

CHARLES DARWIN Descent with modification—

traits are modified, then passed along to offspring and down generations

Natural selection—differences in reproduction of individuals within a population based on characteristics related to survival

CHARLES DARWIN 1859—On the Origin

of Species published. Outlined theory of

evolution (descent with modification) based on natural selection

Created great controversy immediately!

EVOLUTIONARY THEORY There are slight variations in traits of a species. These traits are inheritable. Some of these traits increase an organism’s

chances of survival and reproduction. Those individuals who survive and reproduce

pass along their genetic material (“survival of the fittest”).

The offspring are more likely to have the variation of the trait that allowed better survival & reproduction.

Natural selection—individuals with beneficial traits are more likely to survive and pass on these traits

EVOLUTIONARY THEORY With each generation, there are slight

modifications in traits. Over enough time, modifications

accumulate so that eventually the population is very different from the ancestral organisms.

Darwin theorized a gradual, continual change

EVOLUTIONARY THEORY Origin of life

4.4 billion years ago Began with simple

chemicals, gradually became more complex through bonding

Eventually these formed amino acids

As polypeptides formed, eventually became simple life

Primitive organisms in ancient oceans

EVOLUTIONARY THEORY Natural selection

resulted in more complex single-celled organisms Benefit to working

together These eventually

evolved into multi-celled organisms

Organisms changed based on evolutionary pressures

EVOLUTIONARY THEORY Organisms starting

moving onto land Those who had traits

that increased their survival on land evolved

Organisms continued to evolve and modify based on environmental pressures

Most “fit” of each generation would survive

EVIDENCE—FOSSILS Fossilization

Organism becomes covered in sediments or ash

Minerals absorb into bones & tissues

Pressure builds, furthering conversion to minerals

“Soft” tissues not well preserved

EVIDENCE—FOSSILS Stratification

Layers of rock formed by deposits of volcanic ash, silt, sand, etc.

Older layers deeper, newer layers build up on top

EVIDENCE—FOSSILS Radiometric dating

Measure proportions of an isotope in a mineral

Predictable deterioration of isotope, so based on amounts can determine how long since it was formed

EVIDENCE—FOSSILS Older fossils show fewer variety of

organisms Older fossils show more primitive

features Newer fossils show changes and

progression among characteristics New characteristics appear in newer

fossils The complexity of organisms increases

when looking at newer versus older fossils

EVIDENCE—FOSSILS Transitional forms

“Missing links”ArchaeopteryxEustheopteronSeymouria

EVIDENCE—FOSSILS Problems?

Radiocarbon dating may not be accurate Assuming no or limited “daughter” isotopes in parent Assuming constant rate of decay Inaccuracies have been calculated (dating volcanic rock

known to be 200 years old as billions of years old) Some transitional forms questioned

Archaeopteryx likely an extinct species and not a bird ancestor

Fewer than expected transitional forms Living, unevolved “fossils”

Coelacanth— “extinct” 80 million years ago, rediscovered 1938

EVIDENCE—FOSSILS Problems?

Cambrian “explosion” Sudden appearance of numerous fully-formed

species of organisms No transitional forms prior to that No evidence of Darwininan gradualism

EVIDENCE—ORIGIN OF LIFE Several experiments have created

simple organic molecules under “primitive Earth” conditions“Building blocks” for life

Fossilized bacteria Living multicellular colonies

Portuguese Man o’ War

EVIDENCE—ORIGIN OF LIFE Problems?

Dispute and debate over conditions of primitive Earth

No proven mechanism for evolving from simple compounds to primitive cells

Spontaneous generation? “Life” from “unlife”

Many theories, often conflictingReally an unanswered question

EVIDENCE—ANATOMY Homologous structures—similarities in

body parts between groups. Morphological divergence—Variations in

structures of different species based on a basic form in a common ancestor

EVIDENCE--ANATOMY Problems?

Different genes can produce homologous structures Body segments in fruit

flies and waspsThe same gene can

produce non-homologous structures

EVIDENCE—BIOGEOGRAPHICAL

Similar species in different parts of the worldRheas, emus,

ostriches Common

ancestor, separated because of plate tectonics (movement of sections of the earth’s crust)

EVIDENCE—MUTATIONS Theory—mutations of

DNA (insertion, deletion, inversion, translocation, duplication, etc.) can result in new traits or features. These are random events

If these new features give the organism a survival advantage, they are more likely to be passed along.

EVIDENCE—MUTATIONS Mutations do happen (well established) Mutations can be beneficial

Bacterial resistance to antibioticsSickle cell anemia giving resistance to

malariaResistance to atherosclerosis in Italian

village

EVIDENCE—MUTATIONS Problems?

Virtually all mutations are harmful or neutral Many new traits created in lab are not seen in

the wild (fruit flies) New structures do not mean benefit

Second pair of fruit fly wings lack muscles and harm flight ability

Truly beneficial mutations only found in bacteria & other single-celled organisms

Beneficial “mutations” often can be argued to be recessive traits that already exist

Mutations really beneficial? Sickle-cell anemia

EVIDENCE—DNA & PROTEINS Similar DNA sequences in many species The more closely they appear to be

related, the more DNA is sharedHumans & bananas: 50-60%Humans & worms: 75%Humans & chimpanzees: 98%

The less DNA in common, the more distant the common ancestor

EVIDENCE—DNA & PROTEINS Proteins also show similarities between

speciesCytochrome C (part of electron transfer)

Present in all living organisms, great similarity

Said to have been “conserved” across species as evolution took place

EVIDENCE—DNA & PROTEINS Problems?

Small differences in DNA can mean big differences in appearance & function

Some similarities do not have evolutionary progression (“lower” to “higher” organisms) Cytochrome C

Similarities could result from similar actions & functions, in the same way that sports cars share similarities with each other, but not with SUVs

EVIDENCE—NATURAL SELECTION

Populations evolve, not organisms

Phenotypic variation Morphological—physical

features Physiological—metabolic

activities and products Behavioral—responses to

situations and stimuli

Gene pool—possible trait variations within a population

EVIDENCE—NATURAL SELECTION Mutation changes or creates new alleles Other factors shuffle existing alleles

Crossing over (Meiosis I)Homologous chromosome arrangement

(Meiosis I)Fertilization

Some alleles have greater frequencies in the population than othersRed hairAlbinism

Allele frequencies can change over time

Natural selection— “Survival of the fittest”. Some traits allow individuals to survive or reproduce better than others. These traits therefore increase in a population.

Three typesDirectionalStabilizingDisruptive

EVIDENCE—NATURAL SELECTION

Directional selection—Natural selection “favors” a phenotype, increasing the frequency of this allele

Peppered Moth Two variant phenotypes Pre-industrial, light were

more common After industrial pollution,

dark were more common

EVIDENCE—NATURAL SELECTION

Stabilizing selection—Intermediate forms of a trait are favored, extreme forms are not

Human birth weightVery large or very small babies less likely to

survive compared to average-sized

EVIDENCE—NATURAL SELECTION

Disruptive selection—Extreme forms of variation are favored, intermediate forms selected against

“Darwin’s” finchesSpeculated to be derived from common ancestorDifferent beaks adapted for different foods

EVIDENCE—NATURAL SELECTION

Natural selection (“microevolution”) is readily accepted, even by critics

“Classic” examples not unchallengedQuestions of validity of peppered moth

studiesGalapagos finches show variations in bills,

but return to “normal”

EVIDENCE—NATURAL SELECTION

EVIDENCE—EMBRYOLOGY Ernst Haeckel—in mid 1800s first

theorized that as embryos develop, they go through their evolutionary development

Early embryos look very alike because of common descent

EVIDENCE—EMBRYOLOGY Haeckel faked his drawings! This was detected in his lifetime Recently discussed again But still in some texts (below from 2002)

EVIDENCE—EMBRYOLOGY Most scientists recognize that Haeckel

was wrong Embryos not as similar in early

development

EVIDENCE—EMBRYOLOGY Problems?

Haeckel’s theory disproven, but still sometimes used

Embryos not very similar, easy to distinguish

Some very large differences in embryos, arguably more than similarities

Really no problems?Embryos do share some similar traits (just

not to the degree of Haeckel) Pharyngeal pouches Tails

Development patterns are similar

VERDICT? Scientists overwhelmingly support

evolution Much is still not understood about the

processes involved Many intelligent, non-religious people

have problems with aspects of evolutionary theory

Darwin’s theories have always been challenged

Much evidence for AND against evolution

Decisions need to be made on facts and science

EVOLUTION VS. CREATION Evolution

Evolution is a Fact Creationist Claims TalkOrigins Archive Evolution Evidence

Creationism/Intelligent Design/Anti-Evolution Answers In Genesis (home of the Creation Museum) Institute for Creation Research Science Against Evolution Darwinism Refuted YouTube Videos

Forum presenting both sides in various discussions Main page Thread giving sources for both views