The Origin and Evolutionary Historyof Life

Conditions on Early Earth◦Age of Earth ~4.6 billion years◦Atmosphere had free No O2

Rich in CO2, H2O, CO, H2, N2

Also ammonia (NH3), sulfuric acid (H2S), methane gas (CH4)

◦Presence of oxygen (photosynthesis)◦Increased sun energy ◦Formation of organic compounds◦Sufficient time

Protobionts appeared first. They are considered to have possibly been the precursors to prokaryotic cells.

Then appeared heterotrophic bacteria that fed on organic molecules & carried anaerobic fermentation

Protobiont Heterotrophic Bacteria

Cyanobacteria split water molecules and released oxygen through photosynthesis.

This bacteria accounts for much of the oxygen in our atmosphere.

Endosymbiontic Theory Mitochondria and chloroplasts derived

from prokaryotes A prokaryote ingested but not digested,

some aerobic bacteria Over along time the aerobe became

mitochondria This also happened with a cyanobacteria,

which became chloroplast Reproduced along with host cell

Endosymbiontic Theory

Introduction to Darwinian Evolution

Important Terminology

Evolution Accumulation of inherited changes within

populations over timePopulation

Group of individuals of one species that live in the same geographic area at the same time

Species Group of organisms with similar structure, function,

and behavior capable of interbreeding

Pre-Darwinian Ideas

Aristotle (384–322 B.C.E.) Saw evidence of natural affinities

Leonardo da Vinci (1452–1519) Correctly interpreted fossil rocks

Jean Baptiste de Lamarck (1744–1829) First to propose that organisms undergo change

as a result of natural phenomenon Lamarck ideas discredited when Mendel’s

theories rediscovered around 1900

Darwin & Evolution

Voyage of the H.M.S. Beagle

1831Basis for Darwin’s theory of evolutionDarwin observed similarities between

animals and plants◦Arid Galapagos Islands◦Humid South American mainland

Voyage of the H.M.S. Beagle

Influences on Darwin

Principles of Geology by Lyell Artificial selection

Breeders developing many varieties of domesticated animals in a few generations

Plant varieties, such as kale and broccoli, developed from wild cabbage

Ideas of Thomas Malthus Population growth not always desirable Population increases geometrically; food supply

increases arithmetically

Based on adaptations by organisms over time: Inherited variations favorable to survival

persevere Unfavorable variations are eliminated

Theory of Evolution by Natural Selection

Proposed by both Darwin and WallaceBased on four observations:

1. Genetic variation exists among individuals2. Reproductive ability of species causes its

population to increase3. Organisms compete for resources4. Offspring with most favorable characteristics

is most likely to survive

Genetic variation in emerald tree boas

1. Fossil Record2. Comparative Embryology3. Comparative Anatomy4. Biogeography5. DNA Homology

Direct evidence of evolution comes from fossils

Shows life has evolved through time

Exposed layersofsedimentaryrock

Fossils developin differentways

Fossilintermediatesin whale evolution

Determiningthe ageof fossils:radioisotopedecay

Evidence for evolution from comparative anatomy

Homologous features Derive from same structure in common ancestor

Vestigial structures Remnants of structures indicating adaptation

Homology in plants

Convergent evolution: mammals who eat ants and termites

Vestigial structures

Evidence of Evolution from Biogeography

◦Study of past and present geographic distribution of organisms◦Continental drift has played a major role in

evolution

Continental drift

Evidence for evolution from developmental biology

Proteins and DNA contain record of evolutionary change

Phylogeny Evolutionary history of group of related species

Phylogenetic trees Diagrams showing lines of descent based on

molecular data

Phylogenetictree ofwhales andtheir closestliving relatives

Intergenerational changes in allele or genotype frequencies within a population

Often involves relatively small or minor changes, usually over a few generations

Changes in allele frequencies of a population caused by microevolutionary processes:

1. Nonrandom mating2. Mutation3. Genetic drift4. Gene flow5. Natural selection

Nonrandom Mating

Inbreeding Increases the frequency of similar alleles Prevents genetic variation

Mutation

Source of new alleles in a population

Increases genetic variability acted on by natural selection

Genetic drift

Random change in allele frequencies of a small population

Decreases genetic variation within a population

Changes it causes are usually not adaptive Ex: Polydactyl traits in Northern Amish Communities

Genetic drift

Bottleneck is a sudden decrease in population size caused by adverse environmental factors

Founder effect is genetic drift occurring when a small population colonizes a new area

Gene flow

Movement of alleles caused by migration of individuals between populations

Causes changes in allele frequencies

Natural selection

1. Causes changes in allele frequencies leading to adaptation

2. Operates on an organism’s phenotype3. Changes genetic composition of a population

favorably for a particular environment

Modes of Natural Selection Stabilizing Selection◦ Favors the mean (average individuals)◦ Favors the “already well-adapted organisms”◦ If the environment remains unchanged, the “fittest”

organisms will also remain unchanged Ex: Horseshoe Crabs & Ginkgo Trees(have not changed for

millions of years)

Directional Selection◦Favors one phenotypic extreme over the others

and eventually leads to change in a population.◦It occurs when organisms must adapt to changes

conditions in the environment. Ex: Pesticide & antibiotic resistance (organisms

learn to adapt and withstand a harmful chemical)

Disruptive Selection◦ Favors two or more phenotypic extremes.◦ Ex: African orange butterflies can range in color from orange

to blue. The orange and blue forms mimic foul-tasting species, so predators avoid them. The colors in-between do not ward off predators, so butterflies with those colors are eaten more commonly. As a result, only butterflies with extreme colors (orange & blue) survive.

Modes of Natural Selection

(a) No selection (b) Stabilizing selection

Modes of Natural Selection

(c) Directional selection (d) Disruptive selection

Genetic variation in populations caused by:

1.Mutation2.Sexual reproduction◦ Allows new phenotypes

Speciation & Macroevolution

Reproductive Isolating Mechanisms

Prevent gene flow between species. Two types:

1. Prezygotic Barriers Prevent mating or fertilization.

2. Postzygotic Barriers Reproductive failure after fertilization

1. Temporal Isolation2. Habitat Isolation3. Gametic Isolation4. Begavioral Isolation5. Mechanical Isolation

1. Temporal Isolation

Mating at different times of year

Mating at different times of day

2. Habitat Isolation◦Different habitats in the same area

3. Gametic Isolation◦Incompatible egg and sperm◦Molecular recognition on the surface of the

cells

4. Behavioral (sexual) Isolation◦Required courtship behaviors

The male satin bowerbird builds a bower of twigs (a dark tunnel) to attract females

5. Mechanical Isolation◦Incompatible genital organs

Only small bees can land on the petal of the black sage

Only large bees brush against the stamens of the white sage

Hybrid Inviability Zygote forms, but hybrid embryos die

when genetic regulation fails during development

Hybrid Sterility problems during meiosis cause abnormal

gametesHybrid Breakdown

Hybrid cannot reproduce because of some defect

Formation of New Species: Speciation

When a population becomes reproductively isolated the separated gene pools diverge & genetic exchange stops, as a result a new species is formed

Types of Speciation Mechanisms:1. Allopatric Speciation2. Sympatric Speciation3. Artificial Speciation

Allopatric Speciation a population splits into two geographically

isolated populations (for example, by habitat fragmentation due to geographical changes or social change such as emigration).

Most common form of speciation Genetic drift in small populationsExamples:

1. Galapagos tortoises that live in separate, but nearby islands

2. Squirrel species separated by the Grand Canyon

Abigdoni Tortoises

Chathamensis Tortoises

Porteri Tortoises

Squirrel species separated by the Grand Canyon have diverged in fur color

Sympatric speciationRefers to the formation of two or more

descendant species from a single ancestral species all occupying the same geographic location

Populations diverge and each occupies a new ecological niche

Examples:1. Finches in Galapagos Islands2. Maggot Flies in North America

The finches live in the same habitat but each has a different niche.

Fossil record often lacks transitional forms between two species

Is the fossil record simply incomplete? Or does it accurately reflect evolution as it

really occurs?

Long periods of stasis (~2 My)Punctuated by periods of rapid

speciation (~100,000 y)Triggered by changes in the environmentAbrupt appearance of new species in the

fossil record

Continuous evolution over long periods The traditional view Populations gradually accumulate adaptations Different selective pressures in different

environments

Gradualism

Punctuated Equilibrium

Large-scale phenotypic changes in populations, classified at the species level or higher

Characterized by:1. Appearance of evolutionary novelties2. Adaptive Radiation Patterns3. Mass extinctions

Allometric Growth◦Varied rates of growth for different parts of

the body◦A change in development can result in a new

species when the change is adaptive

Allometric Growth

PaedomorphosisRetention of juvenile characteristics in

the adultA change in the timing of development

Paedomorphosis in an axolotl salamander

Adaptive RadiationSpeciation fills new ecological nichesNew adaptive zones may appear when

the environment changesOne species colonizes an island and

diversifies into new species

Adaptive radiation

Extinction of SpeciesFacilitates evolution by opening adaptive

zonesBackground extinction at a steady rateMass extinctionsFive or six mass extinctions of many

species and higher taxonomic groups Major climate changes Catastrophes such as asteroid impacts

Mass extinction of the archosaurs

The Evolution of Primates

Primate evolution

All Mammals Endothermic (warm blooded) Body hair Feed young with milk from mammary glands Most are viviparous (give live birth)Placental Mammals Placenta exchanges materials between

mother and fetus Newborns are more developed than

marsupials

ALL Primates:Are mammalsHave 5 grasping digitsHave pposable thumb or toeHave long, freely moving limbsHave eyes in front of the headHave large brains

Primate hands and feet

Suborder Prosimii◦Lemurs

Suborder Tarsiiformes◦Tarsiers

Suborder Anthropoidae◦Monkeys, apes, humans

AnthropoidsOld and new world monkeysApes and humansHominoidsApes

Gibbons Orangutans Gorillas Chimpanzees

Humans

New world monkey Old world monkey

Hominids Humans & extinct human ancestors Differences between ape and human skeletons Human adaptations for bipedal life on the ground

Complex curvature of the spine Shorter, broader pelvis Foramen magnum at base of skull First toe aligned with other toes

Human and Gorilla Skeletons

Human and Gorilla Heads