UNIT VIII EVOLUTION

UNIT VIII EVOLUTION

• Big Campbell Ch 22-28, 31

• Baby CampbellCh 13-17

• HillisCh 15-18

I. EVOLUTION - WHAT IS IT?o “Descent with Modification” Earth’s many species are descendants of ancestral species that were diff. than present day species

o Evolution – change in the genetic composition of a population over time.

“Change” – in the genetic composition (the alleles)

Population – group of organisms of one species in a certain habitat that interbreed to produce fertile offspring.

November 24, 1859

II. Hardy-Weinberg Principle

• Predicts allele frequency in a non-evolving population; that is, a population in equilibriumo States that allele frequencies

in a population will remain constant from generation to generation if five conditions are met

o Used to identify/study evolving populations

II. Hardy-Weinberg Principle, cont• Five Conditions for Hardy-Weinberg Equilibrium:

– No Mutations – gene pool is modified if mutations alter alleles or if entire genes are deleted or duplicated

– Random Mating – if individuals mate preferentially within a population (inbreeding), random mixing of gametes does not occur and gene frequency changes.

– No natural Selection – differences in the survival and reproductive success of individuals carry diff. genotypes and can alter allele frequencies.

– Extremely large population size – the smaller the pop the more likely allele frequencies will fluctuate by chance from one generation to the next (genetic drift)

– No gene flow (migration) – moving alleles into or out of populations can alter allele frequencies.

If any of these conditions are not met, evolutionary change will occur!

II. Hardy-Weinberg Principle, cont• Hardy-Weinberg Equation

p = frequency of one allele (A) q = frequency of other allele (a) p + q = 1 (sum of 2 alleles = 1 or 100%)

• Therefore, p = 1 - q q = 1 - p

• Genotype Frequency AA = p2

aa = q2

Aa = 2pq• To determine distribution of genotype frequencies in a population →

p2 + 2pq + q2 = 1

Ex: PKU in the US

• Autosomal Recessive• Occurs 1 in 10,000 births• q2 = .0001• q = .01• p = 1 - .01 = .99• What % of individuals are

carriers for PKU?• 2pq = 2(.99)(.01) = .0198

or 2%

II. Hardy-Weinberg Principle, cont

Hardy-Weinberg Practice Problems1. If you know that you have 16% recessive fish (bb), . . .

• q 2 = • q = • Therefore, p =

To calculate the frequency of each genotype …• p2 =• 2pq =

1. What is the expected percentage of heterozygous fish?

II. Hardy-Weinberg Principle, cont• Hardy-Weinberg Practice Problems, cont

2. If in a population of 1,000, 90 show recessive phenotype (aa), use Hardy-Weinberg to determine frequency of allele combinations.

3. In people light eyes are recessive to dark. In a population of 100 people, 36 have light eyes. What percentage of the population would be …

• Homozygous recessive?

• Homozygous dominant?

• Heterozygous?

II. Hardy-Weinberg Principle, cont4. The ability to roll the tongue is a dominant trait. … 75% of the students at Kingwood Park High School have the ability to roll the tongue. Assuming the student population is 1700,

a) How many students would exhibit each of the possible genotypes?

b) How many students would exhibit each of the possible phenotypes?

III. A HISTORY OF EVOLUTIONARY THEORY

• Aristotle (384-322 BCE)o Scala Naturae

• Carolus Linnaeus (1707-1778)o Taxonomy

III. A HISTORY OF EVOLUTIONARY THEORY, cont

III. A HISTORY OF EVOLUTIONARY THEORY, cont• Charles Darwin (1809-1882)

III. A HISTORY OF EVOLUTIONARY THEORY, cont• Darwin, cont

o Observed many examples of adaptationsInherited characteristics

that enhance organisms’ survival and reproduction

o Based on principles of natural selectionPopulations of organisms

can change over the generations if individuals having certain heritable traits leave more offspring than others

Differential reproductive success


• Darwin’s Conclusions Based on his own observations and

the work of other scientists, Darwin realized … o Members of a population often

vary greatly in their traits.o Traits are inherited from parents to

offspring.o All species are capable of

producing more offspring that their environment can support, therefore …

III. A HISTORY OF EVOLUTIONARY THEORY, cont Darwin concluded …

o Individuals whose inherited traits give them a higher probability of surviving and reproducing in a given environment tend to leave more offspring than other individuals.

o This unequal ability of individuals to survive and reproduce will lead to the accumulation of favorable traits in the population over generations.

Descent with Modification


Human Directed Evolution?•Artificial Selection– Selective Breeding– Transgenic Organisms


• Post-DarwinNeo-Darwinism/Modern Synthesis Theory

Epigenetics

IV. EVIDENCE FOR EVOLUTION

• Direct Observationo Antibiotic/Drug Resistance

IV. EVIDENCE FOR EVOLUTION, cont• Fossil Record

o Succession of forms over timeo Transitional Linkso Vertebrate descent

IV. EVIDENCE FOR EVOLUTION, cont

• Homologyo Homologous structures

o Vestigial organsSnakesCetaceansFlightless birds


o Convergent Evolution Independent evolution of similar features in different lineagesAnalogous structures


• Biogeographyo Geographical distribution of

specieso Continental Drift

Pangaeao Endemic specieso Islands are inhabited by

organisms most closely resembling nearest land mass


• Comparative Embryologyo Pharyngeal

Pouches Gill slits

o Tail


• Molecular Biologyo Similarities in DNA,

proteins, genes, and gene products

o Common genetic code

V. MICROEVOLUTION

• A change in the gene pool of a population over a succession of generations

• Five main causes: Mutation Non-Random

Mating Natural Selection Genetic Drift Gene Flow

V. MICROEVOLUTION, cont• Genetic Drift

o Changes in the gene pool due to chance. o More often seen in small population sizes. o Usually reduces genetic variability. o There are two situations that can drastically reduce population size:

Bottleneck EffectFounder Effect

V. MICROEVOLUTION, cont

• Bottleneck Effect Type of genetic drift resulting

from a reduction in population (natural disaster)

Surviving population is no longer genetically representative of the original population

• Founder Effect Due to colonization by a

limited number of individuals from a parent population

Gene pool is different than source population


• Gene FlowGenetic exchange due to the

migration of fertile individuals or gametes between populations – tends to reduce differences between populations


• MutationsA change in an

organism’s DNA (gametes; many generations); original source of genetic variation (raw material for natural selection)


• Nonrandom Mating Inbreeding – reduces fitness

by putting an individual at a greater risk of harmful recessive traits.

Assortative mating – individuals with similar genotypes/phenotypes mate more frequently than expected


• Natural Selection Blend of Chance and Sorting

chance in the creation of new genetic variations; sorting b/c natural selection favors some alleles over others

Relative fitness – contribution an individual makes to the gene pool of the next generation

Only form of microevolution that adapts a population to its environment

VI. VARIATION IN POPULATIONS

• Genetic Variation is the “substrate” for evolution

• Maintained through … Polymorphism

Coexistence of 2 or more distinct forms of individuals (morphs) within the same population

Geographical Variation Differences in genetic

structure between populations (cline)

VI. VARIATION, cont Mutation and Recombination

Diploidy 2nd set of chromosomes hides

variation in the heterozygote

Balanced Polymorphism Heterozygote Advantage Frequency-Dependent Selection

o Survival & reproduction of any 1 morph declines if it becomes too common

o Parasite/host

VII. A CLOSER LOOK AT NATURAL SELECTION

• Natural Selection Not a random process → Dynamic processIncreases frequency of alleles that provide reproductive

advantageFitness

VII. CLOSER LOOK AT NATURAL SELECTION, cont

Natural selection is the only evolutionary mechanism for adaptive evolution


• Three ways in which natural selection alters variationDirectional

Disruptive

Stabilizing


• Sexual Selection Can result in sexual

dimorphism - secondary sex characteristic distinction

Intrasexual Selection - within the same sex; individuals of one sex compete directly for mates of the opposite sex Intersexual Selection

- individuals of one sex are choosy in selecting their mate (male showiness)

VIII. MACROEVOLUTION

• Macroevolution Refers to the formation of new taxonomic

groups Due to an accumulation of microevolutionary

changes AKA Speciation

• “Species” Morphological Species Concept - charac.

species by body shape & other features; can be applied to sexual & asexual organisms

Ecological Species Concept – views a species in terms of its niche; which is the sum of how members interact w/living & non-living parts of environ; can accommodate sex & asexual

Phylogenetic Species Concept – smallest group of individuals that share a common ancestor; forming 1 branch on tree of life

VIII. MACROEVOLUTION, cont

• Biological Species Concept Described by Ernst Mayr in

1942 A population or group of

populations whose members have the potential to interbreed and produce viable, fertile offspring; in other words, similar organisms that can make babies that can make babies

Can be difficult to apply to certain organisms . . .


• Reproductive Isolationo Prevent closely

related species from interbreeding when their ranges overlap.

o Divided into 2 typesPrezygoticPostzygotic


Prezygotic Reproductive Barriers


Postzygotic Reproductive Barriers

VIII. MACROEVOLUTION, cont• Speciation

o Fossil record shows evidence of bursts of many new species, followed by periods of little chanceKnown as punctuated equilibrium

o Other species appear to change more graduallyGradualism fits model of evolution proposed by Darwin

VIII. MACROEVOLUTION, cont• Modes of Speciation

Based on how gene flow is interrupted

Allopatric Populations segregated by a

geographical barrier; can result in adaptive radiation (island species)

Sympatric Reproductively isolated

subpopulation in the midst of its parent population (change in genome); polyploidy in plants; cichlid fishes

IX. HISTORY OF LIFE ON EARTH

IX. HISTORY OF LIFE ON EARTH, cont

• Formation of Organic Moleculeso Oparin/Haldane Hypothesis

Primitive Earth’s atmosphere was a reducing environment

No O2

Early oceans were an organic “soup”Lightning & UV radiation provided

energy for complex organic molecule formation

o Miller/Urey ExperimentTested Oparin/Haldane hypothesisSimulated atmosphere composed of

water, hydrogen, methane, ammoniaAll 20 amino acids, nitrogen bases,

ATP formedHypothesis was supported

IX. HISTORY OF LIFE ON EARTH, cont

IX. HISTORY OF LIFE ON EARTH, cont• Mass Extinctions

IX. HISTORY OF LIFE ON EARTH, cont• Adaptive Radiation

o Periods of evolutionary change, increased speciation

o Often due to increased ecological niches in communities

o Also seen in organisms with major evolutionary innovations

X. PHYLOGENY

Phylogeny – study of how living things share common evolutionary history (relationships among organisms)- Constructed based on physical structures, behaviors, and biochemical attributes- Commonly used to depict evolutionary history of species, populations, and genes.

X. PHYLOGENY• Taxonomy

Carrolus Linnaeus Binomial

nomenclature – 2 part naming system; genus and species

Taxon (taxa) – which is any group of species that we designate with a name. EX: humans, primates, mammals

KPCOFGS Domain is broadest;

organisms not as closely related

Species is most specific

X. PHYLOGENY, cont• Phylogenetics

Tracing of evolutionary relationships

Illustrated with diagrams known as phylogenetic trees

May portray all life, or a major evolutionary group such as insects or mammals.

Common ancestor is called the “root”

X. PHYLOGENY• Tree Terminology

Root – common ancestor of all organisms in the tree. Node – or split when a single lineage divides into two; giving rise to new lineages Clade

Any taxon that consists of all evolutionary descendants of a common ancestor Clades are sub-categorized as

Monophyletic – Includes ancestral group and all descendantsParaphyletic – Includes ancestral group and some, but not all descendantsPolyphyletic – Includes taxa with multiple ancestors

Sister Clades – any 2 clades that are each other’s closest relatives Sister Species – two species that are each other’s closest relatives

Monophyletic – “single tribe”

Paraphyleticancestral group and some, but not all descendants“beside the tribe”

Paraphyletic

PolyphyleticIncludes taxa with multiple ancestorsEX: Kingdom Protista

Polytomy

• Unresolved pattern of divergence

• More than 2 descendant groups emerge

• Evolutionary relationships are not clear

X. PHYLOGENY, cont

• Evolutionary history of an organism

X. PHYLOGENY, cont• Important to distinguish

between homologies and analogies Homologies are

likenesses attributed to common ancestry

Analogies are likenesses attributed to similar ecological roles and natural selection

• May also be done at a molecular level Known as molecular

systematics

X. PHYLOGENY, cont• Tree Construction, cont

Ancestral Trait – trait from which organisms evolve; found in common ancestor Derived Traits – new traits that evolved after ancestral trait

Synapomorphies – shared among a group of organisms; viewed as evidence for common ancestry of group. EX: vertebral column of vertebrates

May see evolutionary reversals – a character may revert from a derived state back to an ancestral state. EX: finstetrapodsfins in whales/dolphins

Ingroup Groups of organisms being considered, phylogenetically organized

Outgroup Group chosen as point of reference for tree Closely related but diverged before the ingroups

X. PHYLOGENY, cont• Tree Construction, cont

Parsimony Also known as Occam’s Razor Principle that if multiple trees are possible, the correct one is most often the

simplest one; one that takes the fewest assumptions

X. PHYLOGENY, cont

UNIT VIII EVOLUTION

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