Lecture #2 – 1/21/02 – Dr. Kopeny

Much of our knowledge on the history of life on Earth is based on interpretation of fossils -- the

Fossil Record

Evidence from Astrophysics and Geology indicates the Earth is about 4.6 Billion years old

•Atmosphere of early earth probably contained little or no free oxygen (O2):

•Life could only have begun in the absence of free oxygen

Free Oxygen is strongly “reactive”; it oxidizes organic molecules•Oxidize = strip electron (and the associated proton - hydrogen )

•Oxidation “breaks down” molecules – would have oxidized the organic molecules that were the necessary building blocks of life

C6H1206 + 602 6CO2 + 12H20 + ATP

Sugar is oxidized by oxygen; carbon dioxide is formed from the sugar

Oxygen is reduced; it gets the Hydrogens from the sugar…what about the energy-rich electrons?

Think through, review, oxidation/reduction reactions from H.S. Biology or 201 so that you understand the significance of the lack of oxygen at the dawn of life, and the abundance of oxygen later; understand the significance of photosynthesis, fermentation, and aerobic respiration as fundamentally important processes in the evolution and diversification of life

1000

2000

3000

4000

5000

Earth Cools

Photosynthesis evolves

Earth forms 5+ billion years ago

Oldest Eukaryotic fossils

Accum. of atmospheric. O2and evolution of aerobic respiration

Origin of life - anaerobicOldest prokaryotic fossils

Mill

ions

of Y

ears

Ago

(mya

)History of Earth and

Life

~ 2 billion years of a strictly prokaryotic world!!

RNA sequence data shows Bacteria and Archaea diverged early, about 3-2 billion years ago

1000

2000

3000

4000

5000

Earth Cools

Photosynthesis evolves

Earth forms

Oldest Eukaryotic fossils

Substantial atmospheric O2

Origin of lifeOldest prokaryotic fossils

Beginning of Paleozoic EraM

illio

ns o

f Yea

rs A

go (m

ya)

History of Earth and

Life

Symbiotic theory of origin of Eukaryotes

~600 mya ~250 mya ~65 mya

Paleozoic Mesozoic Cenozoic

Today

Phanerozoic Eon (Time of “visible” life)

Paleozoic Era (Time of “Old Animals”)

Mesozoic Era (Time of “Middle Animals”)

Cenozoic Era (Time of “New Animals”)

Paleozoic Era

Precambrian Cambrian

•The Cambrian Period is the oldest geological period from which fossils of relatively complex organisms are represented •Most of the extant (currently living) phyla are represented in the Cambrian, but not before.

~600 mya ~500 mya ~250 mya

PreCambrian Paleozoic Mesozoic Cenozoic

Evolutionary tree of vertebrate animals and close relatives

Width of the track indicates relative species abundance of each group through geological time

Continental Drift and the formation of Pangea

Earths land masses converged to form a “super continent”, Pangea, some 250 million years ago

Paleozoic Era

Mesozoic Era

Cenozoic Era

~250 mya

~65 mya

Pangea

Laurasia

Gondwana

Eras are bounded by major faunal turnovers that are thought to be related to cataclysmic phenomena of global dimensions

A thin band rich in iridium marks the boundary between rocks deposited in the Cretaceous and the Tertiary Periods

H C

Paleozoic Era

Mesozoic Era

Cenozoic Era

Periods of cold climates and glaciations have punctuated Earth’s history

Mass Extinctions and the Geological Time Scale

A Mass Extinction is the widespread extinction of many taxa, globally, that occurs in a relatively brief period of geologic time.

Many of the periods between periods, and between eras, are demarcated by mass extinctions

Two important mass extinctions to be aware of, now, are the:

•Permian Extinctions (“ended” the Paleozoic Era)

•K/T Extinctions (“ended the Mesozoic Era)

Changes in numbers of families of marine invertebrate and vertebrate animals; five major extinctions since the Cambrian Explosion

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Mechanisms of Evolution:How evolution happens to

populations

Three female African Swallowtail Butterflies (Papillio dardanus) from the same population.

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Lecture Outline1. Introductory terms and concepts

2. Darwin, Mendel and the Modern Synthesis

3. Genetic Variation within Populations

4. Genetic composition of a population may change intergenerationally; this is evolution, and one or more processes may contribute to that change

5. The effect of natural selection on frequency distribution of phenotypes and genotypes within a population varies depending on the nature of the selective force or forces operating on individuals

6. We know that variability is often maintained (not lost) over time in populations [ie, recognize a pattern], and we know something about the processes by which variability is maintained.

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Concepts and Terms

Species: A group of populations whose individuals have the potential to interbreed and produce fertile offspring in nature

Population: Localized group of individuals belonging to the same species

Deme: Locally interbreeding group within a population

Gene Flow: Consequence of migration between populations, followed by breeding.

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Homologous pair of chromosomes in diploid parent cell

Homologous Pair of replicated chromosomesSisterChromatids

Homologous chromosomes separate

Sister chromatids separate

Chromosomes replicate

Meiosis: Diploid parent cell to haploid daughter cells (gametes)

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Meiosis I; homologs distributed to daughter cells

Meiosis II; chromatids distributed to daughter cells

Review terms:

-Sexual reproduction-Meiosis, Mitosis-Diploid organism-Homologous chromosomes-Sister chromatids-Gene-Locus-Allele

Sexual Reproduction fosters genetic diversity

•Random selection of half a parents diploid chromosome set to make a haploid gamete

•Fusion of two such haploid gametes to produce a diploid organism

Products of Meiosis are genetically diverse for two reasons

•Crossing over results in recombinant chromatids that contain some genetic material from each chromososome

•It is a matter of chance as to which member of a homologous pair of chromosomes goes to which daughter cellOverview of Meiosis

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Genotype; The genetic constitution governing a heritable trait of an organism

Phenotype: Physical expression of an organism’s genotype

Gene Pool: Sum of all alleles in a population

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How to quantify the genetic structure of a population, for a discrete trait controlled by a single gene locus with one recessive and one dominant allele.:•Allele Frequency

•Genotype Frequency

Phenotypes

Genotypes

Genotype Frequencies

Allele Frequencies

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Consider a population with 500 diploid cats and the single gene locus that controls fur color.•For individuals in this or any other diploid species, how many times is each locus represented?•Two (each individual has two “copies” of the gene).•How many different allelic forms are there in the population?•Two (B, b)•How many alleles in the population?•1000•What defines an individual “homozygous” at a particular locus?Same allele at both loci BB or bb•What defines an individual that is “heterozygous” at a particular locus?Different alleles at the two loci Bb

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Genotype frequencies: BB 36% Bb 48% bb 16%

In a population of 500 individuals, how many individuals have genotype:

BB 500 (.36) = 180 individuals

Bb 500 (.48) = 240 individuals

bb 500 (.16) = 80 individuals

In a population of 500 individuals, how many copies of the B allele are there?

360 alleles from the BB individuals (all the alleles from BB cats)

+ 240 alleles from the Bb individuals (1/2 the alleles from Bb cats)

= 600 copies of the B allele

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Genotype frequencies: BB 36% Bb 48% bb 16%

In a population of 500 individuals, how many individuals have genotype:

BB 500 (.36) = 180 individuals

Bb 500 (.48) = 240 individuals

bb 500 (.16) = 80 individuals

In a population of 500 individuals, how many copies of the B allele are there?

360 from the BB individuals

+ 140 from the Bb individuals

= 500 copies of the B allele

(500)(.36)+

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Consider a population with 500 diploid cats and the single gene locus that controls fur color.

How many copies of the gene for fur color are there? 1000

Consider our genotype frequencies:

BB 36% Bb 48% bb 16%

In a population of 500 individuals, how many individuals are:

BB 500 (.36) = 180 individuals

Bb 500 (.48) = 240 individuals

bb 500 (.16) = 80 individuals

copies of the B allele are there?

(500)(.36)+

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Model of polygenic inheritance based on three genes

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Connecting population genetics and evolutionary change

• Adaptive Evolution

-Of the agents of evolutionary change, only selection is likely to adapt a population to its environment–Adaptive evolution involves random chance in the form of mutation and sexual recombination and probabilistic sorting in terms of the sifting action of natural selection

• Individual Fitness

–Relative contribution of individual to gene pool of the next generation --relative to contribution of other individuals in population. Alternatively – can measure “lifetime fitness” – lifetime contribution to the gene pool)

• Genotype Fitness

–Contribution of genotype at a given locus to the next generation relative to contribution of other genotypes in population

• Fitness Quantified

– Relative scale from 0.0 to 1.0, where individual with greatest contribution in population has fitness of 1.0

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2. Darwin, Mendel and the Modern Synthesis

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R.A. Fisher J.B.S. Haldane Seawall Wright(1890-1962) (1892-1964) (1889-1988)

Architects of the modern synthesis -- theoretical population geneticists whose work reconciled Mendelian theory of heredity with Darwin’s theory of natural selection