Organic evolution
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Organic Evolution. Chapter 6. Evolution - Defined. Evolution – a change in the genetic composition of a population over time. A change in the frequency of certain alleles. On a larger scale, evolution can be used to refer to the gradual appearance of all biological diversity.

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Organic Evolution

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Organic evolution

Organic Evolution

Chapter 6


Evolution defined

Evolution - Defined

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

    • A change in the frequency of certain alleles.

  • On a larger scale, evolution can be used to refer to the gradual appearance of all biological diversity.


Darwin s revolutionary theory

Darwin’s Revolutionary Theory

  • The Origin of Species focused attention on the diversity of life, similarities as well as differences, and the adaptations organisms have for particular environments.


Darwin s revolutionary theory1

Darwin’s Revolutionary Theory

  • Charles Darwin presented evidence that many modern organisms are descended from ancestral species that were different.


Darwin s revolutionary theory2

Darwin’s Revolutionary Theory

  • Prevailing view of the world was that the Earth was only a few thousand years old and that all life had been created at the beginning and remained unchanged.


Pre darwinian evolutionary ideas

Pre-Darwinian Evolutionary Ideas

  • Several ancient Greek philosophers thought life changed through time.

  • Aristotle recognized fossils as forms of ancient life.

    • He developed the scala naturae (scale of nature).

    • Each form of life had a rung on the ladder.

    • Organisms were arranged in order of complexity.

  • The ancient Greeks didn’t propose an evolutionary mechanism.


Pre darwinian evolutionary ideas1

Pre-Darwinian Evolutionary Ideas

  • Lamarck was the first to suggest an explanation for evolution.

    • Inheritance of acquired characteristics

    • Didn’t hold up to testing.


A mechanism for evolution

A Mechanism for Evolution

  • Darwin presented a mechanism for evolution – natural selection.

    • Organisms that are in some way more successful at reproduction will pass on more of their genes.

    • Over time the traits responsible for that success will become widespread in the population.

    • This theory holds up very well!!


Alfred russell wallace

Alfred Russell Wallace

  • Wallace independently developed a theory of natural selection.

  • He sent his manuscript to Darwin, spurring him to finally publish his ideas.

  • Both ideas were presented to the Linnean Society in 1858.

  • Darwin finished On the Origin of Species and published it in 1859.


Uniformitarianism

Uniformitarianism

  • Charles Lyell’s principle of uniformitarianism:

    • Laws of physics & chemistry present throughout history of Earth.

    • Past geological events similar to today’s events.

    • Principles of Geology


Uniformitarianism1

Uniformitarianism

  • Natural forces could explain the formation of fossil-bearing rocks.

  • Lyell concluded the age of the earth must be millions of years.

  • He stressed the gradual nature of geological changes.


Uniformitarianism and the age of earth

Uniformitarianism and the Age of Earth

  • Darwin studied the work of Lyell closely. He took the first volume of Lyell’s Principles of Geology on the Beagle. He received the second volume while on the voyage.

    • He concluded that Earth must be much older than 6000 years.

    • Perhaps these slow changes could work on living things as well…..


Evolution in need of a mechanism

Evolution in Need of a Mechanism

  • Darwin was not the first to have the thought that organisms change through time.

    • His grandfather, Erasmus Darwin, and others suggested that life evolves as environments change.

    • But a mechanism for that change was needed.


Darwin 1809 1882

Darwin (1809 – 1882)

  • Darwin had a lifelong love of nature.

  • His father wanted him to study medicine.

    • This was not what Darwin wanted and he didn’t finish.


Darwin

Darwin

  • After leaving medical school he attended Cambridge University with the intention of entering the clergy.

  • His mentor and botany professor, John Henslow, recommended him for a position as ship’s naturalist aboard the Beagle.


The voyage of the beagle

The Voyage of the Beagle

  • Darwin started out on a five year trip around the world aboard the Beagle in 1831. He was 22.

  • As ship’s naturalist he spent his time on shore collecting thousands of plant and animal specimens and making important observations.


The voyage of the beagle1

The Voyage of the Beagle

  • Darwin saw that the plants and animals that he found in temperate areas of South America were more similar to tropical South American species than they were to temperate European species.


The voyage of the beagle2

The Voyage of the Beagle

  • The fossils he found in South America were more like modern South American species than European species.


The voyage of the beagle3

The Voyage of the Beagle

  • During the voyage he read Lyell’s Principles of Geology.

  • He had Lyell’s ideas in mind as he traveled and observed the geology of South America.


The voyage of the beagle4

The Voyage of the Beagle

  • He experienced an earthquake in Chile and observed that the coastline had risen several feet.

  • He also found marine fossils high in the Andes Mountains.

  • Darwin concluded that the mountains were formed by a series of such earthquakes.


The voyage of the beagle5

The Voyage of the Beagle

  • Darwin became interested in the geographic distribution of organisms after visiting the Galapagos Islands.


After the voyage

After the Voyage

  • After returning, Darwin realized that adaptation to the environment and the origin of new species were closely linked processes.

  • Galapagos finch species have evolved by adapting to specific conditions on each island.


Natural selection

Natural Selection

  • After reading a paper by Thomas Malthus concerning the fact that human populations increase faster than limited food resources, Darwin noticed the connection between natural selection and this ability of populations to overreproduce.


Natural selection1

Natural Selection

  • Only a small fraction of all offspring produced by any species actually reach maturity and reproduce.

  • Natural populations normally remain at a constant size.


Natural selection2

Natural Selection

  • Those that survive may have heritable traits that increased their chances of survival.

    • They will pass those traits on.

    • The frequency of those traits will increase.


Artificial selection

Artificial Selection

  • Artificial selection – people selectively breed organisms with desired traits.

    • Darwin noticed that considerable change can be achieved in a short period of time.


Natural selection3

Natural Selection

  • Natural selection occurs when organisms with particular heritable traits have more offspring that survive & reproduce.


Natural selection4

Natural Selection

  • Natural selection can increase the adaptation of an organism to its environment.


Natural selection5

Natural Selection

  • When an environment changes, or when individuals move to a new environment, natural selection may result in adaptation to the new conditions.

    • Sometimes this results in a new species.


Natural selection6

Natural Selection

  • Individuals do not evolve; populations evolve.

  • Evolution is measured as changes in relative proportions of heritable variations in a population over several generations.


Natural selection7

Natural Selection

  • Natural selection can only work on heritable traits.

    • Acquired traits are not heritable and are not subject to natural selection.


Natural selection8

Natural Selection

  • Environmental factors are variable.

    • A trait that is beneficial in one place or time may be detrimental in another place or time.


Darwinian evolutionary theory evidence

Darwinian Evolutionary Theory: Evidence

  • The main premise underlying evolutionary theory is that the living world is always changing.

  • Perpetual change in form & diversity of organisms over the last 700 million years can be clearly seen in the fossil record.


Fossils

Fossils

  • Fossils are remnants of past life preserved in the earth.

    • Complete remains – insects in amber.

    • Petrified skeletal parts infiltrated with silica or other minerals.

    • Or traces of organisms such as molds, casts, impressions, trackways, or fossilized excrement.


The fossil record

The Fossil Record

  • Fossils provide support for the idea that life changes through time.

    • Fossil intermediates

      • Whales descended from land mammals.

      • Birds descended from one branch of dinosaurs.

    • The oldest fossils are of prokaryotes.


Dating fossils

Dating Fossils

  • Geological time can be measured in sedimentary rock layers.

    • The Law of Stratigraphy

      • Dates oldest layers at the bottom and youngest at the top.

      • Time is divided into eons, eras, periods and epochs.


Dating fossils1

Dating Fossils

  • Radiometric dating methods are based on the decay of naturally occurring elements into other elements.

    • Different methods used for different time periods.


Dating fossils example

Dating Fossils - example

  • 40K has a half life of 1.3 billion years – meaning half of the 40K will have decayed to 40Ar and 40Ca. Half of what remains will decay in the next 1.3 billion years.

  • Measure ratio of remaining 40K to the amount of 40K originally there (remaining 40K plus 40Ar and 40Ca).


Fossil record

Fossil Record

  • The fossil record of macroscopic organisms begins in the Cambrian period: 505–570 MYA.

  • Fossil bacteria and algae, casts of jellyfishes, sponges spicules, soft corals, and flatworms are found in Precambrian rocks.

    • Mostly microscopic


Evolutionary trends

Evolutionary Trends

  • The fossil record shows that species arise and go extinct repeatedly throughout geological history.

  • Trends appear in the fossil record – directional changes in features or patterns of diversity.


Evolutionary trends1

Evolutionary Trends

  • The evolution of horses from the Eocene epoch (57.8 MYA) to the present is a well studied trend.

    • Body size – increasing

    • Foot structure – fewer toes

    • Tooth structure – larger grinding surface


Common descent

Common Descent

  • Darwin proposed that all organisms have descended from a single ancestral form.

  • Life history is shown as a branching tree called a phylogeny.


Homology

Homology

  • The phrase “descent with modification” summarizes Darwin’s view of how Evolution works.

    • All organisms descended from common ancestor.

    • Similar species have diverged more recently.

  • Homology – when similar structures result from shared ancestry.


Anatomical homologies

Anatomical Homologies

  • Homologous structures – variations on a structural theme that was present in a common ancestor.

  • Example – vertebrate forelimbs have different functions, but share the same underlying structure.


Anatomical homologies1

Anatomical Homologies

  • Vertebrate embryos have a tail and pharyngeal pouches.

  • These structures develop into different but homologous structures in adults.

    • Gills in fishes

    • Part of ears & throat in humans.


Ontogeny phylogeny

Ontogeny & Phylogeny

  • There are many parallels between ontogeny (an individual’s development) and phylogeny (evolutionary descent).

    • Embryological similarities

    • Features of an ancestors ontogeny can be shifted earlier or later in a descendant's ontogeny.


Ontogeny phylogeny1

Ontogeny & Phylogeny

  • Heterochrony – evolutionary change in timing of development.

    • Characteristics can be added late in development and features are then moved to an earlier stage.

    • Ontogeny can be shortened in evolution.

    • Terminal stages may be deleted causing adults of descendants to resemble youthful ancestors.

  • Paedomorphosis

    • Retention of ancestral juvenile characters by descendant adults.


Developmental modularity and evolvability

Developmental Modularity and Evolvability

  • Heterotopy– a change in the physical location of a developmental process in an organism’s body.

    • Process must be compartmentalized into semi-autonomous modules to be expressed in new location

    • Ex: Location of toepad development in geckos.


Developmental modularity and evolvability1

Developmental Modularity and Evolvability

  • Evolvability– denotes the great evolutionary opportunities created by semi-autonomous developmental modules whose expression can be moved from one part of the body to another.

    • Allows for “experimentation” with the construction of many new structures.


Vestigial organs

Vestigial Organs

  • Vestigial organs – remnants of structures that served important functions in an ancestor.

    • Remnants of pelvis and leg bones in snakes

    • Appendix in humans


Molecular homologies

Molecular Homologies

  • Similarities can be found at the molecular level, too.

    • The genetic code is universal - it is likely that all organisms descended from a common ancestor.

    • Different organisms share genes that have been inherited from a common ancestor.

      • Often, these genes have different functions, like the mammalian forelimbs.


Homologies the tree of life

Homologies & the Tree of Life

  • Darwin’s evolutionary tree of life can explain homologies.

    • The genetic code is shared by all species because it goes back deep into the ancestral past.

    • More recent homologies are shared by only smaller branches of the tree.


Homologies the tree of life1

Homologies & the Tree of Life

  • Homologies result in a nested pattern.

    • All life shares the deepest layer.

    • Each smaller group adds homologies to those they share with larger groups.


Speciation

Speciation

  • Speciation refers to the formation of new species.

  • Defining a species is difficult…

    • Descent from common ancestral population.

    • Ability to interbreed.

    • Maintenance of genotypic & phenotypic cohesion.

  • Reproductive barriers prevent species from interbreeding.

    • Where do they come from?


Allopatric speciation

Allopatric Speciation

  • Allopatric (another land) populations occupy separate geographic areas.

    • Separated geographically, but able to interbreed if brought together.

  • Over time, reproductive barriers may evolve so that they could not interbreed.

    • Allopatric speciation


Allopatric speciation1

Allopatric Speciation

  • The geographical separation can arise in two ways:

    • Vicariant speciation is initiated when climatic or geological changes fragment a species’ habitat, forming impenetrable barriers.

    • Founder events occur when a small number of individuals disperse to a distant place where no other members of their species exist.


Hybrids

Hybrids

  • Much can be learned by studying what happens when previously isolated populations come into contact again.

    • Hybrids are offspring of members of two closely related species.


Hybrids1

Hybrids

  • Species eventually become different enough that they can’t produce hybrids.

    • Premating barriers prevent mating from occurring in the first place.

    • Postmating barriers impair growth, survival, or reproduction of the offspring.


Sympatric speciation

Sympatric Speciation

  • Sympatric (same land) speciation occurs when speciation occurs in one geographic area – a lake for example.

  • Individuals within the species become specialized on a food type, shelter, part of the lake etc.

    • Eventually reproductive barriers evolve.


Parapatric speciation

Parapatric Speciation

  • Parapatric Speciation – geographically intermediate between allopatric and sympatric speciation.

    • Two species are parapatric if their geographic ranges are primarily allopatric but make contact along a borderline that neither species successfully crosses.


Adaptive radiation

Adaptive Radiation

  • Adaptive radiation – the production of ecologically diverse species from a common ancestral stock.

    • Common in lakes & islands – sources of new evolutionary opportunities.


Adaptive radiation1

Adaptive Radiation

  • Archipelagoes increase opportunities for both founder events and ecological diversification.

    • Entire archipelago isolated from the continent.

    • Each island is geographically isolated from the others.

    • Ex: Galápagos Islands


Gradualism

Gradualism

  • Darwin’s theory of gradualism proposes that small differences accumulate over time producing the larger changes we see over geologic time.

    • Certainly, this process is always at work, but probably does not account for all changes.


Punctuated equilibrium

Punctuated Equilibrium

  • Punctuated equilibrium states that phenotypic evolution is concentrated in relatively brief events of branching speciation followed by periods of stasis.


Populations evolve

Populations Evolve

  • Variation exists within any population.

  • When natural selection acts to favor one trait over another that trait will increase in the population.

  • The population has evolved, not any one individual.


The modern synthesis

The Modern Synthesis

  • Population Genetics – the study of how populations change over time.

    • Dependent on both Darwin’s theory of natural selection and Mendel’s laws of inheritance.

    • All heritable traits have a genetic basis, some are controlled by multiple genes – not as simple as in Mendel’s studies.


The modern synthesis1

The Modern Synthesis

  • The modern synthesis is a comprehensive theory of evolution that brings in ideas from many fields.

    • R. A. Fisher (statistician)

    • J. B. S. Haldane (biologist)

    • Theodosius Dobzhansky (geneticist)

    • Sewall Wright (geneticist)

    • Ernst Mayr (biogeographer)

    • George Gaylord Simpson (paleontologist)

    • G. Ledyard Stebbins (botanist)


Populations

Populations

  • Population – a localized, interbreeding group of individuals of a particular species.

    • Separate populations of a species may be isolated from each other.


Populations1

Populations

  • Sometimes the populations overlap, but little interbreeding occurs.


Microevolution

Microevolution

  • Microevolution – evolutionary changes in the frequency of alleles in a population.

    • Polymorphism – occurrence of different allelic forms of a gene in a population.

    • If there is only one allele for a gene in the population – every individual is homozygous for the trait – it is fixed in the population.

    • All alleles of all genes possessed by all members of a population form a gene pool.


Microevolution1

Microevolution

  • Population geneticists measure the relative frequencies of alleles in the population.

    • Allelic frequency


Genetic equilibrium

Genetic Equilibrium

  • According to Hardy-Weinberg equilibrium, the hereditary process alone does not produce evolutionary change.

    • Allelic frequency will remain constant generation to generation unless disturbed by mutation, natural selection, migration, nonrandom mating, or genetic drift.

      • Sources of microevolutionary change.


Frequency of alleles

Frequency of Alleles

  • Each allele has a frequency (proportion) in the population.

  • Example population of 500 wildflowers.

    • CRCR = red; CRCW = pink; CWCW = white

      • 320 red, 160 pink, 20 white

    • Frequency of CR =

      (320 x 2) + 160 / 1000 = 800/1000 =.8 = 80%


Frequency of alleles1

Frequency of Alleles

  • p is the frequency of the most common allele (CR in this case).

    • p = 0.8 or 80%

  • q is the frequency of the less common allele (CW in this case).

  • p + q = 1

  • q = 1- p = 1 – 0.8 = 0.2 or 20%


Hardy weinberg theorem

Hardy-Weinberg Theorem

  • Populations that are not evolving are said to be in Hardy-Weinberg equilibrium.

  • As long as Mendel’s laws are at work, the frequency of alleles will remain unchanged.


Hardy weinberg theorem1

Hardy-Weinberg Theorem

  • The Hardy-Weinberg theorem assumes random mating.

  • Generation after generation allele frequencies are the same.


Hardy weinberg theorem2

Hardy-Weinberg Theorem

  • At a locus with two alleles, the three genotypes will appear in the following proportions:

  • (p + q) x (p + q) = p2 + 2pq + q2 = 1


Hardy weinberg theorem3

Hardy-Weinberg Theorem

  • Conditions:

    • Very large population

    • No gene flow into or out of the population

    • No mutations

    • Random mating

    • No natural selection

  • Departure from these conditions results in evolution.


Practice with hardy weinberg

Practice with Hardy Weinberg

  • Hardy & Weinberg studied the frequencies of alleles in populations.

    • Frequency – the proportion of individuals in a category in relation to the total number of individuals. 100 cats, 84 black, 16 white – frequency of black = 84/100 = 0.84, white =0.16.

    • Two alleles – p is common, q is less common.

      • p+q = 1


Practice with hardy weinberg1

Practice with Hardy Weinberg

  • (p + q)2 = p2 + 2pq + q2

Individuals homozygous for allele B

Individuals heterozygous for alleles B & b

Individuals homozygous for allele b


Practice with hardy weinberg2

Practice with Hardy Weinberg

  • Used to calculate allele frequencies (p & q) in a simple way.

  • 100 cats, 16 white (bb) so q2 = 0.16

  • q =square root of 0.16 = 0.40.

  • Sincep + q = 1; p = 1 – q = 0.60.

  • p2 = 0.36; so 36 homozygous dominant (BB)

  • 2pq = 0.48; 48 heterozygous (Bb)


Where does variation come from

Where Does Variation Come From?

  • Natural selection acts on the variation that is already present in the population.

    • But, where did that variation come from?


Where does variation come from1

Where Does Variation Come From?

  • Two processes provide the variation in gene pools.

    • Mutation

    • Sexual recombination


Mutation

Mutation

  • New genes or alleles only result by mutations.

  • Mutations are changes in the nucleotide sequence of DNA.


Point mutations

Point Mutations

  • Point mutation – a change in a single base pair.

  • Often harmless

    • Much of the DNA does not code for protein products.

    • Genetic code is redundant.

      • CGU, CGA, CGC, CGG all code for argenine.

  • Occasionally significant

    • Sickle cell disease.


Mutations

Mutations

  • Beneficial mutations of any kind are very rare.

  • Mutations that alter gene number or sequence are almost always harmful.


Gene duplication

Gene Duplication

  • Gene duplication occasionally provides an expanded genome with new loci that may take on new functions as selection continues.

  • New genes can also appear when non-coding introns get shuffled into the coding portion of the genome.


Sexual recombination

Sexual Recombination

  • Sexual recombination is a much more common way of producing variation in populations.

    • Reshuffling of allele combinations already present in the population is how variation is maintained in populations.

    • Sexual reproduction rearranges alleles into fresh combinations every generation.


Natural selection9

Natural Selection

  • When natural selection is occurring, some individuals are having better reproductive success than others.

    • Alleles are being passed to the next generation in frequencies that are different from the current generation.

    • Hardy-Weinberg equilibrium is upset.


Genetic drift

Genetic Drift

  • The smaller the sample, the greater the chance of deviation from expected results.

    • These random deviations from expected frequencies are called genetic drift.

    • Allele frequencies are more likely to deviate from the expected in small populations.


Genetic drift1

Genetic Drift

  • Which allele was lost is due to random chance.

  • Over time, drift tends to reduce genetic variation through random loss of alleles.


The bottleneck effect

The Bottleneck Effect

  • Sometimes a catastrophic event can severely reduce the size of a population.

    • The random assortment of survivors may have drastically different allele frequencies.

    • Bottleneck effect


The bottleneck effect1

The Bottleneck Effect

  • The actions of people sometimes cause bottlenecks in other species.

    • N. California elephant seal population reduced to 20-100 individuals in the 1890s.

    • Current population > 30,000.

    • Variation drastically reduced – 24 genes with 1 allele.

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The founder effect

The Founder Effect

  • Founder effect – When a small group of individuals becomes separated from the population and form a new population, the allele frequencies in their gene pool may be different than the original population.


Gene flow

Gene Flow

  • The population can gain or lose new alleles through gene flow.

  • When individuals move into or out of a population, they may carry the only copy of certain alleles in the gene pool with them.

  • Gene flow usually reduces differences between populations.


Natural selection adaptation

Natural Selection & Adaptation

  • Natural selection is the only one of these ways of altering the gene pool that results in adaptation.

  • Selection depends on variation.


Genetic variation

Genetic Variation

  • Variation in a population is always present.

  • Heritable variation is the raw material of natural selection.


Genetic variation1

Genetic Variation

  • Not all genetic variation is heritable.

  • Environmental influences sometimes effect phenotype.


Polymorphism

Polymorphism

  • Different versions of discrete characters are called morphs.

  • When a population has two or more morphs that are common in the population, it is called polymorphic.

    • This is phenotypic polymorphism


Protein polymorphism

Protein Polymorphism

  • Different allelic forms of a gene code for slightly different proteins – protein polymorphism.

  • If the difference affects the protein’s net electric charge, the different forms can be separated using protein electrophoresis.


Quantitative variation

Quantitative Variation

  • Quantitative traits are those that show continuous variation.

    • Influenced by many genes.

    • Height in humans, tail length in mice

    • When trait values for a population are graphed, they follow a bell shaped curve.


Modes of selection

Modes of Selection

  • Stabilizing – removes the extremes.

  • Directional – variants at one of the extremes are favored.

  • Disruptive – variants at both extremes are favored.


Evolutionary fitness

Evolutionary Fitness

  • Fitness – the contribution an individual makes to the gene pool of the next generation.

  • Relative fitness – the contribution of one genotype relative to the contribution of other genotypes at the same locus.

  • Natural selection acts on phenotypes.


Preserving variation

Preserving Variation

  • Some variation is hidden from the natural selection process in the form of recessive alleles in heterozygotes.

  • Less favorable recessive alleles can be maintained in the population because they do not harm heterozygous individuals.


Sexual selection

Sexual Selection

  • Sexual selection – natural selection for mating success.

    • May result in sexual dimorphism – differences between the sexes.

    • Secondary sexual characteristics – not directly involved in reproduction.


Intrasexual selection

Intrasexual Selection

  • Intrasexual selection – selection within the same sex – results when individuals of one sex are competing with each other for members of the other sex.

    • Features that make the male a better fighter or more intimidating to other males would be favored.


Intersexual selection

Intersexual Selection

  • Intersexual selection – mate choice – individuals of one sex are choosy in selecting a mate.

    • Features that make an individual more attractive to the opposite sex would be favored.


Intersexual selection1

Intersexual Selection

  • Showiness that results from mate choice can be risky.

    • Flashy tails of guppies make them more visible to predators.

  • Benefits of finding a mate outweigh potential costs.

  • Showiness may reflect overall health.


Macroevolution

Macroevolution

  • Macroevolution refers to grand scale events in evolution.

    • Evolution of new structures

    • Major trends in the fossil record


Gould s tiers of time

Gould’s Tiers of Time

  • Stephen Jay Gould recognized three tiers of time for evolutionary processes:

    • Tens to thousands of years – population genetic processes.

    • Millions of years – speciation and extinction can be measured and compared among different groups of organisms.

    • Tens to hundreds of millions of years – marked by episodic mass extinctions.


Speciation and extinction through geological time

Speciation and Extinction Through Geological Time

  • A species has two possible fates:

    • Become extinct or

    • Give rise to new species.

  • Speciation and extinction rates vary among species.

  • Lineages with high speciation and low extinction produce the greatest diversity.


Speciation and extinction through geological time1

Speciation and Extinction Through Geological Time

  • Species Selection

    • Differential survival and multiplication of species based on variation among lineages.

    • Species-level properties include mating rituals, social structuring, migration patterns, geographic distribution, etc.


Mass extinctions

Mass Extinctions

  • Mass extinctions are episodic events where many species go extinct at the same time.

    • Permian extinction – 225 MYA – half the families of shallow-water marine invertebrates and 90% of the marine invertebrate species went extinct over a few million years.

    • Cretaceous extinction – 65 MYA – marks the end of the dinosaurs as well as many other species.


Mass extinctions1

Mass Extinctions

  • Many possible explanations for mass extinctions have been suggested.

  • Alvarez hypothesis – bombardment of the earth by asteroids would send debris into the atmosphere, altering climate.

    • Search for evidence

      • Craters

      • Atypical iridium concentrations


Mass extinctions2

Mass Extinctions

  • Catastrophic species selection would result from selection by these events.

    • Mammals were able to use resources due to dinosaur extinction.

  • Paleontologist Elisabeth Vrba uses term Effect Macroevolution to describe differential speciation and extinction rates among lineages caused by organismal-level properties.


Endurance of darwin s theory

Endurance of Darwin’s Theory

  • The beauty of Darwin’s theory is that it explains so many different kinds of observations: anatomical and molecular homologies that match patterns in space (biogeography) and time (fossil record).

"Nothing in biology makes sense except in the light of evolution." Theodosius Dobzhansky, Geneticist


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