Evolution

Chapters 19 through 22

Learning Objectives

• Compare microevolution to macroevolution

• Define and discuss natural selection• Relate Darwin’s contributions to our

overall understanding of evolution• Compare phenotype and genotype• Apply the Hardy-Weinberg equation to

populations undergoing a shift in equilibrium

Learning Objectives

• Define Convergence, Divergence, Polymorphism, and Sexual Dimorphism

• Discuss speciation

• Explain prezygotic and postzygotic mechanisms for speciation

Nature Changes

• Biogeography – World distribution of organisms– Global exploration raised difficult questions for

“unchanging creation”

• Comparative morphology revealed structural similarities in “dissimilar” anatomies– Vestigial structures currently useless structures

Large, Flightless Birds

Fig. 19-3, p.403Wing of batFlipper of dolphinForeleg of pig

Digits

Humerus

Ulna

Radius

Carpals

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19.2 Darwin’s Journeys

• Darwin saw the world on the voyage of the Beagle

• Darwin used common knowledge and several inferences to develop his theory

• Darwin’s theory revolutionized the way we think about the living world

Darwin and the Beagle

• 1831 voyage of H.M.S. Beagle – Timed well with modern geology

• Darwin observed global biogeography– Fossils similar to extant organisms– Organisms near each other similar even if in

different habitats– Species change after isolation from ancestral

groups

Ancestors and Descendants

Ancestors and Descendants

Morphological Differences

Developing Darwin’s Theory

• Darwin tried to discover how species arise– Finch island biogeography

• Artificial selection– Process of improving organisms by selective

breeding– Darwin tried to reconcile observation of artificial

selection with nature

Struggle for Existence

• Struggle for existence occurs when resources limit reproduction of organisms– All species reproduce faster than needed to

replace parent generation

Natural Selection

• Natural selection increases favorable hereditary traits in successive generations– Adaptive traits are genetic characteristics that

increase likelihood of reproduction– Evolutionary divergence creates new species

over long time periods

Darwin’s Observations and Inferences

Darwin’s Revolutionary Theory

• Darwin provided physical rather than spiritual explanations

• Evolutionary change occurs in populations

• Evolution is a multistage process– Variations, natural selection, inheritance

• Organisms function best in particular environments

Early Theory of Evolution Tests

• Two perceived problems tested theory of evolution:– Darwin used complex trait examples, Mendel

simple traits– Darwin studied gradual evolution, Mendel’s

mutations of simple traits had fast impact

• Population genetics linked Darwinian evolution and Mendelian genetics

Modern Synthesis

• Modern synthesis unified theory of evolution– Combined all areas of biology under evolution– Gradualism more important than dramatic

change

• Microevolution of populations and macroevolution of life history act in concert

Evidence of Evolutionary Change

• Adaptation by natural selection– Long term evolutionary trends across species

(wings)– Short term evolutionary trends within populations

(antibiotic and pesticide resistance)

• Fossil Record– Biological lineages such as birds from dinosaurs

Fig. 19-11, p.412

1. When mosquitoes were first exposed to DDT, only about 5% of the population was resistant and the insecticide killed the remaining 95%.

2. Resistant individuals survived and reproduced,passing the genes for resistance to the next generation.

3. One year later, about50% of the populationwas resistant. The sameconcentration of DDTkilled only 50% of thepopulation.

4. Resistant individuals againsurvived and reproduced.

5. After just a fewmore months, about75% of the populationwas resistant and thesame concentration ofDDT killed only 25% ofthe population.

Per

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Bird Ancestry

Evidence of Evolutionary Change

• Historical biogeography – Study of organismal distribution in relation to

evolutionary history– Island and continental biogeography

• Comparative morphology analyzes extant and extinct structural relationships– Homologous traits similar between species due to

common ancestry– Human and bat forelimbs

Evidence of Evolutionary Change

• Comparative embryology has shown embryos from major groups developmentally similar– Gill pouches in humans and fish

• Genetic code independent evidence of evolutionary relationships– Closely related species have closely related amino

acid, nucleotide sequences

Fig. 19-13, p.414Adult sharkHuman embryo

Differences in Amino Acid Sequences

INSERT FIG 19.14 HERE

Macroevolution

• Microevolution is small changes within a species; creating new alleles

• Macroevolution are large changes over a geological time period that lead to speciation

Phenotype vs. genotype

• Genotype= the genetic makeup of alleles of an individual– Homozygous=pp, qq– Heterozygous=pq

• Phenotype= the expressed alleles– pp=white, qq=black, pq=grey

• Some alleles are dominant, some recessive, and some partial

Hardy-Weinberg Equilibrium

• p2 +2pq + q2 = 1.0

• If a population has 70% p alleles, then q must be at 30%

• Evolution is a process resulting in changes of genetic makeup over time

• Evolutionary agents are those that disrupt the Hardy-Weinberg equation

Speciation

• Speciation – Process of species formation– Inferred by studying products, species

• Microevolutionary processes that lead to population divergence produce new species

Morphological Species Concept

• Based on differences in anatomical features– Only species definition for fossils

• May lead to erroneous conclusions– Some anatomical differences within species– Some species identical in appearance– Not always evolutionarily based

Biological Species Concept

• Based on reproductive isolation– If populations can interbred, they are

members of same species– If populations do not make fertile offspring,

they are different species

• Problems with biological species– Asexual and extinct organisms

Subspecies

Prezygotic Mechanisms

• Ecological isolation from habitats

• Temporal isolation from mating timing

• Behavioral isolation from mating signals, sexual selection

• Mechanical isolation from reproductive structures

• Gametic isolation from gamete incompatibility

Interspecific Hybrids

Postzygotic Isolating Mechanisms

• Interspecies offspring reproductively isolated if less fit than intraspecies offspring

• Hybrid inviability from species hybrids not surviving

• Hybrid sterility from species hybrids

• Hybrid breakdown reduced fitness of F2

Macroevolution

• Microevolution is small changes within a species; creating new alleles

• Macroevolution are large changes over a geological time period that lead to speciation

Convergent/Divergent Evolution

• Convergence=Similar adaptations in distantly-related organisms- Similar selective pressures produce similar adaptations

• Divergence= Very distinct differences in closely related organisms

• Polymorphism- differences within a species

• Sexual Dimorphism- differences between male and females of a species

Adaptive Radiation

• Biodiversity – Number of species in given area

• Adaptive radiation – Group of closely related species occupying

different habitats– Ancestral species move into unfilled adaptive

zone (unoccupied or open from extinction)

Extinctions

• Background extinction rate – Low rate, from environmental change and poor

adaptations– Over global time scales, most species go extinct

• Mass extinctions – High rate over short time– Climate changes from geological activity and asteroid

impacts

Evo-Devo

• Evolutionary developmental biology (evo-devo)– Evolution in genes of embryonic development– Genes of development also regulate

morphology

• Homeotic genes– Control transcription of development genes– Small changes in homeotic genes can

produce large changes in morphology

Homeotic Genes

• Many organisms share common genetic tool-kit for development– Common animal genes for 500 million years– Common genes in animals, plants, fungi and

prokaryotes from earliest life

Hox Genes

• Control animal body plan

• Homeobox – 180-nucleotide sequence– Codes for homeodomain (part of a

transcription factor)