Evolution

Chapters 22 - 26

EvolutionProcesses by which living organisms originated on Earth and have been diversified and modified through sustained changes in form and function. The earliest known fossil organisms are single-celled forms resembling modern bacteria (3.4 bya)Evolution has resulted in successive radiations of new types of organisms, many which have become extinct…and some which are present day flora and fauna…extinction & diversification continue today!

Geologic TimeLife originated 3.4 bya Early Earth Environment lacked free oxygen (Urey-Miller Experiment) concentrations of molecules led to synthesis of active chemical groupings of molecules and interactions of chemical compounds rudimentary genetic system Natural Selection Anaerobic Prokaryotes (bacteria) Blue-Green Algae and aerobic photosynthesis Amalgamation of distinct cell types (first eukaryotes; endosymbiotic theory) blue-green algae evolved into chloroplasts & aerobic bacteria evolved into mitochodria DNA may have arisen through prokaryotic symbionts single-celled eukaryotes developed complex modes of living and advanced types of reproduction multicellular plants and animals w/ free oxygen and food supplies abundant (700 mya) basic body plans of modern animals developed with a remarkable burst of evolutionary diversity (700 – 570 mya) jellyfish and burrowing worm skeletons (570 mya)fishland plants (400 mya)arthropods (some evolving into insects) (360 mya)Dinosaurs and mammals (135 mya)extinction of dinosaurs (65 mya)human evolution (2 mya) *Know general steps (See pbs evolution web site!)

Early Evolutionary StudiesNo scientific explanations of evolution were given until the 18th CenturyThe growth of natural history led to increased knowledge of fossils & living organismsJean Baptiste Lamarck: patterns of resemblance found in various creatures arose through evolutionary modifications of a common lineage…environmental changes evoke in individual animals direct adaptive responses that are passed on as inheritable traits (ex. Giraffe’s long neck)

Early Evolutionary StudiesCharles Darwin (1859) published his theory…offspring inherit a resemblance to their parents not solely due to environment…in nature individual’s with traits that made them better adjusted to their environments or gave them higher reproductive capacities…”higher fitness!”Tenets: 1) more individuals are born than survive; 2) when environmental changes occur, populations require new properties to maintain their fitness; 3) traits are inheritable; 4) either adaptation or extinction occurs

Early Evolutionary StudiesMendel discovered that characteristics are transmitted across generations in discrete units, now known as “genes,” in a predictable manner

Inheritable changes can occur without regard to environment…evolution is driven onward by the random accumulation of favorable mutational changes

Early Evolutionary StudiesHugo de Vries, Thomas Morgan, & William Bateson : natural selection, evolution directed by adaptive fitness, minor role (“mutationists”)

Early Evolutionary StudiesSewall Wright & J.B.S. Haldane, population geneticists, and several others independently developed arguments that when a mutation is immediately favored, its spread within a population depends on: 1) size of population; 2) length of generations; 3) degree to which mutation is favorable; 4) rate at which the same mutation reappears in the descendants

Early Evolutionary StudiesA gene is favorable only under certain environmental conditionsThe total number of genes available for the next generation can be large: genetic variability & the gene poolSexual reproduction ensures that the genes are rearranged in each generation: recombinationWhen a population is stable, the gene frequency remains the same (even though the genes in individuals may be recombined in different ways)When the gene frequencies in the pool change in a sustained manner, evolution is occurring!Mutations provide the gene pool with a continuous supply of new genes; through natural selection, gene frequencies change so that advantageous genes occur in greater proportions

What is considered to be an “advantageous” gene?

Are all acquired traits good, bad, or somewhere in-between?

Early Evolutionary StudiesMathematical support by Hardy and Weinberg (1908)(p + q)2 = p2 + 2pq + q2 = 1, where p = allelic freq. of A and and q = allelic freq. of a in a populationConforms to a Punnet square

Hardy – Weinberg TheoremThe frequency of alleles in a population will remain the same regardless of the starting frequencies (“equilibrium”) if: 1) population is large; 2) matings are random; 3) there are no net changes in gene pool due to mutation; 4) there is no migration of individuals into and out of the population; 5) there is no selection…all genotypes are equal in reproductive success

Hence, evolution does occur if one of the above is violated!

See Population Genetics Lab: H-W Theorem

“Nothing in biology makes sense except in the light of evolution…”

Theodosius Dobzhansky in the “Genetics and Origin of Species,” extended the mathematical arguments with experimental and observational evidence

Demonstrated adaptive genetic changes in large populations of fruit flies as a result of controlled environmental changes…compatible with Darwinian natural selection

Synthetic TheoryErnst Mayer (zoologist) showed that new species usually arise in geographic isolation often following a genetic “revolution” that rapidly changes the gene pool

George Simpson (paleontologist) showed from the fossil record that rates and modes of evolution are correlated

G. Ledyard Stebbins (botanist) showed that plants display evolutionary patterns similar to those of animals

The Synthetic Theory is strongly supported by observation and experiment!

Synthetic TheoryJames Watson, Francis Crick, and Rosalind Franklin demonstrated that DNA is the genetic molecule of inheritanceMutations are known to be changes in the position of a gene, or the information coded in a gene, that can affect the function of the protein for which the gene is responsible.Natural Selection can then operate to favor or suppress a particular gene according to how strongly its protein product contributes to the reproductive success of the organism

Punctuated EquilibriumStephen Jay Gould and Niles Eldredge: Theory of Punctuated Equilibrium (1972)…species undergo most of their morphological modification as they first separate from the parent species showing little change as they produce additional species…gradual change is replaced with stasis punctuated with episodes of speciation (See additional handout)

Evidence for EvolutionPaleontology (fossil record)Comparative anatomyVestigial StructuresComparative Embryology (developmental stages)Comparative Physiology (similar enzymes, hormones, etc…)Taxonomy (classification…see overhead schemas)Biogeography (isolation mechanisms)Genetics (gene mutations, chromosomal rearrangements, chromosome segment doubling produce variations)DNA, RNA, Protein comparative analysisArtificial Selection (See “Dog Evolution” video) See: http://www.pbs.org/wgbh/evolution/

Mechanism of EvolutionPeppered moth example: light v. dark variants; frequency changes due to environmental conditions

Mechanisms of: 1) Geographic Isolation (Darwin’s Galapagos Finches & Hawaiian Fruit Flies); 2) Genetic Drift (gene freq. Change by the chance failure of progeny to reproduce to the exact gene proportions of their parents…natural disaster); 3) Founder effect (migration of individuals to an isolated population); 4) Mutations (seems to be the chief agent); 5) Gene Flow (change in gene pools due to migration of individuals between populations); 6) Nonrandom matings (inbreeding, mate selection); 7) Natural Selection via differential reproductive success (increases freq. Of some alleles and diminishes others)

Selection

Results of EvolutionSpeciation: anagenesis v. cladogenesis

Reproductive barriers separate species (prezygotic, habitats, behavioral, temporal, mechanical, gametic, reduced hybrid viability, reduced hybrid fertility, hybrid breakdown, & introgression)

Pathways of Evolution

Divergent evolution

Covergent evolution

Coevolution

Adaptive radiation

Parallel evolution

Extinction

Paths of Speciation

Phyletic

Allopatric

Parapatric

Sympatric

Hybridization

Adaptive radiation

Acknowledgements

Pictures and graphics from google.com

Information supplemented by: Campbell, et al Biology, Encarta, and Professor Farabee’s web site (http://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookTOC.html)

http://www.pbs.org/wgbh/evolution/