GeneticDiversity kuliah S1

8/10/2019 GeneticDiversity kuliah S1

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Genetic DiversityBiology/Env S 204

Spring 2009


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Genetic diversity

Heritable variation within and betweenpopulations of organisms

Encoded in the sequence of 4 base-pairs that make up DNA

Arises by mutations in genes

and chromosomes


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Genetic Diversity

Very small fraction of genetic

diversity is outwardly expressed Estimated 109different genes across

the Earths biota

Represents a largely untapped genetic

library


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Genetic Diversity

Genetic diversity is the foundation forall higher levels of biodiversity

Genetic diversity provides the recipefor populations and species, which inturn form communities and ecosystems

Genetic variation enables evolutionary

change and artificial selection


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Genetic Diversity

Genetic diversity may have direct economicvalue (genes for disease resistance,

biologically active compounds) But effective conservation for whatever

purpose depends upon accurate, thoughtfulassessment of genetic diversity

Preservation of genetic diversity is usuallya high priority in conservation programs


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Nature of Genetic Diversity

Information for all of life

stored in the structure

of DNA Genetic code or the units

(bases, nucleotides) that

make up DNA areessentially universal


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A length of DNA (often with extraproteins) is a chromosome

A section of DNA along thechromosome that contains theinformation to make a protein (orRNA) is a gene


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chromosome

gene gene

gene gene

Chromosome structure


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DNA Structure

A

T

T

GC

T

G

G

A

CA

T

T

A

A

CG

A

C

C

T

GT

A

A = adenineT = thymine

C = cytosine

G = guanine

Bases:


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A gene may be several hundred to upto about two thousand units (bases)long

A gene contains the information tomake a protein or RNA

A gene is a discrete unit ofhereditary information


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gene protein (enzymes, membranes, etc.)

RNA (essential for productionof proteins)

gene


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A given gene may have more than one form;the different forms of a single gene arecalled alleles.

flower colorgene withtwo alleles


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flower color


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Homozygous(both alleles in anindividual are the same)

Heterozygous(two different allelespresent in an individualfor one gene)

or flower colorgene


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Prokaryotes vs. Eukaryotes

bacteria,archaebacteria

protists, fungi,plants, animals


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Prokaryotes

One-celled, no compartments (nonucleus)

Genetic material in a single, circularchromosome

Therefore only 1 copy of each geneper bacterium

A typical bacterium has 1,000-2,000genes


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Eukaryotes

One-celled or many-celled, withcompartments (e.g., a nucleus is present)

Genetic material in two to many linear,separate chromosomes in the nucleus

Normally two copies of each gene presentin an individual in part of the life cycle

A eukaryote has about 50,000 genes onaverage


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Origin of Genetic Diversity

Mutation = change in the sequence ofbases of DNA along a chromosome

Change in a base or chunks of DNAcan be rearranged

Mutations can occur anywhere along achromosome

This is the ultimate source of allgenetic variation


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Measuring Genetic Diversity

Chromosome = a collection of genesplus extra DNA in between thatdoesnt code for anything

Genes are used in measuring geneticdiversity but

The extra DNA is free to changeand is also useful in assessing geneticdiversity


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Different parts of the DNA evolve atdifferent ratesextra DNA changesfaster than DNA in the genes

Some genes evolve slowly and help inthe study of deep branches of life(ancient lineages)

Extra DNA can change so rapidlythat every individual is distinct(except for clones)


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Within an individual: %heterozygosity (alleles same ordifferent in a given set of genes)

Among individuals in a population:allele frequencies for given genes

Between populations: %heterozygosity, allele frequencies,unique molecular markers


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Measuring diversity betweenevolutionary lineages usuallyinvolves comparing sequences ofDNA and looking for changes in

bases or major rearrangments.


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Loss of genetic diversity = loss ofuseful genetic diversity in theshort term and reduction ofevolutionary options in the long

term.


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Evolutionary Processes

1) Natural Selection

2) Gene Flow

3) Genetic Drift


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Evolutionary Processes1) Natural Selection

A major mechanism of evolution as proposedby Darwin

A filter for genetic variation: the best

adapted individuals survive and reproduce ingreater numbers over time

Not a directed process!

Changes in direction and intensity dependon conditions and time span and availablegenetic diversity


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Evolutionary Processes1) Natural Selection

SURVIVORhttp://science.discovery.com/interactives/literacy/darwin/darwin.html
http://science.discovery.com/interactives/literacy/darwin/darwin.htmlhttp://science.discovery.com/interactives/literacy/darwin/darwin.html


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Evolutionary Processes2) Gene Flow

The exchange of genetic materialwithin a population, betweenpopulations of a species, and even

between species Gene flow among populations of aspecies maintains the integrity of thespecies

Lack of gene flow can lead tospeciation


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Population A

Population B

geneflow

A

B

barrierarises

Species A

Species B

reproductiveisolation

time


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Species A

Species B

geneflow

allopatric speciation =

geographic isolation+

reproductive isolation


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Evolutionary processes2) Gene Flow

Species A

(AA)

X

Species B(BB)

Hybrid AB(infertile,cannot crosswith either

parent either)


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Evolutionary processes2) Gene Flow

Hybrid AB

Chromosomedoubling AABB (now sex

cells can beproduced!)

AA

X

AABB

AAB (infertile)

sex cell A

sexcellAB


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sympatric speciation =reproductive isolation of

parent species fromhybrid derivatives throughhybridization and

chromosome doublingwithout geographic isolation


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Evolutionary Processes3) Genetic Drift

Changes in the gene pool of a smallpopulation due to chance events

Founder effect = one or two

individuals disperse and start a newpopulation with limited geneticdiversity

Bottleneck = extreme reduction inpopulation size and therefore geneticdiversity


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Conservation Genetics

Involves the use of genetic dataand principles to guide

conservation activities

Genetics should be prominent in

the practice of conservation


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1) Rate of evolutionary change in apopulation is proportional to theamount of genetic diversity available

2) Higher genetic diversity is usuallypositively related to fitness

3) Global pool of genetic diversity

represents all of the information forall biological processes (= geneticlibrary)


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Small populations tend to lose

genetic diversity over time!!!


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Habitat fragmentation and destructionnow produce and will continue toproduce small, isolated populations

Understanding the genetic status ofspecies and populations and theconsequences of small population sizes

is vital to conservation, management,and recovery efforts.


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A major goal is to preservenatural patterns of genetic

diversity to the extent possibleto preserve options for futureevolutionary change.

greater prairie chicken example


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Conservation GeneticsExample: Prairie Chickens

35-year study of a remnant population ofprairie chickens in Illinois

In 1962, about 2,000 individuals present;in 1994, fewer than 50

Fertility and hatching rates declined

significantly, as did genetic diversity Translocation program established in 1992

to bring in birds from MN, KS and NE


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Conservation GeneticsExample: Prairie Chicken

By 1994, increased survival of youngprairie chickens was verified

By 1997, there were significant

increases in mean rates of fertilityand hatching

Once the main population in Illinois

became isolated, it began to loseviability and without intervention, itmost likely would have disappeared

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GeneticDiversity kuliah S1