7/31/2019 Genpop Des 2010

1/30

Population Genetics

7/31/2019 Genpop Des 2010

2/30

Populations:

Members of asexually-reproducingspecies are able to

interbreed, producefertile offspring, andhave a shared genepool

Gene pool refers to

the collective groupof alleles of all theindividuals in apopulation

7/31/2019 Genpop Des 2010

3/30

Populations

Different speciesdo not exchange

genes with eachother byinterbreeding

A population is a

group oforganisms of thesame speciesoccupying acertain area

7/31/2019 Genpop Des 2010

4/30

Populations

Members of a populationvary from one another

Variation is the rawmaterial for evolutionarychange

Features that make anorganism suited for itsenvironment so it cansurvive, reproduce, &

pass its alleles onto itsoffspring are calledadaptations

Hooked Beak of Eagle

7/31/2019 Genpop Des 2010

5/30

Populations

Speciation is the splitting of onespecies into two or more species or

the transformation of one speciesinto a new species over time

Speciation is the final result of

changes in gene pool allelic andgenotypic frequencies

7/31/2019 Genpop Des 2010

6/30

Micro- & Macro- Evolution:

Macroevolutionrefers to large

scale evolutionarychanges such asthe formation ofnew groups above

the species level

7/31/2019 Genpop Des 2010

7/30

Micro- & Macro- Evolution:

Microevolutionrefers to smaller

scale changessuch as changeswithin species

7/31/2019 Genpop Des 2010

8/30

Micro- & Macro- Evolution:

In studying evolution at thepopulation level, geneticists focus

on the gene pool When the relative frequency of

alleles in a population changes over

a number of generations, it is calledmicroevolution

7/31/2019 Genpop Des 2010

9/30

Causes of Change in Gene

Pools:

Mutations

Mutations result in theintroduction of newgenes (new geneticinformation) into a genepool

Mutations can bechanges in genes (DNAsequences) or changes

to chromosomes(additions, deletions,substitutions,translocations)

7/31/2019 Genpop Des 2010

10/30

Causes of Change in Gene

Pools:

Gene mutationsprovide new alleles,and therefore are the

ultimate source ofvariation

A gene mutation isan alteration in theDNA

(deoxyribonucleicacid) nucleotidesequence of an allele

7/31/2019 Genpop Des 2010

11/30

Causes of Change in Gene

Pools:

Mutations occur at random

Mutations can be beneficial, neutral, or

harmful Some chromosomal mutations are

alterations in the number ofchromosomes inherited

Others are alterations in arrangement ofalleles on chromosomes due toinversions and translocations

7/31/2019 Genpop Des 2010

12/30

Gene Flow

Gene flow is the movement of genes into(immigration) & out of a population (emigration)

Migration of breeding individuals moves alleles

among populations through interbreeding Gene flow may be agent of microevolution (e.g.

isolated populations with limited gene flowresult in genetic distinctions among groupsliving in different locations)

Continued gene flow tends to decrease thediversity among populations, causing genepools to become similar

7/31/2019 Genpop Des 2010

13/30

Genetic Drift

Genetic driftrefers to changes

in allelefrequencies of agene pool due tochance (random)

events

7/31/2019 Genpop Des 2010

14/30

Genetic Drift

Genetic drift occurs in both large and small populations Larger populations suffers less sampling error Genetic drift causes gene pools of two isolated populations to

become dissimilar as some alleles are lost and other are fixed

Genetic drift occurs when founders start a new population, orafter a genetic bottleneck with interbreeding Founder effect is a case of genetic drift in which rare alleles, or

combinations of alleles, occur in higher frequency in apopulation isolated from the general population (e.g. dwarfismis much higher in a Pennsylvania Amish community due to afew German founders)

Bottleneck effect is genetic drift in which a severe reduction inpopulation size due to natural disaster, predation, or habitatreduction, causes severe reduction in total genetic diversity ofthe original gene pool (e.g. Intense interbreeding in cheetahshas prevented most genotypes from being passed to the nextgeneration)

7/31/2019 Genpop Des 2010

15/30

Non-random Mating

Inbreeding (mating with closeneighbors instead of more distant

members of a population) can effectthe frequency of some genotypes

Causes a reduction in heterozygous

genotypes & an increase inhomozygous genotypes

7/31/2019 Genpop Des 2010

16/30

Non-evolving Populations:

Gene pool of a non-evolving populationremains constant over the generations

The shuffling of genes that accompanies

sexual reproduction does not alter thegenetic makeup of the population (i.e.sexual reproduction alone does not leadto microevolution)

Hardy-Weinberg equilibrium = frequencyof each allele in the gene pool tends toremain constant unless affected by otheragents

7/31/2019 Genpop Des 2010

17/30

Hardy-Weinberg equation :

p2 + 2pq + q2 = 1

where p2 = frequency of homozygous dominant

2pq = frequency of heterozygous

q2 = frequency of homozygous recessive

P + q = 1 so if you know the frequency of one allele, you can calculate the frequency of the other

Example: Using imaginary population of 500 blue-footed boobies, determine the frequencyof each genotype & each allele

Phenotypes No webbing

No webbing

webbing

Genotypes WW Ww ww

# animals (500 total) 320 160 20

Genotype frequencies 320/500 = 0.64 160/500 = 0.32 20/500 = 0.04

Number of alleles ingene pool (1000)

640 W 160W + 160 w = 360 40 w

Allele frequencies (640+160)/1000 = 0.8 W (160 +20)/1000 = 0.2 w

7/31/2019 Genpop Des 2010

18/30

Hardy- Weinberg

Thus the genotypic frequencies will be:WW = p2 = 0.64Ww = 2pq = 2(0.8)(0.2) = 0.32ww = q2 = 0.2

The allele frequencies will be p = 0.8, andq = 0.2; they remain unchanged from theprevious generation so this populationhas not evolved

7/31/2019 Genpop Des 2010

19/30

Five conditions are required for

Hardy-Weinberg equilibrium:

Hardy-Weinberg equilibrium says that something other thansexual reproduction is required to alter the gene frequencies ina population from one generation to the next

The Hardy-Weinberg equilibrium provides basis for comparingidealized, non-evolving populations with actual ones in which

gene pools are changing For a population to be at Hardy-Weinberg equilibrium, it must

satisfy 5 main conditions Population must be very large (no genetic drift) Population must be isolated (no migration or gene flow) No mutation (Rate of mutation does not alter gene pool)

Random mating All individuals are equal in reproductive success (i.e. natural

selection does not occur) These 5 conditions are rarely met

7/31/2019 Genpop Des 2010

20/30

Natural Selection:

Populations mustadapt to their

environment Natural selection

producesadaptive evolution

7/31/2019 Genpop Des 2010

21/30

Natural Selection:

Natural selection requires variation (heritablegenetic differences) in the members of apopulation

Some differences affect how well an organism isadapted to its environment & make them morefit or more likely to reproduce

Fitness is the extent to which an individualcontributes fertile offspring to the nextgeneration & is measured against thereproductive success of other genotypes in thesame environment

7/31/2019 Genpop Des 2010

22/30

Natural Selection

There are three naturaloutcomes of naturalselection --- Stabilizingselection, Directional

selection, & Diversifyingselection

Stabilizing selection(most common) favorsintermediate variations ina population & eliminatesthe extremes (e.g.majority of human birthweights within 6.4-9 lbrange)

7/31/2019 Genpop Des 2010

23/30

Directional selection

Directional selection(common during periodsof environmental changeor during migration to

new habitat with differentenvironmentalconditions) favors oneextreme (e.g. antibioticresistance or a shift ofdark-colored peppered

moths from light-coloredcorrelated withincreasing pollution)

7/31/2019 Genpop Des 2010

24/30

Diversifying selection

(occurs when environmentalconditions are varied in a way that

favors individuals at both extremesof phenotypic range) selects againstintermediate Beaks

7/31/2019 Genpop Des 2010

25/30

Isolation Factors for

Species:

A biological species is a category whosemembers are reproductively isolated from allother such groups

Reproductive isolation occurs when membersof one species can only breed successfully witheach other

Modern biochemical genetics uses DNAhybridization techniques to determine therelatedness of organisms

Reproductive isolating mechanisms are anystructural, functional, or behavioralcharacteristic that prevents successfulreproduction from occurring

7/31/2019 Genpop Des 2010

26/30

Habitat isolation

Habitat isolationoccurs when two

species occupydifferent habitats,even within thesame geographic

range, so thatthey are lesslikely to meet andto attempt to

reproduce

Grand Canyon Isolates Rodent Populations

7/31/2019 Genpop Des 2010

27/30

Temporal isolation

Temporal isolation occurs when twospecies live in the same location,

but each reproduces at a differenttime of year, and so they do notattempt to mate

7/31/2019 Genpop Des 2010

28/30

Behavioral isolation

Behavioralisolation results

from differencesin matingbehavior betweentwo species

7/31/2019 Genpop Des 2010

29/30

Mechanical isolation

Mechanicalisolation is the

result ofdifferencesbetween twospecies in

reproductivestructures orother body parts,so that mating is

prevented

7/31/2019 Genpop Des 2010

30/30

Gamete isolation

Gamete isolation is physical or chemicalincompatibility of gametes of two

different species so that they cannot fuseto form a zygote; an egg may havereceptors only for the sperm of its ownspecies

Zygote mortality is when hybrids(offspring of parents of two differentspecies) do not live to reproduce