Chapter 23 Part 2 Natural Selection, Genetic Drift, and Bottleneck

Info from Class Video

Concept 23.2

Natural Selection, genetic drift, and gene flow can alter allele frequencies in a population

3 major factors alter allele frequencies and bring about most evolutionary change

natural selection

genetic drift

gene flow

Differential success in reproduction results in

certain alleles being passed to next gen in greater proportions

ex: allele that confers resistance to DDT in fruit flies

increased in frequency after DDT widely used in agriculture

adaptive evolution

improvement in the match between organisms and their environment

natural selection can cause

the smaller a sample, the greater the chance of

random deviation from a predicted result

genetic drift

describes how allele frequencies fluctuate unpredictably from one gen to the next

genetic drift tends to reduce

genetic variation through losses of alleles

genetic drift flower example

one gen 3 colors, if herbivore eats all but red, next gen all red bc only red can leave offspring

founder effect

occurs when few indivs become isolated from larger population

allele frequencies in small founder pop can be

different from those in larger parent pop

bottleneck effect

sudden reduction in pop size due to change in environment

bottleneck effect resulting gene pool

may no longer be reflective of original pop’s gene pool

bottleneck effect if population remains small

may be further affected by genetic drift

understanding bottleneck effect can increase

understanding of how human activity affects other species

Case Study

Impact of Genetic Drift on the Greater Prairie Chicken

loss of prairie habitat caused

severe reduction in pop of greater prairie chicken in Illinois

surviving birds had ___ levels of genetic variation

low

only ___% of their eggs hatched

50%

what did the bottleneck effect affect for greater prairie chicken

the range in state of Illinois 1820-1993

how were pop size, number of alleles per locus, and % eggs hatched affected

pop size reduced, # alleles per locus reduced, % eggs hatched reduced

How did birds from Kansas and Nebraska help the pop in Illinois

brought in from other states no bottleneck, mated with birds in Illinois, restored genetic diversity

researchers use what to compare genetic variation in pop before and after bottleneck

DNA from museum specimens

what did results show from DNA of museum specimen

showed a loss of allele frequency at several loci

After introduced other state birds, results:

introduced new alleles, increased egg hatch rate to 90%

4 effects of genetic drift

1. significant in small pops

2. can cause allele freqs change at random

3. can lead to loss of genetic variations within pops

4. can cause harmful alleles to become fixed

gene flow consists of

movement of alleles among populations

alleles can be transferred through

movement of fertile individuals or gametes

ex: pollen

gene flow tends to reduce

variation among populations over time and the fitness of the population

Example gene flow on Parus Major

mating cause gene flow between central and eastern pops

immigration from mainland introduces alleles that decrease fitness on island

NS removes alleles that decrease fitness

result of example gene flow on Parus Major

birds born in central region w high immigration have lower fitness

birds born in east w low immigration have higher fitness

gene flow can decrease or increase fitness of a population

both

example gene flow increase fitness

spread of alleles for resistance to insecticides

insecticides have been used to target

mosquitos that carry West Nile virus and malaria

flow of insecticide resistance alleles into a pop can

increase in fitness

gene flow is an important agent of evolutionary change in

modern human populations

Concept 23.4

Natural Selection is the only mechanism that consistently causes adaptive evolution

evolution by NS involves both

chance and “sorting”

how NS involve chance

new genetic variations arise by chance

how NS involve sorting

beneficial alleles “sorted” and favored by NS

only NS consistently increases

frequencies of alleles that provide reproductive advantage

NS brings about adaptive evolution by acting on

an organism’s phenotype

how are phrases “struggle for existence” and “survival of the fittest” misleading

they imply direct competition among individuals

reproductive success is generally more

subtle, depends on many factors

relative fitness

the contribution an individual makes to the gene pool of the next gen, relative to the contributions of other individuals

selection favors certain genotypes by

acting on the phenotypes of individuals

3 modes of selection

directional, disruptive, stabilizing

directional selection

favors individuals at one extreme end of phenotypic range

disruptive selection

favors indivs at both extremes of phenotypic range

stabilizing selection

favors intermediate variants and acts against extreme phenotypes

striking adaptations have arisen by

natural selection

example of striking adaptations octopuses

can change color rapidly for camouflage

example of striking adaptations snakes

jaws allow them to swallow prey larger than their heads

NS increases the freqs of alleles that

enhance survival and reproduction

adaptive evolution occurs as

match between species and its environment increases

envir can change so

adaptive evolution is a continuous process

why don’t genetic drift and gene flow consistently lead to adaptive evolution

they can increase or decrease the match between an organism and its environment

sexual selection

natural selection for mating success

sexual dimorphism

marked differences between sexes in secondary sexual characteristics

intrasexual selection

direct competition among individuals of one sex (often males) for mates of opposite sex

intersexual selection

aka mate choice

when indivs of one sex (usually females) are choosy in selecting their mates

male showiness due to mate choice can

increase chances of attracting female while

decrease chances of survival

how do female preferences evolve

“good genes” hypothesis

what does good genes hypothesis suggest

if a trait is related to male genetic quality/health, both male trait and female preference for that trait should increase in frequency

Diploidy maintains genetic variation in the form of

recessive alleles hidden from selection in heterozygotes

balancing selection occurs when

NS maintains stable frequencies of two or more phenotypic forms in a population

balancing selection includes (2)

heterozygote advantage

frequency dependent selection

Heterozygote advantage

occurs when heterozygotes have higher fitness than both homozygotes

NS and heterozygote advantage

NS will tend to maintain 2 or more alleles at that locus

what selection can heterozygote advantage result from

stabilizing or directional selection

example heterozygote advantage

mutation in allele codes for part of hemoglobin protein causes sickle cell disease, but also confers malaria resistance

regions where malaria common, selection favors heteros for sickle-cell allele

frequency dependent selection

fitness of phenotype declines if it becomes too common in population

what does selection favor w freq dependent

favors whichever phenotype is less common in a population

example freq dependent selection

results in about equal numbers of “right mouthed” and “left mouthed” scale-eating parasitic fish