Microevolution 2 Flashcards
Do most species exist as many populations which are isolated from each other to some extent?
No. Populations occasionally exchange members
How are most population structured?
Spatially. Individuals tend to cluster in areas of suitable habitat
Local aggregations that regularly exchange members are called
Subpopulations
an evolutionary force that results from migration of individuals or the dispersal of seeds, spores, etc.
Allele Flow
Allele flow can potentially cause evolutionary change, provided that:
The species has multiple subpopulations
There are differences in allele frequency among populations or among subpopulations within populations
Can small amounts of allele flow negate genetic drift?
Yes
If sufficiently strong allele flow occurs, what can it cause allele frequencies in different populations to do?
Converge on a single, population wide mean
In the allele flow equation, this variable = the proportion of migrants exchanged per generation
m
In the allele flow equation, the number of individuals in each population
N
Allele flow will negate the effects of genetic drift if…
If m>(1/2N)
How many migrants every other generation is sufficient to prevent genetic drift from causing evolutionary differences among populations of species or subpopulations within a populations of a species
One migrant every other generation or .5 migrants every generation
Can allele flow oppose selection?
Yes
Allele flow from a large, central population adapted to a different environment might swamp the effects of natural selection,
by causing an influx of less fit alleles every generation to counterbalance the unfit alleles lost to selection.
Two important patterns of nonrandom mating that affect evolution
- Inbreeding (includes selfing)
2. Assortive mating
is common in many species, was formerly common in humans, before the advent of increasingly sophisticated forms of transportation.
Inbreeding
High levels of inbreeding lead to the loss of the ___________, although allele frequencies are not necessarily changed
heterozygous genotype
Inbreeding exposes recessive alleles to selection, since they are more likely to be present in the homozygous state if the population is inbred.
!
How can inbreeding cause a dramatic decline in the fitness of a population, possibly extinction,
Deleterious alleles that are hidden from selection are more likely to be expressed
Are all species affected by inbreeding?
Give an example
No.
Parasitic Hymenopetera
This occurs when individuals choose their mates based on their resemblance to each other at a certain locus or a certain phenotype.
Assortative Mating
This type of assortative mating occurs when like genotypes or phenotypes mate more often than would be expected by chance.
Positive assortative mating
This type of assortative mating occurs when similar genotypes or phenotypes mate less often than would be expected by chance.
Negative Assortative Mating
This group of ppl have a very high positive assortative mating, individuals with achronoplastic dwarfism pair up much more often than would be expected by chance
Dwarfs
IQ
Height
Redheads
IQ: Slight positive assortative mating
Height: Slight positive assortative mating
Redheads: negative assortative mating
This is the differential survival and reproduction of individuals with certain traits.
Natural Selection
What does natural selection act on?
Phenotypes
Does natural selection act on genotypes, and therefore cause change in the frequency of alleles through time?
Yes because phenotypes are in part determined by genotypes at one or several loci
Any allele that affects the ability of an organism to survive and reproduce will be subject to natural selection.
!
In populations, natural selection operates whenever individuals in the population vary in their ability to survive and reproduce.
Natural selection causes evolutionary change whenever there is genetic variation for traits that affect fitness.
2 things that must occur for natural selection to operate
1) The must be variation
2) Some of the variation must affect survival and reproduction of individuals
What must occur for natural selection to cause evolutionary change?
There must also be allelic variation for characteristics that affect fitness
The ability of an individual to survive and make copies of its alleles that are represented in the next generation
Fitness
The fitness of an individual organisms is essentially the same as its…
Lifetime reproductive success
The fitness of a genotype is the average fitness of all the individuals in the population that have that genotype.
!
Is fitness physical performance?
No. Differences in fitness may be due to differences in survivorship, differences in fecundity, or both.
The total number of surviving offspring that an individual produces during its lifetime (its lifetime reproductive success).
Absolute Fitness
An organisms chance of living to a certain age
Age specific survivorship
The number of offspring an organism produces in a certain time interval
Age specific fecundity
Fecundity and survivorship are…..
components of fitness
Absolute fitness is standardized to get
relative fitness
The genotype with the highest absolute fitness has a relative fitness of…
1.0
For every other genotype, their relative fitness is….
absolute fitness of genotype/absolute fitness of fittest genotype
The variable for fitness
w
Is survivorship a component of fitness?
Yes, it is necessary to assume fecundity is identical in the three morphs to calculate relative fitness.
the selection coefficient, is the difference in the fitness of a genotype between its own value, and the ideal of 1.0.
s
So, for a deleterious, recessive allele.
w (AA)=1.0, w (Aa)=1.0, and w(aa)=1-s
Occurs when the most extreme phenotype is the most fit. When applied to a single locus, that means one allele becomes more common until it reaches fixation (I.e., frequency 1.0)
Directional Selection
What does directional selection tend to eliminate over time?
Genetic Variation
If directional selection is to proceed for a long time, new mutations must replace lost genetic variation
In laboratory experiments, directional selection causes rapid change in phenotypes, followed by a plateau, caused by the loss of genetic variation.
The peppered moth, Biston bettularia is an example of
Directional Selection
What characteristic in the peppered moth exhibits directional selection?
Industrial melanism
It has two forms: dark (melanic) and light.
Controlled by a single locus with two alleles.
Melanic is dominant, so that MM and Mm are dark, mm is light
the moth rests on trees during the day, and uses crypsis as protection from predation by birds.
Kettlewell (1955) showed that the two forms differ in their suceptibility to bird predation.
The melanic form was rare in 1848. When it was first reported outside Manchester, it was visible against the lichen-covered trees and often eaten by birds.
Museum collections indicate that by 1898, the melanic form had increased from 98% of the population
What happened?
Soot had darkened the trees, making the light form most visible.
In rural areas with no soot, the melanic form was still rare.
(Since the passage of clean air laws in Britain, the trend has reversed, and the light form is more common once again.)
By imposing directional selection on this species, researchers have documented changes in body size, sternopleural bristle number, and life history characteristics such as age at reproduction.
Drosophilia Melanogaster
Selection in laboratory populations often produces dramatic change quickly, followed by ______ as genetic variation is exhausted
Plateaus
What type of phenotypes are the most fit?
Intermediate
When applied to a single locus, stabilizing selection implies that the heterozygous genotype is most fit, and is called
balancing selection
Generally, stabilizing selection maintains the mean value for the trait, and decreases the variation for the trait (thus, it usually decreases genetic variation).
What about in the sepecial case of balancing for a single locus?
Genetic variation is preserved since both alleles are maintained
It is well known that early mortality is highest for extreme birth weights. Both very small and very large infants suffer high mortality.
Example of stabilizing selection
Females that lay intermediate numbers of eggs have the highest reproductive success. Too many, and the offspring all starve. Too few, and the mother could have laid more. This phenomenen is called _____ and applies to many birds and also parasitoids
Lack optimum
caused by an allele that causes hemoglobin to deform under low oxygen conditions, causing the red blood cell to “sickle”.
Sickle cell anemia
Homozygotes for normal hemoglobin Hb+ Hb+ have no illness.
Homozygotes for the sickle allele HbSHBShave a very serious genetic disease.
Heterozygotes HbS Hb+ appear normal, but occasionally their blood cells sickle under stress. This is not particularly debilitating.
Sickle Cell anemia
(In countries without malaria)) There is strong selection against homozygotes for the sickle cell disease, w(HbSHBS)=0, because, they rarely survive long enough to have many offspring.
The other two genotypes have a the same relative fitness w(Hb+ Hb+)=w(HbS Hb+ )=1, because carriers are essentially indistinguishable from those possessing normal hemoglobin
Is there directional selection for or against the sickle cell allele in countries without malaria?
Against
Heterozygotes for the sickle cell allele have some limited resistance to malaria, because the cells sickle and kill plasmodium within.
The heterozygote is most fit w(HbS Hb+ )=1, w(Hb+ Hb+)=.90, w(HbSHBS)=0.
Selection acts to balance maintain both alleles because the heterozygote is favored, this is an example of balancing selection.
This explains why the original distribution of the sickle cell allele roughly matches the worldwide prevalence of malaria.
Occurs when two or more phenotypes are most fit, but the intermediates have low fitness. Not particularly common in nature.
Disruptive or diversifying Selection
In most cases, disruptive selection does what to the variance for a trait?
How does it affect the mean?
Increases Variance
Doesn’t affect mean
When disruptive selection combines with assortative mating, what type of population has the potential to perform?
Polymorphic population
Selection can be frequency-dependent
Frequency dependent selection where the most common type is the most fit
Frequency dependent selection where the least common type is the most fit
+ frequency dependent selection
- frequency dependent selection
Elderflower orchids have two colors, yellow and purple.
Populations typically have both color morphs, generally with the yellow morph being slightly more common.
Bumblebees are the primary pollinator.
Like many orchids, elderflower orchids are deceptive. They advertize to bees, but offer no nectar reward.
What occurs?
Bumblebees learn to avoid the most common morph, giving an advantage to the least common morph
Despite Darwin’s intuition, many documented examples of natural selection in the natural world show…
rapid evolution and dramatic response to natural selection
The best know study of natural selection in the wild was the study of
Galapagos Finches
What happened to the finch populations in response to drought?
Seed abundance goes down, beak size goes up to get at more difficult seeds. Population size went down because only specialized beaks could survivorship
Alleles may have different fitnesses in different environments. An allele that is favored in one environment may have a disadvantage in another environment.
!
For systems of ________, the genetic environment may affect the fitness of an allele
Epistasis
The frequency of an allele may also affect
Fitness
Ex: Sickle cell anemia, the Lap locus in mussels, color patterns in Heliconia butterflies, chromosomal inversions in Drosophila.
Environmental change may do what to the effects of selection .
Reverse them. This is the rule rather than the exception
There was no special driving force in evolution to produce human beings, or anything like us. This does not exactly make us an “accident:”, more precisely, it makes us one species among billions of potential evolutionary outcomes.
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Selection does not act for the good of the species, nor for the good of the planet.
Is selection weak or strong against rare recessive alleles?
Weak!
Why is selection weak against rare recessive alleles?
Because heterozygotes are likely to exist in the heterozygous state
How does selection act on a new mutation if the mutation is recessive?
It is very weak. Favorable recessive mutations can be lost by genetic drift before they have a chance to spread by selection
In the mutation allele balance equation, what represents the mutation rate of allele A into disadvantageous allele a?
v
v is usually on the order of what magnitude?
10^-8
A mutation-selection balance will be reached where
If the relative fitnesses of genotypes AA and Aa are 1, and the relative fitness of the aa genotype is 1-s
!
a selection coefficient that is a measure of how deleterious the recessive allele is. If it = 1, the allele is lethal, causes sterility, or zero fitness for another reason
s
q*=equilibrium frequency of allele a =
(v/s)^1/2
Sickle cell anemia, thallassemia, and cystic fibrosis are examples of
Balancing selection
What was the likely agent of selection for sickle cell anemia and thallasemia?
What about cystic fibrosis?
Malaria
Typhus
What other thing might have cause sickle cell anemia, thallasemia, Tay Sachs disease and cystic fibrosis to become more common?
Genetic Drift
