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Lecture 4

BIOLOGY 1M03 Lecture 6: BIOLOGY 1M03 Chapter 25 Part 2


Department
Biology
Course Code
BIOLOGY 1M03
Professor
Jon Stone
Lecture
4

Page:
of 4
BIOLOGY 1M03- Chapter 25 Evolutionary Processes Part 2
Experimental Studies of Genetic Drift
Research on genetic drift in small fruit fly populations involved a genetic marker, the
gene for leg-bristle morphology. This gene has two alleles, one resulting in straight leg
(wild-type) bristles and the other in forked (bent) bristles
In 70 among 96 (n= 8, p= 0.5, s= 1) populations studied, genetic drift caused one allele
to be lost
In the lab, genetic drift was found to decrease genetic variation within populations and
increase genetic differences between populations
What Causes Genetic Drift in Natural Populations
Genetic drift is of great concern to conservation biologists because the small
populations found on nature reserves or in zoos are especially susceptible to it
Genetic drift can be caused by any event or process that involves sampling error, not
just the sampling of gametes that occurs during fertilization. Two examples the founder
effect and bottlenecks
Founder Effect on the Green Iguanas of Anguilla
A founder event occurs when a group emigrates to a new area and starts a new
population
If the founding group is small, its allele frequencies probably differ from those in the
source population
This sampling effect on the new population allele frequencies is called a founder effect
Founder effects are especially common in the colonized isolated habitats such as
islands, mountains, caves, and ponds
Each time a founder even occurs, a founder effect is likely to accompany it, changing
allele frequencies through genetic drift
Population Bottleneck
A sudden decrease in population size called a population bottleneck
Population bottlenecks are caused commonly by disease outbreaks and natural
catastrophes
Population bottlenecks lead to genetic bottlenecks- a sudden reduction in allele number
in a population
Genetic drift occurs during genetic bottlenecks, resulting in changes allele frequencies
25.4 Gene Flow
Gene flow (migration), the movement of alleles from one population to another, occurs
whenever individuals leave one population, join another and breed
Gene flow equalizes allele frequencies between the source and recipient populations
Gene flow may be unbiased with respect to fitness
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25.5 Mutation
Although most evolutionary mechanisms genetic diversity, mutation increases genetic
diversity by creating new alleles
Mutation is stochastic and intrinsically unbiased
Most mutations that produce changes in sequences encoding functional products result
in deleterious alleles– alleles that lower fitness; these should be eliminated by natural
selection, so typically are unobserved in populations
Mutations that produce changes in sequences encoding functional products less
commonly produce beneficial alleles; when this occurs, the beneficial alleles should
increase in frequency in a population due to natural selection (and, ultimately,
substitute for the alleles that were present prior to mutation)
Mutation as an Evolutionary Mechanism
As an evolutionary mechanism, mutation effects change relatively slowly compared with
natural selection, genetic drift, and gene flow
Mutation probably introduces new alleles into every individual in every population in
every generation
Highest rate recorded in humans 1 in 10000 gametes
When two gametes unite to form zygote, 1 in 5000 carries mutation
Imagine population with n = 195000 and 5000 newborns
400000 copies for each gene, among which 1 is mutant, new allele
Change would be 1 in 400000 = 2.5 x 10-6
Experimental Studies of Mutation
The bacterium Escherichia coli has been used as a model to study how mutation affects
evolution. In one experiment, Richard Lenski and colleagues set up E. coli populations
and followed them for 10000 generations (periodically storing samples by freezing them
for later analysis).
E. coliis asexual, so mutation is its only source for genetic variation. Although no gene
flow occurred, selection and genetic drift operated in each population
The researchers found that the relative fitness (measured as growth rate) in the
populations increased over time in jumps.
They suggested that this pattern resulted from novel mutations arising and conferring a
fitness benefit.
Thus, –mutation provides the ultimate source for genetic variation; –without mutation,
evolution eventually would stop; –mutation alone usually imparts little effect on allele
frequencies (but can in combination with other factors)
25.6 Biased Mating
In nature, mating may be biased with respect to any particular gene in question.
Mechanisms that violate the Hardy-Weinberg assumption for unbiased mating are
inbreeding, dis- & assortative mating, and sexual selection
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find more resources at oneclass.com
Inbreeding
Inbreeding (mating between relatives) increases homozygosity (homozygous frequency)
and reduces heterozygosity (heterozygous frequency) in each generation.
Inbreeding, as exemplified most-conspicuously in the extreme –by selfing, does not
cause allele frequencies to change in the population as a whole; it changes genotype
frequencies
Inbreeding depression is a decline in average fitness that takes place when
homozygosity increases and heterozygosity decreases in a population
Even though [inbreeding] does not cause evolution directly –because it does not change
allele frequencies – it can speed the rate of evolutionary change by increasing the rate
at which purifying selection eliminates recessive deleterious alleles
Recessive deleterious alleles should be rare and, so, appear predominantly in
heterozygous individuals and, so, evade negative selection
When inbreeding occurs, recessive deleterious alleles are found predominantly in
homozygous individuals and eliminated rapidly via negative selection
Dis and Assertive Mating
Dis- & Assortative mating occurs when mating is biased with respect to traits.
+: like prefers like (assortative)
-: opposites attract (disassortative)
Sexual Selection
Sexual selection occurs when individuals within a population differ in their ability to
attract mates.
Sexual selection favors individuals with heritable traits that enhance their ability to
obtain mates.
Sexual selection may be considered as “a special case of natural selection.”
Sexual selection operates to change allele frequencies.
The fundamental asymmetry of sex (Bateman-Trivers theory) is that, in most species,
females invest more in their offspring than males do.
Therefore, females should be choosy about their mates, while males should be willing to
mate with almost any female (and so could have to compete with each other for mates).
If male fitness is limited by access to mates, then any trait and its underlying alleles that
increase male attractiveness to females or success in male-male competition should
increase rapidly in the population. Thus, sexual selection should operate more strongly
on males than on females
Females may choose mates on the basis of:
Physical characteristics that signal male genetic quality (‘good genes’) and/or
Parental care or resources (to reproduce) provided by males.
Research has shown for example that female zebra finches preferred males with more
colorful beaks and feathers (showing health) and female kiwis mate with males that
take over incubation and care for offspring after females have laid eggs.
find more resources at oneclass.com
find more resources at oneclass.com