Lecture 10 Evolutions of populations Flashcards
Misconception about evolution
Individuals organisms evolve (they do not)
● Natural selection acts on individuals
● Each organism’s traits affect its survival and
reproductive success compared with other individuals.
● The impact of natural selection only becomes apparent in how a population of organisms changes over time.
What was the experiment that showed evolution
Finches survival in drought
● Population of medium ground finches
● Drought results in only 180 survivors out of some 1,200 birds.
● Observation
○ small, soft seeds were scarce,
○ large, hard seeds were more plentiful.
● Birds with larger, deeper beaks were better able to crack and eat
the larger seeds and survived at a higher rate than finches with
smaller beaks.
● beak depth is an inherited trait in these birds, the offspring of
surviving birds also tended to have deep beaks.
slide 5-6
Microevolution
● Smallest scale - change in allele frequencies
(in a population over generations)
● In the finch population, for example
○ Alleles encoding large beaks survived at higher rates than did other
birds—causing those alleles to be more common after the drought
than they had been before it.
3 main mechanisms for evolution
○ Natural selection,
○ Genetic drift (chance events that alter allele frequencies),
○ Gene flow (the transfer of alleles between populations).
Affect the genetic composition of populations
ONLY natural selection consistently improves the degree to which organisms are well suited for life in their environment (adaptation)
Does genetic variation makes evolution possible? What are the 2 things that was concluded in Darwin and Mendel experiment
Genetic variation makes evolution possible
Darwin - observed that individuals differ in their inherited traits and that selection acts on such
differences, leading to evolutionary change.
Mendel - Inheritance in pea plants proposing a model of inheritance in which organisms transmit discrete heritable units (now called genes) to their offspring
Genetic Variation
● Without genetic variation, evolution cannot occur
● Sources:
○ Mutation, gene duplication, or other processes produce new alleles and new
genes.
■ Rapid reproduction increases mutation rates (100,000 genes per
generation in plants or animals)
■ Prokaryotes have more generations per unit of time
■ RNA Virus has higher mutation rates lack of repair mechanisms
○ Sexual reproduction - independent assortment, crossing over and fertilization
○ Changes in chromosome structure or number
Population
- group of individuals of the same species that live in the same area and interbreed, producing fertile offspring
gene pool
population’s genetic makeup: all copies of every type of allele at every locus in all members of the population.
Calculating allele frequencies
● only one allele exists for a particular locus in a population, that allele is
said to be fixed in the gene pool, and all individuals are homozygous
● two or more alleles for a particular locus in a population, individuals may
be either homozygous or heterozygous.
● Each genotype and each allele has a frequency (proportion) in the
population
● Population of 500 wildflower plants with 2 alleles
slide 12-14
The Hardy-Weinberg equation
slide 15
Genetic equilibrium
– A theoretical reference point
– allele frequencies of a population do not change
– Requires five conditions that are never met in nature, so natural populations are never in
genetic equilibrium
5 theoretical conditions of genetic equilibrium
No mutations: the gene pool is modified if mutations occur or if entire genes are deleted or duplicated
Random mating: in individuals mate within a subset of the population, such as near neighbors or close relatives (inbreeding), random mixing or gametes does not occur and genotype frequencies change
No natural selection: allele frequencies change when individuals with different genotypes show consistent differences in their survival or reproductive success
Extremely large population size: in small populations, allele frequencies fluctuate by chance over time (genetic drift)
No gene flow: by moving alleles into or out of populations, gene flow can alter allele frequencies
3 mechanisms directly affect allele frequencies
- Natural selection
- Genetic drift
- Gene flow
- Natural Selection
Differential success in survival and reproduction
slide 19
- Genetic Drift
– Random change in allele frequencies
– Can lead to loss of genetic diversity by causing alleles to become fixed
* An allele for which all members of a population are homozygous
* Especially true in small populations
– The larger the population, the smaller the impact of random changes in allele frequencies
– Example: Allele X occurs at a 10% frequency
* In a population of 10, only one person carries the allele, and if that
person dies, the allele is lost
* In a population of 100, all 10 people who carry the allele would have to
die for the allele to be lost
- can be dramatic when a few individuals rebuild a population or start a new one
slide 21-25
Founder effect and inbreeding
Change in allele frequencies that occurs when a small number of individuals establish a new population
slide 22 and 23
Bottleneck Effect
– Drastic reduction in population size so severe that it
reduces genetic diversity
– Example: Hunting reduced an elephant seal population to 20 (1890s)
– Hunting restrictions (170,000)
– the population is now homozygous at every gene
slide 24 and 25
Effects of Genetic Drift
- Significant in small populations.
- Can cause random changes to allele frequencies
- Can cause loss of genetic variation within populations.
- Can cause harmful alleles to become fixed
Gene flow
Individuals, along with their alleles, move into
and out of populations
– Stabilizes allele frequencies, so it counters the effects of mutation, natural selection, and genetic drift that tend to occur within a population
– Example: blue jays move acorns, and their alleles, among populations of oak trees that would otherwise be genetically isolated
– In plants, gene flow occurs through pollen transfer
– In Gene flow has been observed in humans, which were recently brought together
slide 28
Modes of natural selection
– Directional selection shifts the range of variation in traits in one direction
– Stabilizing selection favors intermediate forms of a trait
– Disruptive selection favors forms at the extremes of a range of variation
slide 29
What is the Effect of Directional Selection?
– Shifts an allele’s frequency in a consistent direction
– Forms at one end of a range of phenotypic variation
– Becomes more common over time
– Bell-shaped curves indicate continuous variation in a butterfly wing-color trait
Example 1 of effects of directional selection: Peppered moths
The peppered moth’s coloration
camouflages it from predatory
birds
* When the air was clean, trees were
light-colored, and so were most
peppered moths
* When smoke from coal-burning
factories changed the environment,
predatory birds ate more white
moths – selection pressure favored
darker moths
* By 1850s, dark-colored moths were
more common
Example 2 of effects of directional selection: Wafarin-resistant rats
– Rats thrive where there are people
– Wafarin became popular in 1950s
– Spreading poison exerts directional selection
– Exposure caused 10 percent of urban rats to
become resistant to Wafarin by 1980s
Stabilizing selection Example: Sociable weavers
– The body weight of sociable weavers is subject to stabilizing selection
– Body weight is a trade-off between risks of starvation and predation
* Leaner birds do not store enough fat to avoid starvation
* Predators select against birds of high body weight
– Birds of intermediate weight have the selective advantage
Disruptive selection
– Mode of natural selection that favor forms of a trait
at both ends of a range
– Midrange forms are eliminated
– Intermediate forms are selected against
slide 34
Example of disruptive selection: African seedcrackers
– Dimorphism in bill size results from competition for two types of food in the dry season
Small-billed birds are better at opening
soft seeds, but large-billed birds are
better at cracking hard seeds
Conditions favor birds with bills that are
either 12 mm wide or wider than 15 mm
Birds with bills of intermediate size are
selected against
How Does Natural Selection Maintain Diversity?
- Selection pressures that operate on natural
populations are complex; an allele may be
adaptive in one circumstance but harmful in
another - Any mode of natural selection may maintain
two or more alleles in a population
Sexual selection
Not always the environment causes selection
– Mode of natural selection in which some individuals of a population out-reproduce others because they are better at securing mates (non random mating)
– The most adaptive forms of a trait are those that help individuals defeat same-sex rivals for mates, or are the ones most attractive to the opposite sex
Sexual dimorphism
– Males and females are
different in size or another
aspect of their appearance
– Individuals (often males) are
more:
* Colorful
* Larger
* More aggressive
slide 38-39
Balancing Selection: Maintaining multiple alleles
Balanced polymorphism
* Maintenance of two or more alleles for a trait at high frequency in a population as a result of natural selection against homozygotes
- Example: the mate preferences of female Drosophila flies
Frequency-dependent selection
The adaptive value of a form of a trait depends on its frequency in the population
slide 41-44
Two opposing selective forces:heterozygous advantage
A. Distribution (by percentage) of people who
carry the sickle-cell allele..
B. Distribution of malaria cases (orange) in Africa, Asia, and the Middle East in the 1920s, before the start of programs to control
mosquitoes, which transmit the parasitic protist that causes the disease. Notice the correlation with the distribution of the sickle-cell allele in A
slide 45
