Allele frequencies and evolution

Flower color is controlled by a single gene with
two alleles in snapdragons. Red is dominant to
white, and heterozygous individuals have pink
flowers. What is the frequency of the recessive
allele in a snapdragon population that consists
of 568 red, 48 white, and 384 pink individuals?

A. 0.24 B. 0.43 C. 0.05 D. 0.76 E. 0.22

0.24

Hardy-Weinberg principle states that allele
and genotype frequencies of populations do
not change when these assumptions are satisfied

• no mutation

• random mating

• no trait advantages

• large population

• no migration

Equations are limited
to a specific situation:

▪ autosomal gene
▪ sexual population
▪ two alleles for gene
▪ diploid organism

Seed color is determined by one gene with two
alleles in peas, and yellow alleles are dominant
to green. If the frequency of the yellow allele is
0.72 in a population in H-W equilibrium, what
percentage of the plants produce yellow seeds?
A. 72% B. 40% C. 92% D. 8% E. 52%

92%

Ear length is controlled by one gene with two
alleles in rabbits, and long ears are dominant
to short ears. If you observe 990 rabbits with
long ears and only 10 rabbits with short ears
in a H-W equilibrium population, how many
rabbits would you expect to be heterozygous?
A. 180 B. 20 C. 810 D. 432 E. unknown

180

Assume hair curliness is controlled by one gene
with two alleles. Dominant alleles promote curly
hair, recessive alleles promote straight hair, and
heterozygotes will have wavy hair. Observations
in a large population are 72.4% straight, 21.2%
wavy, and 6.4% curly hair. Is this population in
H-W equilibrium if a 3% difference between an
observed and expected frequency is significant?
A. yes B. no C. I have no idea what is going on.

no

Evolution

change in allele frequencies over time

Processes that drive evolution correspond to the opposite of the assumptions of the Hardy-Weinberg model.

▪ no mutation → mutation
▪ random mating → nonrandom mating
▪ no trait advantages → selection
▪ large population → genetic drift
▪ no migration → gene flow

Which recessive allele frequency is required for populations in H-W equilibrium to have double the proportion of genotype rr as Rr?
A. 0.70 B. 0.20 C. 0.90 D. 0.50 E. 0.80

0.50

Which of the following genes can be evaluated
for evolution by using a standard H-W model?
A. gene for fruit size in octoploid strawberries
B. gene for X-linked color vision in Minnesota
C. gene with incomplete dominance in orchids
D. gene for ABO blood type in North America
E. gene for heat tolerance in bacterial colonies

gene with incomplete dominance in orchids

C is the correct answer because incomplete dominance would probably be easier to identify heterozygous phenotype

If one of the five thousand people who live on a small island was born with a new mutation for the autosomal TP53 gene (heterozygous), what would be the allele frequency of the mutation?

A. 0.0001 B. 0.02 C. 0.004 D. 0.01 E. 0.0002

0.0001

allele freq.= allele copies/gene copies=1/10000=.0001 

gene flow

the movement of alleles into or out of a population, which could change the allele frequencies if the movement is big enough and bring new alleles to the population

• Movement of alleles into/out of a population through migration.

• Can introduce new alleles and increase variation.

A population of 420 reindeer has a frequency of 0.5 for the dominant allele of a gene. If 140 migrating individuals with a frequency of 0.7 for the dominant allele join the population as 60 migrating reindeer with a frequency of 0.3 for the dominant allele leave the region, what is the new frequency for the dominant allele?

A.0.56 B.52 C.0.6 D.0.54 E.0.58

0.58

genetic drift

any change in allele frequencies of a population due to a random event, this could be anything from gamete sampling to natural disasters

• Strongest in small populations.

• Can eliminate alleles randomly.

Which isolated population of animals would likely be impacted the most by genetic drift?

a.72 sharks

b.536 toads

c.18 whales

d. 70 bears

e. 94 snakes

18 whales

smallest population which would impact their genetic makeup more faster

founder effect

a form of genetic drift

changes in allele frequencies when new populations arise

• A small group starts a new population, carrying a non-representative set of alleles.

• Example: Afrikaner population has high rates of Huntington’s disease due to founders’ alleles.

bottleneck effect

a form of genetic drift

when there is changes in allele frequency due to severe population losses

• Sudden reduction in population due to a random event (e.g., hurricane).

• Survivors’ genes become the entire population’s gene pool.

Willem Schalk van der Merwe was one of the earliest Dutch settlers in South Africa (1658), and researchers believe he is the main source of a high rate of Huntington’s disease among descendants of the Dutch (Afrikaners). If the settlers did not interbreed with others, which evolution mechanism contributed to the high rate of Huntington’s disease observed today?

a. gene flow

b. founder effect

c. bottleneck

founder effect

lack of interbreeding and kept to themselves 

selection

when individuals that have certain heritable traits are better able to survive and produce more fertile offspring than individuals without the traits

artificial selection

breeding organisms together so that it has the traits that u want

fitness

the ability of an individual to have fertile offspring relative to other individuals

organisms that were artificially bred from mustard seed

cabbage, cauliflower, kale, broccoli, and kohlrabi

Which lizard has the highest biological fitness?

Lizard B has the highest offspring to surviving offspring ratio

Natural selection

• the idea of Charles Darwin

• came out in 1850

• idea is just an extension of selective reproduction

  • theory that the environment determines what trait should be heritable because:

    • resources being limited

    • trait variation exists in populations with some of it being heritable

    • individuals with beneficial traits in a specific environment are more likely to survive and reproduce

    • Favors traits that increase survival and reproduction.

    • Only works on heritable traits.

directional selection

pattern in which one extreme trait is favored over another as they provide better fitness

• Favors ONE extreme phenotype

• Shifts the population mean

• Example: darker peppered moths during the Industrial Revolution

stabilizing selection

pattern where the intermediate trait will provide greater fitness

• Favors average phenotype

• Reduces variation

• Example: mice matching the forest floor color

disruptive selection

a pattern in which the extreme traits provide greater fitness

• Favors BOTH extremes

• Intermediate traits are less fit

• Can lead to two distinct morphs (e.g., big alpha males + small sneaking males)

Birds lay from one to a dozen eggs at a time,but robins almost always lay four eggs. This is an example of which category of selection?
A. artificial
B. stabilizing
C. directional
D. disruptive

stabilizing

Fossils show that the ancestors of anteaters had
teeth. Which of the following best explains why
anteaters lack teeth based on natural selection?
A. Eating ants caused a loss of teeth.
B. Teeth were not needed to survive.
C. Loss of teeth occurred randomly.
D. Teeth resulted in fewer offspring.
E. Ancestors did not use their teeth

Teeth resulted in fewer offspring

natural selection is not a targeted thing, if u don’t use one of ur arms that doesn’t mean ull lose it, organisms with a specific trait having more offspring is why traits ill evolve or become the majority

sexual selection

a form of selection that favors individuals with traits that increase the ability to obtain mates (reindeer having big antlers) (sometimes significantly reduce the ability to survive)

Asymmetry of sex

the idea that females will invest more energy into offspring than males

• females produce limited offspring and because of this they should be very choosy on their mate

• males produce unlimited offspring and could mate with any female

  • causes competition between males for females

Does inbreeding by itself cause evolution?

a. always

b. sometimes

c. never

never

Which of the evolution mechanisms below can
increase the number of alleles in a population?
A.geneflow
B.mutation
C.geneticdrift
D.selection

gene flow

mutation

Adaptations are traits that increase the fitness
of individuals in specific environments. Which
evolution mechanism will lead to adaptations?
A. gene flow
B. mutation
C. genetic drift
D. selection

selection

Nonrandom mating

can occur that affect the genetic characteristics of a population

• Mate choice (assortative mating)

• Physical separation

inbreeding

mating of individuals that are closely related to each other is an example

recessive disorders

where you inherit 2 types of nonfunctional genes is more common in inbreeding

allele frequency

how often an allele appears in a population

Allele frequencies change due to

• Natural selection

• Genetic drift

• Founder effects

• Mutations

• Gene flow

p

frequency of dominant allele

q

frequency of recessive allele

genotype frequencies

• p² = homozygous dominant

• 2pq = heterozygous

• q² = homozygous recessive
  And: p² + 2pq + q² = 1

can be acted upon by evolution

only heritable traits

Variation comes from both

• Genetic factors

• Environmental factors

Higher genetic variance

more potential for evolution

Low variation or inbreeding

higher risk of harmful recessive traits appearing (inbreeding depression).

Evolutionary forces that change allele frequencies

• natural selection

• genetic drift

• gene flow

• mutation (source of NEW alleles)

• nonrandom mating

• environmental variance

environmental variance

• Environment influences phenotype (ex: sunlight, temperature-dependent sex determination)

• Leads to geographic variation and clines

Adaptive evolution

caused by natural selection, which increases beneficial alleles.

Large or widely spread populations may experience different evolutionary pressures

different groups within the population may change in different ways

If populations become isolated in any way, their gene pools diverge

speciation becomes more likely

Morphological Species Concept

• Classifies species based on observable traits.

• First formalized by Carl Linnaeus.

• simple concept

Morphological Species Concept is often inaccurate because

• Same species can look very different (e.g., male vs female insects, castes in ants).

• Different species can look very similar (e.g., monarch vs viceroy butterflies).

Biological Species Concept

Most widely used concept

A species = group that can interbreed and produce fertile offspring in nature.

Populations that cannot or do not interbreed are reproductively isolated, meaning they’re separate species.

Examples of Biological Species Concept

• Cats × dogs → cannot mate → different species

• Horses × donkeys → offspring (mules) are infertile → separate species

• Polar bear × brown bear → hybrids exist but are extremely rare → considered separate species

Forms of reproductive isolation

• prezygotic barriers (before fertilization)

• postzygotic barriers (after fertilization)

Forms of prezygotic barriers

• geographic isolation

• temporal isolation

• ecological isolation

• behavioral isolation

• mechanical isolation

• gametic isolation

forms of postzygotic barriers

• hybrid inviability

• hybrid sterility

Geographic isolation

Physical barriers or distance prevent mating
(e.g., island finches, rivers separating insects)

Temporal isolation

Groups reproduce at different times
(e.g., 13-year vs 17-year cicadas)

ecological isolation

Populations live in different environments
(e.g., grassland squirrels vs forest squirrels)

behavioral isolation

Differences in mating behaviors
(e.g., firefly light patterns, bird songs)

mechanical isolation

Physical incompatibilities
(e.g., different reproductive structures; different pollinators in plants)

gametic isolation

Gametes cannot fuse
(e.g., marine organisms with species-specific gamete proteins)

hybrid inviability

Offspring die early or fail to develop properly

hybrid sterility

Offspring survive but are sterile
(e.g., mules have 63 chromosomes → cannot divide evenly during meiosis)

Biological Species Concept cannot apply to

• Asexual organisms (like bacteria)

• Extinct species (only known from fossils)

• Populations that rarely encounter each other

Ecological species concept

based on niche/environment

Phylogenetic species concept

based on shared ancestry

speciation

when one ancestral species splits into two or more distinct species due to long-term reproductive isolation + divergence

phylogenetic trees

Evolutionary relationships among species are shown in

• Hypotheses showing how species are related.

• Now usually built using genetic data (DNA comparisons).

• Example: The three-domain tree (bacteria, archaea, eukarya) was constructed using rRNA genes.

phylogenetics

Study of evolutionary relationships among organisms

taxa

groups shown on the tree

nodes

branching points → represent most recent common ancestors

branches

evolving lineages over time

synapomorphies

Shared, derived traits that are present in an ancestor and all of its descendants but not in more distant ancestors.

• Example: Mammals all have hair and mammary glands — these traits define the group.

Phylogenetic trees sometimes identify traits that are present in lineages that descend from a node or branch

monophyletic groups/clades

consist of a common ancestor and all of the lineages that descend from that ancestor

A group that includes:
✔ A common ancestor
✔ All of that ancestor’s descendants

This is also called a clade.

• Example: Birds, crocodiles, and dinosaurs together form a monophyletic group (Archosaurs).

parsimony

the concept that the simplest explanation is most likely to be
true

The idea that the simplest explanation (requiring the fewest evolutionary changes) is most likely correct.

• Used in building phylogenetic trees — the best tree is the one with the fewest total trait or DNA changes.

homologous traits

Traits that are similar because they were inherited from a common ancestor, even if they now look or function differently.

• Example: Bat wings, human arms, and whale flippers all share the same bone structure.

analogous traits

Traits that look or function the same but evolved independently, not from a shared ancestor.

• Example: Bird wings and insect wings.

convergent evolution

where different organisms independently evolve similar traits that have the same function rather than inherit the traits from a recent common ancestor.

When different species independently evolve similar traits because they live in similar environments or experience similar pressures.

• Example: Sharks (fish) and dolphins (mammals) both evolved streamlined bodies.

divergent evolution

When two or more species evolve different traits from a shared common ancestor because they face different environments or selective pressures.

• Example: Darwin’s finches evolving different beak shapes.