BIO 2200 Unit 1 & 2

Definition of Evolution

A change in the genetic composition of a population from generation to generation

What must occur for selection and evolution to occur?

Evolution can only exist if there is genetic variation

• if all traits were fixed, no variation is present, meaning no evolution or selection can occur

How does speciation occur?

When there is no genetic variation, this can reproductively isolate populations from one another

Fitness

The number of surviving offspring an organism has, quantifies evolution

• Lower fitness means that an organism has fewer offspring

Reproductive success

The number of viable offspring an individual contributes to the next generation

• measures differences within a population

What are the factors that shape fitness

• Probability of an organism surviving to a reproductive age

• Reproduction: Are you able to attract a mate?

• Number of offspring per reproductive event

• Number of reproductive events in your lifetime

• The ability for offspring to survive after birth (viable offspring)

Pre-Darwinism observations

• The Earth was much older than we thought; geological change was very slow, in the millions rather than the thousands

• Fossils of unfamiliar species/species found in unfamiliar places were discovered; the concept of extinction was not formed before this

Survival of the fittest

When populations become too big, not all individuals survive; fittest genotype become more frequent over time

The Origin of Species

• Descent with modification comes from a common ancestor

• Change in phenotype is in response the environment (controversial idea at the time)

• Natural selection is used as a mechanism of evolution

Lamarck’s theory

• Organisms change due to the environment

• Species will move towards increased complexity

• Traits change over time, improvement of these traits are passed onto offspring

• Traits that aren’t used will disappear

How do Lamarckism and Darwinism differ in explaining evolution?

• Lamarckism: Offspring inherit traits acquired during an organism’s life, Ex. A giraffe stretching its neck

• Darwinism: Natural selection favors individuals with heritable traits that enhance survival and reproduction; traits are passed down genetically, not acquired

Why was Lamarck’s theory problematic?

Acquired traits cannot be passed to your offspring, Ex. plastic surgery, an acquired trait, does not get passed onto your offspring

Describe the components of natural selection

• Many offspring are produced, but not all survive

• Traits that vary among individuals within a population may be heritable (genetic variation)

Inferences of natural selection

#1 - Some heritable traits can give individuals a fitness advantage in a particular environment

#2 - Advantageous characteristics that lead to higher fitness will make a trait more common

Can individuals evolve in the context of natural selection?

No, individuals experience selection, but only POPULATIONS evolve, as allele frequencies change over generations

An example of positive selection

Natural selection on lactose intolerance is less because there are other protein sources available; lactase is beneficial if you can digest it

Descent with modifcation

suggests that species change over generations, inheriting traits from common ancestors, gradually accumulating modifications

Homology

Similarities due to common ancestors

Anatomical Homologies

Vestigal structures such as a whale and snake pelvis bones

Developmental homologies

Many embryos look similar in the early stage, showing a shared developmental blueprint

Phenotypic plasticity

A genotype that produces different phenotypes in response to the environment, happens within a single generation

• Not heritable, the environment drives the phenotype

Ex. Daphnia water fleas, predator present/absent

• Predator present: sharper heads & spikier tails

• Predator absent: shorter tails & duller heads

How do evolution and phenotypic plasticity differ?

Evolution: a parent’s phenotype is inherited by their offspring

• Spans over multiple generations, change in genetic composition from generation to generation

Phenotypic plasticity: Not heritable, the environment drives the phenotype

• Usually happens within one generation, the trait becomes fixed, and it cannot revert back

Sources of genetic variation

• Point mutations —> random errors

• Chromosomal mutations —> insertions, deletions, inversions, etc.

• Crossing over during meiosis

• Gene flow —> movement of individuals between populations

• Genetic drift: random changes in allele frequencies, decreases genetic variation

Bottleneck effect

The population is drastically reduced, leading to a loss of genetic diversity

• Think of a bottle inverted

Founder effect

A small group starts a new population with limited genetic diversity

What did Mendel discover through crossing pea plants?

A 3:1 inheritance pattern in the F2 generation, revealing basic mechanics of heritability through dominant and recessive alleles

What are special cases of dominance?

• Codominance: both alleles are expressed (ex. AB blood type)

• Incomplete dominance: Phenotype is a blend of both alleles (ex. red + white = pink flowers)

Does dominance mean an allele is more common or adaptive?

No, dominance only refers to expression, not frequency or evolutionary advantage

How does natural selection affect allele frequencies?

Alleles that increase survival and reproduction become more common over generations

How does genetic drift impact allele frequencies?

Random changes in small populations

How does gene flow impact allele frequencies?

Introduction of new alleles from other populations

What is Hardy Weinberg?

Mathematical models, studying allele frequencies in a population with NO evolution —> helps us predict genotype and allele frequencies over time

Assumptions of Hardy-Weinberg

• No selection —> no evolution

• No mutations

• No migration —> will change the total size of the gene pool

• Large populations

• Random mating

Why is Hardy-Weinberg a null hypothesis for evolution?

The population in HW is not evolving, allele and genotype frequencies remain constant from generation to generation, with no evolutionary forces

• No significant change or effect has occurred, all allele frequencies should remain constant

What does Hardy-Weinberg equilibrium indicate about a population?

Shows expected frequencies of alleles and genotypes in the absence of evolution

• If the actual frequencies change, evolution is occurring, and one or more of the H-W assumptions has been violated

What are the five mechanisms of evolution?

1. Mutation

2. gene flow

3. nonrandom mating

4. Genetic drift

5. Selection

Mutation

Generate new genetic variation that can alter allele frequencies

• Will not cause major phenotype changes unless paired with selection

Gene flow

Immigration/emigration introduced new alleles, changing the gene pool and increasing genetic variation

How are mutation and gene flow similar?

Both generate genetic variation within a population

How does non-random mating/assortative mating impact evolution?

Mating by choice changes genotype frequencies, not allele frequencies (specific alleles, A/a) unless accompanied by selection

• In small populations, it can affect genetic structure

• Same number of alleles are present, but they are just grouped differently

What distinguishes natural selection from other mechanisms?

Natural selection leads to adaptive evolution —> traits that improve fitness increase in frequency, over time

Different distributions of phenotypes are due to

1. Directional

2. Stabilizing

3. Disruptive

Directional selection

Things are improving in one direction, a physical shift

• Beak depth before and after a drought —> shift to the right

Stabilizing selection

Acts on the middle part of the distribution, narrowing the curve, the average is the best

• 10 vs. 1000 geese, smaller chance of being hunted by predators

Disruptive selection

Selection is on both tail ends —> being the common phenotype is unfit, bimodal is preferred

• Large beaks can eat hard seeds, small beaks can eat soft seeds, medium-sized beaks can’t eat either

How does evolution by genetic drift differ from natural selection

• Genetic drift: random change in allele frequency due to chance, especially in small populations

• Natural selection: non-random, traits that increase survival will become more common —> favoring more advantageous traits

Why does genetic drift have a greater impact on smaller populations?

Random chance can more easily shift allele frequencies, possibly reducing variation and leading to fixation of alleles

Allele Fixation

When all individuals have the same allele at a locus, eliminating variation and making natural selection unable to act at that locus

Bottleneck effect in genetic drift

A sharp population decline causes loss of genetic variation; only a few genotypes survive, drastically altering the gene pool

• Sea otters hunted near extinction, reducing genetic diversity

Founder effect in genetic drift

A few individuals colonize a new area, creating a small gene pool that can carry rare/maladaptive traits

• High Huntington’s disease frequency in descendants of Dutch settlers in South Africa

Does evolution always lead to adaptation

NO, only natural selection leads to adaptive evolution; natural selection is the only mechanism that consistently favors traits to improve an organism’s fitness

• Genetic drift and other mechanisms may cause non-adaptive changes

Sexual selection

A type of natural selection that specifically affects an organism’s ability to attract mates & reproduce

Intrasexual selection

Competition within one sex for access to mates —> intra, inside

• Males fighting to be the best male

• Combat: antlers, size, agression

Intersexual selection

One sex (usually females) chooses mates based on traits that indicate fitness —> mate choice between sexes, interview for the best mate

• Courtship: bright feathers, songs, dances

What is demonstrated in sexual selection & why?

Fitness: individuals with more desirable traits are more likely to reproduce, increasing those traits in the population

In the widowbird example, females prefer males with longer tails. What type of selection is this?

Intersexual selection and directional selection

• Longer tails lead to higher mating success

• Females are choosing between males

How does sexual selection differ from natural selection?

• Sexual selection focuses on mating success

• Natural selection focuses on survival and overall reproductive success

Balancing selection

Maintains multiple forms of alleles, which helps maintain genetic diversity rather than just favoring one allele

What are the two main types of balancing selection

Heterozygote advantage and frequency dependent selection

Heterozygote advantage

When heterozygotes have greater fitness than either homozygote

• People with one sickle cell allele (HbA/HbS) resist malaria without severe sickle cell disease. Individuals with both have mild or no symptoms and are resistant to malaria

Frequency-dependent selection

Fitness depends on how common or rare a phenotype is in the population

Negative frequency dependent selection

Favors rare phenotypes, increasing genetic diversity, and maintaining polymorphism (many traits)

Positive frequency dependent selection

Favors common phenotypes, which will decrease genetic diversity

Polymorphism

The presence of multiple phenotypes in a population, maintained by negative frequency dependent selection

How do the orange and gray salamander populations illustrate balancing selection?

If predators focus on one color (e.g., gray), the other (e.g., orange) increases, and vice versa — an oscillating pattern that maintains both traits

Biological Species Concept

Defines a species as a group of interbreeding natural populations that are reproductively isolated from other groups

What does reproductive isolation mean in the context of the Biological Species Concept

Two groups cannot interbreed or produce fertile offspring, indicating that they are separate species —> does not account for hybirds as its own species

Gene flow within a species

Exchange of genetic material among individuals of the same species, which maintains species cohesion

What are the barriers to reproduction, and what is their role in the Biological Species Concept

Mechanisms that prevent interbreeding between different species, which maintains species boundaries

Why is the Biological Species Concept problematic?

• Local variation: geographic differences can affect genotype without creating reproductive isolation

• Asexual reproduction: BSC does not apply to organisms like bacteria or fungi

• Hybrids: some distinct species can produce fertile offspring, which contradicts BSC

Ecological Isolation

Species live/mate in different environments, preventing mating

• Garter snakes: one lives on water, the other on land

Temporal isolation

Breeding at different times or seasons, so there is no interbreeding

Behavioral isolation

Different mating behaviors or signals attract only specific species

• Songs, flashing patterns in fireflies

Mechanical isolation

Morphological differences prevent mating

• Incompatible reproductive organ shapes

Gametic isolation

Sperm cannot fertilize the egg due to incompatibility

Reduced Hybrid Viability

Hybird may not survive to maturity or are weak

Reduced hybrid fertile

A hybrid offspring is sterile and cannot reproduce

• Can grow normally, but cannot have babies

Hybrid breakdown

Hybrids are fertile, but their descendants weak/die out after generations

Hybridization

When reproductive isolation between two species is broken down, allowing them to interbreed

How does reinforcement impact reproductive isolation

Reinforcement strengthens species boundaries, increasing reproductive isolation

• Hybirds tend to have lower fitness, so individuals that don’t hybridize have higher reproductive success —> tradeoff

What is an example of a hybrid that illustrates reduced fitness?

Grolar bears (grizzy + polar bear), not well suited for either environment, represents a fitness middle ground

Fusion

Two species interbreed and have higher fitness, which cna lead to the merging of species or the formation of a new species

What is the effect of fusion on reproductive isolation?

Reproductive isolation will decrease due to higher reproduction rates

Stability

Hybrids are more fit only in specific times and places, so hybridization is limited and does not lead to full species fusion

Hybrid zones

geographic areas where two species meet and produce hybrids —> hybrid fitness will vary depending on the zone

What determines hybrid fitness in hybrid zones

In small or variable hybrid zones, hybrids may be more fit, outside of these zones parents ted to have higher fitness

Allopatric speciation

occurs due to geographic separation, where physical barriers prevent gene flow between populations

Allopatric speciation example

Squirrel species that is split by the grand canyon into north and south rims, which prevents interbreeding

Sympatric speciation

Speciation that occurs within the same geographical area, without physical barriers

• Often due to the emergence of a novel trait

Polymorphism

The presence of two or more distinct forms within a species can contribute to sympatric speciation —> two or more forms within a species with no barrier

How does sympatric speciation occur through sexual selection?

Mutations can lead to assortative mating —> individuals prefer to mate with those that look like them, causing reproductive isolation

Example of sexual selection leading to sympatric speciation

Blue fish only mating with blue fish, leading to reproductive isolation despitve living in the same area

Does sexual selection in sympatric speciation imply one trait is superior?

No, sexual selection is based on choice, not on one trait being better than the other

• Apple & Hawthorn maggot flies —> preference was created by CHOICE

What is an example of choice-driven sympatric speciation in insects?

Apple and hawthorn maggot flies developed mating preferences based on host plant choice

What genetic mechanisms can lead to sympatric speciation without geographic isolation?

Allopolyploidy and Autopolyploidy —> chromosome duplication leads to reproductive isolation

Autopolyploidy

A speciation mechanism caused by a cell division error within a single species, resulting in extra sets of chromosomes

What happens in autopolyploidy during cell division?

A diploid cell forms a tetraploid cell —> the resulting gametes are now diploid, instead of haploid