Biol 213 Midterm 2

Genetic Variation

The raw material required for evolutionary change

Heterozygosity

A population state where individuals have a high proportion of heterozygous loci, containing more genetic variation

Homozygosity

A population state where individuals have a high proportion of homozygous loci, containing less genetic variation

Evolution

Changes in allele frequencies in a population over time

Mechanisms that change allele frequencies over time

selction, genetic drift, gene flow, mutation

Mechanisms that change genotype frequencies over time

Non-random mating

Mutation

The ultimate source of all genetic variation, introducing novel differences in DNA sequences in offspring compared to their parents

SNP (Single Nucleotide Polymorphic)

A variation at a single position in a DNA sequence among individuals in a population

Genetic Drift

Unpredictable changes in allele frequencies in a population over generations due to chance, offspring represent a random sample of the gene pool of prev gen

Genetic drift is minimized

with increasing population size

Neutral Evolution

Evolutionary changes in the frequency of selectively neutral variation (which doesn't affect survival or reproduction), frequency of this not affected by natural selection but is affected by genetic drift

Effects of Genetic Drift

Loss of heterozygosity(reduction in genetic variation), change in allele freq, stronger effect in small pop, losee or fixation of alleles, and divergence over time

Loss of heterozygosity

alleles are lost at loci until only one remains and allele frequencies drift towards fixation or loss

examples of genetic drift

bottleneck effect, and founder effect

Bottleneck Effect

An extreme form of genetic drift that occurs when a population's size is sharply reduced by a sudden event, drastically lowering genetic variation

Founder Effect

An extreme form of genetic drift that occurs when a small group of individuals splits off from a larger population to establish a new colony, resulting in lower genetic diversity [cite: 144, 190]

Gene Flow

The transfer of alleles into (immigration) and out of (emigration) a population

Bidirectional gene flow

The transfer of alleles into and out of a population through both immigration and emigration.

Unidirectional gene flow

The transfer of alleles into a population without any corresponding emigration of alleles out of the population.

Effects of gene flow

genetic difference between pop(homogenizes pop), can restore genetic variation lost to genetic drift in small pop, can slow adaptation to local conditions

Natural Selection

A process where heritable variations in phenotypic traits lead to differences in fitness (differential survival and reproduction) among individuals

Natural Selection evolution Conditions

variation in phenotypic traits, variation must be heritable, variation lead to difference in fitness between individuals

Types of natural selection

directional, stablilzing, disruptive/divergent,

Directional Selection

A type of natural selection where trait values shift progressively toward one direction over time, trait value increases or decreases

Stabilizing Selection

A type of natural selection where intermediate trait values are the most fit, narrowing the trait's frequency distribution

Disruptive (Divergent) Selection

A type of natural selection where trait values shift toward both extremes, decreasing the intermediate values and splitting the population into distinct groups

Frequency-Dependent Selection

A selection process where an individual's fitness is directly tied to how common or rare its phenotype/genotype is in the population

Positive Frequency-Dependent Selection

A form of selection where the most common genotypes or phenotypes have the highest relative fitness

Negative Frequency-Dependent Selection

A form of selection where rare genotypes or phenotypes have the highest relative fitness (e.g., host-parasite immune dynamics)

Negative Frequency-Dependent Selection (Host-Parasite Example)

Parasites target the common host phenotype, driving hosts to mate with rare immune genotypes so offspring have diverse antigen proteins

Beak Depth Evolution (Daphne Major)

During the 1977 drought, finches with deeper beaks survived better because they could crack the remaining large, hard seeds, leading to the evolution of larger average beak size

1977 Drought

A severe environmental event on Daphne Major that caused a sharp decline in seed abundance and finch populations, leaving mostly large, hard seeds

Peter and Rosemary Grant

Researchers who conducted a famous long-term case study on natural selection using Darwin's finches

Natural Selection (Three Conditions)

1. Variation in phenotypic traits, 2. Heritability of the variation, 3. Variation leads to differences in fitness (differential survival/reproduction)

Adaptation

A heritable trait that increases an organism's fitness in a particular environment

Adaptive Evolution

Consistent evolutionary change driven by natural selection that makes an organism more suitable to its habitat

Sexual Selection

Differential reproductive success driven specifically by variation in success at obtaining mates

Two types of Sexual selection

intersexual, and intrasexual

Intersexual Selection

A form of sexual selection where individuals of one sex (usually females) exercise choice over their mates from the opposite sex

Intrasexual Selection

A form of sexual selection involving direct competition between individuals of the same sex (usually male-male competition) to gain access to mates

Non-Random Mating

A phenomenon where the likelihood of mating depends on phenotypic traits, which changes genotype frequencies without altering allele frequencies on its own

Assortative Mating

Mating patterns based directly on phenotypic similarity or dissimilarity

Positive Assortative Mating

Mating between phenotypically similar individuals, which increases homozygosity at the associated loci

Negative Assortative Mating

Mating between phenotypically different individuals, which increases heterozygosity at the associated loci

Inbreeding

Mating with oneself (selfing) or with closely related individuals, which is highly inevitable in small populations and increases homozygosity

Inbreeding Depression

The reduction in fitness of inbred individuals caused by increased homozygosity expressing harmful recessive alleles

Condition 1: Phenotypic Variation (Finch Case Study)

Daphne Major birds in 1976 showed clear phenotypic variation in beak depth, ranging from 6 mm to 14 mm

Condition 2: Heritability (Finch Case Study)

Finch beak depth is highly heritable, showing a strong positive correlation between midparent and midoffspring beak depth (h² = 0.74)

Condition 3: Fitness Differences (Finch Case Study)

Variation in beak depth directly influenced survival during the drought

Evidence of Finch Evolution (1976 vs. 1978 offspring)

The offspring of drought survivors (hatched in 1978) had significantly larger average beak sizes than the generation hatched before the drought (1976), proving the trait evolved

Long-term Beak Evolution Trends

Daphne Major finch traits are dynamic

Asymmetry of Sex (Females vs. Males)

Females are limited by resource-intensive, expensive, and few eggs (not mates)

Bateman's Principle

The prediction that sexual selection is more potent in driving phenotypic evolution in males than females because males are limited by mates and females are limited by eggs

Male Competition vs. Female Choosiness

Two evolutionary outcomes of Bateman's principle where males actively fight for access to females and females are highly selective in picking the "best" males

Genetic drift tends to reduce genetic variation by

Increasing homozygosity & Decreasing heterozygosity (Random loss of alleles increases homozygosity and decreases heterozygosity).

Negative assortative mating is expected to:

Increase heterozygosity (Mating between individuals with different phenotypes increases heterozygous genotypes).

Female mate choice is an example of:

Intersexual selection (Individuals of one sex choosing mates of the opposite sex based on specific traits).

Continual gene flow between two populations generally causes them to become:

More genetically similar (Transfer of alleles homogenizes allele frequencies between populations over time).

A population of birds experiences selection against individuals with both very large and very small beaks. Which type of selection is beak size under in this population?

Stabilizing selection (Selection against both extremes favors intermediate phenotypes).

Which of the following scenarios best represents a founder effect?

A small number of birds colonize a newly formed volcanic island (A small group colonizes a new area with a fraction of the original genetic diversity).

Rare colour morphs of a prey species survive better because predators preferentially search for the common morph. This is an example of:

Negative frequency-dependent selection (Phenotypes have higher fitness when rare and lower fitness when common).

A small population of frogs receives several migrants each generation from a much larger neighbouring population. Which outcome is most likely?

Greater genetic similarity between the populations (Gene flow reduces genetic divergence between populations).

A mutation causes male birds to develop exceptionally bright feathers. Males with brighter feathers attract more mates but are also more visible to predators. Why might this mutation still increase in frequency in the population?

Increased mating success outweighs the survival cost (Higher reproductive success compensates for predation risks).

A mutation arises that is neutral in effect (i.e., has no impact on fitness). It is observed to increase in frequency in the population. Which mechanism is most likely the cause of this increase in frequency?

Genetic drift (Neutral mutations are unaffected by selection; changes in frequency are driven by chance).

Mountain Lion Eye Color Practice Problem

If a mountain lion population has only 1 eye color allele (A_light), eye color is likely not evolving due to either drift or selection because there is no genetic variation to act upon

Tree Height Selection Practice Problem

Trees that are too short cannot compete for sunlight, while trees that are too tall break in the wind, meaning stabilizing selection favors intermediate height

Speciation

Simply the evolutionary process by which one original species splits over time to become two distinct species

Species Concepts

morphological, phlyogenetic, Biological and more

Morphological Species Concept

A concept defining a species as a group of individuals that share distinct morphological (physical) characteristics in common

Morphological Species Concept (Pros)

It applies easily to both sexual and asexual species, is simple to understand, and represents how most species have historically been classified

Morphological Species Concept (Cons)

It relies heavily on subjective criteria, and fails when dealing with convergent evolution, cryptic species, or high phenotypic variation within a single species

Phylogenetic Species Concept

A concept defining a species as a group of related individuals that form a clade and share a certain pre-determined degree of genetic relatedness

Phylogenetic Species Concept (Pros)

It applies to both sexual and asexual species, utilizes multiple data types (including molecular/DNA sequences), and is highly precise and objective

Phylogenetic Species Concept (Cons)

It can be very expensive and time-consuming to analyze, and a high degree of genetic relation doesn't necessarily reflect the organism's physical biology

Biological Species Concept

A concept defining a species as a group of individuals that can mate and produce viable, fertile offspring, but are reproductively isolated from other groups

Biological Species Concept (Pros)

It is experimentally testable, objective, and directly takes into account the actual reproductive biology of the organisms

Biological Species Concept (Cons)

It cannot be applied to fossil records and is entirely non-applicable to organisms that reproduce asexually

Reproductive Isolation

The existence of biological barriers that prevent individuals of different species from producing viable, fertile offspring

Pre-zygotic Isolation Barriers

Barriers that act before fertilization by making mating physically impossible or by preventing successful fertilization if mating does occur

Post-zygotic Isolation Barriers

Barriers that act after a zygote forms by ensuring the hybrid offspring are either completely non-viable or completely sterile

Reduced Hybrid Viability

A post-zygotic barrier where the offspring of two genetically distinct populations fail to survive or develop properly during their life cycle

Reduced Hybrid Fertility

A post-zygotic barrier where hybrid offspring are born perfectly healthy but are completely sterile or have severely reduced fertility (e.g., mules)

Hybrid Breakdown

A post-zygotic barrier where the first generation of hybrids is healthy and fertile, but their descendants accumulate severe health defects and the lineage becomes inviable

Two-Group Frog Breeding Practice Problem

Two identical-looking frog groups living in the same lake might be prevented from breeding by pre-zygotic barriers like temporal isolation (different breeding times) or behavioral isolation (different mating calls)

General Mechanism of Speciation

Occurs when populations become isolated with no gene flow, allowing mutations, genetic drift, and natural selection to differentiate them independently over time

Allopatric Speciation

The formation of a new species that occurs because a physical geographic barrier divides a population, entirely cutting off gene flow

Sympatric Speciation

The formation of a new species that occurs without any physical geographic barrier, taking place while populations inhabit overlapping ranges

Sympatry by Habitat Differentiation

Sympatric speciation driven by populations occupying the same geographical range but developing separate ecological niches (e.g., apple maggot flies preferring apples vs. hawthorns)

Polyploidy and Speciation

An unintentional doubling of chromosome numbers due to a mistake in meiosis

Polyploidy Reproductive Isolation Mechanism

When a haploid gamete and a diploid gamete mate, they form a triploid zygote whose homologous chromosomes cannot line up properly during meiosis, resulting in sterile gametes with uneven chromosomes

What evolutionary mechanism(s) cause(s) isolated populations to diverge over time?

Mutation, drift, and natural selection (Isolated populations without gene flow accumulate differences via these three mechanisms).

True or false: Polyploidy can immediately cause reproductive isolation.

True (Chromosomal doubling prevents proper pairing in meiosis during cross-mating, instantly creating a reproductive barrier).

Sympatric speciation differs from allopatric speciation because:

It does not require a physical barrier (Occurs in overlapping geographic ranges, unlike allopatric speciation).

True or false: The morphological species concept can be applied to both sexual and asexual organisms.

True (It groups organisms by shared physical traits rather than interbreeding capability).

Which of the following is a prezygotic reproductive barrier?

Behavioural isolation (Prevents mating or fertilization before a zygote can form).

Which species concept generally cannot be applied to fossil organisms?

Biological species concept (Relies on observing interbreeding and producing fertile offspring, which cannot be tested in fossils).

Two populations of squirrels become separated when a river changes course. Over thousands of generations, mutations, natural selection, and genetic drift cause them to diverge until they can no longer interbreed. This is an example of:

Allopatric speciation (Speciation driven by geographic separation by a physical barrier).

Two species of frogs live in the same pond. One breeds in March while the other breeds in June. What reproductive barrier is at play here?

Temporal isolation (Species reproduce at different times, seasons, or years).

Gametic Isolation

A prezygotic barrier where the sperm of one species is unable to fertilize the eggs of another species.

Mechanical Isolation

A prezygotic barrier where structural/morphological differences prevent successful mating or pollen transfer.