Hardy-Weinberg Principles & Phylogenetics: Evolutionary Genetics & Tree Building

Hardy-Weinberg Equilibrium

A principle that describes the genetic variation in a population that is not evolving.

Hardy-Weinberg assumptions

Conditions that must be met for Hardy-Weinberg equilibrium to be valid, including no mutation, no selection among genotypes, no gene flow, random mating, and an infinite population size.

Diploidy

A condition that hides recessive alleles in a population, helping to maintain genetic variation.

Fluctuating selection

A phenomenon where different traits are preferred at different times due to environmental changes, preventing the fixation of one particular allele.

Heterozygote advantage

A situation where heterozygotes maintain multiple alleles in a population if they have higher fitness than either homozygote.

Evolutionary constraints

Factors that limit evolutionary possibilities, including physical and chemical laws, lack of genetic variation, evolutionary history, and trade-offs.

Trade-offs

A situation where a trait that improves fitness in one scenario can reduce fitness in another scenario.

Natural selection

A process that can reduce variation in a population, while variation serves as 'fuel' for evolution.

Genetic variation

The diversity of alleles and genotypes within a population.

Fitness

The ability of an organism to survive and reproduce in its environment.

Mutation

A change in the DNA sequence that can lead to genetic variation.

Gene flow

The transfer of genetic material between populations, which can affect allele frequencies.

Random mating

A mating system where individuals pair by chance, not according to their genotypes or phenotypes.

Population size

The number of individuals in a population, which is assumed to be infinite for Hardy-Weinberg equilibrium.

Phylogenies

Used to reconstruct evolutionary history

Phylogenetic Trees

Horizontal branching + branch length convey information; Vertical position and order do not

Clade

Taxon that includes all evolutionary descendants (from node to tip)

Sister taxa

Lineages that share a most recent common ancestor

Sister clades

Lineages that share a most recent common ancestor

Synapomorphy

Used to identify clades + revealed shared patterns of evolutionary history

Derived traits

Indicate evolutionary relationships

Homologous traits

Shared similar evolutionary origins [divergent evolution]

Analogous traits

Evolved independently for SAME function [convergent evolution]

Evolutionary reversal

Lineage reverts to an ancestral trait

Homoplasy

Similar trait that evolved INDEPENDENTLY (either via convergent evolution or through evolutionary reversal)

Building Phylogenetic Trees

[commonly used genetic traits are either morphological (size, shape etc) or molecular (DNA & protein sequences)]

Identify synapomorphies

Shared derived traits among a group; This will also help identify common ancestors

Parsimony principle

Best phylogenetic tree requires the FEWEST evolutionary changes

Stabilizing selection

Babies with extremely high or low birth weight are at a higher risk of health complications and mortality. As a result, there is a selective advantage for newborns with intermediate birth weights, associated with better survival rates and overall health.

Directional selection

Before the Industrial Revolution, most moths in England had light-colored wings with dark speckles, which provided camouflage against light-colored tree bark and lichens. However, as industrialization progressed, soot and pollution darkened the tree's bark, making the light-colored moths more visible to predators. This change resulted in selection of favoring dark-colored moths, due to better camouflage.

Disruptive selection

Galapagos finches have different beak sizes that are adapted to their specific feeding habits. During a drought, many food sources are lost, leaving mostly seeds of variable sizes. Birds with small beaks are better adapted to feeding smaller seeds, and birds with large beaks are better adapted to opening large, tough seeds. Meanwhile, birds with intermediate beaks will struggle to feed on either of the seeds.

Population genetics example

In a population of 500 rabbits, a gene that determines fur color has two alleles, B (for brown fur), and b (for white fur).

Allele frequency calculation

In a population of 1000 wildflowers, the allele for purple petals (P) is dominant over the allele for white petals (p). In this population, 360 wildflowers have purple petals, whereas 640 have white petals.

Frequency of homozygous recessive genotype (pp)

Calculated as 640/1000 = 0.64

Allele frequency for P

Calculated as p = (1 - 0.8) = 0.2

Expected number of wildflowers with purple petals

360, calculated from total frequency = 0.36 * 1000

Frequency of homozygous recessive genotype (bb)

Calculated as 900/2500 = 0.36

Allele frequency for B

Calculated as p = (1 - 0.6) = 0.4

Expected number of heterozygous beetles (Bb)

Calculated as 1200, from Bb = 2pq = 20.40.6 * 2500

Common ancestor relationship

The common ancestor shares DNA with each of the other organisms

Monophyletic group

Composed of most recent common ancestors and all of its descendants

Synapomorphic trait example

Venom

Non-synapomorphic traits for lineage

Detachable tail, feathers

Traits possessed by bald eagle

Jaws, lungs, gizzard, feathers

Common ancestor of bald eagle

A

First trait to evolve

Lungs

Paraphyletic group

Group that contains common ancestor and some descendants

Monophyletic group definition

Group of organisms that includes an ancestor and all its descendants

Polyphyletic group

Group that does not include the common ancestor

Taxon

Any named group of organisms at any level

Node in phylogenetics

Point at which lineage branches out from a common ancestor

Plesiomorphy

an ancestral trait, originally present in the ancestor of a given group

apomorphy

derived trait; evolved later and is different from the ancestor