bio: microevolution

Definition of Fossils: Remains or imprints of organisms that lived in the past.
Importance of Fossils:
- Indicate that physical forms of organisms have changed over time.
- Highlight that most living organisms today differ significantly from past organisms.
- Provide evidence for extinction; approximately 99% of species that ever lived are extinct.
- Offer a historical sequence of organism changes over time.
Examples of Fossil Evidence:
- Modifications in jaw structures observed from early dinosaurs like Dimetrodon to modern Eutherian mammals.
- Gradual accumulation of bone modifications leading to mammalian evolution.

Definition: Similar characteristics in different organisms due to common ancestry.
Examples of Homologous Structures:
- The limb bones of humans, cats, whales, and bats:
  - Humans: Walk on two legs.
  - Cats: Walk on four legs.
  - Whales: Swim with flippers.
  - Bats: Fly with wings.
- Despite differing functions, all have structurally similar limb bones.
Developmental Homology:
- Early developmental stages of different organisms may show similarities that are absent in adults.
- Vestigial Structures: Remnants of structures that were functional in ancestors (e.g., post-anal tails and pharyngeal pouches in embryos).
- These structures show common ancestry but have little to no function in current organisms.

Concept: Represents the evolutionary history of life, illustrating relationships among species based on homologous structures.
Components of the Evolutionary Tree:
- Nodes: Represent most recent common ancestors of the branching species.
- Adaptations: Certain traits or physical characteristics can be plotted on the tree (marked by vertical lines).
- Time Axis: X-axis represents evolutionary time; modern time to ancient time.
Interpreting the Tree:
- Once a trait evolves, all organisms that branch after that point share this trait.
- Example: Appearance of the amnion occurred before a key branching point, leading to all subsequent branches having this trait.
Understanding Relatedness:
- The closer the branching point, the more related the organisms are.
- Identifying closely related species involves tracing back to the nearest shared branching point on the tree.
Practical Questions:
- Example Questions:
  - What species have amnion? Group is referred to as amniotes.
  - Which group is most closely related to crocodiles?: Related group includes ostriches and hawks.
  - Determine relationships by following branches back to their shared nodes.

Evolution occurs within a population, not within an individual.
Population: A group of individuals of the same species living in the same area and interbreeding.
Gene Pool: All copies of every type of allele present in all members of the population.

Alleles: Different versions of a gene (e.g., dominant "D" and recessive "d" alleles).
Genotype: The genetic makeup of an individual (e.g., "dd").
Microevolution involves changes in the frequency of alleles in a population over time.

Populations can be physically isolated, reducing interbreeding and important for speciation.
Bringing individuals from different populations together may allow for interbreeding, but usually, physical separation prevents it.

Changes in populations can be measured over successive generations through allele frequencies.
Example: Insects (like roaches) with a particular gene pool and alleles (e.g., "B" and "b").

Genetic Drift: Random selection events that change allele frequencies without regard to traits.
Natural Selection: Selection events eliminating non-resistant individuals, leading to higher frequencies of beneficial traits (e.g., insecticide resistance).
Gene Flow: Movement between populations can introduce new alleles, making populations more alike.

Pesticide Resistance: Pesticides kill a majority of pests, but resistant individuals survive and reproduce, increasing resistance in the population.
Antibiotic Resistance: Similar processes lead to antibiotic-resistant bacteria (e.g., MRSA), creating public health challenges.

To quantify evolution within a population, we can use the Hardy-Weinberg equation.
Hardy-Weinberg Principle: In a sexually reproducing diploid population, allele and genotype frequencies remain the same (equilibrium) unless influenced by outside forces.
If frequencies change, evolution is occurring.

Large Population: The population must be very large to avoid genetic drift.
No Gene Flow: No individuals should migrate in or out of the population.
No Mutations: No new alleles can be introduced through mutations.
Random Mating: Individuals must pair by chance, not by selective choice.
No Natural Selection: All individuals must have equal chances of survival and reproduction.

Alleles:
- Dominant (Big W) = Non-webbed foot
- Recessive (Little w) = Webbed foot
Starting with a population of 500 individuals, we find the genotype frequencies:
- Homozygous Dominant (WW): 320
- Heterozygous (Ww): 160
- Homozygous Recessive (ww): 20Total: 500
Calculating Allele Frequencies:
- Total alleles = 500 individuals * 2 = 1000 alleles
- Big W alleles = (320 * 2) + (160 * 1) = 800
- Little w alleles = (20 * 2) = 40Big W frequency (p) = 800/1000 = 0.8; Little w frequency (q) = 200/1000 = 0.2

Using Punnett Squares, we predict frequencies of offspring genotypes based on original allele frequencies:
- Hom. Dominant (WW) = p^2 = 0.64
- Heterozygous (Ww) = 2pq = 0.32
- Hom. Recessive (ww) = q^2 = 0.04

If genotype frequencies in offspring differ from parental generation, evolution is happening. If they are the same, no evolution has occurred.