Speciation and Phylogenetic Trees

Speciation and Genetic Processes

• Sympatric speciation occurs within the same geographic area.
• Random mating prevents genetic isolation, while nonrandom mating leads to it.

Factors Affecting Speciation

• Polyploidy, a condition of having more than two sets of chromosomes, can cause speciation due to errors in meiosis/mitosis.
• Autopolyploidy occurs when an organism gets extra chromosomes from its own species.
• Allopolyploidy occurs when an organism gets chromosome sets from different species, accounting for 80% of plant species speciation.

Anatomy of a Phylogenetic Tree

• Tips represent extant taxa, not ancestors of the other taxa on the tree.
• Branches represent evolutionary lineages.
• Branch points (nodes) signify the most recent common ancestor of diverging lineages.
• The root represents the last common ancestor of all taxa on the tree.
• Sister taxa are the closest taxa that share a recent common ancestor.
• Synapomorphies possessed by the most recent common ancestor, are shared by two or more taxa
  • Examples include hair and lactation in mammals.
• The "one snip test" determines if a group is a clade: cutting below a point removes all descendants of a common ancestor.
• A monophyletic group (clade) includes an ancestor and all its descendants.
• A paraphyletic group includes an ancestor and some, but not all, of its descendants.
• A polyphyletic group includes distantly related species but not their most recent common ancestor.
• Taxa order at tips is not significant, only the branching order matters.
• Branches on a tree can rotate without changing the tree's meaning.

Species Concepts

• The phylogenetic species concept identifies species based on shared evolutionary history and common ancestry.
• Species identified on phylogenetic trees as a monophyletic group (clade or lineage).
• Different species will have different synapomorphies because they lack gene flow, resulting in independent evolution.
• Useful for living, asexual, and fossil organisms.
• May lead to recognizing more species than other concepts.
• Reproductive isolation prevents species from interbreeding.

Prezygotic Barriers

• Prezygotic barriers impede mating or hinder fertilization.
  • Habitat isolation: Species occupy different habitats.
  • Temporal isolation: Species breed at different times.
  • Behavioral isolation: Unique courtship rituals prevent mating.
  • Mechanical isolation: Physical barriers prevent mating.
  • Gametic isolation: Gametes cannot fuse to fertilize

Postzygotic Barriers

• Postzygotic barriers prevent hybrid zygotes from developing into viable, fertile adults.
  • Reduced hybrid viability: Hybrid offspring survival is impaired.
  • Reduced hybrid fertility: Hybrids are sterile due to meiosis failure.
  • Hybrid breakdown: Later-generation hybrids are not viable.

How Speciation Occurs

• Genetic isolation and divergence are key in speciation.
• Allopatric speciation begins with geographic isolation of populations.
  • Dispersal: Movement of individuals to a new location.
  • Vicariance: Physical splitting of a habitat.

Sympatric Speciation

• Example: Killer whales may be undergoing sympatric speciation into resident (fish-eating) and transient (mammal-eating) ecotypes due to ecological differences.
• Polyploidy: Condition of possessing more than two complete sets of chromosomes due to error in mitosis or meiosis
• Microevolutionary Forces: Genetic Drift

Allele Frequency Calculation

• To calculate the frequency of an A allele in a sample of 50 individuals (10 AA, 30 Aa, 10 aa):

• p = [(2 x number of AA individuals) + number of Aa individuals] / (2 x total number of individuals).

• p = [(2 x 10) + 30] / 100 = 0.5

• The frequency of the AA genotype in the sample is .2, or 10/50 individuals.

Hardy-Weinberg Equilibrium

• Hardy-Weinberg equation: p² + 2pq + q² = 1, where p and q are allele frequencies and p², 2pq, and q² are genotype frequencies.
• Conditions required for Hardy-Weinberg equilibrium:
  • No mutations
  • Random mating
  • No natural selection
  • Extremely large population size
  • No gene flow.

Using the Hardy-Weinberg Equation:

• Determine allele frequencies (p and q).
• Calculate expected genotype frequencies (p², 2pq, q²) under Hardy-Weinberg equilibrium.
• Sample the population and calculate actual genotype frequencies.
• Compare actual and expected frequencies.
  • If different, the population is evolving; if the same, the population is in equilibrium and not evolving.

Microevolutionary Forces

• Genetic drift: Chance events cause unpredictable allele frequency fluctuations.
  • Reduces allele diversity by allele loss or fixation, leading to increased homogeneity.
  • Significant in small populations.
• Founder effect: A few individuals start a new, isolated population.
  • Allele frequencies in the new population differ from the source population.
• Bottleneck effect: Population size drastically reduces due to environmental change.
  • The remaining gene pool likely does not reflect the original population.
• Alleles can be tranferred through the movement of fertile individuals or gametes
• Makes populations more similar to each other over time

Natural Selection

• Natural selection consistently causes adaptive evolution.
• Other mechanisms will be neutral or bad

Speciation Overview

• Microevolution: Changes in allele frequency within a population over time.
• Macroevolution: Broad evolutionary changes above the species level.
• Genetic isolation: Barriers to gene flow that isolate populations.
• Genetic divergence: Accumulation of differences between isolated populations due to mutation, selection, and drift.

Species Identification Criteria

• Biological species concept
• Morphological species concept
• Phylogenetic species concept

Biological Species Concept

• Pros: Relatively easy and consistent.
• Cons: Not applicable to fossils or asexual organisms; some species can interbreed in captivity but not in nature.

Morphospecies Concept

• Pros: Easy to apply; usable for living organisms and fossils.
• Cons: Subjective.

Explore sympatric speciation that occurs within the same geographic area, polyploidy, autopolyploidy and allopolyploidy. Dive into anatomy of phylogenetic tree and understand tips, branches, branch points, root, sister taxa and synapomorphies.