Darwinian Evolution: Natural Selection, Speciation, and Mechanisms

Posted on Mar 24, 2025 in Biology

Lesson 1: Darwin and Natural Selection

Evidence for Evolution:

• Selection is a plausible mechanism for species change.
• The fossil record shows species change over time (e.g., whale hind legs).
• Homology of forelimb structure and comparative embryology suggest common ancestry.
• Biogeography showed patterns of distribution of organisms consistent with ancestor-descendant relationships (e.g., beak sizes).

Evolution of drug resistance and viruses is important to our health and society.

The State of Knowledge Before Darwin:

1. Scientists recognized that organisms were adapted to their environments.
2. Variation among individuals within a species was considered unimportant (Darwin: variation is critical).
3. The constancy of species had been questioned.
4. Shared common ancestry of species had been suggested.
5. A role of the environment in driving species change had been suggested. (Natural selection is “blind” and “uncaring”).

Jean-Baptiste Lamarck contributed the concept of “Inheritance of acquired characteristics” (e.g., giraffe neck growing, buff baby). Thomas Malthus, an economist who studied population growth, influenced Darwin with the idea that there is a “struggle for existence,” and populations will increase faster than their resources, leading to suffering and death.

Darwin observed the fauna of South America (toothless mammals), the rugged Andes Mountains (evidence of recent violent geological activity), and finches with many different beaks, suggesting evolution from a common ancestor. THE STRUGGLE FOR EXISTENCE LEADS TO SURVIVAL OF THE FITTEST. = Darwin’s mechanism. Alfred Russel Wallace was about to publish his own theory on evolution by natural selection, but Darwin did it first.

Origin of Species (1859) – 2 Themes: Descent with modification and Survival of the fittest

Essential Elements of Evolution by Natural Selection:

1. Phenotypic variation.
2. Heritability.
3. Differential survival/reproduction of phenotypes.

Fitness = the number of offspring produced by an individual.

Types of Selection: 1- Directional, 2- Stabilizing, 3- Disruptive

Lesson 2: Hardy-Weinberg Equilibrium

Evolution = change in allele and genotype frequencies in a population (a species that shares a gene pool) over time.

p + q = 1, where p = frequency of the dominant allele and q = frequency of the recessive allele.

p² + 2pq + q² = 1

Lesson 3: Mechanisms of Evolution

Conditions Required to Maintain Hardy-Weinberg Equilibrium:

1. All individuals survive and reproduce equally well (i.e., no natural selection).
2. There is no immigration or emigration.
3. There is no mutation.
4. The population is large.
5. Mating is random.

Gene flow = the exchange of alleles among populations (emigration or immigration changing allele frequency) makes allele frequencies between populations more similar to each other.

Genetic drift = the effect of chance on allele frequency in small populations. It can reduce genetic variation and sometimes lead to fixation.

Bottleneck = a temporary period of small population size, like a disaster, in which subsequent recovery maintains the change in allele frequency.

Founder effect = effect on (“change” in) allele frequency of the founding of a new population by a small number of colonists, which promotes drift. An example of this is the Amish and their six-fingered dwarfism.

Non-random mating = mating based on genotype; affects the frequency of genotypes in a population, but does not change allele frequency in the gene pool.

Positive assortative mating = preferring mates with a phenotype similar to their own. Increases frequency of homozygous genotypes.

Negative assortative mating = preferring mates with a different phenotype than their own. Increases frequency of heterozygous genotypes.

Lesson 4: Speciation

Microevolution = evolutionary change in populations (within a species). Macroevolution = the history of the diversification of life (above species level).

Species Concepts:

• Morphological Species Concept: Species are defined on the basis of measurable morphological differences. Easy, but polymorphism makes it difficult (e.g., male or female ducks, caterpillars to butterflies).
• Biological Species Concept: Species are groups of interbreeding natural populations that are reproductively isolated from other such groups (these groups must produce viable, fertile offspring and cannot produce viable fertile offspring with other groups). Good because of polymorphism, but reproductive isolation makes it hard to demonstrate and impractical (e.g., North and South blue whales), also no fossil evidence, and doesn’t apply to asexual animals.
• Recognition Species Concept: Organisms that recognize each other as mates.
• Ecological Species Concept: Share the same niche or adaptive zone.

Reproductive Isolating Mechanisms:

• Prezygotic: Prevents formation of hybrid zygotes.
• Postzygotic: Barriers result from low fitness of hybrids.

Prezygotic Barriers:

1. Ecological/Habitat isolation = species in different niches or habitats never meet.
2. Temporal isolation = species habitats overlap, but they mate at different times.
3. Behavioral isolation = do not recognize each other as mates because of different behavioral rituals.
4. Mechanical isolation = structural differences in genitalia prevent copulation.
5. Gametic isolation = incompatibility between gametes of different species prevents hybrid zygotes from forming (protein recognition).

Postzygotic Barriers:

1. Reduced hybrid viability = hybrid zygotes fail to develop or fail to reach sexual maturity.
2. Hybrid breakdown = offspring of hybrids have reduced vitality or fertility.

Hybrids are sterile because they can reach sexual maturity, but homologous chromosomes cannot pair during Prophase I, so meiosis stops and no gametes are produced.

Speciation: The formation of new species by the division of a single gene pool into two pools capable of independent evolution.

Polyploid = organism with more than 2 sets of chromosomes (e.g., 4n)

Autopolyploidy = they have extra set(s) of chromosomes from the same species. Results in rapid speciation. Allopolyploidy = they have more than two sets of chromosomes all derived from different species.

Steps of Allopolyploidy:

1. Hybridization between two species produces sterile offspring.
2. Chromosome doubling restores fertility.

Instantaneous speciation = rapid formation of a new species like auto- or allopolyploidy. Gradual speciation = slow reproductive (genetic) isolation over a long time.

Allopatric = isolation due to a geographic barrier. Sympatric = isolation without a geographic barrier.

Sexual Selection: Sometimes, the ability to mate can have a stronger effect on fitness than escaping predation.

• Intersexual selection = female mate choice (selection between sexes).
• Intrasexual selection = male competition for access to mates (selection within sex).

Lesson 6: Darwin’s Limits of Natural Selection

1. The paradox of variation for natural selection: evolution requires variation, but natural selection eliminates variation – Shouldn’t all the variation eventually run out?
2. What is the mechanism by which traits are inherited? Mendel’s work helps resolve these issues.

Mechanisms that Preserve/Promote Polymorphisms in a Population:

1. Meiosis + Diploidy – independent assortment, crossing over, random fertilization, heterozygosity
2. Epistasis – one locus can “protect” alleles at another locus from selection
3. Disruptive selection in “patchy” environments

Heterozygote advantage = higher fitness for heterozygotes, like in genetic diseases.

Homologous = traits similar due to common ancestry. Analogous = traits between species due to common function. Vestigial = traits inherited from ancestral forms but are no longer functional.