LEC 6

Evolution

A process driven by natural selection, resulting in changes to populations of organisms over time and leading to speciation.

Darwin’s observation on the Galápagos Islands

1. Common Ancestry: The finches on the archipelago bore resemblance to mainland finches, suggesting a shared common ancestor.

2. Morphological Diversity: Variance in morphology, especially beak size and form, among the species indicated adaptation to different environmental niches.

3. Natural Selection: These observations led Darwin to postulate that natural selection was a mechanism driving evolutionary change

Trait

A heritable characteristic that can influence the survival and reproduction of organisms within a particular habitat, encompassing physical, physiological, and behavioral attributes.

Alleles

Distinct forms of a gene (e.g., M, m).

Gene Pool

The complete set of alleles within a population.

Individual Fitness

The relative genetic contribution of an individual to the gene pool of the next generation.

Genotype

The genetic makeup of an organism.

Phenotype

Observable attributes, including behavior, morphology, and physiology.

In the phenotypic variation equation: VP = VG + VE , what does VP mean?

Phenotypic variance

In the phenotypic variation equation: VP = VG + VE , what does VG mean?

Variance due to genetic effects

In the phenotypic variation equation: VP = VG + VE , what does VE mean?

Variance due to environmental effects

What does the Hardy-Weinberg principle state?

That allele and genotype frequencies in a population will remain constant from generation to generation in the absence of other evolutionary influences.

What are the evolutionary influences according to the Hardy-Weinberg principle?

• No mutation. No new alleles are generated by mutation, nor are genes duplicated or deleted.

• Random mating. ...

• No gene flow. ...

• Very large population size. ...

• No natural selection.

Bottleneck

A drastic reduction in population size, leading to reduced genetic diversity.

Founder Effects

Occurs when a small group of individuals establishes a new population, carrying only a fraction of the genetic variation present in the source population.

Natural Selection

Non-random evolutionary process in which individuals with beneficial phenotypic traits tend to survive and reproduce at higher rates.

Key Principles of Natural Selection

1. More offspring are produced than can survive.

2. Traits exhibit variation within populations and can be heritable.

3. Certain heritable traits confer advantages, enhancing individual fitness.

4. Traits associated with increased fitness become more frequent over generations.

Directional Selection

Favors an extreme phenotype, effectively shifting trait distributions; as seen in the increased frequency of deeper beaks in medium ground finches due to drought conditions.

Stabilizing Selection

Intermediate phenotypes increase in frequency, whereas extreme phenotypes decrease; as illustrated by egg sizes in Ural owls where very large or very small eggs exhibited lower hatching rates.

Disruptive Selection

Extreme phenotypes are favoured over intermediate phenotypes. In the case of medium ground finches, partners exhibit non-random mating based on beak size preferences.

Adaptation

Results from evolutionary processes where natural selection leads to alterations in anatomy, physiology, or behaviour, enhancing an organism's ability to survive in its environment.

How do the mechanisms of evolution work together?

They act symbiotically to shape biodiversity, influence species interactions, and foster adaptations critical for survival in changing environments.

What are the mechanisms of evolution?

Genetic mutation, gene flow, genetic drift, and natural selection.

Speciation

The process by which two or more species arise from a common ancestor.

What are the characteristics of speciation?

• Genetically different from one another.

• Can no longer breed.

Species

A name and classification that humans give to living things, they can be grouped based on: morphological, biological, or phylogenetic criteria

Morphological Species Concept

Species are grouped by morphological similarities or structural traits.

Biological Species Concept

Species are grouped by their ability to interbreed, produce viable offspring, as well as their reproductive isolation from other such groups.

Isolating Mechanisms

Factors that prevent two species from producing viable offspring

Prezygotic Mechanisms

Reproductive barriers that occur before fertilization, preventing mating or fertilization between different species, eg. mechanical, ecological, temporal, and behavioural barriers

Postzygotic Mechanisms

Reproductive barriers that occur after fertilization, affecting the viability or fertility of hybrid offspring, such as hybrid inviability (zygotes do not reach maturity) or hybrid sterility (offspring reach maturity but cannot reproduce)

Phylogenetic Species Concept

Allopatric Speciation

Definition: Occurs when geographically isolated populations are formed.

Mechanism: When a single population becomes spatially subdivided into multiple subpopulations.

Parapatric Speciation

Definition: Occurs in adjacent populations with limited overlap and reduced gene flow.

Mechanism: Results from local adaptation and reduced mating between distant individuals across environmental gradients.

Sympatric Speciation

Definition: Occurs within the geographical range of the ancestral population.

Mechanism: Results from non-spatial isolating mechanisms like positive assortative mating.