Natural and Artificial Selection

• Natural selection is a process where individuals with certain inherited traits survive and reproduce at higher rates.

Types of Natural Selection

• Stabilizing selection favors intermediate phenotypes.
• Disruptive selection favors extreme phenotypes.
• Directional selection favors one extreme phenotype.

Stabilizing Selection

• It acts against both extreme phenotypes.
• An example is the birth mass of human babies, where intermediate weights have higher survival chances.

Disruptive Selection

• It acts against intermediate phenotypes.
• Bird beak size exemplifies this where small and large beaks are favored.
• Birds with medium beaks cannot crack large seeds or reach small seeds.

Directional Selection

• It acts against the other extreme and intermediate phenotypes.
• Peppered moths exemplify this where the color shifted from light to dark during the Industrial Revolution.

Artificial Selection

• It is a process where humans alter an organism's genotype by choosing favored characteristics.
• This produces new strains for specific purposes.
• Selective breeding is an example seen through domesticating plants and animals.

Inbreeding

• It involves selective reproduction between individuals with similar genotypes or are closely related.
• This can include breeding between siblings or offspring with one parent.
• Self fertilization in many plants and marriage between cousins in humans are examples.
• Some disadvantages are: a decrease in genetic variations, hybrid vigor, poor growth, less fertile individuals, and less resistance to disease.

Outbreeding

• It involves mating between individuals that are not closely related or distantly related.
• The hybrid has greater adaptation to environmental changes.
• An example is crossing oil palm Pisifera with oil palm Dura in Malaysia, which produces hybrid oil palm Tenera.
• Disadvantages include: hybrids are often sterile because they do not have sets of homologous chromosomes and will only become fertile after chromosome doubling.

Definition of Species

• A species is a population able to interbreed in nature as well as produce viable, fertile offspring.
• They are reproductively isolated from other populations.
• Two populations are distinct species if they do not interbreed in nature, or if they mate but produce sterile offspring.

Speciation

• This is an evolutionary process where one species splits into two or more species.

Allopatric Speciation

• It occurs when a population is separated by geographical barriers.
• Gene flow is prevented due to spatial separation leading to reproductive isolation.

Sympatric Speciation

• It occurs when a population is reproductively isolated within the same geographical area.
• This does not involve geographical barriers.
• It involves biological barriers (e.g. polyploidy) and reproductive isolating mechanisms.

Reproductive Isolation

• Reproductive isolation is the inability of a species to breed successfully with related species.
• Reproductive isolation can occur before fertilization (pre-zygotic) or after fertilization (post-zygotic).

Pre-Zygotic Isolation

• This prevents mating between individuals or fertilization between gametes.
• Therefore there is no formation of a zygote.
• Temporal isolation: Different species reproduce at different times
• Habitat Isolation: Different species live in different habitats within the same geographical area
• Behavioral Isolation: Different animal species have different courtship patterns
• Mechanical Isolation: Different animal species have different genital structures or different plant species have different floral structures.
• Gametic Isolation: Fertilization cannot occur between gametes from different species

Post-Zygotic Isolation

• Fertilization occurs but there is no formation of viable, fertile offspring.
• Hybrid inviability where hybrids are not viable or die.
• Hybrid sterility where hybrids develop into sterile adults.
• Hybrid breakdown leads to viable and fertile F1 generation, but the next generation is sterile.

Genetic Drift

• Genetic drift is a change in allele and genotype frequencies of a gene pool due to chance rather than selection.
• Founder effect occurs when a new colony is started by a few individuals, leading to reduced genetic variation
• Bottleneck effect is a drastic decrease in population due to adverse environmental factors.

Hybridization

• It refers to allopolyploidy.
• It occurs when different species interbreed, generally producing sterile hybrids.
• Chromosomes cannot pair up during meiosis because they are not homologous.
• After chromosome doubling, the sterile hybrid changes to a fertile polyploid.

Adaptive Radiation

• Adaptive radiation refers to the evolution of diverse species from a common ancestor.
• It is due to adaptation to various new environmental conditions.
• Finches of Galapagos Islands exemplify this.