Bio111: Chapter 17

Reproductive Isolation Mechanisms

Reproductive isolation prevents different species from interbreeding and maintaining separate gene pools. It is a key process in speciation. These mechanisms can be prezygotic (before fertilization) or postzygotic (after fertilization).

1. Geographical Isolation

   • Populations are separated by physical barriers (e.g., mountains, rivers, oceans).

   • Example: Two populations of squirrels separated by the Grand Canyon.

2. Ecological Isolation

   • Populations occupy different habitats within the same area and rarely encounter each other.

   • Example: One species of bird lives in trees, while another lives in grasslands.

3. Behavioral Isolation

   • Differences in courtship or mating behaviors prevent interbreeding.

   • Example: Different bird species have unique mating songs or dances.

4. Temporal Isolation

   • Populations reproduce at different times (e.g., different seasons, times of day).

   • Example: One species of frog breeds in the spring, another in the summer.

5. Mechanical Isolation

   • Physical differences in reproductive structures prevent successful mating.

   • Example: Flowers with differently shaped structures only allow specific pollinators.

6. Gametic Incompatibility

   • Sperm and egg cannot fuse due to biochemical differences.

   • Example: Sea urchin species release sperm and eggs into water, but fertilization occurs only within the same species.

7. Hybrid Inviability (Postzygotic)

   • Zygote forms but fails to develop properly, leading to an unviable offspring.

   • Example: Sheep and goats can mate but produce embryos that don’t survive.

8. Hybrid Infertility (Postzygotic)

   • Hybrids are sterile and cannot reproduce.

   • Example: A mule (horse × donkey) is sterile.

Allopatric vs. Sympatric Speciation

1. Allopatric Speciation

   • Speciation occurs due to geographical isolation.

   • Physical barriers (e.g., mountains, rivers) separate populations, leading to genetic divergence over time.

   • Example: Darwin’s finches evolved on separate Galápagos Islands.

2. Sympatric Speciation

   • Speciation occurs without physical separation.

   • Driven by factors such as behavioral differences, ecological niches, or polyploidy (common in plants).

   • Example: Cichlid fish in the same lake diversify by feeding in different areas.

Extinction

The complete loss of a species from Earth.

• Can occur due to natural events (e.g., asteroid impacts, volcanic eruptions) or human activities (e.g., habitat destruction, climate change).

• Example: The extinction of the dodo bird due to human hunting and habitat loss.

• Extinction plays a significant role in shaping biodiversity and opening ecological niches for adaptive radiation.

Adaptive Radiation vs. Bilateral

1. Adaptive Radiation

   • The rapid diversification of a single ancestral species into multiple species adapted to different ecological niches.

   • Example: Darwin’s finches evolved different beak shapes to exploit various food sources.

2. Bilateral Symmetry

   • A characteristic of organisms with body plans that can be divided into mirror-image halves along one plane.

   • Example: Humans, insects, and fish exhibit bilateral symmetry.

   • Not directly related to adaptive radiation but is a key evolutionary trait.

Species Definitions

1. Biological Species Concept

   • Defines a species as a group of individuals that can interbreed and produce viable, fertile offspring.

   • Limitation: Doesn’t apply to asexual organisms or fossils.

2. Morphological Species Concept

   • Defines a species based on physical traits (e.g., size, shape, structure).

   • Limitation: Subjective, as individuals of the same species can look different.

3. Genealogical (Phylogenetic) Species Concept

   • Defines a species as the smallest group of individuals sharing a common ancestor on the evolutionary tree.

   • Useful for identifying species using genetic data.

Instantaneous Speciation

Speciation that occurs rapidly, often in one generation, typically through:

• Polyploidy: Doubling of chromosome numbers, common in plants.

• Hybridization events leading to a reproductively isolated population.

• Example: Certain plant species can hybridize to form a new species in a single generation.

Speciation Requires What 2 Things?

1. Reproductive Isolation: Prevents gene flow between populations.

2. Genetic Divergence: Accumulation of genetic differences through mutation, natural selection, or genetic drift that leads to new traits incompatible with the original population.