Speciation and Polyploidy in Plants

Unit 1: Evolution and Biodiversity

• Topics covered include: What is Evolution?, Phylogenetic Trees, Evolution of Populations, Origin of Species, History of Life, and Biodiversity (Bacteria, Archaea, Protists, Fungi, & Plants).

Today's Topic: Origin of Species (Ch. 24)

• Focuses on the process of speciation, where one species evolves into two.
• Geographic isolation and accumulated genetic differences are key factors.
• The biological species concept defines a species as groups of possibly interbreeding populations reproductively isolated from others.
• Reproductive isolating mechanisms (pre-zygotic and post-zygotic) maintain genetic differences.
• Systematics aims to build monophyletic evolutionary trees (phylogenies) representing evolutionary history.

Speciation

• Speciation, the process of one species evolving into two, usually results from geographic isolation.
• The process leads to accumulated genetic differences amongst populations.
• The biological species concept defines a species as a group of interbreeding populations reproductively isolated from other groups.
• Reproductive isolating mechanisms categorize differences as pre-zygotic or post-zygotic barriers.
• Systematics creates monophyletic evolutionary trees called phylogenies.

Synopsis for Today

• Understanding sexual selection.
• Identifying reasons for maintaining polymorphisms in populations.
• Differentiating species patterns and speciation processes.
• Understanding how scientists define species.
• Describing the ways speciation can occur.

Flightless Cormorant (Phalacrocorax harrisi)

• Example of a species.

Are These Different Species?

• Questions arise regarding whether organisms are of different species.

Are These Different Species? (Additional Questions)

• How do scientists recognize and name species?
• How is the process of speciation defined?
• Is there a connection between species definition and speciation process?

Patterns of Natural Selection

• Natural selection can modify populations in three ways. -Directional selection shifts the mean. -Disruptive selection increases variance. -Stabilizing selection decreases variance.

Speciation Links Microevolution and Macroevolution

• Microevolution (changes in allele frequencies over generations) leads to macroevolution (large-scale changes in populations/species).
• The peppered moth (Biston betularia) provides an example.

What Is a Species?

• Species are often distinguished by morphology (physical structure), physiology (function), biochemistry, behaviors, and genetic sequences. -Differences in geographic distributions, songs, and behaviors prevent interbreeding.

What about... Homo sapiens?

• All humans have a common ancestor (~200,000 years ago) originating in Africa.
• Sub-Saharan Africa has greater genetic diversity than other populations.
• No biological impediments to interbreeding exist between human groups.
• Humans’ genomes are largely identical, though individuals are unique.
• “Race” is under genetic control, not ancestry.

Biological Species Concept

• Species are groups of actually or potentially interbreeding natural populations reproductively isolated from other groups.
• Viable, fertile offspring must be produced (at least potentially).
• Breeding with other species is prevented.

The Species "Problem"

• Additional species definitions include the morphological (physical differences) and the ecological (unique environmental niche). The phylogenetic species concept focuses on monophyletic groups only.

Biological Species Concept (Mule)

• Mules, produced from a horse and a donkey, are not considered a species according to the biological species concept because they are infertile.

Morphological Species Concept

• Species are distinguished by unique combinations of physical traits (Historically used by Linnaeus).
• May be subjective in trait choice. Non-morphological differences and convergent evolution can lead to misinterpretations of ancestries.

Phylogenetic Species Concept

• Species are identified as monophyletic groups via shared, derived traits (unique to the descendants of a common ancestor)
• Requires phylogenies (evolutionary trees). Many more species may exist than currently recognized.

Speciation due to Selection

• Directional and disruptive selection can lead to speciation.
• Reproductive isolation is required.

Reproductive-Isolating Mechanisms

• Prezygotic barriers impede mating or fertilization (e.g., habitat, temporal, behavioral, mechanical, gametic).
• Postzygotic barriers reduce hybrid viability or fertility (e.g., reduced hybrid viability, reduced hybrid fertility, hybrid breakdown).

Pre-zygotic Barriers

• Ecological (habitat) isolation: Populations occupy different habitats, even in the same area.
• Temporal isolation: Populations breed during different times of day or year.
• Behavioral isolation: Species have unique courtship rituals or behaviors to attract mates.
• Morphological (mechanical) isolation: Species have incompatible reproductive structures.
• Gametic isolation: Species have incompatible eggs and sperm.

Post-zygotic Barriers

• Reduced hybrid viability: Hybrid offspring die before reproductive maturity.
• Reduced hybrid fertility: Hybrid offspring are infertile.
• Hybrid breakdown: First generation hybrids are fertile, but subsequent generations are infertile or have reduced viability.

Modes of Speciation: Allopatric

• Geographical isolation is most common.
• A physical barrier splits populations, leading to different evolutionary pressures & reproductive isolation.
  • Uplifting mountains, new river formation, lowering of water level.

Allopatric Speciation: Snapping Shrimp

• 15 pairs of snapping shrimp (Alpheus species) are separated by the Isthmus Panama.
• Originated 9–13 million years ago.

Modes of Speciation: Sympatric

• Speciation occurs without geographical isolation.
  • Polyploidy (genome duplication, common in plants).
  • Strong divergent selection (habitat differentiation or sexual selection).

Polyploidy

• Autopolyploidy (self): two or more chromosome sets from the same species.
• Allopolyploidy (other): two or more chromosome sets from different species.
• Polyploidy is important in plant breeding. Examples include grapes, citrus, apples, wheat, potato, cotton, etc.

Sexual Selection

• Hypothesis: Females mate based on male coloration, leading to reproductive isolation.
• Sympatric cichlid species in Lake Victoria illustrate this.

Hybrid Zones

• Regions where two species meet and interbreed, producing hybrid individuals.
• Genetic exchange between species can continue or might be limited by reduced viability/fertility of hybrid offspring.
  • Altitudinal gradients, maintaining species separation.

Hybrid Zones: Formation and Outcomes

• Isolation of a population
• Barrier to gene flow
• Hybrid zone creation
  • Reinforcement: Selection against hybrids.
  • Fusion: Hybrids become part of one species.
  • Stability: Hybrids continue to form in a specific zone.

How Fast is Speciation?

• Gradualism: Species arise gradually through the accumulation of traits, observable in the fossil record.

Why Aren't There More Intermediate Fossils?

• Taphonomy (fossil record formation): Not all organisms become fossils.
• Allopatric origin of new species
• Rates of speciation are not always slow.

Punctuated Equilibrium Model

• Species remain constant for long periods, then experience rapid divergence.

Next: Adaptive Radiation & Phylogeny

• Evolutionary diversification into many new species.
• Tree depicts the divergence from a common ancestor into various groups (e.g., vertebrates, hinged jaws, four walking legs, amniotic egg, hair).

This quiz explores key concepts in speciation and polyploidy, particularly in the context of plant biology. Questions range from the definitions and implications of polyploidy to the mechanisms of speciation and reproductive isolation. Test your understanding of these fundamental biological principles!