18.12 Species Diversity and Genetic Diversity

Species Diversity

Definition

• Species Richness: The number of species in a community.
• Species Diversity: Includes both species richness and species evenness (how evenly organisms are distributed among species).
• Greater diversity = more species and more balanced populations.

Key Points

1. Ecosystem Stability:

• High species diversity: More resilient to changes in environmental factors.
• Low species diversity: Dominance of a few species makes ecosystems more vulnerable to disruptions.

1. Examples of Limited Diversity:

• Natural Pine Forests (Florida):
  • Dominated by a few pine species; other species are rare.
• Temperate Forests (Canada):
  • Similarly dominated by a small number of species.

1. Tropical Biodiversity:

• Tropics are global biodiversity hotspots:
  • Favorable conditions:
  • No frost, snow, or extreme temperatures.
  • Year-round intense sunlight.
  • Reduced energy expenditure for warmth (birds/mammals).
  • Examples:
  • Central America: ~1,500 bird species.
  • Northwest Territories (Canada): ~300 bird species.

Genetic Diversity

Definition

• Genetic Diversity: The diversity of alleles within the genes of a species’ genome.
• Individuals of the same species share the same genes but may have different alleles (variations of a gene).
• Greater genetic diversity improves the species’ ability to adapt to changing conditions.

Key Points

1. Assessing Genetic Diversity:

• Measured by:
  • Proportion of genes with different alleles.
  • Number of alleles for each gene.
  • Frequency of heterozygous individuals (carrying different alleles for a gene).

1. Visible Genetic Diversity:

• Examples in cultivated plants and domesticated animals:
  • Different types of roses or dog breeds.
• Natural Populations:
  • Genetic differences may be less obvious but exist within and between populations.
  • Figure 18.20: Variation in Cepaea nemoralis snail shells (color and banding) due to genetic differences.

1. Importance of Genetic Diversity:

• Adaptation:
  • Helps populations survive and adapt to changes in:
  • Biotic factors: Competition, predation, new diseases.
  • Abiotic factors: Temperature, salinity, humidity, rainfall.
• Prevents extinction due to environmental stressors.

Threats to Genetic Diversity

1. Artificial Selection (Selective Breeding):

• Reduces genetic diversity:
  • Focus on a few desirable traits → Loss of rare alleles.
  • Makes populations less adaptable to environmental changes or diseases.

1. Habitat Destruction:

• Reduces population size:
  • Leads to genetic bottlenecks (loss of genetic variation due to reduced population).
  • Fewer alleles in surviving individuals → Reduced ability to adapt.

1. Release of Farmed Fish into the Wild:

• Reduces genetic diversity in wild populations:
  • Farmed fish are often bred for uniform traits.
  • Interbreeding with wild fish dilutes genetic variation.
  • Wild populations may lose unique adaptations necessary for survival.

Questions and Answers

1. Why are the snails in Figure 18.20 members of the same species?

• They share the same genes and can interbreed to produce fertile offspring, despite phenotypic differences (e.g., shell color and banding).

1. How to Investigate Genetic Diversity in Snails:

• Step 1: Collect a population sample.
• Step 2: Analyze DNA:
  • Identify allele frequencies for genes controlling shell color/banding.
  • Measure proportion of heterozygous individuals using molecular techniques (e.g., PCR and gel electrophoresis).

1. Effects on Genetic Diversity:

• Artificial Selection:
  • Reduces diversity by favoring specific traits.
• Habitat Destruction:
  • Causes loss of alleles due to population decline (genetic bottlenecks).
• Farmed Fish Release:
  • Reduces wild population diversity by introducing uniform farmed alleles, weakening local adaptations.

Key Terms

• Species Diversity: Variation in species within an ecosystem, considering richness and evenness.
• Genetic Diversity: Variation in alleles within the genome of a species.
• Allele: Different forms of a gene.
• Heterozygous: An individual with two different alleles for a specific gene.

Summary

• Species Diversity and Genetic Diversity are essential for ecosystem stability and species survival.
• High species diversity supports resilience, while genetic diversity ensures adaptability.
• Human activities, such as selective breeding and habitat destruction, significantly threaten both forms of diversity, underscoring the need for conservation efforts.