Surviving in the Environment: Evolution and Adaptation

Overview of Survivorship and Adaptation in Environmental Contexts

• Survival and Adaptation

  • Survival relies on the ability to cope with environmental stress.

Environmental Stress

• Definition: Conditions that exist outside of optimal living conditions.

• Sources: Can originate from both abiotic (non-living aspects such as temperature and pH) and biotic (living components such as relationships with other organisms) factors.

• Example: Intertidal Zones

  • Air/water interface scenario.

  • Challenges faced:

  • Desiccation (drying out).

  • Temperature fluctuations (extremes ranging from heat to cold).

  • Salinity (changes in salt concentration).

  • Predation (threats from predators that might emerge from water or land).

Individual Organism Responses to Environmental Stress

• Organisms generally employ one of two strategies in response to environmental stress:

  1. Avoidance: Moving away from stressful conditions.

  2. Tolerance: Remaining in place to endure stressful conditions.

  • Example: Trees in boreal forests are capable of withstanding temperatures that can fall below $-50^{ ext{°C}}$ in winter and rise to $30^{ ext{°C}}$ in summer.

  • Many organisms use physiological or behavioral mechanisms to tolerate stress, termed acclimatization (reversible physiological changes).

    • Examples:

      • Growing thicker fur for colder winters.

      • Shedding fur during the summer months.

      • Utilizing basking behaviors to regulate body temperature.

Case Study: Frozen Frogs

• Understanding Frozen Frogs on TLC's Series

  • Wood frogs exhibit the ability to survive being completely frozen, allowing them to resume normal life functions upon thawing.

  • They can freeze up to 60% of their body tissues, halting their hearts and blood flow during winter months.

  • Wood frogs possess natural cryoprotectants in their cells, preventing cellular damage during freezing.

  • Extra-cellular fluid crystals are formed while the organisms remain alive.

Developmental Responses: Plasticity

• Plasticity: A form of tolerance to stress that results in non-reversible changes.

  • Developmental Plasticity: The use of environmental cues to activate alternate genes controlling development, mainly happening during embryonic or juvenile stages.

  • Example 1: Temperature-dependent sex determination in reptiles.

  • Example 2: Nutrition-dependent caste determination in insects.

• Phenotypic Plasticity: The ability of the same genotype to yield different phenotypes based on varying environmental influences without altering genetic structure.

  • Environmental cues affect the rate or degree of expression of developmental genes.

Evolution and Individual Responses

• Individual responses to environmental stress can lead to changes in population characteristics over generations via natural selection.

• Proposed by Darwin and linked to Mendelian genetics, suggesting that adaptations accumulate over time as heritable traits (behavioral, morphological, physiological) that improve survival and reproductive potential.

• Result in increased fitness of organisms within their environments, leading to population evolution (change in allele frequencies).

Important Genetic Terms in Evolution

• Descent with Genetic Modification: Inheritance of traits affected by genetic variations.

• Key Genetic Concepts:

  • Somatic Cells vs. Sex Cells

  • DNA and Chromosomes

  • Gene vs. Allele

  • Genotypic variations:

  • Homozygous vs. Heterozygous:

    • Homozygous: Having two identical alleles for a particular gene (e.g., $AA$, $oo$).

    • Heterozygous: Having two different alleles for a particular gene (e.g., $AB$, $Ao$, $Bo$).

  • Phenotype vs. Genotype:

  • Phenotype: The observable physical or biochemical characteristics of an organism.

  • Genotype: The genetic constitution of an organism.

Case Study: The Peppered Moth

• Observation of Evolution:

  • Original phenotype was peppered, but mutations led to a black phenotype.

  • Variations in fitness influenced by environmental changes led to natural selection favoring one phenotype over another.

• Evolution can be conceptualized as genetic change, characterized by alterations in the frequencies of alleles and genotypes over time.

Mechanisms of Evolution

1. Mutations:

   • Spontaneous alterations in DNA sequences (substitutions, deletions, additions) that serve as a powerful source of variation.

   • Rare events that may impact fitness positively, negatively, or neutrally.

2. Natural Selection:

   • Survival of the fittest; organisms that are better adapted to their environments exhibit increased fitness, leading to trait inheritance in subsequent generations.

3. Anthropogenic Influences (Artificial Selection):

   • Human-induced changes in trait inheritance through selective breeding practices, especially noticeable in domestication.

4. Sexual Reproduction:

   • Assortative Mating: Preference for genetically similar partners, promoting homozygosity.

   • Disassortative Mating: Preference for genetically diverse partners, enhancing heterozygosity.

5. Gene Flow:

   • Movement of genetic material between populations can introduce new alleles into a population, impacting genetic diversity over time.

6. Genetic Drift:

   • Evolutionary change due to random sampling effects within populations, notably prevalent in small populations.

7. Bottleneck and Founder Effects:

   • Bottleneck Effect: Occurs when large reductions in population sizes limit genetic diversity. Example: Florida panthers dwindling to a size of six individuals.

   • Founder Effect: Establishment of new populations by a small number of individuals, potentially leading to reduced genetic variability.

Accumulation of Differences in Populations

• Ecotypes: Subpopulations exhibiting unique adaptations to their environmental circumstances, leading to minor morphological or physiological changes.

  • Severe geographic barriers can facilitate speciation when there is no gene mixing.

• Biological Species Concept: Communicates that species are groups capable of interbreeding and producing fertile offspring, emphasizing the importance of geographical isolation in initiating speciation processes.

• Evolution can ultimately lead to diverse species, driven by mutations, natural selections, and environmental pressures.

Summary

• The coping mechanisms towards environmental stress lead to a web of responses that are interconnected across multiple scales of biological organization.

• Notable Transition:

  • From the ancient survival aspects to contemporary understanding regarding evolution and adaptation.