5.2 Adaptation and fitness concepts

Natural selection shapes organisms through adaptation, enhancing their fitness in specific environments. This process drives evolutionary change, increasing beneficial traits' frequency in populations over time. Adaptations can be morphological, physiological, or behavioral.

Fitness measures an organism's reproductive success relative to others. It involves survival, mating, and offspring production. Absolute fitness quantifies offspring numbers, while relative fitness compares genotypes within a population. Understanding fitness is crucial for grasping evolutionary dynamics.

Adaptation and Natural Selection

Role of adaptation in evolution

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• Adaptation: heritable trait enhancing organism's fitness in environment results from natural selection acting on genetic variation
• Provides survival and reproductive advantages increases frequency of beneficial alleles in populations over time drives evolutionary change and species diversification
• Examples: morphological (camouflage in prey species), physiological (salt glands in marine birds), behavioral (courtship displays in birds of paradise)

Fitness and reproductive success

• Fitness measures individual's reproductive success relative to others in population reflects ability to survive and produce viable offspring
• Higher fitness leads to increased reproductive output more offspring surviving to reproduce themselves
• Components: viability (survival to reproductive age), mating success (ability to acquire mates), fecundity (number of offspring produced)
• Fitness landscape conceptualizes fitness in relation to genotype or phenotype

Absolute vs relative fitness

• Absolute fitness: average offspring produced by individuals with specific genotype measured as ratio of surviving offspring to initial number of individuals (genotype produces average 2.5 offspring per individual)
• Relative fitness: compares genotype fitness to others in population expressed as ratio highest fitness genotype assigned value of 1 (genotype A fitness 0.8, genotype B fitness 1, A is 80% as fit as B)

Mechanisms and Limitations of Adaptation

Accumulation of beneficial mutations

• Genetic variation sources: mutations (random DNA sequence changes) and recombination (genetic material shuffling during meiosis)
• Beneficial mutations: rare fitness-increasing mutations spread through populations via natural selection
• Accumulation process:
  1. Gradual increase in beneficial allele frequency
  2. Selection acts on existing variation each generation
• Factors influencing adaptation rate: population size (larger populations have more potential for beneficial mutations), generation time (shorter allows faster adaptation), selection strength (stronger pressure leads to faster adaptation)
• Observed adaptations: antibiotic resistance in bacteria, industrial melanism in peppered moths

Limitations and trade-offs of adaptation

• Limitations: genetic constraints (limited variation available), developmental constraints (physical or physiological limitations), historical constraints (evolutionary baggage from ancestral forms)
• Trade-offs: competing demands on limited resources or energy improvement in one trait may cost another
• Types: life history (reproduction vs longevity), physiological (growth rate vs immune function)
• Shape evolution: prevent "perfect" adaptations lead to specialized adaptations for specific environments maintain genetic variation in populations
• Examples: increased egg production vs reduced flight capability in chickens, antibiotic resistance vs reduced growth rate in bacteria
• Evolutionary compromises: jack-of-all-trades vs master of none, generalist vs specialist strategies