LECTURE NOTES - Week 3

Coping with Environmental Variation - Energy

The Evolution of Photosynthesis Changed the Trajectory of Life on Earth

Classic Photosynthesis (C3)

Photo/Chemo Autotroph or Heterotroph

• Key distinction: 

  • Autotrophs synthesize organic compounds from CO₂ + an energy source

• Photosynthesis:

  • Light + 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

• Chemosynthesis:

  • CO₂ + 4H₂S + O₂ → CH₂O + 4S + 3H₂O

• Photoheterotrophs:

  • Carbohydrate + O₂ → malate + CO₂ + energy

  • CO₂ + pyruvate + ATP (from photons) → malate + ADP = P_i

Not All Plants Are Fully Autotrophic

And Not All Animals Are Fully Heterotropic

• Kleptoplasty:

  • A digestive tubule cell of the sea slug Elysia clarki, packed with chloroplast taken from green algae

  • C = chloroplast

  • N = cell nucleus

How is Photosynthesis Affected By Environmental Conditions?

• Light Availability is Key

Plastic (Acclimatory) Responses Differ From Adaptive Responses to Light Availability

• Plant economic trade-offs dictate the sets of traits that are possible

Temperature Stress

• Species differ in their thermal optima (adaptive trait differences)

• Ecotypes differ in their plastic responses to growth conditions (example of plastic and adaptive responses)

Another Problem: RuBisCO is a Sloppy Enzyme

• ~25% of reactions by RuBisCO in C3 plants lead to oxygenation not carboxylation

• This product cannot be used by the Calvin-Benson cycle

• Much energetic waste due to photorespiration

C4 Photosynthesis - Function Separated by Space

Why Use C4? 

• Better in hot environments

• This pathway also greatly increases water use efficiency

Crassulacean Acid Metabolism (CAM) Function Separated by Time

• ~10,000 spp across 33 families

• Photosynthetic rates relate to the ability to store the 4-C organic acid so many CAM plants are succulent

Water Availability a Driving Force Behind CAM Evolution

• Distribution of CAM species

• Facultative CAM

C3 VS. C4 VS. CAM

• C3 = 

• C4 = Space

• Cam = Time

Evolution and Ecology

Evolution

• Definition: 

  • A change in the heritable characteristics of a population over successive generations

• The process

  • Genetic changes (changes in allele frequency)

• The outcome:

  • The accumulation of differences from an ancestral form (descent with modification)

Definitions

• Gene:

  • A distinct sequence of nucleotides forming part of a chromosome

• Allele:

  • One of two or more forms of a gene that result in the production of different versions of the protein that the gene encodes

• Genotype:

  • The genetic make-up of an individual

• Phenotype:

  • The observable characteristics of an organism

Evolution is a Change in Allele Frequencies

• A change over time in the frequencies (proportions) of different alleles in a population

• Hardy-Weinberg principle - allele and phenotype frequencies in a population will remain constant from generation to generation in the absence of other evolutionary influences

Results in Descent with Modification

• Populations experiencing different environmental conditions accumulate differences over time due to natural selection

• When a new species arises, it differs from its ancestors (modification)

• Although there are differences in the new species, there are also similarities (descent)

• Natural selection:

  • The process by which individuals with certain genetically determined characteristics survive and reproduce more successfully than other individuals because of those characteristics

Natural Selection Occurs at the Population NOT Individual Level

• Individuals have a trait or they don’t

• Within a population, individuals with favored traits have more offspring

• Across generations, an increasing proportion of the population will have the traits selected for by natural selection

Sources of Allelic Variation

Natural Selection Increases the Frequencies of Advantageous Alleles and Decreases the Frequencies of Deleterious Alleles

Genetic Drift

• Change in an allelic frequency due to change event

• Hardy-Weinberg probabilities are based on infinite (large populations); in small populations, the ratios often can’t hold (e.g., if there are 3 offspring, a 1:2:1 ratio is impossible)

• The effect of genetic drift is much larger in small populations

• Bottom line:

  • Higher risk of loss of genetic diversity in small populations

• Loss of alleles from small populations means loss of evolutionary potential - loss in resilience

Gene Flow

• Definition:

  • Transfer of alleles from one population to another via the movement of individuals or gametes

• Outcomes:

  • 1) Genetic homogenization among populations

  • 2) Introduction of new alleles into the population (functionally similar outcome as mutation)

Gene Flow Can Also Have Detrimental Impacts in Isolated Populations

• The world is patchy

• Many sub-populations, with local adaptations to local conditions

• Gene flow or isolation?

• Both inbreeding and outbreeding can lead to reduced fitness

• If the environments are too different, hybrid offspring with intermediate characteristics are NOT favored

• Adaptive Evolution:

  • A process of changes driven by natural selection in which traits that confer survival or reproductive advantages tend to increase in frequency over time

Climate Change Signatures in Adaptive Evolution

• AdhS = an allele of the alcohol dehydrogenase gene that codes for a form of the Adh enzyme that is more effective in warmer temperatures

Adaptive Evolution Will Not Always Lead to a Perfect Organism - Environment Match

• Lack of genetic variation - beneficial allele is not present

Evolutionary History

• Natural selection can only act on the traits that are present

Ecological Compromises

• Ecological trade-offs:

  • One function reduces the ability to perform another

• No species/organism will be perfect

• Adaptations can be compromises

Speciation

• Diversity of life reflects both speciation and extinction rates

Mass Extinctions

Joint Effects of Ecology and Evolution

Examples of Human-Induced Evolution

Life History

Life History

• The overall pattern in the timing and nature of life history events averaged across all the individuals in the species

• Division of energy into pools of growth, reproduction, and survival

• Within species variation determined by genetics and/or environment

Life History and Natural Selection

• Genetic variation in life history traits is what natural selection acts on

• Within a species, certain traits will lead to improved survival and reproduction and will be favored

• Evolution toward maximum fitness but no perfect life history

• Phenotypic plasticity:

  • The ability of a single genotype to produce different phenotypes under different environmental conditions

• Allocation: 

  • The relative amounts of energy or resources that an organism devotes to different functions

Phenotypic Plasticity is Not Always Continuous

• A single genotype can also produce discrete types or morphs

How Do You Demonstrate Whether a Response is Adaptive?

• Not all plastic responses confer a fitness advantage

Mode of Reproduction - A Basic Life History Trait

The Cost of Sex (AKA The Cost of Males)

• Only half of the genetic information of an individual is transmitted to the next generation 

• Loss of favorable gene combinations

• Slower growth rate of sexual populations

Then Why Is Sexual Reproduction So Common?

• The genetic variation generated by sex (recombination) is beneficial in challenging environments

Life History Trade-Offs

• Resource acquisition strategies

• Reproductive strategies

How Many “Kids” Is Too Many?

• “Lack clutch size” - the maximum number of offspring a parent can successfully raise to maturity

• Energy, resources, time, loss of opportunity to engage in other activities

• Too many offspring = low survival

And When There Is No Parental Care?

• Turns out there are still tradeoffs

• Size vs. number

High Fecundity Is Also Costly For The Reproducing Individual

R/K Selection

Live Fast And Die Young Or Slow And Steady Wins The Race?

Charnov’s Dimensionlass Ratio: Comparing Life Histories