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Summary - Class notes - Life history of Aquatic Organisms
Knowledge clip 'natural selection' 1
- Natural selection leads to reproduction of the fit enough
- Variation in species is necessary (morphological, physiological, behavioural, etc)
- at least part of the variation is genetic
- individuals with traits that are more suited to environment are more likely to survive and reproduce
- You need to survive to reproductive age ánd produce a lot offspring during entire life
- suitability to environment can be dynamic (gold/black fish)
Knowledge clip 'natural selection' 2
- Natural selection through environment influences phenotype
- This selection influences the genotype of the next generation because of differential reproduction of the more suited organisms, these genotypes are expressed in phenotype again.
- Adaptation as a process (functional response to some problem set by the environment, leading to greater fitness) (used for changes within a population)
- Adaptation as an attribute, derived homologous trait (functional response to an environmental challenge) (used for species)
- Adaptation older than the species (ancestor) (fins in wales/dolphins/ -> cetacea)
Life histories - the sequence of events related to survival and reproduction that occur from birth through death
The classical theory treats life history evolution as an optimization problem: given particular ecological factors (such as predation, nutrition) that affect an organism's probability of survival and reproduction, and given limiting constraints and trade-offs intrinsic to the organism, what are the optimal values and combinations of life history traits that maximize reproductive success?
Boundary conditions: 1) how extrinsic, environmental factors affect survival and reproduction 2) how intrinsic connections among life history traits (trade-offs) and other constraints limit whether and how life history traits can evolve
2 conditions (genotypic and phenotypic) are necessary for natural selection to occur.
1) heritable variability for the trait in question (genotype)
2) individuals must vary in fitness (phenotype)
analysis of the evolution of fitness components is called life history evolution.
Principal life history traits are:
Size at Birth, Growth Pattern, Age at Maturity, Size at Maturity,
Number,size, sex ratio of offspring,
Age- and size-specific reproductive investments
Age- and size-specific mortality schedules
Length of life
Some trade-offs (numerous happen between traits) are:
Current reproduction and survival
Current reproduction and future reproduction
Number, size and sex of offspring
Stearns (1992) developed a framework in which all life histories can be understood as variations of a few general themes. The four elements:
2. quantitative genetics and reaction norms
4. lineage-specific elements
Demography (to understand life history)
Phenotypic plasticity: the ability of a single genotype (or clone) to produce different phenotypes across different environments.
Plasticity of a specific genotype can be described by a mathematical function called a reaction norm (a line or curve that relates the phenotypes produced by this genotype to changes in the environment it experiences)
Darwinian demons don't exist.
(organisms that start to reproduce as soon as they are born, produce an infinite number of offspring and live forever)
Can't happen because resources are finite, life history traits are subject to intrinsic trade-offs.
Natural selection cannot maximize life history traits beyond certain limits:
these limits are called evolutionary constraints.
A trade-off exists when an increase in one life history trait (improving fitness) is coupled to a decrease in another life history trait (reducing fitness), so there's a balance (can also involve more than 2 traits). on genetic level thought to be caused by alleles with antagonistic pleiotropic (one gene controls more than one phynotypic trait) effects or by linkage disequilibirum (non-random association of alleles at different loci... Loci are said to be in linkage disequilibirum when the frequency of association of their different alleles is higher or lower than what would be expected if the loci were independent and associated randomly) between loci.
Most important trade-offs:
- current reproduction vs survival
- current vs future reproduction
- current reproduction vs parental growth
- current reproduction vs parental condition
- number versus size of offspring
Sometimes it is better to speak of the fitness of genes (in stead of organisms)
r is called the Malthusian parameter in genetics (growth rate) and intrinsic rate of natural increase in demography.
Another fitness measure sometimes used is R0 the per-generation rate of multiplication.
Traits are a mixture of adaptation and constraint. Many definitions of adaptation exist:
Functional definition: an adaptation is a change in a phenotype that occurs in response to a specific environmental signal and has a clear functional relationship to that signal that results in an improvement in growth, survival, or reproduction.
Example: daphnia with elongate helmets
Phylogeneticdefinition: we only apply the word adaptation to homologous traits that are derived, uniqueto a single species, and functionally associated with a change in habitat and selection pressure that is also unique to that species.
This phylogenetic definition ignores variation within populations and
lineage-specificselection. It rules out convergence as evidence for adaptation :/
Adaptation through lineage-specific selection pressure: selection pressure that must be common to the whole lineage is not something generally associated with a habitat, but a pattern of age- and size-specific mortality and fecundity.
Two microevolutionary definitions of adaptation: The first defines an adaption as the state of a trait predicted to be in that state and no other by an optimality model, but only where that prediction has been tested by using mutations or phenocopies to perturb the phenotype away from the optimal state and thus demonstrate that the fitness of the perturbed phenotypes is lower than the fitness of the optimal one. (this has been tested through clutch size manipulations)
The second requires one to show that every time a given factor changes in the environment, a trait evolves to the same new state, and that every time the environmental factor changes back to original state, the trait does too. (never been tested)
Traits are a mixture of adaptation and constraint. Many definitions of constraints exist:
Casual phylogenetic definition: Any pattern or state that can be attributed to phylogeny, as opposed to recent
microevolutionwithin the currently existing population, is a constraint.
Biomechanical definition: is relatively uncontroversial. It asserts that organisms are constrained to obey the laws of physics and chemistry.
Systems definition of constraint: Comes in several versions. The most basic constraint is the requirement that each stage of development must proceed from where the last one left off. Genes act through proteins that change the properties of cels, cells are the key players in development and cells interact in processes constrained by physics. Vagus nerve behind 6th 'gill arch (ductus arteriosus in mammals).
A small portion of phenotype space is available for exploration by gene substitution.
Developmental constrants are biases on the production of variant phenotypes or limitations on phenotypic variability caused by the structure, character, composition, or dynamics of the developmental system.
The origin of constaint in the fixation of key traits; fixation leads to progressive integration and irreversible change. Once one trait becomes fixed, other functionally related traits are less free to vary. Constraints are the functional connections between fixed traits. First it is difficult, then impossible for evolution to proceed in reverse to the original state.
Nothing in biology makes sense except in the light of evolution (T. Dobzhansky)
Difference between functional and phylogenetic definitions of adaptation
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