5 - Modern Synthesis Flashcards
Evolution
Changes in gene frequencies in populations between generations
Evolutionary change in a phenotype
Reflects changes in underlying genetic composition of the population
Level of natural selection
Individuals
Level of evolution
Populations
Population genetics
Study of changes in gene frequencies in populations under natural selection
Modern synthesis
The synthesis of genetics and Darwinian evolutionary theory
DIscontinuous
If a trait controlled by one gene with 2 alleles, will not not be continuous (yellow or geen)
Continuous variation
If trait is controlled by multiple genes (e.g. human height)
Environmental variation
- Phenotypes will be
affected differently by development and
environment (e.g.,
prenatal nutrition) - Overlap in range of variation for phenotypes
of discrete genotypes - Explains blending of
phenotypes without
blending genotypes
Maintenance/generation of variation
- Mutation
- Recombination
- Sexual reproduction
Mechanism of inheritance
- Discontinuous (e.g. Mendelian traits)
- Continuous (multi-locus or polygenic traits)
Mechanisms of evolution
- Natural selection (reduces variation, removes disadvantageous ohenotypes)
- Mutation (increases variation)
- Gene flow (maintains variation)
- Genetic drift (changes allele frequencies through chance, usually decreasing variation)
Evolutionary constraints
- A factor limiting the adaptive potential of the phenotype
- A limit on the direction, nature, rate and amount of evolutionary change
- Can prevent evolution of optimal adaptations, leading to maladaptive traits
Three broad types of evolutionary constraints
- Historical constraints
- Developmental/genetic constraints
- Material constraints
HIstorical constraints
- The adaptive potential of a population depends on the history of that population
- e.g. Genetic drift, disequilibrium, local and global optimal adaptations
Fixation
All individuals have identical alleles at a locus
When is genetic drift most rapid
If small populations are isolated
Founder effect
Establishment of a new population by a few original founders which carry only a small fraction of the total genetic variation of the parental population (not indicative of the original population)
Example of founder effect
- Remote south atlantic island colonised by british then abandoned
- Gene pool traces to 15 individuals
- One carried allele recessive eye disease
- Disease became common due to inbreeding
Is genetic drift selection
no, is unpredictable
Disequilibrium
- Selection produces optimal adaptations only at equilibrium (reach stabilizing selection for an optimum)
- Populations may still be undergoing selection
Disequilibrium example
- Hunter gatherers had little access to fat, salt and sugar
- Agriculture and trade make these nutrients more readily available (Change in the environment where trait originally evolved)
- Excess salt, fat, sugar causes health problems
Local vs global optima
- Small improvements to existing phenotype, based on which phenotype increases fitness
- Existing phenotype a function of phylogenetic
history - Change occurs until an optimum is reached
- Once reached, cannot get to another optimum
without steps that reduce fitness - That optimum may be
local, not global
Pleiotropic
- Genes that affect multiple aspects of the phenotype
- Causes certain characters to be correlated
Developmental constraints
- Variation is patterned by normal genetic and
developmental processes - Some processes are used multiple times over the course of an organism’s development
- Genes that affect multiple aspects of the phenotype are pleiotropic
Material constraints
- Adaptive potential is limited by the laws of
chemistry and physics, - Requires organisms to be able to function properly in their environments (e.g. gravity)
Material constraints example
- To provide the same support as an animal half that size, bones must be over 4-times thicker
- Places upper limit on size of animals on land, as the bones quickly become too heavy to move!
