the evolution of populations once you understand genetics… it all makes sense!
TRANSCRIPT
The Evolution of Populations
• Once you understand Genetics…
• it all makes sense!
Population genetics• Population: a localized group of
individuals belonging to the same species
• Species: a group of populations whose individuals have the potential to interbreed and produce fertile offspring
• Gene pool: the total aggregate of genes in a population at any one time
• Population genetics: the study of genetic changes in populations
• “Individuals are selected, but populations evolve.”
Hardy-Weinberg Theorem• “The frequencies of alleles in
a population will remain constant .”
• Serves as a model for the genetic structure of a non-evolving population (equilibrium)
• 5 conditions:• 1- Very large population size• 2- No migration• 3- No net mutations• 4- Random mating• 5- No natural selection
Hardy-Weinberg Equation
• p=frequency of one allele (A); • q=frequency of the other allele (a);
p+q=1.0 (p=1-q & q=1-p)• And if (p+q)2 also =1.0, then….
• p2 + 2pq + q2 = 1.0• P2=frequency of AA genotype • 2pq=frequency of Aa plus aA genotype • q2=frequency of aa genotype;
Microevolution• A change in the gene
pool of a population over a succession of generations
• Genetic drift: changes in the gene pool of a small population due to chance (usually reduces genetic variability)
• There are 2 main types of Genetic Drift:
• Founder & Bottleneck Effects
Genetic Drift:
• The Bottleneck Effect: a reduction in population (natural disaster)
• the surviving population is no longer genetically representative of the original population
• Kills “non-selectively”
Genetic Drift continued….
• Founder Effect: a cause of genetic drift attributable to colonization by a limited number of individuals from a parent population
• Again, not likely to have allele frequencies of the original population
• Darwin’s Finches
Microevolution continued….
• GENE FLOW:• genetic exchange due to the
migration of fertile individuals or gametes between populations
• reduces differences between populations.
• Can even merge populations
Microevolution continued…• Mutations: • a change in an organism’s
DNA (gametes; many generations)
• Mutation rates are typically only 1:10,000 gametes so….
• It would take 2000 generations just to change allele frequencies by 1%.
• Has very little effect over the short term, but…
• ..is the original source of genetic variation
Microevolution continued….
• Nonrandom mating: inbreeding and assortive mating (both shift frequencies of different genotypes)
• ASSORTATIVE MATING: When individuals mate with those with similar phenotypes.
• INBREEDING: Individuals tend to mate with those in their own proximity.
Changes genotypic frequencies (p2, 2pq,q2)
……but NOT allele frequencies! (total p & q)
Microevolution continued….
• Natural Selection: differential success in reproduction; only form of microevolution that adapts a population to its environment
“..if you accept microevolution, you get macroevolution for free.”
Population variation• Polymorphism: coexistence of
2 or more distinct forms of individuals (morphs) within the same population
• Geographical variation: differences in genetic structure between populations (cline)
• Variation Generated by:• Mutation- usually harmful but can be
beneficial in a changing environment.• Genetic Recombination- actually
provides nearly all variation in most populations
Variation preservation• Prevention of natural selection’s
reduction of variation• Diploidy 2nd
set of chromosomes hides variation in the heterozygote
• Balanced polymorphism • 1- heterozygote advantage
(hybrid vigor; i.e., malaria/sickle-cell anemia);
• 2- frequency dependent selection (survival & reproduction of any 1 morph declines if it becomes too common; i.e., parasite/host)
Natural selection• 3 types:• A. Directional ex: European Pepper Moth
• B. Diversifyingexample: Tadpole sizes
• C. Stabilizingex: Human Birthweight
Fitness:• Measured by the relative
contribution that an individual makes to the gene pool of the next generation.
• RELATIVE FITNESS: measured by the “selection coefficient”the most successful variety will have a coeffient of “1”; all others a decimal fraction of 1.the new generation will produce a multiple of its fitness
Sexual selection
• Sexual dimorphism: secondary sex characteristic distinction
• Sexual selection: selection towards secondary sex characteristics that leads to sexual dimorphism
Does Evolution Fashion Perfection?• …of course not. WHY?
• Organisms are locked into historical constraints.
• Adaptations are often compromises.
• (seal bodies, mole appendages)
• Not all evolution is adaptive. • (genetic drift, for example)
• Selection can only edit variations that already exist.
• (New genes are not formed “on demand”)