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”)