16-1 Genetic Equilibrium

Chapter 16Population Genetics and

Speciation

Terms to Know Population genetics – study of

evolution from a genetic point of view

Microevolution – evolution at the genetic level

Bell Curve – most members of the population have similar traits; only a few are at the extremes– Can SHIFT over time!!!

Variations of Traits Within a Population

Causes of Variation1. Mutations

1. TAGATC -> TAAATC2. Recombination (crossing-over and

independent assortment)1. (during meiosis – see pic!)

– Random pairing of gametes– Which sex cells combine successfully

The Gene Pool Gene pool = the total genetic

information available in a population Allele Frequency = number of a

certain allele / total number of alleles in a population– Calculated by … # of allele A

total # of A and a

What is the allele frequency?

Half a population of four o clocks are red/ half are white.

What is the frequency of the r allele?– 0.50 or 50%

The Gene Pool Phenotype Frequency = # of

individuals with a phenotype / total # of individuals within the population

The Gene Pool Phenotype Frequency = # of individuals

with a phenotype / total # of individuals within the population

– # red flowers/ Total # of individuals

Hardy – Weinberg Genetic Equilibrium

Ideal hypothetical population that is not evolving (ie not changing over time)

5 criteria (must be met)1. No net mutations occur2. No one enters or leaves the population3. The population is large4. Individuals mate randomly5. Selection does not occurWhy might a population never be in HW genetic equilibrium???

16-2 Disruption of Genetic Equilbrium

1. Mutation2. Gene flow – the process of genes

moving from one population to the next- Immigration- Emigration

3. Genetic Drift – allele frequencies in a population change usually because the population is small

4. Nonrandom mating – mate selection is influenced by geographic proximity, mates with similar traits, and sexual selection

5. Natural Selection – some members are more likely to survive and reproduce- stabilizing selection- disruptive selection- directional selection

Types of Selection Stabilizing

selection - individuals with average forms of a trait have the highest fitness– Ex. Large lizards

will be spotted by predators; small lizards can’t run fast enough to get away from predators

Types of Selection Disruptive Selection –

individuals with either extreme of the trait has the greatest fitness

– Ex. White moths on white trees cannot be seen; Dark moths on dark trees cannot be seen; medium colored moths will be seen on both trees (eaten by birds)

Types of Selection Directional

selection – more extreme form of a trait has the greatest fitness

– Ex. Anteaters with the longest tongues will get the most food

16-3 Formation of Species

The Concept of Species Speciation – the process of species

formation Morphology – internal and external

appearance of an organism Species – morphologically similar

and can interbreed to produce fully fertile offspring

Morphological vs Biological Species Concept

2 competing concepts (ideas) on how to determine different species– Morph = based solely on appearance

– Bio = based on who can reproduce with who successfully (not useful for extinct or asexual organisms)

– What do we use today? Mix of BOTH!!!

Isolation and Speciation Geographic isolation – physical

separation of members of a species– Ex. River dries up into several small

pools; fish diverged enough to be considered separate species

Reproductive Isolation – species become genetically isolated – Ex. The offspring of interbreeding

species may die early or may not be fertile

– Ex. Different mating times

Rates of Speciation Gradualism – speciation occurs at a

regular, gradual rate Punctuated Equilibrium – sudden, rapid

change then long periods of no change