Population Genetics and Speciation Chapter 16

Variation of Traits within a Population  Microevolution: is the evolution that occurs within a population or a change in the gene pool over a succession of generations  Macroevolution is evolutionary change on a grand scale, encompassing the origin of novel designs, evolutionary trends and adaptive radiation and mass extinction

Macroevolution  Novel designs like feather & wings  Trends like increasing brain size in mammals  Adaptive radiation is seen in flowering plants  Mass extinctions like the dinosaurs

Causes of Variation?  Mutation  Recombination  Random pairing of gametes

The Gene Pool  Definition: the total genetic information available in a population  Allele frequency: determined by dividing the number of a certain allele by the total number of alleles of all types in the population  Ex. Two alleles A, a. If in a set of 100 gametes, half are carrying allele A, then the frequency of A is.5 or 50 %. The total of all allele types must add up to 100.

Hardy-Weinberg Theorem  Before we can look at microevolution we must consider the H-W theorem. But first….. A few definitions:  Population-all the members of a single species occupying a particular area at the same time.  Species-organisms that share a common gene pool, interbreed with one another  Gene Pool- total of all the genes of all the individuals in a population.

Hardy-Weinberg Genetic Equilibrium

5 conditions that must be met to maintain equilibrium  Population must be large  Population must be isolated from others  No mutations  Random mating must occur  No natural selection can occur

Getting the Hardy-Weinberg Equilibrium Formula  In a wildflower population there are two alleles for color. A-pink and a-white.  500 plants = 1000 alleles  20 of those plants are white = 40 a alleles  480 of those plants are pink  320 are AA = 640 A alleles  160 are Aa = 160 A alleles and 160 a alleles

 So the frequency of allele A is 800/1000 =.8 = 80%  The frequency allele a is 200/1000 =.2 = 20%  p = the frequency of allele A  q = the frequency of allele a  p + q = 1 ( = 1)

 If you consider genotypic frequencies  AA = 320/500 =.64 = 64%  Aa = 160/500 =.32 = 32%  Aa = 20/500 =.04 = 4% Hardy-Weinberg equation: p 2 + 2pq + q 2 = 1 (frequency of (frequency of (frequency of AA genotype) Aa genotype) aa genotype) AA genotype) Aa genotype) aa genotype) For our example: = 1

Disruption of Genetic Equilibrium  Evolution is the change in a populations’ genetic material over generations, that is, a change of the population’s allele frequencies or genotype frequencies.  ANY exceptions to the five conditions necessary for H-W equilibrium can result in evolution.

Causes of Microevolution  Genetic drift  Gene flow  Mutations  Nonrandom mating  Natural selection  If any of these occur then equilibrium is NOT present in the population!!

Gene Flow  Populations may gain or lose alleles by gene flow. This is genetic change due to the migration of fertile individuals or gametes between populations  Ex. Human moving around the world.

Mutations  A change in an organism’s DNA

Genetic Drift-changes in a gene pool of a small population due to chance  Two situations that can lead to genetic drift:  Bottleneck effect: disasters such as earthquakes or floods reduce the pop. drastically, killing victims unselectively- reduces genetic variability.  Founder effect: a small number of individuals colonize an isolated island, lake or other new habitat-reduced genetic variability.

Nonrandom mating or assortive mating  Individual select mates because of a particular phenotype.  Ex. Cardinal with the brightest red feathers.  Peacocks with the most “eyes” in tail.

Natural Selection  Differential success in reproduction because an organism is more fit for their environment.  Which colored dot “mouse” became most common in your Adaptation Activity?

Types of Selection  Stabilizing: individual with the average form of a trait have the highest fitness.

 Disruptive Selection: individual with either extreme variation of a trait have a greater fitness than individual with the average form of the trait.

 Directional Selection: individual that display a more extreme form of a trail have greater fitness than individuals with an average form of a trait.

Formation of Species  Biological concept of Species:  a population of organisms that can successfully interbreed but cannot breed with other groups.

It all begins with being isolated.  Geography: barriers formed by canyons, mountains, water or deserts (cities and highways) can divide or fragment and isolate parts of populations from each other.  Natural selection and genetic drift cause the two subpopulations to diverge, eventually making them incompatible for mating.

Allopatric Speciation  New species arise because of geographic isolation.  More likely to happen in small populations where gene pool will change quickly.

Reproductive Isolation  May happen in the absence of geographic barriers.  May be caused by disruptive selection  Temporal: different breeding times  Behavioral: different mating calls.

Sympatric Speciation  Two species develop reproductive isolation within the same geographic area by occupying different niches.

Rates of Speciation  Gradualism: speciation occurs at a regular, gradual rate. Change happens slowly  Punctuated equilibrium: sudden, rapid change followed by long periods of equilibrium or little change.