Evolution as Genetic Change Chapter 16-1 & 16-2 Evolution as Genetic Change

Gene Pool All the different genes/alleles in a population The allele frequency is the number of times an allele is present in a population Relative frequency is comparing two or more allele frequencies for the same gene. (relative means you can’t give exact numbers only more/less, higher/lower, older/younger,) COMBINATION OF ALL ALLELES = 100

Sources of Genetic Variation Mutations- when the sequence of DNA is changed a new allele or phenotype can be produced. This can change the allele frequency of the population. They may make an individual more or less fit or have no effect.

Sources of Genetic Variation Continued… Gene Shuffling- This occurs during crossing over of meiosis Genes are switched on chromosomes, results in all eggs and sperm being genetically different. Which means siblings born will not usually be identical. This does not change the allele frequency of the population

Natural Selection Never acts directly on genes Can only affect which individuals will Survive and reproduce OR Die without reproducing

If an individual dies without reproducing Its alleles are not contributed to the population’s gene pool

Each time an individual produces offspring Its alleles: Are added to gene pool Can increase in frequency

Natural Selection on single gene traits (traits controlled by one gene) Can be shown on a bar graph because only a few phenotypes are possible. Graph shows the frequency of the phenotypes. Dominant traits do not always show up more often.

Natural Selection on Polygenic Traits More complex Polygenic traits produce a range of phenotypes --Can usually fit a bell curve Over time, Natural Selection Can affect the distribution of phenotypes three ways: Directional selection Stabilizing selection Disruptive selection

Pushes the bell curve in one direction. One extreme phenotype has highest fitness and over time increases and becomes the new High frequency phenotype, while the phenotype at the other extreme is lost.

Avgerage phenotype has highest fitness, while BOTH extremes have low fitness. Causes a loss in both extreme phenotypes and an increase in the average. Loss of genetic variation

Disrupts the bell curve and splits it down the center. Phenotypes at BOTH extremes increase and phenotype in the middle Decreases. Creates two distinct phenotypes in the population.

Genetic Drift – a change in gene pool due to chance(random) Individuals that carry a particular allele may reproduce more offspring than others by chance NOT by natural selection. Over time a series of these occurrences can cause an allele to increase in frequency within the population. More likely to occur in a small population because laws of probability are less accurate in small populations

Founder Effect If a small group is separated by chance from a population, the allele frequency of the new, smaller population is very likely to be significantly different from the original population it came from.

Founder Effect in an insect population

Evolution vs. Genetic Equilibrium Evolution- changes allele frequencies over time More fit phenotypes increase, while less fit phenotypes decrease Ex. Long necked giraffes more fit, so they’re population increased, while shorter necked giraffes decreased in number.

Genetic Equilibrium- situation in which allele frequencies remain constant. Ex. The number of long, medium and short necked giraffes would never change. How likely is genetic equilibrium?? NOT LIKELY!

Hardy-Weinberg Principle States that allele frequencies in a population will remain constant unless one or more factors cause those frequencies to change.

To maintain Genetic Equilibrium Five Conditions are required: Random mating- WHY? All individuals in the population would have an equal opportunity to reproduce, therefore and equal opportunity for their alleles to be added to the gene pool. Large population- WHY? small changes have less of an effect. No immigration or emmigration- WHY? Immigration brings in new alleles and emmigration causes alleles to be removed from the population. No mutations- WHY? Mutations cause new alleles, which causes changes the allele frequencies No natural selection- WHY? Mating wouldn’t be random!

Why is understanding genetic equilibrium important? Understanding what happens when there NO change helps scientists understand how evolution works.