Evolution of Populations Ch 16 notes Evolution of Populations

Gene Pool All the genes present in a population. Ex.) The alleles for a trait can be dominant or recessive. The phenotype can be ruled by dominance or blending. .

Relative Frequency The relative frequency of an allele is how often the allele is found in the gene pool. Bb bb bb bb Bb Relative frequency is B=2/10 b=8/10 B = 2 b = 10

Genetic definition of Evolution Evolution is a change in the relative frequency of alleles in a population. Then Now B=2/10 b=8/10 B=4/8 b=4/8 Bb bb bb bb Bb Bb Bb Bb Bb B = 2 b = 10

Sources of Genetic Variation 1) Mutations- changes DNA 2) Gene Shuffling from Sexual Reproduction- Mixes up DNA 8.4 million ways to shuffle the 23 chromosomes in a gamete Also crossing-over happens. Changing or mixing up the DNA causes the new organism to have a different phenotype from its parents.

The number of phenotypes produced for a given trait depends on how many genes control the trait. Skin color=lots of phenotypes=lots of genes controlling the trait. Widows peak=2 phenotypes=controlled by 1 gene.

Single-Gene Trait Either one has a widow’s peak or they don’t because it is controlled by only one gene. W w Homologous Chromosomes WW or Ww= widow’s peak ww=no widow’s peak

Polygenic Traits Traits controlled by two or more genes have lots of phenotypes Ex. Are skin color and hair color. There are a lot of different colors of skin and a lot of different heights in people. If we graph this information it will give us a bell-shaped curve.

Natural Selection on Single-gene traits If one phenotype is more fit, then that animal will survive and be able to reproduce. Ex. Black moth vs. White moth lab

Natural Selection on Polygenic Traits When traits are controlled by more than one gene, the effects of natural selection are more complex. Fitness can vary when there is so much variation in a trait.

Natural Selection can affect the distribution of phenotypes in any one of 3 ways. Directional Selection When individuals at one end of the curve have higher fitness than individuals in the middle or at the other end. Stabilizing Selection When individuals near the center of the curve have higher fitness than at either end. Disruptive Selection When individuals at the upper and lower ends of the curve have higher fitness than individuals near the middle.

Directional Selection If having a darker phenotype makes the moth more fit, then over time the population will shift from lighter to darker and the graph’s curve will shift. peppered moth story

Directional Selection The Galapagos Finches as studied by Peter and Rosemary Grant Studied the finches for more than 20 years. Having a large beak is the most beneficial to the birds. The Grants observed the average beak size in the population increase dramatically over time. This is different from the slow, gradual evolution that Darwin envisioned.

Stabilizing Selection Babies too small or too big die. Usually, only the average sized babies live to pass on their genes.

Disruptive Selection

Genetic Drift Natural Selection is not the only cause of evolution. Sometimes an allele can become more or less common by chance- this is Genetic Drift. Founder effect: This is when a few individuals colonize a new habitat. Their alleles may not be in the same relative frequency as the parent population from which they came –thus they have changed the allele frequency for this new population in their new habitat.

Hardy-Weinberg Principle To understand how evolutionary change happens it helps to theorize what would happen if no change took place. Scientists ask themselves “Are there any conditions under which evolution does not occur?” In 1908, 2 scientists –Hardy and Weinberg proposed this principle to answer that question.

5 conditions are required for no change to take place. Hardy-Weinberg Principle 5 conditions are required for no change to take place. 1) There must be random mating. 2) The population must be very large 3) There can be no movement into or out of the population 4) No mutations 5) No natural selection

Hardy-Weinberg Principle Godfrey Hardy (1877-1947) Wilhelm Weinberg (1862-1937) Their theory mathematically proves that evolution MUST occur.

If these conditions are not met, the population will evolve. 1.   mutation is not occurring 2.  natural selection is not occurring 3.  the population is infinitely large 4.  all members of the population breed 5.  all mating is totally random 6.  everyone produces the same number of offspring 7.  there is no migration in or out of the population

The Process of Speciation Speciation is the formation of new species. How can a change in the relative frequencies of alleles in a population lead to a new species? If organisms in a population are different enough (caused by different allele combinations) that they no longer reproduce with one another, then they are a different species.

Hardy and Weinberg went on to develop a simple equation that can be used to discover the probable genotype frequencies in a population and to track their changes from one generation to another.  This has become known as the Hardy-Weinberg equilibrium equation.  In this equation (p² + 2pq + q² = 1), p is defined as the frequency of the dominant allele and q as the frequency of the recessive allele for a trait controlled by a pair of alleles (A and a).

Isolating Mechanisms Reproductive Isolation- has occurred when members of two populations (that were previously members of the same species) cannot interbreed and produce fertile offspring.

Isolating Mechanisms Behavioral Isolation- Occurs when two populations have the ability to mate, but don’t because of some behavior change. Ex. The Eastern and Western Meadowlarks can mate, but don’t because they use different songs to attract mates. Neither responds to each other’s songs. Eastern Western

Isolation Mechanisms Geographic Isolation- two populations are separated by geographic barriers such as rivers, mountains, or bodies of water. They could mate, but physical cannot get near each other.

Isolating Mechanisms Temporal Isolation- 2 or more species reproduce at different times. For example when flowers live in the same area, but release pollen at different times and, therefore, do not pollinate each other.

Observing speciation Viruses evolve to resist our vaccines and bacteria evolve to resist our antibiotics Plants – both flowers and vegetables.