Mechanisms of Evolution Mechanisms of Evolution There are several: 1. Natural Selection 2. Gene Flow 3. Genetic drift 4. Mutations 5. Non-random mating Artificial Selection Domesticated breeds have not always been in their current form. This change has been achieved by repeatedly selecting for breeding the individuals most suited to human uses. This shows that selection can cause evolution. Genetic Variation individuals in a species carry different alleles (An allele is an alternative form of a gene (one member of a pair) that is located at a specific positionon on a specific chromosome. Any change in gene (and allele) frequencies within a population or species is Evolution Allele Frequency – proportion of gene copies in a population of a given allele 1. Natural Selection: Affects variation in a population as the better adapted (more fit) individuals to their environment survive and reproduce, passing on their genes to the successive generations increasing the frequency of favourable alleles in the population. Nature “selects” which organisms will be successful Imagine that green beetles are easier for birds to spot (and hence, eat). Brown beetles are a little more likely to survive to produce offspring. They pass their genes for brown coloration on to their offspring. So in the next generation, brown beetles are more common than in the previous generation. Natural Selection Dark Pepper Moths http://www.youtube.com/watch?v=LyRA8 07djLc&feature=related 4 Steps of Natural Selection: 1. In nature , more offspring are produced than can survive. 2. In any population, individuals have variation. 3. Individuals with advantageous variations survive and pass on their variations to the next generation. 4. Overtime, offspring with certain advantageous variations make up most of the population 2. Gene Flow: Is the movement of alleles into or out of a population (immigration or emigration). Gene flow can introduce new alleles into a gene pool or can change allele frequencies. The overall effect of gene flow is to counteract natural selection by creating less differences between populations. Example: Plant pollen being blown into a new area Gene flow is what happens when two or more populations interbreed. This generally increases genetic diversity. Imagine two populations of squirrels on opposite sides of a river. The squirrels on the west side have bushier tails than those on the east side as a result of three different genes that code for tail bushiness. If a tree falls over the river and the squirrels are able to scamper across it to mate with the other population, gene flow occurs. The next generation of squirrels on the east side may have more bushy tails than those in the previous generation, and west side squirrels might have fewer bushy tails. Gene Flow Some individuals from a population of brown beetles might have joined a population of green beetles. That would make the genes for brown beetles more frequent in the green beetle population. 3. Genetic Drift The change in allele frequencies as a result of chance processes. These changes are much more pronounced in small populations. Directly related to the population numbers. Smaller population sizes are more susceptible to genetic drift than larger populations because there is a greater chance that a rare allele will be lost. Imagine that in one generation, two brown beetles happened to have four offspring survive to reproduce. Several green beetles were killed when someone stepped on them and had no offspring. The next generation would have a few more brown beetles than the previous generation—but just by chance. These chance changes from generation to generation are known as genetic drift. In a population of 100 bears, suppose there are two alleles for fur color: A1 (black) and A2 (brown). A1 has a frequency of .9, A2 a frequency of .1 (1.0 = 100%). The number of individuals carrying A2 is very small compared to the number of individuals carrying A1, and if only fifty percent of the population survives to breed that year, there's a good chance that the A2s will be wiped out. Examples of Genetic Drift A) The Founder Effect: A founder effect occurs when a new colony is started by a few members of original population. Small population that branches off from a larger one may or may not be genetically representative of the larger population from which it was derived. Only a fraction of the total genetic diversity of the original gene pool is represented in these few individuals. For example, the Afrikaner population of Dutch settlers in South Africa is descended mainly from a few colonists. Today, the Afrikaner population has an unusually high frequency of the gene that causes Huntington’s disease, because those original Dutch colonists just happened to carry that gene with unusually high frequency. This effect is easy to recognize in genetic diseases, but of course, the frequencies of all sorts of genes are affected by founder events. Examples of Genetic Drift B) Population Bottleneck: Occurs when a population undergoes an event in which a significant percentage of a population or species is killed or otherwise prevented from reproducing. •The event may eliminate alleles entirely or also cause other alleles to be overrepresented in a gene pool. Bottleneck = any kind of event that reduces the population significantly..... earthquake....flood.....disease.....etc.… An example of a bottleneck: Northern elephant seals have reduced genetic variation probably because of a population bottleneck humans inflicted on them in the 1890s. Hunting reduced their population size to as few as 20 individuals at the end of the 19th century. Their population has since rebounded to over 30,000 but their genes still carry the marks of this bottleneck. They have much less genetic variation than a population of southern elephant seals that was not so intensely hunted. 4. Mutations Are inheritable changes in the genotype. Provide the variation that can be acted upon by natural selection. Mutations provide the raw material on which natural selection can act. Only source of additional genetic material and new alleles. Can be neutral, harmful or beneficial( give an individual a better chance for survival). Antibiotic resistance in bacteria is one form. Mutation is a change in DNA the hereditary material of life. An organism’s DNA affects how it looks, how it behaves, and its physiology—all aspects of its life. So a change in an organism’s DNA can cause changes in all aspects of its life. Somatic mutations occur in nonreproductive cells and won’t be passed onto offspring. For example, the golden color on half of this Red Delicious apple was caused by a somatic mutation. The seeds of this apple do not carry the mutation. The only mutations that matter to large-scale evolution are those that can be passed on to offspring. These occur in reproductive cells like eggs and sperm and are called germ line mutations. A single germ line mutation can have a range of effects: 1. No change occurs in phenotype. 2. Small change occurs in phenotype. 3. Big change occurs in phenotype. Some really important resistance in insects are sometimes caused by single mutations1. A single mutation can also have strong negative effects for the organism. Mutations that cause the death of an organism are called lethals —and it doesn't get more negative than that. Causes of Mutations DNA fails to copy accurately. External influences can create mutations. Mutations can also be caused by exposure to specific chemicals or radiation. 5. Non-Random Mating In animals, non-random mating can change allele frequencies as the choice of mates is often an important part of behavior. Many plants self-pollinate, which is also a form of non-random mating (inbreeding). Sexual reproduction results in variation of traits in offspring as a result of crossing over in meiosis and mutations Genetic shuffling is a source of variation. Sexual selection occurs when certain traits increase mating success. There are two types of sexual selection. – intrasexual selection: competition among males https://www.youtube.com/watch?v=Ez7RUSCUhzk – intersexual selection: males display certain traits to females https://www.youtube.com/watch?v=vR6gVWPke1Q Modes of Selection Stabilizing Selection, extreme varieties from both ends of the frequency distribution are eliminated. The frequency distribution looks exactly as it did in the generation before Directional Selection - individuals at one end of the distribution of beak sizes do especially well, and so the frequency distribution of the trait in the subsequent generation is shifted from where it was in the parental generation Diversifying (disruptive) Selection - both extremes are favored at the expense of intermediate varieties. This is uncommon, but of theoretical interest because it suggests a mechanism for species formation without geographic isolation The Hardy-Weinberg Theorem • Used to describe a non-evolving population. • Shuffling of alleles by meiosis and random fertilization have no effect on the overall gene pool. • Natural populations are NOT expected to actually be in Hardy-Weinberg equilibrium. 42 The Hardy-Weinberg Theorem •Deviation from Hardy-Weinberg equilibrium usually results in evolution •Understanding a non-evolving population, helps us to understand how evolution occurs 43 Hardy-Weinberg Theorem 1. Large population. The population must be large to minimize random sampling errors. 2. Random mating. There is no mating preference. For example an AA male does not prefer an aa female. 3. No mutation. The alleles must not change. 4. No migration. Exchange of genes between the population and another population must not occur. 5. No natural selection. Natural selection must not favor any particular individual. Hardy-Weinberg Theorem In each cell, chromosomes come in pairs, so the possible combinations are AA, Aa, and aa. Since A is dominant, people with AA or Aa in their cells will be right-handed. AA or Aa - dominant phenotype. (Aa - carriers of recessive trait. AA - non-carriers). aa - recessive phenotype (left-handed people). Homozygote is an organism with pair of identical alleles (in this case AA or aa). Heterozygote is an organism with two different alleles (Aa). Hardy-Weinberg Theorem p = frequency of A q = frequency of a Note: p+q = 1 = 100% total number of alleles. p² = frequency of AA q² = frequency of aa 2pq = frequency of Aa because Note: p² + 2pq + q² = 1 100% total number of cases. Examples: 1. Suppose there are 30 students in a class and 3 are lefthanded. How many right handed students carry a recessive allele? Solution: The frequency of aa is 3/30 students. So, q² = 3/30 = 0.1. Hence, q = .316. From that we get that p = 1 - q = .684. To find how many students carry a recessive allele, we need to find the frequency of recessive allele 2pq. 2pq = 2 x .684 x .316 = .432 = 43.2% So, the number of right-handed students who carry a recessive allele is 43.2% of 30 =.432 x 30 = 13 students. Note: The frequency of right-handed students (AA or Aa) is 27/30 = 0.9 = p² +2pq = 1 - q². 2. Experiments have been performed in which you find that 41% of the population displays the recessive trait. Determine the percentage of the population that are carriers of the recessive trait? That are homozygous? 3. If the frequency of the recessive allele is .32, what percentage of population display the dominant phenotype?