Biology 15.2 How Populations Evolve How Populations Evolve

 When Mendel’s work was rediscovered in 1900, biologists began to study how the frequency of alleles changes in a population.  Specifically, they wondered if the dominant alleles would replace recessive alleles in a population.  Remember that alleles create the genotypes that affect which phenotypes, or traits, become visible in a population.

How Populations Evolve  In 1908, GH Hardy and Wilhelm Weinberg independently demonstrated that dominant alleles do not automatically replace recessive alleles.  Using algebra, and simple applications of the theories of probabilities, they proved that the frequency of alleles in a population does not change.  The ratio of heterozygous individuals to homozygous individuals does not change from generation to generation unless the population is acted upon by processes that favor particular alleles.  Their discovery, called the Hardy- Weinberg principal, states that the frequencies of alleles in a population do not change unless evolutionary forces act on the population.

Hardy-Weinberg Principal  The Hardy-Weinberg principal holds true for any population as long as the population is large enough that it’s members are not likely to mate with relatives and as long as evolutionary forces are not acting on it.  There are 5 principal evolutionary principals: Mutation Gene flow Nonrandom mating Genetic drift Natural selection Hardy and Weinberg

Hardy-Weinberg Principal: Mutation Mutation:  Although mutation from one allele to another can eventually change allele frequencies, mutation rates in nature are very slow.  Most genes mutate only about 1 to 10 times per 100,000 cell divisions so mutation does not change allele frequencies except over very long periods of time.  Furthermore, not all mutations result in phonotype changes.  Mutation is however, the source of variation and thus makes evolution possible. Hardy and Weinberg

Hardy-Weinberg Principal: Gene Flow Gene flow:  The movement of individuals from one population to another can cause genetic damage.  The movement of individuals to or from a population, called migration, causes gene flow.  Gene flow is the movement of alleles into or out of a population.  Gene flow occurs because new individuals (immigrants) add alleles to the population and departing individuals (emigrants) take alleles away. Gene flow of brown beetles Into a green beetle community

Hardy-Weinberg Principal: Nonrandom mating Nonrandom mating:  Sometimes individuals prefer to mate with others that live nearby or are of their own phenotype, a situation called nonrandom mating.  Mating with relatives (inbreeding) is a nonrandom mating that causes a lower frequency of heterozygotes than would be predicted by the Hardy-Weinberg principal.  Inbreeding does not change the frequency of alleles, but it does increase the proportion of homozygotes (AA or aa genotypes) in a population. When brown beetles breed with Only other brown beetles. They Increase the alleles for brown In the population

Hardy-Weinberg Principal: genetic drift Genetic drift:  In small populations, the frequency of an allele can be greatly changed by a chance event. For example; poaching, disease or a fire can reduce a population to a few survivors.  When an allele is found in only a few individuals, the loss of even one individual from the population can have major affects on the allele’s frequency.  Because this sort of change, an allele frequency appears to occur randomly, as if the frequency was drifting, it is called genetic drift.  Small populations that are isolated from each other can differ greatly as a result of genetic drift. Genetic drift: When the beetles reproduced, just by random luck more brown genes than green genes ended up in the offspring. In the diagram above, brown genes occur slightly more frequently in the offspring (29%) than in the parent generation (25%).

Hardy-Weinberg Principal: Natural Selection Natural Selection:  Natural selection causes deviations from the Hardy-Weinberg proportions by directly changing the frequency of the alleles.  The frequency of an allele will increase or decrease depending on the allele’s effects on survival and reproduction.  If an allele makes a species less visible to a predator, than it’s survival chance will go up and the allele will increase in the population.  Conversely, if an allele makes a species more visible to a predator, it’s survival chance will go down and the allele will decrease as the species will not survive to pass it on. Natural selection: Beetles with brown genes escaped predation and survived to reproduce more frequently than beetles with green genes, so that more brown genes got into the next generation.

Natural Selection and Phenotypes How Natural selection Acts on phenotypes  Natural selection constantly changes populations through actions on individuals within the population.  It does act directly on genes. It enables individuals who have favorable traits to reproduce and pass on those traits to their offspring.  This means natural selection acts upon the phenotypes; not the genotypes. The phenotype for brown coloration in beetles allows brown beetles to survive and increase.

Natural Selection and Phenotypes How Selection Acts:  Think carefully how natural selection might operate on an allele mutation.  Only characteristics that are expressed can be targets of natural selection. Therefore, selection can not operate against rare recessive genes, even if they are unfavorable.  Only when an allele becomes common enough that heterozygous individuals come together and produce homozygous offspring does natural selection have an opportunity to act. The phenotype for brown coloration in beetles allows brown beetles to survive and increase.

Natural Selection and Phenotypes Why Genes Persist:  To better understand this limitation on natural selection, consider this example.  If a recessive allele (a) is homozygous (aa) in 1 out of 100 individuals, (appearing visibly in only 1 person)  and if 18 out of 100 individuals will be heterozygous (Aa) (a dominant gene overpowering the recessive gene)  these 18 will therefore carry the recessive allele (a) hidden within their genetic code. The phenotype for brown coloration in beetles allows brown beetles to survive and increase.

Natural Selection and Phenotypes Why Genes Persist:  So natural selection in this example can act on only 1 out of the 19 individuals that carry the allele ( the 1 homozygous individual it can act on but the 18 heterozygous it can not).  That leaves 18 individuals who still carry the gene in a recessive hidden form.  Many human diseases have frequencies similar to this. For example, about 1 in 25 Caucasians have a copy of a defective gene for Cystic Fibrosis but show no visible symptoms.  Only homozygous individuals, 1 in 2,500, show the disease and thus perish from it. So natural selection only works against this 1 in 2,500. The phenotype for brown coloration in beetles allows brown beetles to survive and increase.

Natural Selection and Phenotypes Why Genes Persist:  Genetic conditions like Cystic Fibrosis or sickle Cell Anemia are not eliminated from the population because very few of the individuals bearing the alleles express the recessive phenotype.  The condition lies hidden until two individuals who both carry the rare recessive trait have offspring.  Thus natural selection does not eliminate it because all those who have it hidden as a recessive allele still pass it on. Natural selection can not eliminate a recessive hidden gene. The phenotype for brown coloration in beetles allows brown beetles to survive and increase.

Natural Selection and Distribution of Traits Natural selection and the Distribution of Traits:  Natural selection shapes populations affected by phenotypes that are controlled by one or by a large number of genes.  A trait that is influenced by several genes is called a polygenic trait.  Human height and human skin color are examples of polygenic traits; influenced by dozens of genes at once.  Natural selection can change the allele frequencies of many different genes covering a trait. The phenotype for brown coloration in beetles allows brown beetles to survive and increase.

Natural Selection and Distribution of Traits Natural selection and the Distribution of Traits:  Natural selection can change the allele frequencies of many different genes governing a single trait, influencing most strongly those genes that make the strongest contribution to the phenotype.  Biologists measure a polygenic trait by measuring each individual in a population.  These measurements are than used to calculate the average value of a trait for the population as a whole. The phenotype for brown coloration in beetles allows brown beetles to survive and increase.

Natural Selection and Distribution of Traits Natural selection and the Distribution of Traits:  Because genes can have many alleles, polygenic traits tend to exhibit a range of phenotypes clustered around an average value.  If you were to plot the height of everyone in your class on a graph, the values would probably form a hill-shaped curve called a normal- distribution curve.

Natural Selection and Distribution of Traits Natural selection and the Distribution of Traits:  When selection eliminates one extreme from a range of phenotype possibilities, the alleles promoting this extreme become less common in the population.  In one experiment, when fruit flies raised in the dark were exposed to light, some flew toward the light and some did not.  Only those flies with the strongest tendency to fly towards the light were allowed to reproduce.  After 20 generations, the average tendency to fly toward the light increased. This is an example of directional selection.

Natural Selection and Distribution of Traits Natural selection and the Distribution of Traits:  In directional selection, the frequency of a particular trait moves in one direction in a range.  Directional selection plays a role in the evolution of single gene traits, such as pesticide resistance in insects. Distribution curves before and after Directional selection

Natural Selection and Distribution of Traits Natural selection and the Distribution of Traits:  When selection reduces extremes in the range of phenotypes, the frequencies of the intermediate phenotypes increase.  As a result, the population contains fewer individuals that have alleles promoting extreme types.  In a stabilizing selection, the distribution becomes narrower, tending to stabilize the average by increasing the proportion of similar individuals.  Stabilizing selection is very common in nature.