MICROEVOLUTION AND THE GENETICS OF POPULATIONS

The Scale of Evolution

Microevolution• Microevolution occurs

over a relatively short period of time within a population or species. The Grants observed this level of evolution in Darwin’s finches which will be discussed later in this lesson.

Macroevolution• Macroevolution occurs

over geologic time above the level of the species. The fossil record reflects this level of evolution. It results from microevolution taking place over many generations and will be discussed in the next lesson of this chapter.

Microevolution Explained• Microevolution is how individual

traits within a population change over time.

• In order for a population to change, some things must be assumed to be true.

• In other words, there must be some sort of process happening that causes microevolution.

• The five ways alleles within a population change over time are natural selection, migration (gene flow), mating, mutations, or genetic drift.

• One common misconception about evolution is the idea that individuals can evolve. Individuals do not evolve.

• SPECIES EVOLVE…

Genes in Populations• Genes do not change over time.• Individual adaptations help species survive in the environment.• Evolution takes a long time, spanning several generations, to happen• Mutations or adaptations do not equal evolution. • There are no species alive today that have individuals that live long enough to see all of

evolution happen to its species. A• new species may diverge from an existing species’ lineage, but this was a buildup of new

traits over a long period of time and did not happen spontaneously in an instant.• So if individuals cannot evolve, then how does evolution happen? • Populations can evolve. • The unit of evolution is the population. • A population consists of organisms of the same species that live in the same area and can

interbreed. • Populations of individuals in the same species have a collective gene pool in which all future

offspring will draw their genes from. • This allows natural selection to work on the population and determine which individuals are

more “fit” for their environments. • The aim is to increase those favorable traits in the gene pool while weeding out the ones that

not favorable. • Natural selection cannot work on a single individual because there are not competing traits in

the individual to choose between.

Gene Pool• A population consists of many

genotypes; together they make up the population’s gene pool.

• The gene pool consists of all the available genes of all the members of the population that are able to be passed down from parents to offspring.

• For each gene, the gene pool includes all the different alleles for the gene that exist in the population.

• The more diversity there is in a population of a species, the larger the gene pool.

• The gene pool determines which phenotypes are visible in the population at any given time.

• The gene pool can change in an area due to migration of individuals into or out of a population.

• If individuals that have certain traits are the only ones in the population and they emigrate to a different population, the gene pool shrinks and those traits are no longer available to be passed down to offspring.

• If individuals that have different traits immigrates into a population, they increase the gene pool and a new type of diversity can be seen within the population in that area as they interbreed with the others who already life there.

• The size of the gene pool directly affects the evolutionary trajectory of that population.

• The Theory of Evolution states that natural selection acts on a population to favor the desirable traits for that environment while simultaneously weeding out the unfavorable characteristics.

• As natural selection works on a population, the gene pool changes.

• Favorable adaptations become more plentiful and the less desirable traits become fewer or even disappear from the gene pool completely.

Allele Frequencies• Allele frequency or genetic

variation is how often an allele occurs in a gene pool relative to the other alleles for that gene.

• Gene allele frequencies determine genetic variation and the distinct traits that can be passed on from parents to offspring.

• Gene variation is important to the process of natural selection.

• Natural selection is the result of the interactions between genetic variations in a population and the environment.

• The environment determines which variations are more favorable. More favorable traits are thereby passed on to the population as a whole.

• Genetic variation occurs mainly through DNA mutation, gene flow (movement of genes from one population to another) and sexual reproduction.

• Due to the fact that environments are unstable, populations that are genetically variable will be able to adapt to changing situations better than those that do not contain genetic variation.

Hardy and Weinberg and Microevolution• The focus of Hardy's and Weinberg's work was on very small

changes at a gene level either due to chance or other circumstances that changed the gene pool of the population.

• The frequency at which certain alleles appeared changed over generations.

• This change in frequency of the alleles was the driving force behind evolution at a molecular level, or microevolution.

• Since Hardy was a very gifted mathematician, he wanted to find an equation that would predict allele frequency in populations so he could find the probability of evolution occurring over a number of generations.

• Weinberg also independently worked toward the same solution. The Hardy Weinberg Equilibrium Equation used the frequency of alleles to predict genotypes and track them over generations.

The Hardy Weinberg Equilibrium Equation• In order for this equation to work, it is assumed that all of the following

conditions are not met at the same time: (1) Mutation at a DNA level is not occurring. Therefore, no new alleles are being created. (2) Natural Selection is not occurring. Thus, all members of the population have an equal chance of reproducing and passing their genes to the next generation. (3) The population is infinitely large. (4) All members of the population are able to breed and do breed. (5) All mating is totally random. This means that individuals do not choose mates based on genotype. (6) All individuals produce the same number of offspring. (7) There is no emigration or immigration occurring. In other words, no one is moving into or out of the population.

THE EQUATIONGenotype Frequencies in a Hardy-Weinberg Equilibrium Population (for one gene with two alleles, A and a)--------------------------------------------------------------------------------------------------Genotype Genotype Frequency--------------------------------------------------------------------------------------------------

AA p2Aa 2pqaa q2

-------------------------------------------------------------------------------------------------- (p = frequency of A, q =frequency of a): p2 + 2pq + q2 = 1. p + q = 1 • If p = 0.4, what is the frequency of the AA genotype using the Hardy-

Weinberg Equilibrium Equation: p2 + 2pq + q2 = 1?

THE SOLUTIONGenotype Frequencies of the AA genotype when p = 0.4 using the Hardy-Weinberg Equilibrium Equation (for one gene with two alleles, A and a)-----------------------------------------------------------------------------------------------------------Genotype Genotype Frequency-----------------------------------------------------------------------------------------------------------

AA p2= 0.16 or 16%Aa 2pqaa q2

----------------------------------------------------------------------------------------------------------- (p = frequency of A, q =frequency of a): p2 + 2pq + q2 = 1. p + q = 1• CHECK YOUR WORK:• Since p + q = 1 and p= 0.4; q = 0.6• p2 + 2pq + q2 = 1• (0.4)2 + 2(0.4 x 0.6) + (0.6)2 = 1

The Hardy-Weinberg Theorem• Hardy-Weinberg Equilibrium Equation is commonly referred to

as the Hardy-Weinberg Theorem and is regarded as the founding principle of population genetics.

• Its mathematical equation shows that allele frequencies do not change in a population if certain conditions are met and the population remains in genetic equilibrium, and under these conditions evolution cannot occur.

• Such a population is said to be in Hardy-Weinberg equilibrium. • However, if the conditions are not met then evolution can

occur.

Forces of Evolution

• From the theorem, we can infer factors that cause allele frequencies to change.

• These factors are the forces of evolution. • There are five such forces: (1) natural selection (2) migration (gene flow) (3) mating (4) mutations (5) genetic drift

Natural Selection

Natural selection is the main mechanism for microevolution. Alleles that produce favorable adaptations are more likely to be passed down to future generations because they allow those individuals to live long enough to reproduce. As a result, unfavorable adaptations are eventually bred out of the population and those alleles are removed from the gene pool. Over time, changes in allele frequency become more apparent when compared to previous generations.

TYPES OF NATURAL SELECTION(1)Directional Selection

Directional because of the shape of the approximate bell curve that is produced when all individuals' traits are plotted. Instead of the bell curve falling directly in the middle of the axes on which they are plotted, it skews either to the left or the right by varying degrees. In other words one of two extreme phenotypes is selected for or favored over another. This phenomena is usually seen in environments that have changed over time. Changes in weather, climate, or food availability lead to directional selection.Directional selection curves are most often seen when one coloring is favored over another for a species. This could be to help them blend into an environment, camouflage themselves from predators, or to mimic another species to trick predators. Other factors that may contribute to one extreme being selected for over the other include the amount and type of food available.

(2) Disruptive Selection

Disruptive selection is also named for the way the bell curve skews when individuals are plotted on a graph. To disrupt means to break apart and that is what happens to the bell curve of disruptive selection. Instead of the bell curve having one peak in the middle, disruptive selection's graph has two peaks with a valley in the middle of them.The shape comes from the fact that both extremes are selected for during disruptive selection, in other words the phenotypes in the middle of the range are selected against. The median is not the favorable trait in this case. Instead, it is desirable to have one extreme or the other, with no preference over which extreme is better for survival. This is the rarest of the types of natural selection.Disruptive selection can lead to speciation (the appearance of new species), and form two or more different species in areas of drastic environmental changes.Like directional selection, disruptive selection can be influenced by human interaction. Environmental pollution can drive disruptive selection to choose different colorings in animals for survival.

(3) Stabilizing Selection

The most common of the types of natural selection is stabilizing selection. In stabilizing selection, the median phenotype is the one selected for during natural selection. This does not skew the bell curve in any way. Instead, it makes the peak of the bell curve even higher than what would be considered normal.Stabilizing selection is the type of natural selection that human skin color follows. Most humans are not extremely light skinned or extremely dark skinned. The majority of the species fall somewhere in the middle of those two extremes. This creates a very large peak right in the middle of the bell curve. This is usually caused by a blending of traits through incomplete or codominance of the alleles.Diversity in a population is decreased due to stabilizing selection. However, this does not mean that all individuals are exactly the same. Often, mutation rates in DNA within a stabilized population are actually a bit statistically higher than those in other types of populations. This and other kinds of microevolution keep the population from becoming too homogeneous.

Migration (Gene Flow)• Migration is the movement of individuals into or out of a

population. Just like the migration of birds from the north to the south in the winter, organisms will sometimes change their locations and therefore change the gene pool of that population. Gene flow occurs when individuals move into or out of a population. If the rate of migration is high, this can have a significant effect on allele frequencies.

• During the Vietnam War in the 1960s and 1970s, many American servicemen had children with Vietnamese women. Most of the servicemen returned to the United States after the war. However, they left copies of their genes behind in their offspring. In this way, they changed the allele frequencies in the Vietnamese gene pool. Was the gene pool of the American population also affected? Why or why not?

Mating• Many species are not selective when it comes to mating. Asexual

reproduction essentially clones the parent by copying its alleles without any sort of mating between individuals. Some species that use sexual reproduction will choose any available individual that is available as a partner with no regard for which characteristics they show. This keeps the alleles that are being passed down from generation to generation random.

• However, many animal species are selective when finding a mate. These individuals look for particular traits in a mate that will translate to an advantage for their offspring. Since this mating is no longer random, many undesirable alleles are bred out of the population over several generations. This makes the gene pool shrink and fewer traits available for the next generation, causing microevolution.

Mutations• Mutations change alleles by changing the actual DNA of the organism,

thus creating genetic variation in a gene pool. • It is how all new alleles first arise. There are several types of mutations

that can occur and have varying degrees of change that accompany them.

• Alleles may not necessarily change if the change in DNA is small, like a point mutation, but could be lethal to organisms if it has a profound change, like a frame shift mutation.

• Cells are equipped with a system of checkpoints to make sure mutations do not occur and are corrected if they do. Mutations within populations that actually change the gene pool are rare.

However, mutations provide the genetic variation needed for other forces of evolution to act. Mutations result in genotypic variations within organisms and these variations in genotypes sometimes lead to variations in the phenotypes on which evolutionary forces act.

Genetic Drift• Unlike natural selection, genetic drift is a random, chance

event that depends solely on statistical chance instead of desirable traits being passed down to offspring.

• Genetic drift changes allele frequencies in small populations with small gene pools.

• The smaller a population, the more likely it is to see significant microevolution related differences over generations.

There are two special conditions under which genetic drift occurs. They are called bottleneck effect and founder effect.

Bottleneck Effect• The bottleneck effect occurs when a larger population shrinks

significantly in size in a short amount of time. Usually, this decrease in population size is generally due to a random environmental affect like a natural disaster or spread of disease. This rapid loss of alleles makes the gene pool much smaller and some alleles are completely eliminated from the population.

Founder’s Effect• Another cause of genetic drift is called founders effect. The

root cause of Founders Effect is also due to an unusually small population.

• However, instead of a chance environmental effect reducing the numbers of available breeding individuals, the founders effect is seen in populations who have chosen to stay small and do not allow breeding outside of that population.

Lesson Summary• Microevolution occurs over a short period of time in a

population or species. Macroevolution occurs over geologic time above the level of the species.

• The population is the unit of evolution. A population’s gene pool consists of all the genes of all the members of the population. For a given gene, the population is characterized by the frequency of different alleles in the gene pool.

• The Hardy-Weinberg theorem states that, if a population meets certain conditions, it will be in equilibrium. In an equilibrium population, allele and genotype frequencies do not change over time. The conditions that must be met are no mutation, no migration, very large population size, random mating, and no natural selection.

• There are five forces of evolution: natural selection, migration (gene flow),mating, mutation, and genetic drift. Natural selection for a polygenic trait changes the distribution of phenotypes. It may have a stabilizing, directional, or disruptive effect on the phenotype distribution.