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Home , Adaptive radiation, Allele frequency, Darwin (unit), Disruptive selection, Evolution, Genetic drift

Chapter 8: Lesson 8.3: and the of ​ ​ Microevolution refers to varieties within a given type. Change happens within a group, but the descendant is clearly of the same type as the . This might better be called variation, or , but the changes are "horizontal" in effect, not "vertical." Such changes might be accomplished by "," in which a trait ​ ​ ​ ​ within the variety is selected as the best for a given of conditions, or accomplished by "artificial selection," such as when produce a new of dog.

Lesson Objectives ● Distinguish what is microevolution and how it affects changes in populations. ● Define pool, and explain how to calculate frequencies. ● State the Hardy-Weinberg theorem ● Identify the five forces of evolution.

Vocabulary ● adaptive ● ● migration ● allele ● ● ● artificial selection ● Hardy-Weinberg theorem ● natural selection ● ● ● genetics ● ● microevolution ● ●

Introduction knew that heritable variations are needed for evolution to occur. However, he knew nothing about Mendel’s laws of genetics. Mendel’s laws were rediscovered in the early 1900s. Only then could scientists fully understand the process of evolution. Microevolution is how traits within a population change over . 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 within a population change over time are natural selection, migration (gene flow), mating, , or genetic drift.

The Scale of Evolution We now know that variations of traits are heritable. These variations are determined by different alleles. We also know that evolution is due to a change in alleles over time. How long a time? That depends on the scale of evolution. ● Microevolution occurs over a relatively short period of time within a population or . The ​ Grants observed this level of evolution in Darwin’s which will be discussed later in this lesson. ● Macroevolution occurs over geologic time above the level of the species. The record reflects ​ this level of evolution. It results from microevolution taking place over many and will be discussed in the next lesson of this chapter.

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Genes in Populations One common misconception about evolution is the that can evolve. Individuals do not evolve. Their do not change over time. Individuals can only accumulate that help them survive in the environment. Evolution takes a long time, spanning several generations, to happen. While it is possible for individuals to mutate and have changes made to their DNA, this does not mean the individual has evolved. So if individuals cannot evolve, then how does evolution happen? Populations can evolve. The unit of evolution is the population. A population consists of of the same species that live in the same area and can interbreed. In terms of evolution, the population is assumed to be a relatively closed group. This means that most mating takes place within the population. Populations of individuals in the same species have a 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 The genetic makeup of an individual is the individual’s . A population consists of many . Altogether, 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 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 out of or immigrate into a new population their genes will travel with them. The size of the gene pool directly affects the evolutionary trajectory of that population. As natural selection works on a population, the gene pool changes. The favorable adaptations become more plentiful and the less desirable traits become fewer or even disappear from the gene pool completely. Populations with larger gene pools are more likely to survive as the environment changes than those with smaller gene pools. For example, in populations, individuals that are resistant are more likely to survive any sort of medical intervention and will live long enough to reproduce. Therefore, the gene pool has now changed to include only bacteria that are antibiotic resistant.

Allele Frequencies or is how often an allele occurs in a gene pool relative to the ​ other alleles for that gene. In genetic variation, the genes of organisms within a population change. ​ ​ Genetic variation occurs mainly through DNA mutation, gene flow (movement of genes from one population to another) and sexual . Due to the 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. Look at the example in Table 8.3. The population in the table has 100 members. In a sexually ​ ​ reproducing species, each member of the population has two copies of each gene. Therefore, the total number of copies of each gene in the gene pool is 200. The gene in the example exists in the gene pool in two forms, alleles A and a. Knowing the genotypes of each population member; we can count the ​ ​ ​ ​ number of alleles of each type in the gene pool. The table shows how this is done.

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Table 8.3: Number of Alleles in a Gene Pool (for one gene with two Alleles, A and a) ​ ​ ------Genotype Number of Individuals Number of Allele A Number of Allele a ​ ​ in the Population Contributed to the Contributed to the with that Genotype Gene Pool by that Gene Pool by that Genotype Genotype ------AA 50 50 × 2 = 100 50 × 0 = 0 Aa 40 40 × 1 = 40 40 × 1 = 40 aa 10 10 × 0 = 0 10 × 2 = 20 Totals 100 140 60 ------

Let the letter p stand for the frequency of allele A. Let the letter q stand for the frequency of allele a. We ​ ​ ​ ​ ​ ​ ​ ​ can calculate p and q as follows: ​ ​ ​ ​ ● p = number of A alleles/total number of alleles = 140/200 = 0.7 ​ ​ ​ ● q = number of a alleles/total number of alleles = 60/200 = 0.3 ​ ​ ​ ● Notice that p + q = 1. ​ ​ ​ ​

Evolution occurs in a population when allele frequencies change over time. What causes allele frequencies to change? That question was answered by Godfrey Hardy and Wilhelm Weinberg in 1908.

Hardy and Weinberg and Microevolution Hardy was an English mathematician. Weinberg was a German doctor. Each worked alone to come up with the founding principle of . Today, that principle is called the Hardy-Weinberg theorem. It shows that allele frequencies do not change in a population if certain conditions are met. Such a population is said to be in Hardy-Weinberg equilibrium. The conditions for equilibrium are:

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 . 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 list above describes causes of evolution. If all of these conditions are met at the same time, then there is no evolution occurring in a population. Since the Hardy Weinberg Equilibrium Equation is used to predict evolution, a for evolution must be happening. However, when all these conditions are met, allele frequencies stay the same. Genotype frequencies also remain constant. In addition, genotype frequencies can be expressed in terms of allele frequencies, as Table 8.4 shows. ​ ​ 233

Table 8.4: Genotype Frequencies in a Hardy-Weinberg Equilibrium Population ​ ​ (for one gene with two alleles, A and a) ------Genotype ------AA p2 ​ Aa 2pq ​ aa q2 ​ ------Hardy and Weinberg used mathematics to describe an equilibrium population (p = frequency of A, q = frequency of a): p2 + 2pq + q2 = 1. In Table above, if p = 0.4, what is the frequency of the AA genotype? A video explanation of the Hardy-Weinberg model can be viewed at here (14:57). ​ ​

The Hardy-Weinberg Theorem Today the 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 The conditions for Hardy-Weinberg equilibrium are unlikely to be met in real populations. The Hardy-Weinberg theorem also describes populations in which allele frequencies are not changing. By definition, such populations are not evolving. How does the theorem help us understand evolution in the real world? 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: natural selection, migration (gene flow), mating, mutations, and genetic drift.

Natural Selection Natural selection is the main mechanism for microevolution. Natural selection occurs when there are differences in among members of a population. As a result, some individuals pass more genes to the next generation. This causes allele frequencies to change. The example of sickle- anemia is described in Figure below and Table below. It shows how natural selection can keep a harmful allele in a gene pool. You can also watch a video about natural selection and sickle-cell anemia at this link. ​ ​

Here’s how natural selection can keep a harmful allele in a gene pool: ● The allele (S) for sickle-cell anemia is a harmful autosomal recessive. It is caused by a mutation in the normal allele (A) for (a on red blood cells). ● is a deadly tropical . It is common in many African populations.

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● Heterozygotes (AS) with the sickle-cell allele are resistant to malaria. Therefore, they are more likely to survive and reproduce. This keeps the S allele in the gene pool. The sickle-cell example shows that fitness depends on . It also shows that fitness may depend on the environment. What do you think might happen if malaria was eliminated in an African population with a relatively high frequency of the S allele? How might the fitness of the different genotypes change? How might this affect the frequency of the S allele? Sickle-cell trait is controlled by a single gene. Natural selection for polygenic traits is more complex, unless you just look at phenotypes. Three ways that natural selection can affect phenotypes are shown in Figure below. You can also watch an animation comparing the three ways at the link. ​ ​

Below is more about the types of selection known today. 1. Artificial Selection Artificial selection is not a type of natural selection, obviously, but it did help obtain data for his of natural selection. Artificial selection mimics natural selection in that certain traits are chosen to be passed down to the next generation. However, instead of or the environment in which the species being the deciding factor for which traits are favorable and which are not, it is that do the selecting of traits during artificial selection.

2. Directional Selection Directional selection occurs when one of two extreme phenotypes is selected for. This shifts the distribution toward that extreme. This is the type of natural selection that the Grants observed in the size of Galápagos finches.

--Example of Directional Selection: Charles Darwin studied what was to become known as directional selection while he was in the Galapagos Islands. The beak length of the Galapagos finches changed over time due to available sources. When there was a lack of to eat, finches with larger and deeper survived because they could crack . Over time, as insects became more plentiful, directional selection favored finches with smaller and longer beaks. All of the finches probably descended from one that arrived on the islands from South America. Until the first bird arrived, there had never been on the islands. The first bird was a eater. It evolved into many species. Each species was adapted for a different type of food. This is an example of . This is the process by which a single species evolves into many new species to fill available niches.

Figure 8.25: Galápagos Finches ​

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3. Disruptive Selection Disruptive selection occurs when phenotypes in the middle of the range are selected against. This results in two overlapping phenotypes, one at each end of the distribution. An example is . This refers to differences between the phenotypes of males and of the same species. In humans, for example, males and females have different heights and body shapes. Like directional selection, disruptive selection can be influenced by interaction. Environmental can drive disruptive selection to choose different colorings in for survival.

--Example of Disruptive Selection: ​ One of the most studied examples of disruptive selection is the case of London’s peppered moths. In rural areas, the peppered moths were almost all a very light . However, these same moths were very dark in color in industrial areas. Very few medium colored moths were seen in either location. It seems that the darker colored moths survived predators in the industrial areas by blending into the polluted surroundings. The lighter moths were seen easily by predators in industrial areas and were eaten. The opposite happened in the rural areas. The medium colored moths were easily seen in both locations and were therefore very few of them left after disruptive selection. Figure 8.26: London’s Peppered Moths ​

4. Stabilizing Selection The most common of the types of natural selection is stabilizing selection. Stabilizing selection occurs when phenotypes at both extremes of the phenotypic distribution are selected against. This narrows the range of variation. 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. --Example of Stabilizing Selection: ​ Many human characteristics are a result of stabilizing selection. Human birth weight is not only a polygenic trait, but it is also controlled by environmental factors. Infants with average birth weight are more likely to survive than a baby that is too small or too large. The bell curve peaks at a birth weight that has the minimum rate.

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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. essentially clones the parent by copying its alleles without any sort of mating between individuals. Some species that use 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 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 Mutation creates new genetic variation in a gene pool. It is how all new alleles first arise. In sexually reproducing species, the mutations that matter for evolution are those that occur in . Only these mutations can be passed to offspring. For any given gene, the chance of a mutation occurring in a given is very low. Thus, mutations alone do not have much effect on allele frequencies. However, mutations provide the genetic variation needed for other forces of evolution to act.

Genetic Drift Genetic drift is a random change in allele frequencies that occurs in a small population. When a small number of parents produce just a few offspring, allele frequencies in the offspring may differ, by chance, from allele frequencies in the parents. This is like tossing a coin. If you toss a coin just a few , you may, by chance, get more or less than the expected 50 percent or tails. In a small population, you may also, by chance, get different allele frequencies than expected in the next generation. In this way, allele frequencies may drift over time. There are two special conditions under which genetic drift occurs. They are called bottleneck effect and founder effect. 1. Bottleneck effect occurs when a population suddenly gets much smaller. This might happen because of a natural disaster such as a forest fire. By chance, allele frequencies of the survivors may be different from those of the original population. 2. Founder effect occurs when a few individuals start, or found, a new population. By chance, allele frequencies of the founders may be different from allele frequencies of the population they left. An example is described in Figure below.

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Figure 8.27: Founder Effect in the Amish Population. The Amish population in the U.S. and Canada had a small ​ number of founders. How has this affected the Amish gene pool?

Eyewitness to Evolution In the 1970s, went to the Galápagos Islands. They wanted to re-study Darwin’s finches. They spent more than 30 years on the project. Their efforts paid off. They were able to observe evolution by natural selection actually taking place. While the Grants were on the Galápagos, a drought occurred. As a result, fewer seeds were available for finches to eat. Birds with smaller beaks could crack open and eat only the smaller seeds. Birds with bigger beaks could crack and eat seeds of all sizes. As a result, many of the small-beaked birds died in the drought. Birds with bigger beaks survived and reproduced (see Figure 8.28). Within 2 ​ ​ years, the average beak size in the finch population increased. Evolution by natural selection had occurred. Figure 8.28: Evolution of Beak Size in Galápagos Finches ​ ​ ​ 238

Can Microevolution Lead to Macroevolution? No matter how controversial the Theory of Evolution is in some circles, it is rarely argued that microevolution happens in all species. There is pretty significant amounts of evidence that DNA changes and in turn can cause small changes in the species, including thousands of years of artificial selection via breeding. However, the opposition comes when scientists propose that microevolution over very long periods of time can lead to macroevolution. After all, thousands of years of breeding different species has not led to completely new species being formed. Doesn't that prove that microevolution does not lead to macroevolution? Proponents for the idea that microevolution leads to macroevolution point out that not enough time has gone by in the scheme of the history of on to show if microevolution does lead to macroevolution. The bottom line is that this is one controversy that has not been solved. Both sides have legitimate arguments for their causes. It may not be solved within our lifetimes. It is important to understand both sides and make an informed decision based on the evidence that fits in with your beliefs. Keeping an open while remaining skeptical is often the hardest thing for people to do, but it is necessary when considering scientific evidence.

Basics of Microevolution Microevolution is the changes in species at a molecular, or DNA, level. All species on Earth have very similar DNA sequences that code for all of their characteristics. Small changes can happen through mutations or other random environmental factors. Over time, these can affect the available traits that can be passed down through natural selection to the next generation. Microevolution is rarely argued and can be seen through breeding experiments or studying population biology in various areas.

Changes in Species Species do change over time. Sometimes these are very small changes caused by microevolution, or they may be larger morphological changes described by Charles Darwin and now known as macroevolution. There are different ways species change based on geography, reproductive patterns, or other environmental influences. Both proponents and opponents of the microevolution leading to macroevolution controversy use the idea of to support their arguments. Therefore, it does not really settle any of the controversy.

Basics of Macroevolution Macroevolution was the type of evolution Darwin described in his time. Genetics and microevolution were not discovered until after Darwin died and published his plant experiments. Darwin proposed that species changed over time in and . His extensive study of the Galapagos finches helped shape his Theory of Evolution through Natural Selection, which is now most often associated with macroevolution.

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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 , 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 distribution.

References/ Multimedia Resources

● Bailey, Regina. "The Importance of Genetic Variation." About.com Biology. Web. 16 June 2014. ​ ​ ● Scoville, Heather. "Can Microevolution Lead to Macroevolution?" About.com Evolution. Web. 16 ​ ​ June 2014. ● Scoville, Heather. "Gene Pool." About.com Evolution. Web. 16 June 2014. ​ ​ ● Scoville, Heather. "Genetic Drift." About.com Evolution. Web. 16 June 2014. ​ ​ ● Scoville, Heather. "Processes of Microevolution." About.com Evolution. . Web. 16 June 2014. ​ ​ ● Scoville, Heather. "3 Types of Natural Selection (and 1 Type of Not-So-Natural Selection)." About.com Evolution. Web. 16 June 2014. ​

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