Chapter 8: Evolution Lesson 8.3: Microevolution and the Genetics of ...

Chapter 8: Evolution Lesson 8.3: Microevolution and the Genetics of Populations

Microevolution refers to varieties within a given type. Change happens within a group, but the descendant is clearly of the same type as the ancestor. This might better be called variation, or adaptation, but the changes are "horizontal" in effect, not "vertical." Such changes might be accomplished by "natural selection," in which a trait within the present variety is selected as the best for a given set of conditions, or accomplished by "artificial selection," such as when dog breeders produce a new breed of dog.

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

Vocabulary

? adaptive radiation ? allele frequency ? directional selection ? disruptive selection ? gene flow ? gene pool ? genetic drift ? Hardy-Weinberg theorem ? macroevolution ? microevolution ? migration ? population genetics ? stabilizing selection

Introduction Darwin 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 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.

262

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 species. The

Grants observed this level of evolution in Darwin's finches which will be discussed later in this lesson. 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.

Genes in Populations One common misconception about evolution is the idea that individuals can evolve. Individuals

do not evolve. Their genes do not change over time. Individuals can only accumulate adaptations 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. In other words, 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. 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 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. Therefore, only populations can evolve using the mechanism of natural selection. The science that focuses on evolution within populations is population genetics. It is a combination of evolutionary theory and Mendelian genetics.

Gene Pool The genetic makeup of an individual is the individual's genotype. A population consists of many

genotypes. 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 determines which phenotypes are visible in the population at any given time. For a given gene, the population is characterized by the frequency of the different alleles in 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 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. The favorable adaptations become more plentiful and the less desirable traits become fewer or even disappear from the gene pool completely.

263

Populations with larger gene pools are more likely to survive as the environment changes than those with smaller gene pools. Larger populations with more diversity will tend to have at least some desirable characteristics within the population as the environment changes and requires new adaptations. A smaller and more homogeneous gene pool puts the population at risk for extinction if there are few or no individuals have the genes for the traits necessary to survive the change. For example, in bacteria populations, individuals that are antibiotic 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 Allele frequency or genetic variation 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. 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. The genetic variations that arise in a population happen by chance, but the process of natural selection does not. 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.

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.

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.

264

Hardy and Weinberg and Microevolution Charles Darwin's Theory of Evolution touched briefly on favorable characteristics being passed

down from parents to offspring, but the actual mechanism for that was flawed. Gregor Mendel did not publish his work until after Darwin's death. Both Hardy and Weinberg understood that natural selection occurred because of small changes within the genes of the species.

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 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 mechanism 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.

Table 8.4: Genotype Frequencies in a Hardy-Weinberg Equilibrium Population (for one gene with two

alleles, A and a)

----------------------------------------------------------------------------------------------------------------------------- ---------------------------

Genotype

Genotype Frequency

----------------------------------------------------------------------------------------------------------------------------- ---------------------------

AA

p2

Aa

2pq

aa

q2

--------------------------------------------------------------------------------------------------------------------------------------------------------

(p = frequency of A, q =frequency of a): p2 + 2pq + q2 = 1.

Remember in Table 8.3 we saw that p + q = 1

In Table 8.4, if p = 0.4, what is the frequency of the AA genotype using the Hardy-Weinberg Equilibrium Equation: p2 + 2pq + q2 = 1? See Table 8.5 on the next page for the solution.

265

Table 8.5: Genotype 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

2pq

aa

q2

----------------------------------------------------------------------------------------------------------------------------- ---------------------------

(p = frequency of A, q =frequency of a): p2 + 2pq + q2 = 1.

Remember in Table 8.3 we saw that 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

Summary of what the Hardy-Weinberg Equilibrium Equation shows: 1. p = the frequency or percentage of the dominant allele in decimal format 2. q = the frequency or percentage of the recessive allele in decimal format 3. Since p is the frequency of all dominant alleles (A), it counts all of the homozygous dominant

individuals (AA) and half of the heterozygous individuals (Aa) 4. Likewise, since q is the frequency of all recessive alleles (a), it counts all of the homozygous

recessive individuals (aa) and half of the heterozygous individuals (Aa) 5. Therefore, p2 stands for all homozygous dominant individuals 6. q2 stands for all homozygous recessive individuals 7. 2pq is all heterozygous individuals in a population 8. Everything is set equal to 1 because all individuals in a population equals 100%

For a further explanation of this theorem, see .

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 Charles Darwin was not the first scientist to explain evolution, or that species change over time.

However, he gets most of the credit simply because he was the first to publish a mechanism for how evolution happened. This mechanism is what he called Natural Selection. As time passed, more and

266

 ................
................

In order to avoid copyright disputes, this page is only a partial summary.

Google Online Preview   Download