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Biology LibreTexts

Evolution and Population Genetics

Review

Haploid, Diploid

Diploid cells (2N) have two complete sets of chromosomes.   The body cells of animals are diploid.

Haploid cells have one complete set of chromosomes.  Some organisms are haploid. Animals are diploid but their gametes (sperm and eggs) are haploid.

Mitosis

Mitosis is a type of cell division that results in daughter cells that are identical (genetically) to the parent cell. If the parent cell is diploid, the two daughter cells will be diploid. Similarly, a haploid cell that divides by mitosis will produce two haploid daughter cells.

The diagram shows how chromosome movement results in two daughter cells with chromosomes that are identical to the parent cell.

 

 Below: The single-stranded chromosomes in the two daughter cells will later become double-stranded. The two resulting strands (chromatids) are identical.

Meiosis

Meiosisis a type of cell division in which the daughter cells have 1/2 the number of chromosomes as the parent cell. If the parent cell is diploid, the daughter cells will each be haploid.

Meiosis has two separate divisions resulting in four daughter cells. The first division is shown below.

Each of the two cells produced by the first division (shown above) divides again (shown below). Notice that the second meiotic division is like mitosis.

     

Evolution and Population Genetics

Evolution Occurs in Populations

Species

There is not a good definition of species; perhaps the concept of species is artificial but it is useful because it allows people to classify organisms.

Most biologists would agree that members of a sexually-reproducing species are able to interbreed and have a shared gene pool.

Different species do not exchange genes with each other; they do not interbreed.

This definition of species is based on sexual reproduction and therefore does not work with prokaryotes or other asexual species.

Population

A population is often described as an interbreeding group of organisms (the same species) that occupies a particular area.

The size of the area is somewhat arbitrary. There could be a population of fish in an aquarium and a population of fish in a lake.

Gene Frequency and Evolution

Gene frequency refers to the proportion of alleles that are of a particular type. For example, if 60% of the alleles in a population are "a" and 40% are "A", then the gene frequency of "a" is 0.6 and the gene frequency of "A" is 0.4.

On a small scale, evolution involves changes in gene frequencies.

Population Model

A population is a group of interbreeding organisms that occupies a particular area.

Initial Population

Circles are used to represent genes in this diagram of a population. Individuals are diploid, so two circles are used to represent an individual.

Gene Frequencies in the Model Population

In the population above, 33% of the genes for eye color in a population are "A" and 67% are "a".  The frequency of "A" is therefore 0.33 and the frequency of "a" is 0.67.

Gametes

During meiosis, "AA" individuals will produce all "A" gametes. Similarly, 1/2 of the gametes produced by an "Aa" individuals will be "A" and the other half will be "a"; "aa" individuals will produce all "a" gametes.

Individual

Gametes

AA

all A

Aa

1/2 A, 1/2 a

aa

all a

The proportion of A and a in the gametes will be the same as in the population. In the example population we have been using, suppose that each individual produces four gametes.

In reality, males produce many millions of gametes and females produce relatively few. This is not a concern for our model because in either case, the gene frequency of the gametes will be the same as that of the population that produced them.

The gene frequency of "A" and "a" in the gamete pool will remain 0.33 and 0.67.

Gene frequency: The next generation

Because the gene frequency in the gamete pool did not change, the gene frequency in the population the next generation remains the same.

The Hardy-Weinberg law states that under certain conditions (discussed below), the gene frequency of a population does not change from generation to generation.

Should There Be Fewer Recessive Alleles?

The population model described above predicts that gene frequencies will not change from one generation to the next even if there are more recessive alleles.

There is sometimes a misconception among students beginning to study genetics that dominant traits are more common than recessive traits. It isn't true. For example, blood type O is recessive and is the most common type of blood. Huntington's (a disease of the nervous system) is caused by a dominant gene and the normal gene is recessive. Fortunately, most people are recessive; the dominant is uncommon.

The misconception comes from the observation that in a cross of Aa X Aa, 3/4 of the offspring will show the dominant characteristic. However, the 3:1 ratio comes only if the parents are both Aa. If there are many recessive genes in a population, then most matings are likely to be aa X aa or aa X Aa and most offspring will be aa or Aa. 

Natural Selection

Natural selection is a mechanism that produces changes in the gene frequency from one generation to the next. As a result, organisms become better adapted to their environment.

Natural selection occurs because

  1. Individuals within a population vary; they are not all identical.
  2. The traits that vary are heritable.
  3. Some variants are "better" than others.

The "better" individuals will have more success reproducing; they will have more offspring.
In successive generations, more offspring will have the better trait.

These items are discussed below.

Variation

Sexual reproduction promotes genetic variation.

For many traits that occur in a population, individuals are often not all identical.  For example, if running speed were measured, some individuals would likely be able to run faster than others but most individuals would probably be intermediate.  

If number of individuals is plotted against the trait in question (running speed for example), a graph like the one shown is often produced.

We would get a similar bell-shaped curve if we plotted height, weight, performance on exams, or almost any other characteristic.
Some Variants are Better

Some individuals are bound to be better than others. Perhaps their body structure allows them to escape predators better or to find food faster or to better provide for their young. For example, suppose that the faster-running animals diagrammed below are better able to escape predators than the slower ones. You would expect that more of the faster ones would survive and reproduce than the slower ones.

The slower rabbits will not reproduce as much because predators kill them more than they kill the faster rabbits.

Traits Are Heritable

Those individuals that reproduce more will pass their superior genes to the next generation. Individuals that reproduce less as a result of "poorer genes" will not pass those genes to the next generation in high numbers. As a result,  the population will change from one generation to the next. The frequency of individuals with better genes will increase. This process is called natural selection.

Fitness

We often hear natural selection described as "survival of the fittest." The word "fitness" used in a biological context means "reproductive." It does not have anything to do with physical fitness or strength. In the example above, it is the fastest rabbits that reproduce the most, not the strongest.

Natural Selection Produces Evolutionary Change

If the conditions discussed above are met, the genetic composition of the population will change from one generation to the next. This process is called natural selection.

The word "evolution" refers to a change in the genetic composition of a population. Natural selection produces evolutionary change because it changes the genetic composition of populations.

A variety of other mechanisms can also produce evolutionary change. For example, suppose that 65% of the eye-color genes in a population were for individuals with blue eyes and 35% of the genes were for brown eyes. If most of the immigrants entering the population carried the blue gene, the overall composition might change from 65% blue to 70% blue.

Although natural selection affects individuals, it is important to note that multicellular organisms cannot change their genes. Changes in the genetic composition of a population occur as a result of changes in reproduction or survival of individuals.

Example of Natural Selection: Industrial Melanism

Kettlewell studied the peppered moth (Biston betularia) from insect collections in England. He observed that in polluted areas, most of the peppered moths were the dark form. In clean areas, most were the pale form.

During the early 1800's, the dark form comprised less than 2% of the population and the pale form made up more than 98%. During the 1800's the dark form increased in frequency in urban areas.

Kettlewell suggested that dark moths survived better in polluted areas because they were more difficult for avian (bird) predators to see on the darkened tree trunks. Similarly, he suggested that light-colored moths were more difficult to see in unpolluted areas because the tree trunks were light-colored.

To test this, he released moths of each type (light and dark) in both polluted and unpolluted areas. In  the unpolluted area, he recaptured 13.7% of the light moths and 4.7% of the dark moths. In the polluted area, he recaptured 13% of the light and 27.5% of the dark moths.

Sexual Reproduction and Evolutionary Change

Variation

Individuals with in a population usually are not all identical and much of this variation is due to genetic differences among individuals.

Sexual reproduction acts to increase variation in populations by shuffling genes. Offspring have some genes from each of two different parents and therefore are not identical clones of their parents. The increased variation due to sexual reproduction allows natural selection (and thus evolution) to produce changes in populations as described above.

Ultimately, all variation in a population comes from changes in the DNA. These changes are called mutations.

Recombination during sexual reproduction promotes variation. Sperm and eggs (gametes) are produced by a type of cell division called meiosis. During meiosis, crossing-over and independent assortment act to shuffle the genes before gametes are produced.

Fluctuating environments

Evolutionary change due to natural selection would not be necessary if the environment never changed and the organisms within the environment were optimally adapted to the environment. For example, imagine a plant that is adapted to an environment that has an average annual  rainfall of 100 cm. If the climate were to change so that the amount of rainfall decreased, individuals that could tolerate less rain would survive and reproduce better, thus establishing their drought-tolerant genes in subsequent generations. If there was no variation in the plant population, there would not be any drought-tolerant individuals and the species would likely go extinct in areas of decreased rainfall.

Sexual reproduction therefore, enables species to survive in fluctuating or changing environments because it promotes variation, which in turn allows natural selection.

Other Forces that Change Gene Frequencies

Migration can change the gene frequency of a population if the migrants have a different gene frequency than that of the population they are leaving or entering.

The founder effect occurs when the gene frequency of a newly established population is somewhat different from the parental population. This may be due to the small sample of founding individuals.

The sample-size phenomenon can be illustrated by flipping a coin. The expected number of "heads" from flipping a coin is 50% but if a coin is flipped only 4 times, you may get all "heads" or all "tails". If the coin is flipped 1000 times, the actual number of "heads" and "tails" will probably not deviate much from 50%. Thus, the larger the sample size of emigrants, the more likely it is to reflect the population from which it is leaving.

Below: The population on the right was formed from a few individuals emigrating from the population on the left.

During a bottleneck, a large population undergoes a decrease in size so that relatively few individuals remain. Because there are few individuals, the gene frequency is more likely to drift.

Below: The gene frequency of the initial population (left) changes because many of the individuals have died. The population on the right is the same population after the bottleneck has occurred.

Genetic drift refers to random fluctuations in the gene frequency of a population. This is more likely to occur in a small population. As with bottlenecks and the founder effect, it is a sample-size phenomenon. The smaller the population, the more likely that gene frequencies are likely to fluctuate from generation to generation.

Mutation changes gene frequencies when genes of one type ("A" for example) mutate to another type ("a" for example).

The rate of mutation may be low but mutations that confer a strong advantage can have a large affect.

Nonrandom mating can affect genotype frequencies (p2, 2pq, or q2) but often has less affect on gene frequencies (p and q).

Natural selection (discussed above) changes gene frequencies when genes or gene combinations are more likely to result in greater reproductive success of the individual that possesses them.

Conditions Necessary for Hardy-Weinberg Equilibrium

Notice that the gene frequency the next generation is the same as that of the initial population. The Hardy-Weinberg principle states that if the following conditions are met, the gene frequency of a population will not change from generation to generation:

  • No migration
  • Large population size (no founder effect, no bottlenecks, no genetic drift)
  • No mutation
  • Random mating
  • No selection

Model Chromosomes

The drawings of chromosomes below will be cut out and used in class for reviewing mitosis and meiosis in the "Review" section at the beginning of this page.

Be sure that you can do the following using these models of chromosomes:

  • Create a haploid cell.
  • Create a diploid cell.
  • Simulate mitosis in a diploid cell.
  • Simulate mitosis in a haploid
  • Simulate meiosis in a diploid cell.
  • Use the models to create two gametes: an egg and a sperm.
  • Simulate the fusion of the two gametes to create a fertilized egg (called a zygote).