In conclusion, heritable variations, which Darwin knew were necessary for natural selection, are determined by the variety of alleles, or different forms of genes, which build proteins. The environment may add to the variety of phenotypes by influencing their expression, but the genes themselves remain unchanged. Only genes and their alleles can be sources of heritable variations in traits.
Sources of Variation
Genetic variation results from the diversity of alleles. How do alleles form?
You may recall that mutations change the sequence of nucleotides in DNA. Mutations can alter single nucleotides or entire chromosomes Figure below, and they are the sole source of new alleles.
Figure 13.6
The ultimate source of genetic variation, random mutations are changes in nucleotide sequences of DNA. They may involve only a single base pair - as in (A), or many - as in chromosomal mutations (B).
Even a single nucleotide substitution can cause major changes in an organism; Figure below shows the molecular and cellular effects of the well-known substitution mutation which causes sickle-cell anemia in humans.
Figure 13.7
Mutations in DNA cause changes in the amino acid sequence of proteins, which in turn cause changes in traits. The disease Sickle Cell Anemia results from a single base-pair substitution in the gene for Hemoglobin. The single amino acid change dramatically alters the shape and function of the protein - and of red blood cells, which each contain 280,000 molecules of hemoglobin.
Caused by ultraviolet or ionizing radiation, certain chemicals, or viruses, mutations occur entirely by chance; they are not in any sense goal-directed. Usually, they reduce an organism’s fitness - its ability to survive and reproduce. However, many mutations are neutral; they have no effect on an organism’s fitness. A few may actually improve fitness. Sickle-cell hemoglobin (Hemoglobin S – see Figure above) prevents malarial infection, so in equatorial environments where malaria is prevalent, one copy of the Hemoglobin-S allele increases survival and reproduction.
New alleles arise only by chance mutations, yet without them, there would be no diversity and no evolution. Although they are rare due to repair mechanisms, mutations provide the creative potential for adaptation to environmental change.
Rates of mutation depend largely on reproductive rate, and are highest in bacteria and viruses. HIV, for example, can produce over one trillion new viruses per day, and each replication of its genome provides an opportunity for mutation. Their high mutation rates explain why viruses and bacteria so often become resistant to drug treatments.
Mutations resulting in heritable variation for multicellular organisms happen constantly, but at a lower rate. In part, this is because mutations in body cells do not affect the DNA in eggs and sperm. While this prevents many damaging mutations from dooming offspring, it also reduces diversity and the potential for adaptation to changing environments. Sexual reproduction, however, compensates at least in part for this loss.
Figure 13.8
Sexual reproduction cannot produce new alleles, but it does shuffle alleles into unique combinations, adding to the raw material for natural selection.
Although sexual reproduction cannot produce new alleles, meiosis and fertilization shuffle alleles from past mutations into new combinations Figure above. As Mendel demonstrated, genes (which he called “factors”) segregate and sort independently during the formation of eggs and sperm (return to the Mendelian Genetics if needed), and fertilization is random. The result is that – by chance – each offspring has a unique combination of alleles – a tremendous source of variation and raw material for natural selection.
Populations and Gene Pools
Individuals do not evolve. Natural selection may affect an individual’s chance to survive and reproduce, but it cannot change the individual’s genes. However, a population – a group of organisms of a single species in a certain area – evolves when natural selection imposes differential survival on individuals within it. Population genetics studies populations at the level of genes and alleles in order to discover how evolution works.
If we consider all the alleles of all the genes of all the individuals within a population, we have defined the gene pool for that population. Gene pools contain all the genetic variation – that raw material for natural selection – within a population. The gene pool for a rabbit population, for example, includes alleles which determine coat color, ear size, whisker length, tail shape, and more (Table below). If a population geneticist wants to focus on the variation in an individual gene, she/he may look at the gene pool of all the alleles for that gene alone.
Consider a population of 100 rabbits, including 10 albinos and 90 brown rabbits. The proportion of each phenotype is a measure of variation in this population, but it does not show the true genetic diversity. The gene pool does. Two populations of 10 white and 90 brown rabbits – with identical phenotypic diversity - could differ, because brown rabbits could be either homozygous or heterozygous.
Rabbits, like humans, are diploid. Therefore, each population’s gene pool for coat color contains 200 alleles. The 10 albinos in each population contribute a total of 20 b alleles to each gene pool. However, if population #1 has 40 heterozygotes and population #2 just 20, their gene pools differ significantly.
In population #1, 50 homozygous dominant individuals contribute 100 B alleles, and the 40 heterozygotes contribute 40 more. The heterozygotes also contribute 40 albino b alleles. Thus, population #1 contains a total of 140 B alleles and 60 b alleles.
Population #2, on the other hand, has 70 homozygous dominant rabbits, which add 140 B alleles to the 20 from the heterozygotes – a total of 160 B alleles. Only 40 b alleles are present in the gene pool, so genetic diversity is lower. The use of gene pools allows population geneticists to see variation and the influence of natural selection in detail.
Table of Phenotypes and Genotypes in Two Rabbit Populations Phenotypes Number of each
phenotype
Genotypes Number of each
genotype
Alleles contributed
to gene pool
Total gene pool
Rabbit Population #1
Rabbit Population #2
As Darwin saw, populations are the units of evolution. As Mendel showed but Darwin could not have seen, heritable units, which we now know as alleles, determine the amount of variation within a population. A measure of this variation is the population’s gene pool. Once again, individuals cannot evolve, but it is the individual organism that adapts to its environment. It is the gradual accumulation of a series of adaptations - through genetic change - in a lineage from the population that is evolution.
Allele Frequencies
Our analysis of the rabbit populations (Table above) showed that the relative numbers of various alleles are the best measure of variability in a gene pool (or a population). Population geneticists calculate the proportions, or frequencies, of each allele in order to study changes in populations. In fact, a change in allele frequency is the most precise measure of the process of evolution.
An allele’s frequency is the fraction (expressed as a decimal) of a population’s gene pool made up of that particular allele. In rabbit population #1 above, the gene pool for coat color included 200 alleles, 140 of which were B. The frequency of allele B for population #1 is 140/200 = 0.7. Because frequencies must total 1.0, and the only other allele is b, the frequency of allele b is 0.3. (Alternatively, we can calculate the frequency of allele b by dividing the number of this allele by the total number of alleles: 60/200 is again 0.3.) In population #2, 160 of 200 alleles are B, so the frequency of allele B is 0.8, and the frequency of allele b is 0.2.
If all the members of a rabbit population (#3) are homozygous brown, the frequency of B is 1.0, the frequency of b is 0, and there is no diversity. Allele B is said to be fixed, and until a mutation occurs, no possibility for evolution (with respect to this particular gene) exists. A population with frequencies of 0.5 for B and 0.5 for b
would have maximum diversity for this two-gene system.
Any change in those frequencies across generations would reflect evolution of the population – the subject of the next lesson.
Lesson Summary
Population genetics studies populations at the level of genes and alleles in order to discover how evolution works.
Genes - segments of DNA - determine traits by coding for enzymes that control chemical reactions.
Many genes have several different forms known as alleles.
Alleles are responsible for the variation in traits.
The environment can affect the expression of some genes, but variations induced by the environment are not heritable.
Mutations – changes in single nucleotides or entire chromosomes - are the sole source of new alleles.
Ultraviolet or ionizing radiation, chemicals, or viruses cause mutations - in an entirely random way.
Rates of mutation are high in rapidly reproducing viruses and bacteria, allowing them to adapt quickly to changing environments. An example is drug resistance.
Rates of heritable mutations are lower in multicellular organisms because only germ cell mutations are passed on to offspring.
Sexual reproduction recombines existing alleles through meiosis and random fertilization, so that each offspring is unique.
Mutations and sexual reproduction provide the variety which is the raw material for natural selection.
Individuals experience natural selection, but not the genetic changes of evolution.
Populations evolve through genetic change.
A population’s gene pool includes all the alleles of all the genes of all the individuals within it.
A gene pool for a single gene includes all the alleles of that gene present in all individuals.
Analysis of a gene pool can reveal variation which is not visible in phenotypes.
Changes in allele frequency measure the process of evolution.
Allele frequency is the decimal fraction of a population’s gene pool made up of that particular allele.
A gene pool containing only one type of allele is fixed; it lacks variation and potential for evolution, at least until mutation occurs.
For a gene with two alleles, allele frequencies of 0.5 indicate maximum variation.
Any change in allele frequency reflects evolution of a population.
Review Questions
Describe the relationship between genes and traits.
Explain the goal of population genetics.
Differentiate between genes and alleles.
Distinguish environmental effects on gene expression from allelic variations in genes.
Describe the relationship between mutations, alleles, variation, natural selection, and chance.
Compare rates of mutation in microorganisms to those in multicellular organisms.
Explain why populations, but not individuals, can evolve.
Distinguish between a population’s gene pool and a gene pool for a single gene.
Explain how to determine allele frequencies.
Evaluate phenotype, genotype, and allele frequencies as measures of variation, as raw material for evolution, and as a measure of evolution.
Further Reading / Supplemental Links
http://library.thinkquest.org/19037/population.html
https://www3.nationalgeographic.com/genographic/
http://darwin.eeb.uconn.edu/simulations/simulations.html
http://chroma.gs.washington.edu/outreach/genetics/sickle/sickle-bean.html
Vocabulary
allele
An alternative form or different version of a gene.
allele frequency
The fraction (usually expressed as a decimal) of a population’s gene pool made up of a particular allele.
evolution
A change in the frequency of an allele (or genetic makeup) in a population.
fitness
The ability of an organism with a certain genotype to survive and reproduce, often measured as the proportion of that organism’s genes in all of the next generation’s genes.
gene
A segment of DNA which codes for a protein or RNA molecule; a unit of inheritance.
gene pool
Within a population, the sum of all the alleles of all the genes of all the individuals.
genotype
The genetic makeup of an organism; specifically, the two alleles present.
heterozygous
Describes a genotype or individual having two different alleles for a gene.
homozygous
Describes a genotype or individual having two copies of the same allele for a gene.
mutation
A change in the nucleotide sequence of DNA or RNA.
natural selection
The process by which a certain trait becomes more common within a population, including heritable variation, overproduction of offspring, and differential survival and reproduction.
phenotype
The physical appearance of an organism determined by a particular genotype (and sometimes also by the environment).
population
A group of organisms of a single species living within a certain area.
population genetics
The study of the evolution of populations at the level of genes and alleles.
sexual reproduction
Two-parent gamete based reproduction, involving meiosis and fertilization.
Points to Consider
Imagine how Darwin felt, knowing that traits were passed on to offspring and that heritable variations “somehow” appeared. What key discoveries now explain these facts?
As we noted in the last chapter, Theodosius Dobzhansky is famous for his statement, “Nothing in biology makes sense except in the light of evolution.” Do you agree that people cannot understand biology without understanding evolution?
How does evolutionary theory “make sense of” your similarities to your parents and siblings? Your differences from them? Similarities among all humans? Differences among us?
Lesson 13.2: Genetic Change in Populations
Lesson Objectives
Compare and relate macroevolution to microevolution.
Define microevolution in terms of allele frequencies.
Define genetic equilibrium for a population.
List the five conditions for genetic equilibrium according to the Hardy-Weinberg model.
State and explain the generalized equation for Hardy-Weinberg equilibrium.
Explain how to use the Hardy-Weinberg equation to solve for allele or genotype frequencies.
Discuss the reasons why the 5 conditions for Hardy-Weinberg equilibrium are rarely met.
Explain how mutation disrupts genetic equilibrium.
Predict the possible effects of mutation, and analyze the probability of each type of effect.
Contrast mutation in microorganisms to mutation in multicellular organisms.
Define gene flow.
Describe two possible effects of gene flow on the genetics of a population.
Define genetic drift.
Describe three possible effects of genetic drift on populations and/or specific alleles.
Explain and give an example of the bottleneck effect.
Clarify and give an example of the concept of “founder effect.”
Compare and contrast genetic drift and natural selection as causes of evolution.
Discuss natural selection and evolution in terms of phenotypes and allele frequencies.
Explain the distribution of phenotypes for a trait whose genetic basis is polygenic.
Using a normal distribution of phenotypic variation, interpret directional, disruptive, and stabilizing patterns of selection.
Define fitness as it relates to natural selection.
Describe how natural selection can sometimes lead to the persistence of harmful or even lethal allele.
Analyze the logic of kin selection.
I
ntroduction
How many people in the United States carry the recessive allele for the lethal genetic disease, cystic fibrosis?
Why must we treat AIDS patients with multi-drug “cocktails?”
Why do we consider Northern Elephant seals endangered, even though their population has risen to 100,000 individuals?
Why don’t South African Cheetahs reject skin grafts from unrelated individuals?
What were the probable effects on human evolution of the six years of “volcanic winter” caused by the “megacollosal” eruption of Mt. Toba 70,000 years ago?
Why do humans vary in skin color?
Why doesn’t natural selection eliminate certain lethal genes?
This lesson will introduce you to our current understanding of the causes of evolution and show you how scientists use this understanding to solve problems and answer puzzling questions like those above.
Microevolution
In an earlier chapter, we defined microevolution as evolution within a species or population, and macroevolution as evolution at or above the level of species. The differences between the two are time and scale. Microevolution can refer to changes as small as a shift in allele frequencies for a single gene from one generation to the next. In contrast, macroevolution describes changes in species over geologic time. Many biologists consider evolution to be a single process; they regard macroevolution as the cumulative effects of microevolution. The History of Life chapter discussed patterns and processes of macroevolution; this lesson will focus on genetic changes within populations and species. At this level, we can see how evolution really works – a view that Darwin never had.
CK-12 Biology I - Honors Page 57