Lesson Summary
Macroevolution – change in species over geologic time – is the cumulative effect of microevolution.
Microevolution – evolution within species or populations – can be measured as a generation-to-generation change in allele frequencies.
Non-evolving populations have constant frequencies of alleles and genotypes – genetic equilibrium.
The Hardy-Weinberg model holds for equilibrium populations under 5 conditions:
no mutation
no migration
very large population size
random mating
no natural selection
A generalized form of the Hardy-Weinberg equation for a gene pool of two alleles at equilibrium is:
p2 + 2 pq + q2 = 1
frequency of genotype BB frequency of genotype Bb frequency of genotype bb
where p is the frequency of one allele and q the frequency of the other allele.
In nature very few populations meet the Hardy-Weinberg requirements for equilibrium, due to constant mutation, gene flow, small populations, nonrandom mating, and environmental change.
Random, spontaneous mutations constantly generate new alleles in a gene pool, destabilizing genetic equilibrium and creating the potential for adaptation to changing environments.
In multicellular organisms, only mutations that affect germ cells become part of the gene pool.
Because of extensive “junk” DNA, repair enzymes, and multicellularity, many mutations do not reach the gene pool.
Many mutations are harmful, disrupting protein function.
Because the genetic code is redundant and some amino-acid substitutions may not change protein function, many mutations have no effect on an organism’s fitness.
Even neutral mutations hold potential for future selection if the environment changes.
A few mutations may be advantageous, improving or changing protein function.
In microorganisms mutation rates are much higher due to rapid reproduction and small genomes.
Mutation, together with recombination of existing alleles provides the diversity which is the raw material for natural selection and evolution.
The movement of genes from one population to another (gene flow) can change allele frequencies.
Gene flow can accelerate the spread of successful alleles or reduce differences between populations.
In small populations random variations in allele frequencies can significantly influence the “survival” of any allele, regardless of its adaptive value; this phenomenon is genetic drift.
Genetic drift can result in extinction of an allele or an entire population.
Alternatively, genetic drift can lead to rapid evolution of a population.
In the bottleneck effect, a catastrophe or disease or overhunting dramatically reduces a population’s size and genetic variation, increasing its susceptibility to the effects of genetic drift.
In the founder effect, a small group leaves a larger population to colonize a new area. Again, genetic drift may lead to loss of genetic diversity, extinction, or rapid evolution.
Genetic drift leads to evolution in populations of small size, but results are mostly due to chance.
Natural selection occurs in populations of any size, and results are more likely to adapt a population to its environment.
Natural selection acts on phenotype variations, so may include environmental effects which are not heritable.
Evolution is a result of natural selection, and is measured as a change in allele frequencies.
For a trait whose genetic basis is polygenic, the pattern of phenotypic variation usually forms a bell curve about an average value.
Directional selection results in a shift of allele frequencies toward one extreme.
Disruptive selection favors both extremes over the average phenotypic value.
Stabilizing selection maintains or narrows existing variation in phenotype.
Fitness is the ability of an organism with a certain genotype to reproduce successfully.
Because alleles often affect more than one trait in different ways, or have different effects in different environments, natural selection can sometimes lead to the persistence of harmful or even lethal allele.
Kin selection involves the sacrifice by an individual of his/her reproductive potential in order to help a close relative reproduce successfully.
Review Questions
Define microevolution in terms of allele frequencies.
Describe genetic equilibrium, including the Hardy-Weinberg conditions.
In the United States, about 1 in every 12 black people of African descent carry one copy of the allele for sickle-cell anemia. Assuming the five conditions of Hardy-Weinberg equilibrium hold (we’ll consider why they probably don’t in question #4), calculate the probability that a member of the next generation will suffer from sickle-cell anemia (have the homozygous Hemoglobin S genotype).
Discuss the reasons why the 5 conditions for Hardy-Weinberg equilibrium are probably not met by the black population for the Hemoglobin alleles.
Analyze the possible effects of mutation, and the probability and importance of each.
Describe two possible effects of gene flow on the genetics of a population.
Describe three possible effects of genetic drift on populations and/or specific alleles.
Why do we consider Northern Elephant seals endangered, even though their population has risen to 100,000 individuals?
Use the distribution of phenotypes for a trait whose genetic basis is polygenic to interpret directional, disruptive, and stabilizing patterns of selection.
Describe how natural selection can sometimes lead to the persistence of harmful or even lethal allele. Include an example.
Further Reading / Supplemental Links
“The Gene School:”
http://library.thinkquest.org/19037/population.html
http://genetics-education-partnership.mbt.washington.edu/class/activities/HS/sickle-bean.htm
http://www.koshland-science-museum.org/teachers/idactivity-act018.jsp
http://chroma.gs.washington.edu/outreach/genetics/sickle/index.html
http://www.umsl.edu/~biology/hwec/MO-STEP/lessons/naturalsel.html
http://www.radford.edu/~rsheehy/Gen_flash/popgen/
http://www.phschool.com/science/biology_place/labbench/lab8/intro.html
http://sps.k12.ar.us/massengale/lab_8_sample2_ap_population_gene.htm
http://www.bgsu.edu/departments/chem/faculty/leontis/chem447/PDF_files/Jablonski_skin_color_2000.pdf
https://www3.nationalgeographic.com/genographic/
http://darwin.eeb.uconn.edu/simulations/simulations.html
http://www.berkeley.edu/news/media/releases/2005/03/02_turkeys.shtml
http://essp.csumb.edu/eseal/kristi_west/history.html
Vocabulary
adaptive radiation
Relatively rapid evolution of several species from a single founder population to several to fill a diversity of available ecological niches.
allele frequency
The fraction (usually expressed as a decimal) of a population’s gene pool made up of a particular allele.
bottleneck effect
The loss of diversity resulting from a drastic reduction in population size and subsequent genetic drift.
directional selection
Selection which favors one side of a phenotypic distribution – one allele or one extreme of a normal distribution.
disruptive selection
Selection which favors the two extremes of a phenotypic distribution – the ends of a bell curve, or the homozygous phenotypes, as opposed to the average, or heterozygous phenotype.
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.
founder effect
The loss of genetic diversity resulting
from colonization of a new area by a small group of individuals which have broken off from a larger population.
gene flow
The net movement of genes into or out of a population through immigration or emigration.
genetic drift
Random changes in allele frequencies in small populations.
genetic equilibrium
State of a population in which allele and genotype frequencies remain constant from one generation to the next – a non-evolving population.
Hardy-Weinberg model
Describes a population at genetic equilibrium, meeting five conditions: no mutation, no migration, very large population size, random mating, and no natural selection.
kin selection
Behaviors which sacrifice reproductive success or even survival to promote the survival and reproduction of close relatives who share a significant proportion of the same genes.
macroevolution
Evolution at or above the species level.
microevolution
Evolution within a species or population, also defined as a generation-to-generation change in allele frequencies for a population.
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.
polygenic trait
Traits that are influenced by more than one gene.
stabilizing selection
Selection which favors the average or heterozygous phenotype, resulting in no change or in a narrowing of the distribution of phenotypes.
Points to Consider
This lesson discussed probable past microevolution of alleles for genes for human skin color and hemoglobin. For what other genes (or heritable traits) can you suggest past selective pressures? Do you think certain human genes (or heritable traits) may be at genetic equilibrium? Give some examples, and explain your reasoning.
Some people suggest that we humans have removed ourselves from natural selection. Do you agree?
What are some consequences of understanding that chance variations and natural selection can result in the persistence of lethal alleles, such as the alleles for sickle-cell anemia and cystic fibrosis?
Do you think it is important that people understand the biological basis of skin color? Explain
Lesson 13.3: The Origin of Species
Lesson Objectives
Recognize that new discoveries since Darwin have added an understanding of speciation to evolutionary theory.
Explain the concept of a species.
Define the biological species concept and analyze its usefulness.
Compare the biological species concept to morphological, genealogical, and ecological concepts.
Analyze the reasons why biologists consider all humans to be members of the same species.
Define speciation.
Describe two conditions that can lead to speciation.
Explain the results of speciation in terms of adaptation, chance, and changes in the environment.
Distinguish allopatric from sympatric speciation.
Describe an experiment which demonstrated allopatric speciation.
Describe two general types of reproductive isolation.
Explain how polyploidy can result in sympatric speciation.
Discuss the use of hybridization to form new crop species.
Analyze the importance of environmental complexity to sympatric speciation for animals.
Compare and contrast the gradualist and punctuated equilibrium models of evolutionary change.
Describe conditions that could increase the rate of speciation.
Describe circumstances that could lower the rate of speciation.
Species Concept
Darwin avoided the use of the word evolution in his major work about evolution. The word “evolve” appears once, at the very end. He titled his work The Origin of Species – a process or group of processes now called speciation. What exactly is a species? How did the millions of species which exist on earth today arise? Will they (we!) continue to change? How quickly? These are questions Darwin sought to answer – and the questions we will explore in this lesson on evolution. Darwin’s work opened the door to life’s history. This lesson will look at the details and modifications of his ideas which research has made clear in the 150 years since he published The Origin.
What is a Species?
A toddler readily recognizes that she/he is surrounded by distinct groups of individuals which we call “kinds” of plants and animals. Biologists refer to kinds of living things as a species - groups of individual organisms which are very similar. If we look closely, however, variations make it quite difficult to decide where to draw the lines between species. For example, are a St. Bernard and a Chihuahua similar enough to group within the same species? Just how similar must two organisms be for biologists to decide they are members of the same species? The answer to this question is not clear, even to biologists who specialize in classification. We will explore several, because how we define a species can help to clarify how species develop.
The Biological Species Concept
A widely used definition of species is the biological species concept (Figure below), first proposed by evolutionary biologist Ernst Mayr in 1942. Mayr’s concept begins with the idea that members of a species must be similar enough that they could interbreed. Because all dogs - from a St. Bernard to a Chihuahua – are capable of interbreeding, biologists consider all dogs to be members of the same species. If you are familiar with mules, however, you know that this definition needs clarification. If horses and donkeys mate, they produce mules, but mules are sterile and cannot continue to interbreed. Therefore, the biological species concept becomes organisms similar enough to interbreed and produce fertile offspring. Horses and donkeys, therefore, are not members of the same species. As you may know, wolves and dogs can interbreed to produce viable hybrids with fertile offspring; surely, wolves and dogs are not members of the same species? The last part of the definition addresses this problem, and the complete definition becomes:
Figure 13.28
The species is the smallest group or level used to classify living things. As for all levels, the goal of classification is to show evolutionary relationships. A biological species is defined as a group of organisms similar enough to reproduce and have fertile offspring under natural conditions. A mating between a horse and a donkey produces a mule, but the mule is sterile, so the horse and donkey are not considered members of the same species. On the other hand, dogs and wolves can interbreed successfully, but because they do not do so in nature, they are not classified as members of the same species.
A biological species is a group of organisms similar enough that they could interbreed and produce fertile offspring under natural conditions.
This definition serves the goal of defining members of a species as individuals which are still undergoing evolution – they form a distinct yet potentially common gene pool. You will learn much more about classification in the next chapter, but here it is important to realize that one of the primary goals of classification is to show evolutionary history – patterns of common ancestry. This makes the biological species – a functionally reproducing unit – an important foundation of classification. Closely related species descended from relatively recent common ancestors, and distantly related species descended from more distant common ancestors. The emphasis the biological species concept places on successful reproduction fits this goal of classification quite well.
Another advantage of the biological concept is that it has the potential to explain how speciation occurred – that is, how two closely related groups of organisms became different species through reproductive isolation. Members of two species may appear very similar, yet fail to interbreed because of reproductive barriers. Barriers
to reproduction may either prevent mating, or prevent development of a fertilized egg after mating. Elaborate courtship behaviors, including the blinking pattern of fireflies or the songs of birds, are often required to elicit mating behavior – and limit mating to members of a species (Figure below). Different breeding seasons, such as flowering dates, can also prevent interbreeding. Molecular differences between even closely related species may prevent sperm or pollen from actually fertilizing eggs. Once the eggs are fertilized, chromosomes may be so incompatible that mitosis and meiosis cannot proceed normally; if the zygote cannot develop, offspring do not survive. All of these barriers between species work to ensure successful reproduction within species, keeping specific, useful adaptations “in the family,” so they are a logical way for us, too, to distinguish members of one species from members of another.
Figure 13.29
The Western Meadowlark (left) and the Eastern Meadowlark (right) appear morphologically identical. However, geography and songs serve as reproductive barriers to interbreeding, so the two are considered to be separate species.
Although the biological species concept is extremely helpful in evolutionary thinking it has serious limitations for practical use. For organisms that reproduce asexually – including all bacteria and viruses - the definition is entirely unworkable. Nor can we detect whether or not fossil organisms would have been able to interbreed – whether or not they coexisted. Biologists must rely on structure and (if available) biochemical similarities to classify fossils and most microorganisms.
CK-12 Biology I - Honors Page 60