The development of molecular genetics has revealed the record of evolution left in the genomes of all organisms (Figure below). It also provides new information about the relationships among species and how evolution occurs.
Molecular genetics provides evidence of evolution such as:
the same biochemical building blocks – such as amino acids and nucleotides - are responsible for life in all organisms, from bacteria to plants and animals
DNA and RNA determine the development of all organisms
the similarities and differences between the genomes, the gene sequences of each species, reveal patterns of evolution.
Figure 7.24
This is a map of the genes on just one of the 46 human chromosomes. Similarities and differences between the genomes (the genetic makeup) of different organisms reveal the relationships between the species. The human and chimpanzee genomes are almost identical- just about 1.2% differences between the two genomes. The complexity of the map signifies close evolutionary relationships when the genomes are highly similar.
Lesson Summary
Fossil evidence, depicted by the skeletal fragments, demonstrates evolutionary milestones.
Fossils and the rocks they are embedded in provide evidence of how life and environmental conditions have changed throughout Earth’s history.
The fossils and the order in which fossils appear is called the fossil record.
Geologists use a method called radiometric dating to determine the age of rocks and fossils in each layer of rock.
Radiometric dating has been used to determine that the oldest known rocks on Earth are between 4-5 billion years old. The oldest fossils are between 3-4 billion years old.
Body parts that do not serve any function are called vestigial structures.
Vestigial structures indicate that two species have a recent common ancestor.
The similarities between embryos suggests that animals are related and have common ancestors.
The same biochemical building blocks – such as amino acids and nucleotides - are responsible for life in all organisms, from bacteria to plants and animals.
DNA and RNA determines the development of all organisms.
The similarities and differences between the genomes, the gene sequences of each species, reveal patterns of evolution.
Review Questions
What are the different kinds of evidence of evolution?
How do geologists determine the age of rocks and fossils?
What is an embryo?
What is a vestigial structure?
What is an example of a vestigial structure?
What is a genome?
What is the most convincing evidence of evolution?
How do the embryos of different species support the idea of evolution?
Further Reading / Supplemental Links
Stein, Sara, The Evolution Book, Workman, N.Y., 1986.
Yeh, Jennifer, Modern Synthesis, (From Animal Sciences).
Darwin, Charles, Origin of the Species, Broadview Press (Sixth Edition), 1859 .
Ridley, Matt, The Red Queen: Sex and the Evolution of Human Nature. Perennial Books, 2003.
Ridley, Matt, Genome, Harper Collins, 2000.
Sagan, Carl, Cosmos, Edicions Universitat Barcelona, 2006.
Carroll, Sean B., The Making of the Fittest: DNA and the Ultimate Forensic Record of Evolution, Norton, 2006.
Dawkins, Richard, The Blind Watchmaker, W.W. Norton & Company, 1996.
Dawkins, Richard, The Selfish Gene, Oxford University Press, 1989.
Diamond, Jared, The Third Chimpanzee: The Evolution and Future of the Human Animal, HarperCollins, 2006.
Mayr, Ernst, What Evolution Is, Basic Books, 2001.
Zimmer, Carl, Smithsonian Intimate Guide to Human Origins, Smithsonian Press, 2008.
http://en.wikipedia.org/
Vocabulary
embryo
An animal or plant in its earliest stages of development, before it is born or hatched.
embryology
The study of how organisms develop.
fossil
The preserved remains or traces of animals, plants, and other organisms from the distant past; examples include bones, teeth, impressions, and leaves.
fossil record
Fossils and the order in which fossils appear; provides important records of how species have evolved, divided and gone extinct.
genetics
The scientific study of heredity.
genome
All of the genes in an organism.
paleontologists
Scientists who study fossils to learn about life in the past.
radiometric dating
A method to determine the age of rocks and fossils in each layer of rock; measures the decay rate of radioactive materials in each rock layer.
vestigial structure
Body part that has lost its use through evolution, such as a whale’s pelvic bones.
Points to Consider
How do you think new species evolve?
How long do you think it takes for a new species to evolve?
Lesson 7.3: Macroevolution
Lesson Objectives
Students will understand the differences between macroevolution and microevolution.
Students will understand that speciation is the formation of new species.
Students will understand the mechanisms of speciation.
Check Your Understanding
Why can’t an individual person evolve? Why can only groups evolve over many generations?
What causes a species or a population to evolve?
Introduction
Small changes or large changes, how does evolution occur? It is easy to think that many small changes, as they accumulate over time, may gradually lead to a new species. Or is it possible that due to severe changes in the environment, large changes are needed to allow species to adapt to the new surroundings? Or are both probable methods of evolution?
Microevolution and Macroevolution
Microevolution
You already know that evolution is the change in species over time, due to the change of how often an inherited trait occurs in a population over many generations. Most evolutionary changes are small and do not lead to the creation of a new species. These small changes are called microevolution.
An example of microevolution is the evolution of pesticide resistance in mosquitoes. Imagine that you have a pesticide that kills most of the mosquitoes in your state one year. As a result, the only remaining mosquitoes are the pesticide resistant mosquitoes. When these mosquitoes reproduce the next year, they produce more mosquitoes with the pesticide resistant trait. This is an example of microevolution because the number of mosquitoes with this trait changed. However, this evolutionary change did not create a new species of mosquito, because the pesticide resistant mosquitoes can still reproduce with other mosquitoes if they were put together.
Macroevolution
Macroevolution refers to much bigger evolutionary changes that result in new species. Macroevolution may happen:
when many microevolution steps lead to the creation of a new species,
as a result of a major environmental change, such as volcanic eruptions, earthquakes or an asteroid hitting Earth, which changes the environment so much that natural selection leads to large changes in the traits of a species.
After thousands of years of isolation from each other, some of Darwin’s finch population, which was discussed in the Evolution by Natural Selection lesson, will not or cannot breed with other finch populations when they are brought together. Since they do not breed together, they are classified as separate species.
Genotype or Phenotype?
Natural selection acts on the phenotype - the traits or characteristics - of an individual, not on the underlying genotype. For many traits, the homozygous genotype, AA for example, has the same phenotype as the heterozygous Aa genotype. If both an AA and Aa individual have the same phenotype, the en
vironment cannot distinguish between them. So natural selection cannot choose a homozygous individual over a heterozygous individual. If homozygous recessive aa individuals are selected against, that is they are not well adapted to their environment, acting on the phenotype allows the a allele to be maintained in the population through heterozygous Aa individuals.
Carriers
Because natural selection acts on the phenotype, if an allele is lethal in a homozygous individual, aa for example, it will not be lethal in a heterozygous Aa individual. These heterozygous Aa individuals will then act as carriers of the a allele. This allele is then maintained in the population's gene pool. The gene pool is the complete set of alleles within a population.
Tay-Sachs disease is an autosomal recessive genetic disorder. It is caused by a genetic defect in a single gene with one defective copy of that gene inherited from each parent, rr for example. Affected individuals usually die from complications of the disease in early childhood. Affected individuals must have unaffected parents, each being a carrier of the defective allele, so the parents are heterozygous Rr. This lethal allele is maintained in the gene pool through these unsuspecting heterozygous individuals; they do not show any symptoms of the disease, so most individuals do not get tested to see if they are carriers.
Figure 7.25
Tay-Sachs disease is inherited in the autosomal recessive pattern. Each parent is an unaffected carrier of the lethal allele.
Hardy-Weinberg Equilibrium
The Hardy-Weinberg model (sometimes called a law) states that a population will remain at genetic equilibrium - with constant (unchanging) allele and genotype frequencies and no evolution - as long as five conditions are met:
No mutation (no change in the DNA sequence)
No migration (no moving into or out of a population)
Very large population size
Random mating (mating not based on preference)
No natural selection.
These five conditions rarely occur in nature. For example, it is highly unlikely that new mutations are not constantly generated. If these five conditions are met, the frequencies of genotypes within a population can be determined given the phenotypic frequencies.
The Hardy-Weinberg Equation
For example, let's use a hypothetical rabbit population of 100 rabbits (200 alleles) to determine allele frequencies for color:
9 albino rabbits (represented by the alleles bb) and
91 brown rabbits (49 homozygous [BB] and 42 heterozygous [Bb]).
The gene pool contains 140 B alleles [49 + 49 + 42] (70%) and 60 b alleles [9 + 9 + 42] (30%) – which have gene frequencies of 0.7 and 0.3, respectively.
If we assume that alleles sort independently and segregate randomly as sperm and eggs form, and that mating and fertilization are also random, the probability that an offspring will receive a particular allele from the gene pool is identical to the frequency of that allele in the population:
BB: 0.7 x 0.7 = 0.49
Bb: 0.7 x 0.3 = 0.21
bB: 0.3 x 0.7 = 0.21
bb: 0.3 x 0.3 = 0.09
If we calculate the frequency of genotypes among the offspring, they are identical to the genotype frequencies of the parents. There are 9% bb albino rabbits and 91% BB and Bb brown rabbits. Allele frequency remains constant as well. The population is stable – at a Hardy-Weinberg genetic equilibrium.
A useful equation generalizes the calculations we’ve just completed. Variables include
p = the frequency of one allele (we’ll use allele B here) and
q = the frequency of the second allele (b in this example).
We will use only two alleles (so p + q must equal 1), but similar equations can be written for more alleles.
Allele frequency equals the chance of any particular gamete receiving that allele. Therefore, when egg and sperm combine, the probability of any genotype is the product of the probabilities of the alleles in that genotype. So:
Probability of genotype BB = p X p = p2 and
Probability of genotype Bb= (p X q) + (q X p) = 2pq and
Probability of genotype bb = q X q = q2
We have included all possible genotypes, so the probabilities must add to 1.0. In our example 0.49 + 2(0.21) + 0.9 = 1. Our equation becomes:
p2 + 2 pq + q2 = 1
frequency of genotype BB frequency of genotype Bb frequency of genotype bb
This is the Hardy-Weinberg equation, which describes the relationship between allele frequencies and genotype frequencies for a population at equilibrium.
Genetic Drift
Recall that the third requirement for Hardy-Weinberg equilibrium is a very large population size. This is because variations in allele frequencies that occur by chance are minimal in large populations. In small populations, random variations in allele frequencies can significantly influence the "survival" of any allele. Random changes in allele frequencies in small populations is known as genetic drift. As the population (and therefore the gene pool) is small, genetic drift could have substantial effects on the traits and diversity of a population. Many biologists think that genetic drift is a major cause of microevolution.
The Origin of Species
The creation of a new species is called speciation. Most new species develop naturally, but humans have also artificially created new subspecies, breeds, and species for thousands of years.
Natural selection causes beneficial heritable traits to become more common in a population, and unfavorable heritable traits become less common. For example, a giraffe’s neck is beneficial because it allows the giraffe to reach leaves high in trees. Natural selection caused this beneficial trait to become more common than short necks.
As new mutations (changes in the DNA sequence) are constantly being generated in a population's gene pool, some of these mutations will be beneficial and result in traits that allow adaptation and survival. Natural selection causes evolution through the genetic change of a species as the beneficial traits become more common within a population.
Artificial selection is when humans select which plants or animals to breed to pass specific traits on to the next generation. A farmer may choose to breed only the cows that produce the best milk (the favored traits) and not breed cows that do not produce much milk (a less desirable trait). Humans have also artificially bred dogs to create new breeds (Figure below).
Figure 7.26
Artificial Selection: Humans used artificial selection to create these different breeds. Both dog breeds are descended from the same wolves, and their genes are almost identical. Yet there is at least one difference between their genes that determine size.
Reproductive Isolation
There are two main ways that speciation happens naturally. Both processes create new species by isolating groups (populations) of the same species from each other. Organisms can be reproductively isolated from each other either geographically or by some behavior. Over a long period of time (usually thousands of years), each population evolves in a different direction. One way scientists test whether two populations are separate species is to bring them together again. If the two populations do not interbreed and produce fertile offspring, they are separate species.
Geographic Isolation
Allopatric speciation happens when groups from the same species are geographically isolated physically for long periods. Imagine all the ways that plants or animals could be isolated from each other:
a mountain range
a canyon
water such as rivers, streams, or an ocean
a desert
Charles Darwin recognized that speciation could happen when some members of a species were isolated from the others for hundreds or thousands of years. Darwin had observed thirteen distinct finch species on the Galápagos Islands that had evolved from the same ancestor. Several of the finch populations evolved into separate species while they were isolated on separate islands. Scientists were able to determine which finches had evolved into distinct species by bringing mem
bers of each population together. The birds that would not or could not interbreed were regarded as separate species.
A classic example of geographic isolation is the Abert squirrel, shown in Figures below) and below). When the Grand Canyon in Arizona formed, squirrels from one species were separated by the giant canyon that they could not cross. After thousands of years of isolation from each other, the squirrel populations on the northern wall of the canyon looked and behaved differently from those on the southern wall. North rim squirrels have white tails and black bellies. Squirrels on the south rim have white bellies and dark tails.
Figure 7.27
Abert Squirrel on the southern rim of the Grand Canyon
Figure 7.28
Kaibab squirrel (a subspecies of Aberts) found on northern rim of the Grand Canyon
Isolation without Physical Separation
Sympatric speciation happens when groups from the same species stop interbreeding, because of something other than physical separation, such as behavior. The separation may be due to different mating seasons, for example. Sympatric speciation is more difficult to identify.
Some scientists suspect that two groups of orcas (killer whales) live in the same part of the Pacific Ocean part of the year, but do not interbreed. The two groups hunt different prey species, eat different foods, sing different songs, and have different social structures.
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