17.1 How New Species Evolve
1. Macroevolution is evolution on a large scale.
2. Macroevolution involves the splitting of one species into two or more species, called speciation.
A. What is a Species?
1. Taxonomists are scientists who classify living organisms into groups.
2. These classification groups can change over time, as new information unfolds.
3. Three species concepts are the morphological species concept, the phylogenetic species concept, and the biological species concept.
4. The morphological species concept states that each species is defined by one or more distinct physical characteristics called diagnostic traits that distinguish one species from another.
a. An advantage of this species concept is that it can be used to diagnose new species in the fossil record.
b. A disadvantage is some species do not have many measurable traits, and some trait differences are subtle and difficult to detect.
c. Cryptic species look almost identical but are different in other traits.
5. The phylogenetic species concept states that an evolutionary family tree is used to identify species based on a common ancestor.
a. An advantage of defining species according to this concept is that it does not rely only on morphological traits to define a species.
6. The biological species concept says that the members of one species interbreed and have a shared gene pool, and each species is reproductively isolated from every other species.
a. An advantage to this species concept is that it can designate species even when trait differences may be difficult to find.
b. A disadvantage is that this concept cannot be applied to asexually reproducing organisms, organisms only known by the fossil record, or species that could possibly interbreed if they lived near one another.
B. Reproductive Isolating Mechanisms
1. For two species to be separate, gene flow must not occur between them.
2. A reproductive isolating mechanism is any structural, functional, or behavioral characteristic that prevents successful reproduction from occurring.
3. Prezygotic (“before formation of a zygote”) isolating mechanisms are anatomical or behavioral differences between the members of two species that prevent mating or make it unlikely fertilization will take place if mating occurs.
a. Habitat isolation occurs when two species occupy different habitats, even within the same geographic range, so that they are less likely to meet and to attempt to reproduce.
b. Temporal isolation occurs when two species live in the same location, but each reproduces at a different time of year, and so they do not attempt to mate.
c. Behavioral isolation results from differences in mating behavior between two species.
d. Mechanical isolation is the result of differences between two species in reproductive structures or other body parts, so that mating is prevented.
e. Gamete isolation includes incompatibility of gametes of two different species so they cannot fuse to form a zygote; an egg may have receptors only for the sperm of its own species or a plant stigma prevents completion of pollination.
4. Postzygotic (“after formation of a zygote”) isolating mechanisms prevent development of a hybrid after mating has taken place.
a. Hybrid inviability is when hybrids (offspring of parents of two different species) do not live to reproduce.
b. Hybrid sterility occurs when the hybrid offspring are sterile (e.g., mules).
C. The Anatomy of Speciation (Evolution reading)
1. Phylogeny is a tool evolutionary biologists use to investigate the history of evolution among organisms.
2. The following are some important terms used with a phylogenetic tree:
a. Node: the point where two lineages intersect, and represent a shared common ancestor.
b. Root: the origin of species’ shared common ancestry.
c. Extinction: an extinct taxon that is represented by a shortened branch on the phylogenetic tree.
d. Monophyletic group (monophyly): also known as a clade. A group of species and their common ancestor.
17.2 Modes of Speciation
1. Speciation is the splitting of one species into two more species or the transformation of one species into a new species over time.
a. Researchers recognize two modes of speciation: geographic isolation and reproductive isolation.
A. Allopatric speciation
1. Allopatric speciation occurs when new species result from populations being separated by a geographical barrier that prevents their members from reproducing with each other.
2. First proposed by Ernst Mayr of Harvard University.
3. While geographically isolated, variations accumulate until the populations are reproductively isolated.
4. Examples of Allopatric Speciation
a. An ancestral population of Ensatina salamanders migrated from northern California to southern California. The Central Valley prevented gene flow between the eastern and western populations of these salamanders. Genetic differences increased resulting in two distinct forms of Ensatina salamanders.
b. Green iguana of South America is believed to be the common ancestor for the marine iguana on the Galápagos Islands (to the west) and the rhinoceros iguana on Hispaniola (to the north). A few of these iguanas may have swam to the islands and over time formed populations separate from each other and from the parent population of South America.
c. Many sockeye salmon in Washington State were introduced into Lake Washington when some colonized an area of the lake near Pleasure Point Beach and others migrated into the Cedar River. Because of the difference in water current, over time, the salmon differed in shape and size due to the demands of reproducing in the different water currents.
5. Reinforcement of Reproductive Isolation
a. A side effect to adaptive changes involving mating is reproductive isolation.
b. As populations become reproductively isolated, postzygotic isolating mechanisms may arise before prezygotic isolating mechanisms.
c. An example is a horse reproducing with a mule, a sterile donkey is produced.
d. Natural selection would favor any variation in populations that prevents the occurrence of hybrids when they do not have offspring.
e. Reinforcement refers to the process of natural selection favoring variations that lead to reproductive isolation.
B. Sympatric Speciation
1. Sympatric speciation would occur when members of a single population develop a genetic difference (e.g., chromosome number) that prevents them from reproducing with the parent type.
2. A polyploid is a eukaryote with three or more complete sets of chromosomes.
a. Polyploidy is predominantly seen in plants and makes a significant contribution to the evolution of new plants.
b. A polyploid plant can reproduce with itself, but cannot reproduce with the 2n population.
c. The two types of polyploidy are autoploidy and alloploidy.
Autoploidy occurs when a diploid plant produces diploid gametes due to nondisjunction during meiosis.
A triploid (3n) plant is sterile because the chromosomes cannot pair during meiosis.
If two diploid gametes fuse, the plant is a tetraploid (4n) and the plant is fertile, so long as it reproduces with another of its own kind.
Alloploidy requires two different but related species of plants to hybridize.
When hybridization occurs, it is followed by chromosome doubling.
However the offspring that has parents with different numbered pairs of chromosomes will be sterile.
An example of alloploidy can be seen in the wheat plant used to produce bread. The parents of the present day bread wheat had 28 and 14 chromosomes. The hybrid with 21 chromosomes is sterile, but bread wheat with 43 chromosomes is fertile since the chromosomes can pair during meiosis.
C. Adaptive Radiation
1. Adaptive radiation is a type of allopatric speciation and occurs when a single ancestral species gives rise to a variety of species, each adapted to a specific environment.
2. An ecological niche is where a species lives and how it interacts with other species.
3. The case of Darwin’s finches illustrates the adaptive radiation of 13 species from one founder mainland finch.
4. On the Hawaiian Islands, a wide variety of honeycreepers descended from one goldfinch-like ancestor; Hawaii is also the home of the silversword plants that radiated from ancestral tarweeds.
D. Convergent Evolution
1. Convergent evolution occurs when a similar biological trait evolves in two unrelated species as a result of exposure to similar environments.
2. Analogous traits are those that evolve convergent in two unrelated lineages because of a response to a similar lifestyle or habitat.
3. Homologous traits are those that are similar because they evolved from a common ancestor.
Principles of Macroevolution
Macroevolution is evolution of new species and higher levels of classification.
Some evolutionists support a gradualistic model of macroevolution, meaning that speciation occurs after populations become isolated, with each group continuing slowly on its own evolutionary pathway.
According to this model, ancestral species gradually gives rise to two separate species.
This model suggests that it is difficult to indicate when speciation occurred because there would be so many transitional links.
Other evolutionists support a punctuated equilibrium model to explain the pace of evolution.
According to this model, periods of equilibrium (no change) are punctuated (or interrupted) by speciation.
This model suggests that transitional links are less likely to become fossils and less likely to be found.
Speciation is more likely to involve only an isolated population at one locate, because a favorable genotype could spread more rapidly within such a population.
These two models could both assist in interpretation of the fossil record. For example, some species may fit into one model, and other species fit into the other model.
Genetic Basis of Beak Shape in Darwin’s Finches (Nature of Science reading)
The finches on the Galapagos Islands are an example of species originating from a common ancestor.
Each type of finch adapted to a particular way of life, and their beak size and shape was related to their diet.
DNA sequencing supports that Darwin’s finches are closely related to one another.
In 2006, two genes that are responsible for finch beak shape were discovered.
The gene for bone morphogenic protein 4 (Bmp4) determines how deep or tall the beak will be.
The gene for calmodulin (CaM) regulates how long a beak will grow.
Developmental Genes and Macroevolution
Genes can bring about radical changes in body shapes and organs.
The Pax6 gene is involved in eye formation in all organisms.
Homeotic (Hox) genes determine the location of repeated structures in all vertebrates.
2. Gene expression influences organisms’ developmental processes.
a. These genes can bring about changes in body shapes and organs.
b. Despite millions of years of divergent evolution, all animals share the same control switches for development.
3. Development of the Eye
Eyes of species vary in size, compound or simple, etc.
Despite these differences, there is one gene, Pax6, required for eye formation.
The gene Pax6 was discovered by Walter Gehrig and colleagues in 1994.
Interestingly, the mouse Pax6 gene can cause an eye to develop in the leg of a fruit fly.
4. Development of Limbs
The Tbx5 gene helps in the development of limbs in humans and wings in birds.
Tbx5 triggers different genes in birds and humans, which may explain why the same protein is used in developing limbs in humans and wings in birds.
Development of Overall Shape
Hox genes control the number and appearances of repeated structures along the main body axes of vertebrates.
Shifts when the Hox gene is expressed can explain why some vertebrates, like the snake, have hundreds of vertebrae, and others, like the chick only have seven.
An altered expression of a particular gene can reduce the pelvic-fin bud in the embryo.
Natural selection can lead to major skeletal changes in a relatively short period of time.
Human DNA base sequencing is similar to that of chimpanzees, mice, and all vertebrates.
Scientists predict that differential gene expression and/or new functions for “old” genes will explain how humans evolved.
Macroevolution Is Not Goal-Oriented
The evolution of the horse, Equus, represents a gradual, straight-line evolution until its goal, the modern horse, has been achieved.
The trends seen in the evolution of the horse are: overall size, toe reduction, and change in tooth size and shape.
However, based on fossils, it is easier to see that the horse lineage is not a straight-line evolution, but rather forming a thick bush of many equine species.
One may have deducted that since the only genus that remains is Equus and the other genera have become extinct, that evolution was directed towards producing Equus. However, each of the ancestral species was adapted to its environment.
Adaptation occurs because the members of a population with an advantage are able to have more offspring than other members.
Natural selection is not goal-oriented, but rather opportunistic.