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THE PROCESS OF SPECIATION
Definitions of biological evolution
We begin with two working definitions of biological evolution, which capture these two
facets of genetics and differences among life forms. Then we will ask what is a species, and
how does a species arise?
Definition 1:
Changes in the genetic composition of a population with the passage of each
generation
Definition 2:
The gradual change of living things from one form into another over the course of
time, the origin of species and lineages by descent of living forms from ancestral
forms, and the generation of diversity
Note that the first definition emphasizes genetic change. It commonly is referred to as
microevolution. The second definition emphasizes the appearance of new, physically distinct
life forms that can be grouped with similar appearing life forms in a taxonomic hierarchy. It
commonly is referred to as macroevolution.
What is a species?
Biological species concept: This concept states that "a species is a group of actually or
potentially interbreeding individuals who are reproductively isolated from other such
groups." Two individuals belong to the same species if their gametes can unite with
each other under natural conditions to produce fertile offspring. This means that gene
flow is possible.
 This concept emphasized that a species is an evolutionary unit. Members share genes
with other members of their species, and not with members of other species.
 The biological species concept helps us ask how species are formed, because it focuses
our attention on the question of how reproductive isolation comes about.
Types of reproductive isolation
There are many barriers to reproduction. Each species may have its own courtship displays,
or breeding season, so that members of the two species do not have the opportunity to
interbreed. Or, the two species may be unable to interbreed successfully because of failure of
the egg to become fertilized or to develop.
This suggests a simple and useful dichotomy, between pre-mating or prezygotic (i.e., prezygote formation) reproductive isolating mechanisms, and post-mating or postzygotic
isolating mechanisms. Remember that a zygote is the cell formed by the union of two
gametes and is the basis of a developing individual.
Prezygotic isolating mechanisms
1. Ecological isolation: Species occupy different habitats. The lion and tiger overlapped
in India until 150 years ago, but the lion lived in open grassland and the tiger in
forest. Consequently, the two species did not hybridize in nature (although they
sometimes do in zoos).
2. Temporal isolation: Species breed at different times. In North America, five frog
species of the genus Rana differ in the time of their peak breeding activity.
3. Behavioral isolation: Species engage in distinct courtship and mating rituals.
4. Mechanical isolation: Interbreeding is prevented by structural or molecular
blockage of the formation of the zygote. Mechanisms include the inability of the sperm
to bind to the egg in animals, or the female reproductive organ of a plant preventing
the wrong pollinator from landing.
** All of the above prevent the formation of hybrid zygotes. **
Postzygotic isolating mechanisms
1. Hybrid inviability. Development of the zygote proceeds abnormally and the hybrid is
aborted. (For instance, the hybrid egg formed from the mating of a sheep and a goat
will die early in development.)
2. Hybrid sterility. The hybrid is healthy but sterile. (The mule, the hybrid offspring of
a donkey and a mare, is sterile; it is unable to produce viable gametes because the
chromosomes inherited from its parents do not pair and cross over correctly during
meiosis (cell division in which two sets of chromosomes of the parent cell are
reduced to a single set in the products, termed gametes).
3. Hybrid is healthy and fertile, but less fit, or infertility appears in later generations (as
witnessed in laboratory crosses of fruit flies, where the offspring of secondgeneration hybrids are weak and usually cannot produce viable offspring).
** Post-zygotic mechanisms are those in which hybrid zygotes fail, develop
abnormally, or cannot self-reproduce and establish viable populations in nature. **
The Hardy-Weinberg Theorem
Once the gene pool has been isolated, a splinter population can easily follow with changes in
the allele frequencies. However, if the gene pool remains constant, no speciation will occur.
This theorem states that if the following characteristics exist in a population, the gene pool
will therefore remain constant and no speciation will occur:
 the population must be very large so that if an event occurs that kills many organisms,
the gene pool will not be affected greatly.
 the population must be isolated and no immigration and no emigration may occur.
 there are no changes in the gene pool due to mutations.
 mating must be random so that certain alleles will not be favored over others.
Therefore, all genotypes are equal in reproductive success, and there is no natural selection.
If any one of these characteristics are violated, speciation may occur.
Causes of Gene Pool Variation
Genetic Drift
Genetic drift occurs when chance events cause the frequency of alleles in a small population
to drift randomly with each generation. The only way for genetic drift to be ruled out
completely is for the population to be infinitely large. There are two types of genetic drift:
the Bottleneck Effect and the Founder Effect. The Bottleneck Effect is when disasters,
such as earthquakes or fires, destroy much of the population randomly. The resulting
population may have a dramatic change in allele frequency. Certain alleles will be over
represented and others will be under represented. The Founder Effect occurs when a few
individuals from a population migrate to an island or a new habitat. Then the new colony
may have a change in the gene pool from the larger population.
Gene Flow
Gene flow is when a population gains or loses alleles through migration of individuals to
different populations. This can occur through immigration, the inflow of individuals into a
population, or through emigration, the out flow of individuals from a population. Unlike
genetic drift, gene flow thens to reduce differences between populations.
Mutation
A mutation in the chromosomes of one individual may change the gene pool by substituting
one allele for another. This new allele may serve as the basis for natural selection to occur.
Nonrandom Mating
In most populations, animals will mate with other members that live close to them causing
inbreeding. Inbreeding will cause certain types of alleles to become more frequent due to the
mating of closely related individuals with similar genotypes.
Natural Selection
Natural selection is the differential success of certain characteristics in reproduction which
results in disproportionate numbers of alleles being passed along to each generation. This
may be because a certain characteristic is better for surviving predators, the environment, or
disease.
Different Modes of Speciation (page 574)
There are two different modes of speciation that may occur: allopatric speciation and
sympatric speciation.
Allopatric Speciation
The most common way for species to split, especially in animal species, is when the
population becomes geographically isolated into two populations. Allopatric speciation
occurs when the two different populations are segregated by a geographical barrier. The
barriers may range from mountains and islands to lakes or forests.
Geographic isolation leads to reproductive isolation. Once two populations are
reproductively isolated, they are free to follow different evolutionary paths.
Sympatric Speciation
The second mode of speciation is called sympatric speciation. This occurs when two
populations overlap in their ranges and much gene flow occurs.
 Occurs without geographical isolation.
 Reproductive barriers arise within the range of one species/population.
 Less common than allopatric.
It relies on the development of reproductive isolation mechanisms to allow divergence of the
two sub-populations. This reproductive isolation can be a result of a number of factors,
including competition for resources, disruptive selection, and sexual selection.
Some cases of sympatric speciation seem to arise because of the occurence of "polyploidy".
Polyploidy, common in plants, is a condition where there are extra sets of chromosomes.
Suppose that chromosome number is doubled to 4N as the result of a mistake in cell
division. Then, if a 4N plant mates with another 4N plant, the offspring will also be 4N. But,
if a 4N plant mates with a 2N plant, the offspring will be 3N. It turns out that these
individuals are usually sterile. So, the 4N "mistake" plant is now reproductively isolated
from the original 2N population.
To review: allopatric (geographic) speciation is the differentiation of physically isolated
populations to the point that reunion of the two populations does not occur if contact is reestablished.
Speciation as a gradual process
Our understanding of speciation arising from reproductive isolation and the gradual
evolution of reproductive isolating mechanisms should help us to appreciate why the
biological species concept, and the test of reproductive isolation, may sometimes fail.
If speciation is a gradual process, species may not yet be fully separated. A continuum must
exist from species that are in the process of splitting into two, to species that are fully
formed.