Speciation:
Review:
1) Biological species concept: A population of organisms that can produce fertile offspring.
2) Population: A group of organisms that is of the same species and is in one area.
3) Gene Pool: All the hereditary material of a population.
4) Speciation: Long term change in the gene pool. A change in the gene frequency of a population
which splits a population and which occurs over a long period of time. Speciation is the formation
of a new species by the isolation of the gene pools of related populations.
5) Races: Members of the same population that differ somewhat genetically but can still interbreed.
6) Subspecies: Races that are different phenotypically.
Races and subspecies can become full species if geographic barriers arise preventing gene flow.
7) Hybrid: The offspring of two species which is not able to have offspring. This organism should
be sterile.
Even though many populations live side by side, they may become reproductively and genetically
isolated and cannot produce fertile offspring. Any factor that impedes two species from producing
fertile hybrids contributes to reproductive isolation. A geographic barrier is not a form of
reproductive isolation.
If populations cannot exchange genes, their gene pools are isolated. This raises two questions:
1) How did one gene pool split off from the other?
2) How can two species that are similar to each other inhabit the same place at the same time and
remain reproductively isolated?
How are the animals isolated and how is the gene flow prevented?
Allopatric Speciation (geographic isolation): Populations in one geographic area are split into two
or more geographically separated populations. Initially this blocks gene flow between two parts of
a population.
For example, a glacier cuts across a plain. A small number of individuals colonize the new habitats
on either side of the glacier. The populations are geographically isolated. Other examples include
islands and island like situations such as, lakes for fish, the break up of pangea into continents,
mountain tops for animals and plants, forest extent for small mammals, etc.
The populations may begin to diverge genetically under the pressure of different selection forces. If
enough time passes and the selective pressures are significant, the isolated gene pools of the
populations diverge. Differences accumulate such that microevolution occurs, causing changes in
the respective phenotypes. Even if the small isolated populations were reunited with the parent
population, interbreeding under natural conditions would no longer occur.
Allopatric speciation usually occurs at the fringe of the parent population. Peripheral groups are
good candidates for speciation for three reasons:
1) The gene pool of the peripheral group is probably different
from the parent group
initially.
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2) Unless the peripheral group is large, the gene pool is likely to change because of genetic
drift, new mutations and lack of gene flow.
3) Evolution caused by selection may take a different direction in the peripheral groups
from the parent population.
Adaptive Radiation: The emergence of a number of species from a common ancestor, as the
population spreads to new environments.
Darwin's finches represent an important example of speciation. The different finch species live on
different islands and, therefore, cannot interbreed. Different environmental pressures and thus
different selective pressures occur and the populations differ from each other. Darwin was able to
identify 13 different species of finches on the Galapagos Islands; all have apparently risen from one
ancestral species. The finches were not able to mate, so the genes were isolated. Natural selection
occurred on each island, selecting for different genes. Eventually, the gene pools diverged.
A hybrid organism is the offspring of two parents from different species. Hybrids can occur in
animals (mule), but they are more common in plants.
The gene frequencies in a population start to change over a period of time depending on the
environment and selection pressures. Once the gene frequencies change, the gene pool changes,
and new species develop.
Once two animal populations have become separated and have evolved into separate species, why
can't the different species reproduce? Why can't the different species have fertile offspring? There
are two categories of isolating mechanisms, which prevent the formation of fertile offspring. These
are Prezygotic isolating mechanisms and Postzygotic isolating mechanisms. These categories can
be altered slightly and termed pre-mating and post-mating isolating mechanisms.
1) Prezygotic Isolating Mechanisms: These occur before the zygote is formed. They prevent the
fertilization of the mother's egg.
a) Habitat differences: Two species may inhabit different habitats. They never encounter
each other to mate.
b) Differences in breeding times: Two species may reproduce at different times.
c) Mechanical differences: Organisms genitals work on the lock and key method. If the
male and female genitals do not fit, mating will not occur.
d) Behavioral differences: Courtship behavior may differ. If so, no mating will occur.
e) Gametic differences: Even if the gametes meet, they rarely
fuse to form a zygote.
The sperm may not be able to survive the environment of the female reproductive tract, the
gametes may not recognize each other, or the sperm may not fuse with
the ovum.
2) Postzygotic isolating mechanism: These prevent successful reproduction after fertilization of the
mother's egg has occurred.
a) The zygote does not survive the gestation period.
b) The offspring are sterile. The incompatible chromosomes undergo meiosis. However,
whenever the offspring mates, the sperm/egg will not be able to combine with the
egg/sperm of the other mate.
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c) Hybrid Breakdown: Hybrids are fertile but produce infertile or inviable offspring in the
F2 generation.
Sympatric speciation refers to the production of a new species within the parent population, ie.
Without geographical isolation. Sympatric speciation usually involves the incomplete separation of
homologous chromosomes (Nondisjunction) or the homologous chromosomes do separate properly
but cytokinesis does not occur.
A new species forms by polyploidy (multiplication of chromosomal numbers ie. 2n ---> 3n or 4n)
when the chromosomes fail to segregate during meiosis.
AUTOPOLYPLOIDY is the doubling of the chromosome number within a population and hence
the formation of a new species. Tetraploid plants can only mate with tetraploid plants and produce
fertile offspring. If they mate with diploid individuals, the offspring are sterile because synapsis of
homologous pairs does not occur properly.
Hybrids are usually sterile, since the chromosomes cannot pair up in meiosis (no homologues). If
polyploidy occurs within the hybrid, its chromosomes can line up during meiosis. They are,
however, reproductively isolated from their parents.
If two species contribute gametes, a doubling of the chromosome number in the hybrid offspring
can generate a new species. This is called ALLOPOLYPLOIDY. In allopolyploidy, two species
interbreed and combine their chromosomes. Usually the organism will be sterile because the
chromosomes cannot synapse properly in meiosis. If the chromosome number first doubles, and
nondisjunction creates a new set of homologues, then the organism can self fertilize to create fertile
offspring.
Sympatric speciation through polyploidy is a common occurrence in plants.
Some polyploidy plants are able to survive harsher environments better than nonpolyploids. Some
polyploid plants that have an odd number of chromosome sets cannot produce sexually, but survive
through vegetative propagation, ie. Bananas have flowers but the fruit yields no seeds.
Introgression: Alleles from different species cross the reproductive barrier and enter the gene pool.
This usually occurs in plants, ie. Corn has some alleles that can be traced back to wild grass. A
fraction of the grass and corn hybrids backcrossed with corn plants. Thus the genes ended up in the
corn gene pool.
Genetic Mechanisms of Speciation:
When two populations adapt to different environments, they accumulate differences in frequencies
of genotypes and phenotypes. In the course of the gradual adaptive divergence of the two gene
pools, reproductive barriers between the two populations evolve. Hence, two species arise.
These reproductive barriers can come about without being favored by natural selection.
Reproductive isolation is usually a consequence of the divergence of the two populations in
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response to other selective pressures.
Postzygotic barriers may be attributed to differences in regulatory genes that control development.
Prezygotic barriers can come about as products of the gradual genetic divergence of two
populations.
We’ve only discussed the Biological Species Concept. However, scientists can use other types of
species concepts.
1. Biological Species Concept: this emphasizes reproductive isolation. Members of the same
species interbreed and produce fertile offspring, but not with other species.
2. Morphological Species Concept: this emphasizes measurable anatomical differences
between species. Define species by morphological criteria.
3. Recognition Species Concept: mating adaptations are emphasized here. Individuals
recognize certain characteristics of suitable mates. This tends to focus on traits that are
subject to natural selection.
4. Cohesion Species Concept: maintains that species are discrete phenotypic entities.
Organisms that reproduce asexually fit into the cohesion concept. This concept also
recognizes that some hybrids are fertile and have fertile offspring.
5. Ecological Species Concept: this emphasizes the role of the species (niche) in their
environment.
6. Evolutionary Species Concept: evolutionary lineages and ecological roles are emphasized.
Gradual and Punctuated Speciation:
Gradual: The rate of change in the composition of a gene pool is relatively constant and gradual.
Large changes occur by the accumulation of many small ones over long periods of time.
Punctuated Equilibrium: Changes occur in relatively short spurts interspersed among long time
periods or relatively little change. Speciation events occur rapidly on a geologic (evolutionary) time
scale. These rapid changes interrupt or end long periods of little change in species' gene pools.
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