8.2 & 8.3 Speciation and Patterns of Evolution
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Scientists know that species continue to evolve.
Microevolution: Changes in gene (allele) frequencies and phenotypic traits that occur within a
population and species. (Changes that occur with a species).
o Example: The average beak size of the ground-finches increased over time
Adaptive radiation: the relatively rapid evolution of a single species into many different species, filling a
variety of previously unfilled ecological niches.
o Example: after the ground finches populated the different islands, they evolved over time to
form new species of tree finches.
Speciation: the formation of a new species
Species: All members of a population that can interbreed successfully under natural conditions.
(offspring must be able to interbreed successfully as well)
o When two organisms cannot interbreed, they are said to be reproductively isolated
o Populations of different species that do not exchange genetic information have separate gene
pools.
o Biologist today compare the genetic makeup of different populations to determine relatedness
and species type
Reproductive isolating Mechanisms….
A reproductive isolating is any behavioural, structural, or biochemical trait that prevents individuals of
different species from reproducing successfully together. Can lead to speciation.
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Prezygotic mechanism is a reproductive isolating mechanism that prevents interspecies mating and
fertilization (For example ecological isolations, temporal isolation and behavioural isolation).
o Temporal: time separation (mating season)
o Ecological isolation: space differences (occupy different habitats)
o Mechanical: differences in structure (differences in morphological features, making the two
species incompatible)
o Behavioural: differences in mating behaviour ( different courtships, or different mating clues to
find and attract mate)
o Gametic isolation: fertilization does not happen (male gametes cannot recognize and fertilize
the egg)
Example:
o Asian lions were once common in and lived in open grasslands, while Asian tigers preferred
forests = prezygotic ecological isolation
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Postzygotic mechanism is a reproductive isolating mechanism that prevents maturation and
reproduction in offspring from interspecies reproduction.
o Zygotic mortality: mating a fertilization are possible, but the zygote does not survive
o Hybrid inviability: a hybrid individual develops but dies before or soon after birth
o Hybrid infertility: the hybrid remains healthy, but cannot reproduce (sterile)
Example:
o Zebroids the hybrid of zebras and horses are infertile= hybrid infertility
MODES OF SPECIATION:
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Reproductive isolating mechanisms
There are 2 modes of speciation;
allopatric speciation and sympatric
speciation
Allopatric Speciation:
Allopatric speciation is the formation of a
new species as a result of evolutionary
changes following a period of geographic
isolation
Once separated, populations can no longer
exchange genetic information
Over generations these populations
become less similar, because any mutation
that occurs is not shared between
populations, environmental conditions
differ and eventually they could develop
reproductive isolating mechanisms.
Examples:
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Separation of Pangea
Mountain ranges forming ( As the Rocky
mountains formed, the West side has a
moderate climate with heavy precipitation
and the East side has dry and hot summers
Glacier passes
Urbanization (highway production)
Emigration to a remote island (Galapagos)
Isthmus of Panama forming 2 million years ago (A thin strip of land that separates the Caribbean and pacific
ocean, many aquatic organisms affected)
Sympatric Speciation:
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Sympatric speciation is the evolution of populationswithin the same geographic location.
Can be gradual or sudden or can be a result of polyploidy or hybrid formation.
Gradual sympatric speciation:
o Example: The hawthorn fly is native to North America. The original population of hawthorn
flies laid their eggs in the small fruits of hawthorn trees. Between 1800 and 1850, after the
introduction of apple trees to North America, some of these flies began laying their eggs on
apples. Today, the species consists of two populations. One population, now called apple
maggot flies, feeds almost exclusively on apples, while the other feeds almost exclusively on
hawthorns.
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Sudden Sympatric speciation
o a single mutation can render an individual unable to reproduce with other members of the
population. If two such individuals share the same mutation or if a single individual is able to
reproduce asexually, a reproductively isolated population, a new species, may result.
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Polyploidy and hybrid formation
o Polyploidy can result in sympatric speciation. Recall that mutations causing polyploidy
double the number of chromosomes in an individual. Polyploids produce fertile offspring
when mated with each other but produce sterile offspring when mated with the original
species.
Types of Evolution….
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Divergent Evolution: the large scale evolution of a group into many different forms.
o In order to fill different ecological niches species evolve into having a variety of different forms and
functions
o Example: deer mice, flying squirrels, porcupines and beavers all come from closely related species
but have evolved into different roles to fill ecological niches.
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Convergent Evolution: the evolution of similar traits in distantly related species.
o Due to similar environments organisms have evolved similar structures and appearances
o Example: Similar body shapes can be found in sharks and dolphins.
o Example: Cacti and euphorbia have evolved similar features in response to their hot and dry
environments
Coevolution: a process in which one species evolves in response to the evolution of another species.
o Species that exist in linked/ symbiotic relationships may evolve in response to changes in the other
o Example: Brazil nut trees have evolved extremely hard protective shells and the agouti is the only
mammal that has evolved teeth and jaws strong enough to bite open the shell.
o The Madagascar long spurred orchid has nectar at the end of a 30cm spur. It is pollinated by the
hawk moth, which has evolved to have a 30cm long tongue to reach the nectar.
Human Influence:
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Human activities influence the evolution of species.
o Agricultural expansion
o Building roads
o Habitat fragmentation (building roads, cities,
dams, fences etc.)
By constructing corridors between habitat fragments, human impact can be decreased
Avoiding Extinction
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New species evolve and living species become extinct.
This general trend, however, has been sharply reversed on at least five occasions, when mass
extinction events have taken place.
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Mass extinction events occur on a global scale and are biologically traumatic. The diversity of life on
Earth plummets. Each past mass extinction event has been followed by the recovery of species
diversity over a period of millions of years.
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Past mass extinction events were caused by actions that altered Earth’s biosphere in a profound
way. E.g. An asteroid impact or a series of large volcanic eruptions
Today, biologists around the world are deeply concerned about the increasing rate of species
extinction. The primary threats to species are habitat loss and degradation, the introduction of
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invasive species, overharvesting, pollution, and climate change—all caused by humans. • Primary
forests are being lost at a rate of 6 million hectares per year.
• Present extinction rates are estimated at 1000 times the natural rate.
• About 70 % of the world’s coral reefs are threatened or severely damaged.
• An estimated 22 % of all plant species are at risk.
• Of 47 677 species that have been assessed, 17 291 are in danger of extinction.
Questions:
1. Extinction events have occurred over the entire course of Earth’s existence. What has changed that is
causing scientists concern?
2. Explain which type of reproductive isolating mechanism is at work in each of the following situations:
a) Mules, the hybrid offspring of mating between horses and donkeys, are sterile.
b) The Pacific Wrasse and the Caribbean Wrasse are separated by the isthmus of panama (land joining
North and South America)
c) Female fireflies identify males by the pattern of light flashes they produce. Each species has a
unique pattern of flashes.
d) Male geese have a penis, while male herons do not. Male herons are unable to fertilize female
geese.
e) When pollen grains from white pine trees land on the female cones of red pine trees, fertilization
does not occur.
3. Plant and animal breeders try to keep their breeding lines “pure” to prevent unwanted crosses with their
breeding stock. How is this human activity analogous to the conditions that cause allopatric speciation?
4. Would you expect to fi nd more unique species on a large remote island or on a smaller island that is close to a
large continent? Explain your reasoning.