Download chapter24 Origin of Species

Chapter 24
THE ORIGIN OF SPECIES
Summary of Chapter 24, BIOLOGY, 12TH ED Campbell, by J.B. Reece et al. 2014.
I. WHAT IS A SPECIES?
Species comes from the Latin word meaning "kind" or "appearance".
Speciation is the process by which one species splits into two or more species.
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Microevolution is the change in allele frequency over time.
Macroevolution refers to the broad pattern of evolution above the species level.
The evolution of mammals and the evolution of angiosperms are examples of macroevolution.
Speciation is the process by which a species splits into two species.
1. BIOLOGICAL SPECIES CONCEPT
A species is a population of organisms that have the potential to interbreed with one another in
nature to produce viable, fertile offspring, but cannot produce viable, fertile offspring with
members of another species.
The emphasis is reproductive isolation.
A. Reproductive isolation
Prezygotic and postzygotic barriers
Prezygotic barriers prevent mating between species or hinder the fertilization of ova if
members of different species mate. Reduces the chances of forming hybrids.
1. Habitat isolation: live in the same locality but in different habitats, e.g. primarily aquatic
while the other mostly terrestrial.
2. Behavioral isolation: differences in courtship or life style.
3. Temporal isolation: reproductive period occurs at different time of the year.
4. Mechanical isolation: the anatomy prevents mating.
5. Gametic isolation: gamete recognition is based on the presence of specific molecules
on the coats around the egg, which adhere only to complementary molecules in the
sperm.
Postzygotic barriers prevent the hybrid zygote to develop into a viable, fertile adult.
1. Reduced hybrid viability: hybrid zygote dies in the early stages of development or fails to
reach sexual maturity.
2. Reduced hybrid fertility: hybrid does not produce functional gametes.
3. Hybrid breakdown: offspring of hybrids fail to produce functional gametes or do not reach
sexual maturity.
B. Limitations of the Biological Species Concept
The biological species concept cannot be applied to fossils or to organisms that reproduce only
asexually, e.g. prokaryotes.
The species delineation is not clear in some cases due to gene flow between populations in the
wild. This is a common phenomenon between some sea gull species and many plant species.
See pages 502 and 503.
2. ALTERNATE DEFINITIONS
The ecological species concept defines the species in terms of its ecological role in the
environment, its niche in the environment.
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It includes species that reproduce asexually.
The pluralistic species concept states that the factors that define a species vary.
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In some cases, reproductive isolation is a useful concept. In other cases ecological niche
is appropriate.
The morphological species concept is commonly used to distinguish specie. It uses
appearance and structures to separate species.
The phylogenetic species concept defines a species as the smallest group o individuals that
share a common ancestor.
II. MODES OF SPECIATION
Speciation can take place with or without geographic separation.
Speciation is the evolution of a new species from an ancestral population.
1. ALLOPATRIC SPECIATION
Allopatric speciation occurs when one population becomes geographically isolated from the
rest of the species and subsequently diverges.
A. The Process of Allopatric speciation
It is promoted by the creation of geographical barriers, e.g. mountains, rivers, migration to new
areas, etc.
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It is the most common method of speciation.
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Animal species use it almost exclusively.
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Genetic drift and natural selection act on the gene pool.
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Genetic drift tends to result in rapid changes in allele frequencies in small isolated
populations.
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The chances of allopatric speciation increase if the population is small and isolated.
B. Evidence of Allopatric Speciation
Adaptive radiation occurs in groups of islands (archipelagos) where several species evolved
from a single common ancestral population, and occupy different ecological niches.
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Recolonization of the first island leads to several closely related species living
together but occupying different niches, e. g. Galapagos finches.
2. SYMPATRIC SPECIATION
It occurs in the same physical location.
A. Polyploidy
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It is common in plants but very unclear in animals.
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Polyploidy is a major factor on plant evolution.
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Spontaneous doubling of chromosomes before meiosis has been documented in plants and
a few animal species.
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An autopolyploid is a polyploid that has an extra set of chromosomes all derived from a
single species. The offspring may be viable and fertile.
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When polyploidy occurs in conjunction with hybridization is called allopolyploidy. Two
species contribute to the making of the hybrid.
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Doubling of chromosomes (polyploidy) follows hybridization of two species.
Allopolyploids are reproductively isolated from both parents. They have different number of
chromosomes.
Many of our food plants are polyploids.
B. Sexual Selection
Sympatric speciation occurs in animals but it is not well understood.
It relies on genetic mechanisms other than polyploid.
Mutations and behavior are probably important. See page 509, mate selection in cichlids.
C. Habitat Differentiation
Other mechanisms at play could be change in habitat, food source or other resource not used
by the parent population.
Example of the apple-maggot fly on page 510.
III. HYBRID ZONES
A hybrid zone is a region in which members of different species meet and mate producing
some hybrid offspring.
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Hybrid zones are areas of geographical overlap where interbreeding occurs.
1. PATTERNS WITHIN HYBRID ZONES.
The species involved have an incomplete reproductive barrier.
Reproductive isolation breaks down in hybrid zones.
Hybrid zones are usually narrow presumably because the hybrids are not well adapted to either
parental environment.
There might be an increase in embryonic death of hybrids or morphological deformities.
The hybrids produce few viable offspring with members of the parent species.
Outside the hybrid zone, gene flow may be difficult due to poor adaptation to the environment,
e.g. natural selection in action.
2. HYBRID ZONES OVER TIME
A. Reinforcement: Strengthening the Reproductive Barrier.
Hybrids are often less fit than members of the parent species.
Natural selection reinforces the Prezygotic reproductive barriers
B. Fusion: Weakening Reproductive Barriers
If the barriers to reproduction are not strong, gene flow will increase with time and the gene pool
of both populations become increasingly alike.
Eventually the two species fuse into one thus reversing the process of speciation.
C. Stability: Continued Formation of Hybrid Individuals.
Many hybrid zones are stable in that hybrid offspring continue to be produced over time.
When members of both parent populations continue to migrate into the hybrid zone, hybrids will
be produced even if the hybrids fail to reproduce successfully in a significant number.
IV. PUNCTUATED EQUILIBRIUM
Speciation can occur rapidly or slowly and can result from changes in a few or many genes.
1. THE TIME COURSES OF SPECIATION
A. Patterns in the Fossil Record.
Paleontologists rarely find fossils that show a gradual transition from one species to the next.
New species appear in the fossil record rather abruptly.
According to this model, there are long periods of stasis (equilibrium) when no evolution occurs,
punctuated or interrupted by short periods of rapid speciation possibly triggered by changes in
the environment. This called punctuated equilibrium.
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Questions the idea that the fossil record is as incomplete as it initially appeared.
Allopatric and sympatric speciation can occur in very short time.
It accounts for the abrupt appearance of new species in the fossil record.
Transitional forms are absent for the most part.
Speciation can occur relatively rapid once it has begun.
Gradualism proposes that evolution occurs continually over long periods of time.
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Populations gradually diverge from one another by the accumulation of adaptive
characteristics in each population.
It is rarely observed in the fossil record because most organisms decompose without a
trace.
There are transitional forms.
B. Speciation Rates.
See the example of Helianthus anomalus on page 514.
2. STUDYING THE GENETICS OF SPECIATION
By studying the genetics of speciation (as in hybrid zones) the causes of reproductive isolation
can be better understood.
In some cases, the evolution of reproductive isolation results from the effects of a single gene.
See examples on pages 515 and 516.