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The Origin of The Species

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I.  The biological species concept

A.     Modern taxonomists determine species by differences in physiology, biochemistry, behavior, and genetic makeup.

B.     The biological species concept

1.      Defines a species as a population or group of populations whose members have the potential to interbreed with one another in nature to produce viable, fertile offspring but who cannot successfully interbreed with members of other species.

2.      A biological species is the largest unit of a population in which genetic exchange is possible and that is isolated from other populations.

3.      Members of a species are conspecific and are united by being reproductively compatible.

4.      The biological species concept does not work in all situations like asexual reproduction.

5.      It is also unable to group extinct forms of life.

6.      Lastly, it will not work if two populates are geographically segregated from each other.

II.  Reproductive barriers

A.     Any factor that impedes two species from producing viable, fertile hybrids contribute to reproductive isolation.

B.     Prezygotic barriers

1.      Impede mating between species or hinder the fertilization of ova if members of different species attempt to mate.

2.      Habitat isolation

a.      Two species that live in different habitats within the same area may encounter each other rarely.

3.      Behavioral isolation

a.      Special signals that attract mates are probably the most important reproductive barriers among closely related animals.

4.      Temporal isolation

a.      Two species that breed during different times of the day, different seasons, or different years cannot mix their gametes.

5.      Mechanical isolation

a.      Closely related species may attempt to mate but fail to consummate the act because they are anatomically incompatible.

6.      Gametic isolation

a.      Even if the gametes of different species meet, they rarely fuse to form a zygote.

b.      Gamete recognizition may be based on the presence of a specific molecule on the coats around the eggs that complement molecules on the sperm.

C.    Postzygotic barriers

1.      If a sperm cell from one species fertilizes an ovum of another species than these barriers prevent the hybrid zygote from developing into a viable, fertile adult.

2.      Reduced hybrid viability

a.      When prezygotic barriers are crossed and hybrid zygotes are formed, genetic incompatibility between the two species may abort development of the hybrid at some embryonic stage.

3.      Reduced hybrid fertility

a.      Even if two species mate and produce hybrid offspring that are vigorous, reproductive isolation is intact if the hybrids are completely or largely sterile.

b.      Maybe be caused if a failure of meiosis to produce normal gametes in the hybrid if the chromosomes of the two species differ.

4.      Hybrid breakdown

a.      In some cases where species cross-mate, the first generation hybrids are viable and fertile, but when these hybrids mate with one another or with either parent species, offspring of the next generation are feeble or sterile.

D.    Introgression

1.      This is the transplantation of alleles between species.

2.      It occurs when the two species hybridize and a fraction of the hybrids manage to cross with another species.

3.      This increases the reservoir of genetic variation.

III.  Geographical Isolation can lead to the origin of species.

A.     Two modes of speciation

1.      Allopatric speciation is where the populations are segregated by a geographical barrier. 

2.      Sympatric speciation is where a subpopulation becomes reproductively isolated in the midst of its parent population.

B.     Allopatric speciation

1.      Geographical barriers

a.      Geological processes can fragment a population into two or more isolated populations.

b.      A small population may become geographically isolated when individuals from the parent population colonize to a new location.

2.      Condition Favoring Allopatric Speciation

a.      The geographical isolation of a small population usually occurs at the fringe of the parent populationís range.  The splinter population or peripheral isolate is a good candidate for allopatric speciation because

1.      The gene pool of the peripheral isolate probably differs from that of the parent population.  If the population is small, the founder effect will result.

2.      Until the peripheral isolate becomes a large population, genetic drift will continue to change its gene pool at random.  New mutations or combination of existing alleles become fixed.

3.      Evolution caused by natural selection may take a different direction in the peripheral isolate than in the parent population.  Because the peripheral isolate is on the frontier, it will probably encounter selection factors different that those of the parent population.

3.      Adaptive Radiation on Island Chains

a.      Allopatric speciation has occurred on island chains where organisms that have strayed or become passively dispersed from their parent populations have founded new populations that evolved in isolation.  See page 443

b.      Multiple invasions of islands by peripheral isolates of species from neighboring islands would eventually lead to the coexistence of several species on each island.  The islands are far enough apart to permit populations to evolve in isolation but close enough together for occasional dispersion events to occur.

C.    Sympatric Speciation

1.      New species arise within the range of parent populations.  Reproductive isolation evolves without geographical isolation. 

2.      This may occur in a single generation if some genetic change results in a reproductive barrier between the mutants and the parent population.

3.      Many plant species have their origins in accidents during cell division that result in extra sets of chromosomes called polyploidy.

4.      An autopolyploidy is an individual that has more than two chromosome sets all derived from a single species.  These plants may breed by self-fertilization or by fertilizing other autopolyploidy plants.  They cannot interbreed successfully with diploid plants of the original population. 

5.      If the diploid plant is 2n and polyploidy takes place, the resulting plant is 4n.  If the 4n plant tries to interbreed with another 2n plant, the resulting offspring is 3n and sterile.

6.      Allopolyploid is more common than autopolyploidy.  It refers to the contribution of two different species to a polyploid hybrid. 

7.      This begins when two different species interbreed and combine their chromosomes.  Interspecific hybrids are usually sterile because the haploid set of chromosomes from one species cannot pair during meiosis with the haploid set from the other species.

8.      Though infertile, a hybrid may actually be more vigorous than its parents and propagate asexually.

9.      This accounts for 25%-50% of the current plant species.

IV.  Population genetics can account for speciation

A.     Speciation by Adaptive Divergence

1.      When two populations adapt to disparate environments, they accumulate differences in genetic allele frequencies and genotypes.

2.      In the course of this, reproductive barriers between the two populations may evolve coincidentally

3.      Reproductive barriers can arise without being favored directly by natural selection.

4.      Reproductive isolation is usually a secondary consequence of divergence of the two populations as they adapt to their separate environments.

B.  Speciation by Shifts in Adaptive Peaks

1.      An adaptive peak represents an equilibrium state where the gene poll has allele frequencies that optimize the populationís success in that environment.

2.      Even in a stable environment, several adaptive peaks from a given population are possible but natural selection will tend to maintain the population at a singe peak.

3.      To reach an alternative adaptive peak by some change in the overall gene pool, a population must go through a period where the average fitness of the individuals is low.

4.      If a slight change in the frequency of alleles at one or more loci drives a population off an adaptive peak, natural selection will usually push the population back to its original peak.

5.      If the environment changes, the adaptive peak is redefined.

6.      A population that survives in this new environment must climb another adaptive peak through microevolution of its gene pool.

7.      Peak shifts may cause the bottleneck or founder effect.

B.        Hybrid Zones and the Cohesion Concept of Species

1.      What happens when two closely related populations that have been allopatric from some time come back into contact?  Three outcomes are possible.

a.      The two populations may freely interbreed and speciation has not occurred during geographical isolation

b.      Evolutionary divergence during the period of allopatry resulted in reproductive barriers that keep the gene pools of the two populations separate and speciation has occurred.

c.      The formation of a hybrid zone or a region where two related populations that diverged after becoming geographically isolated make secondary contact and interbreed where their geographical ranges overlap.

V.  The theory of Punctuated Equilibrium

A.     The traditional evolutionary tree that diagrams the descent of species from ancestral forms sprouts branches that diverge gradually with each new species evolving continuously over long spans of time.

B.     However, paleontologists rarely find gradual transitions of fossil forms.

C.    Advocates of punctuated equilibrium  have redrawn the evolutionary tree to represent he fossil evidence for evolution occurring in spurts of relatively rapid change.

D.    The species undergoes most of their morphological modifications as they first bud from parent species and then change little even as they produce additional species.

E.     The fossil record indicates that successful species last for a few million years on average.