originea speciilor

©2001 Timothy G. Standish

Genesis 124 And God said, Let the earth bring

forth the living creature after his kind, cattle, and creeping thing, and beast of the earth after his kind: and it was so.

25 And God made the beast of the earth after his kind, and cattle after their kind, and every thing that creepeth upon the earth after his kind: and God saw that it was good.


Timothy G. Standish, Ph. D.

The OriginThe OriginOf SpeciesOf Species


Observing SpeciationObserving Speciation"The evolutionary divergence of a single species into two has never been directly observed in nature, primarily because speciation can take a long time to occur.”

Darren E. Irwin, et al. 2001 Speciation in a ring, Nature 409:333-337.


What is a Species?What is a Species?Six major concepts:1 A - Morphospecies - If it looks different, it is a different species1 B - Cohesion - Defined by an integrated complex of genes and set of

adaptations2 A - Biological - Reproductively isolated groups of organisms2 B - Recognition - If two organisms don’t recognize one another as potential

mates, they are different species3 Ecological - If they do not occupy the same niche, they are not the same

species4 Evolutionary - If they share the same common ancestor and niche, they are

related and may be the same species


EvolutionEvolutionMicroevolution - Changes in

allele frequency over time (Population genetics)

Macroevolution - Accumulation of novel genetic changes in a population until it becomes a new species


How Species EvolveHow Species Evolve Anagenesis - (an = without, genesis = beginning) Over

time the environment in which a species lives changes and the species continually adapts to the new environment. Thus the species changes over time and eventually becomes a new species

Cladogenesis - (clad = branch, genesis = beginning) As new niches become available, members of existing species move into exploit them. As these individuals adapt to their new environment, they become distinctly new species


How Species EvolveHow Species Evolve

CladogenesisAnagenesis Morphology

Time

Morphology

Time


Where Speciation OccursWhere Speciation OccursAllopatric Speciation - Speciation that does

not occur in the same place. First two populations are separated, then they change and become different species.

Sympatric Speciation - Speciation in the same place. Species arise within the same population due to something other than a physical reproductive barrier.


Reproductive BarriersReproductive Barriers If a species is to be produced, some sort of

reproductive barrier needs to come into play between two populations of the same species

Reproductive barriers fall into two classes: Prezygotic - Those that occur before a zygote is

produced Post zygotic - Those that prevent the offspring of

two species (mule) from reproducing


Physical Reproductive BarriersPhysical Reproductive Barriers If a population is separated into two populations by

a physical barrier the Hardy-Weinburg assumption of random mating will be violated

If different selective pressures are brought to bare on the separate populations, they will develop different allelic frequencies

Evolutionary theory extrapolates from here to say that they will form new species and if they drift enough new genera and so on


Prezygotic BarriersPrezygotic Barriers Habitat isolation - If they live in different places, they can’t mate Behavioral isolation - If species recognition is behavior based,

organisms with different behaviors will not mate (i.e. eastern and western meadowlarks are identical in almost all things except song)

Temporal isolation - If they breed at different times, they will not breed with each other

Mechanical isolation - Need any more be said? Gametic isolation - Gametes have complex recognition

mechanisms so that gametes from one species will rarely fuse with those of another species


Post Zygotic BarriersPost Zygotic Barriers Inviable Hybrids - Hybrids may develop from a zygote

formed from the sperm of one species and the egg of another, but they are weak inferior creatures and may not even survive until birth

Infertile Hybrids - Hybrids may be hardy creatures, but they are incapable of reproduction, frequency due to difficulties in producing gametes due to strange chromosome combinations resulting from meiosis

Hybrid Breakdown - At first hybrids are fairly successful, but over the course of several generations problems develop


Hybrid breakdown

Barriers To Hybrid FormationBarriers To Hybrid FormationHabitat isolation

Behavioral isolationTemporal isolation

Mechanical isolation

Gametic isolation

Infertile hybridsInviable hybrids

++

Happy Hybrid

PrezygoticBarriers

PostzygoticBarriers


Sympatric Speciation:Sympatric Speciation:AutopolyploidyAutopolyploidy

Somatic nondisjunction

Diploid plant2n = 4

Tetraploid cells develop into flowers

Tetraploid flowers make diploid gametes

Self fertilization results in tetraploid offspring which cannot interbreed with the original diploid species


2n=10 hybrid(fertile)

Mitotic nondisjunction produces diploid cells capable of producing

fertile gametes

Sympatric Speciation:Sympatric Speciation:Allopolyploidy - Scenario 1Allopolyploidy - Scenario 1

Plant species A2n = 4

Plant species B2n = 6

1n=3 gamete

1n=2 gamete

1n=5 hybrid(infertile)

Gametes combine to make a hybrid


Sympatric Speciation:Sympatric Speciation:Allopolyploidy - Scenario 2Allopolyploidy - Scenario 2

Plant species A2n = 4

Plant species B2n = 6

Unreduced 2n gamete

Normal gamete

1n=3 gamete

Gametes combine to make a hybrid 1n=7 hybrid

(infertile)

Meiotic nondisjunction produces unreduced

gameteUnreduced

gamete

2n=10 hybrid(fertile)


Tempo Of EvolutionTempo Of Evolution

Morphology

Time

Punctuated Equilibrium

Morphology

Time

Gradualism


Edredge on Punctuated EquilibriaEdredge on Punctuated Equilibria"At the core of punctuated equilibria lies an empirical observation: once evolved, species

tend to remain remarkably stable, recognizable entities for millions of years. The observation is by no means new, nearly every paleontologist who reviewed Darwin's Origin of Species pointed to his evasion of this salient feature of the fossil record. But stasis was conveniently dropped as a feature of life's history to be reckoned with in evolutionary biology. And stasis had continued to be ignored until Gould and I showed that such stability is a real aspect of life's history which must be confronted-and that, in fact, it posed no fundamental threat to the basic notion of evolution itself. For that was Darwin's problem: to establish the plausibility of the very idea of evolution, Darwin felt that he had to undermine the older (and ultimately biblically based) doctrine of species fixity. Stasis, to Darwin, was an ugly inconvenience."

Eldredge N., "Time Frames: The Rethinking of Darwinian Evolution and the Theory of Punctuated Equilibria", Simon & Schuster: New York NY, 1985, pp188-189


The Rate of EvolutionThe Rate of Evolution Sometimes evolution has occurred at an amazingly rapid rate: Drosophila pseudo-obscura, a native species, has declined since 1978

when the European species Drosophila subobscura was introduced in Chili

In Europe D. subobscura exhibits an increase in wing size as one goes from south to north

A south to north wing size gradient went unobserved when D. subobscura was studied around 1989, but a decade later a difference in wing size distribution mimicking that seen in European flies was evident.

Thus this wing size difference must have evolved in a decade or less.


The Rate of EvolutionThe Rate of Evolution Galapagos finches are also known to have evolved

very rapidly in nature After a drought in 1978, a dramatic shift in beak

size was observed in a local population of finches


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