No Slide TitleGenesis 1
24 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.
©2001 Timothy G. Standish
©2001 Timothy G. Standish
Observing 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.
©2001 Timothy G. Standish
What is a Species?
Six major concepts:
A - Morphospecies - If it looks different, it is a different species
B - Cohesion - Defined by an integrated complex of genes and set of adaptations
A - Biological - Reproductively isolated groups of organisms
B - Recognition - If two organisms don’t recognize one another as potential mates, they are different species
Ecological - If they do not occupy the same niche, they are not the same species
Evolutionary - If they share the same common ancestor and niche, they are related and may be the same species
©2001 Timothy G. Standish
Microevolution - Changes in allele frequency over time (Population genetics)
Macroevolution - Accumulation of novel genetic changes in a population until it becomes a new species
©2001 Timothy G. Standish
How 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
©2001 Timothy G. Standish
Where Speciation Occurs
Allopatric 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.
©2001 Timothy G. Standish
Reproductive 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
©2001 Timothy G. Standish
Physical 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
©2001 Timothy G. Standish
Prezygotic 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
©2001 Timothy G. Standish
Post 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
©2001 Timothy G. Standish
Barriers To Hybrid Formation
Somatic nondisjunction
Diploid plant
2n = 4
Self fertilization results in tetraploid offspring which cannot interbreed with the original diploid species
©2001 Timothy G. Standish
Plant species A
©2001 Timothy G. Standish
Unreduced gamete
2n=10 hybrid
©2001 Timothy G. Standish
Edredge 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
©2001 Timothy G. Standish
The 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.
For references on this see:
http://www.newscientist.com/news/news.jsp?id=ns222252
©2001 Timothy G. Standish
The 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
©2001 Timothy G. Standish