Speciation Species One or more populations of individuals that can inter breed under natural conditions and produce fertile offspring. Different species may arise from genetic divergence between populations Genetic Divergence Genetic divergence is the process whereby local units of a population become reproductively isolated from other units and thus experience changes in gene frequencies between the groups
If the environments are different between isolated units then natural selection, mutations, and genetic drift will work independently on each. This will speed up genetic divergence and speciation. Isolating mechanisms Mechanisms that lead to, or cause genetic divergence by preventing interbreeding between two groups. As long as two groups do not interbreed their gene pools will continue to drift further and further apart. The longer two species are genetically isolated from each other the more different they become
from each other. Isolating mechanisms Isolating mechanisms can be divide into two different types. Isolating mechanisms that occur before fertilization are called Prezygotic mechanisms Isolating mechanisms that occur after fertilization are called postzygotic mechanisms Types of Prezygotic Isolating mechanisms Behavioral isolation: Potential mates meet but cannot figure out what to do about it because patterns of courtship may be
altered to the extent that sexual union is not achieved Temporal isolation: (Time) Different groups overlap in range but may not be reproductively mature in the same season. Mechanical isolation: Potential mates attempt engagement but sperm cannot be successfully transferred . This may be due to differences in reproductive organs. Types of Prezygotic Isolating mechanisms cont. Gametic isolation: Sperm is transferred but sperm and egg are incompatible. Ecological isolation: potential mates never meet because they live in different habitats
Types of Postzygotic isolating mechanisms Zygotic mortality: Egg is fertilized but zygote does not develop properly dies before birth because parents are genetically incompatible. Hybrid inviability: Hybrid very weak and cant live outside the uterus. Hybrid offspring: Hybrid is sterile. Speciation Speciation occurs when a species gives rise to one or more different species. There are three main speciation patterns Allopatric speciation
Sympatric speciation Parapatric speciation Allopatric Allopatric (Allo= different, Patric=homeland) Population are separated due to geographical barriers Rivers, earthquakes, continental drift, glaciation, archipelagos cause allopatric speciation. Examples:
Cave fish, Darwins finches Antelope squirrels of the Grand Canyon Isthmus of Panama Sympatric speciation Sympatric (Sym = same, Patric=homeland) speciation occurs in the same geographical region without physical isolation. A new species can arise in a single generation if a genetic change produces a reproductive barrier between mutants and the parent population. Example: Accidents during cell division that result in extra sets of chromosomes (Polyploidy). Self-fertilization can give rise to new individuals that are unable to mate and form
fertile off-spring with the parent species. Parapatric Speciation Parapatric (Para = near, Patric=homeland) neighboring populations become distinct species while maintaining contact through Hybrid zones If hybrid zone is removed through increased natural selection, natural disasters or some other means the extremes of a population fail to mate. Examples 1 Toad population along the north rim of the Grand 2 Dogs Canyon.
Branching and unbranched evolution Cladogenesis: Branched evolution. Occurs as populations split and become reproductively isolated from each other. Anagenesis: Unbranched evolution. Occurs as changes in allele frequency and morphology accumulate over long periods of time. New species do not live within the
same time period B A C A B C Related species are only seen in the fossil record
Transitional Forms Species which are intermediate in body form between two groups of organisms. Species that are intermediate in time in the fossil record. Archaeopteryx Archaeopteryx Archaeopteryx lived after the development of dinosaurs but before that advent of birds.
Archaeopteryx contains features that are both characteristic of reptilian dinosaurs as well as modern birds. Some features are half way inbetween. Feature Birds Archaeopteryx Coelurosaurs Body covering
Feathers Feathers Scales (?) Metatarsals Fused Partly fused Not fused
Bones Hollow and Hollow, not pneumatic Not hollow or pneumatic pneumatic Clavicles Fused (wishbone) Fused (wishbone)
Not fused Sternum Large, keel Small, no keel Small, no keel Abdominal ribs Absent
Present present The tempo of speciation Phyletic gradualism: New species develop slowly and gradually as an entire species changes over time. This idea was asserted by Darwin. Punctuated equilibrium: This theory states that new species arise suddenly and rapidly as small subpopulations of a species split from the populations of which they were a part.
Tissue Systems. Plants have 3 major tissue systems: Dermal. Covers outer surface of plant. Vascular. Found in every root, shoot and leaf. System is continuous (all plant parts joined by vascular tissues)
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