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Origin of Species
• The term species refers to individuals in a population that are free
to breed and that produce viable offspring, without outside
intervention, that can carry on the lineage. Speciation, or how
species arise, can be further broken down using the concepts of
micro and macroevolution.
– microevolution - species arise through a divergence of breeding
populations that subsequently change the gene frequencies in a gene
– macroevolution - evolutionary change above the species level such as
the evolution of birds from dinosaurs
Biological Species
• A biological species cannot (or does not) breed with members of a
different population. The concept hinges on the concept of
reproductive isolation or the existence of biological factors that
inhibit or prevent reproduction.
Prezygotic Barriers
• prezygotic barriers - factors that prevent mating or inhibit
– habitat isolation - species that rarely come into contact due to
occupation different habitats
– temporal isolation - time of day
– behavioral isolation - courtship rituals that attract mates are different
– mechanical isolation - morphological differences (parts don't fit)
– gametic isolation - sperm of one species cannot penetrate egg of
Postzygotic Barrier
• postzygotic barrier - prevents a hybrid from developing or to
become fertile as an adult
– reduced hybrid viability - genes of the different parents impair the
development and viability of the offspring
– reduced hybrid fertility - adult hybrid is sterile (mule)
– hybrid breakdown - fertility is decreased with each subsequent
generation (more common in plants)
Factors that lead to speciation
• allopatric speciation - occurs when a population becomes geographically
isolated over a long period of time.
Factors that lead to speciation
• sympatric speciation - occurs between members in a breeding
population. Mechanisms include chromosomal changes and nonrandom mating that changes gene flow within a population.
– polyploidy having more than 1 set of chromosomes
• autopolyploildy - more than 2 sets of chromosomes from a single species.
More common in plants.
• allopolyploidy - two different species interbreed creating a hybrid with an odd
# of chromosomes. The offspring then can mate with each other but not with
the parental generation forming a new species
– Parapatric or habitat differentiation - occurs when a portion of a
populations starts to exploit a resource in the habitat which the parent
generation cannot use. This can lead to differential sexual selection
and speciation.
Factors that lead to speciation
• Adaptive radiation - the evolution of many diversely adapted
species from a common ancestor upon introduction to a new
environment (founder effect). Ex: HI islands and introductions of
species. Displays the model of gradualism.
Changes that lead to macroevolution.
• shows model of punctuated equilibrium - new species is formed
from parent then is stable (stasis) for many years
• evolutionary novelties through genes that control development heterochromy
– allometric growth changes - affected by genes that control body
proportioning by affecting rate and timing
– paedomorphosis - an alteration in the timing of juvenile and adult
characteristics (especially in species that have distinct morphological
– homeotic gene changes - alter body plan (where wings or appendages