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Classification System Kingdom: Animalia Phylum: Chordata Class: Mammalia Order: Primate Family: Hominidae Genus: Homo Species: sapiens Varieties and Breeds
Defining Species Evolution creates new species, but … What is a species? Its not as straightforward a question as most believe.These are members of different species - eastern (left) and western (right) meadowlark.
What is a Species?There is only one extant (existing) human species.
What is a Species?And these are all members of a single species.
Determining What Is and What Isn’t a Distinct Species Can Be Difficult Northern spotted owl (left) and barred owl (right).
What is a Species?Lets start with the most widely accepted definition:A species is a group of individuals capable ofinterbreeding to produce fertile offspring.This is thebiologicalspecies concept.Like all attemptsto define aspecies, it has aproblem.
Hybrid InfertilityHorse X Donkey = MuleCannot Reproduceand produceFertile offspring.
So a better definition of a species might be: A group of individuals capable of interbreeding to produce offspring that are fertile and these offspring, in turn, can produce offspring that are fertile.
Another problem with the Biological Species ConceptFor asexually-reproducing organisms, like these bacteria,what constitutes a species?
How Many Species Are There? We don’t know.About 2 million species have been described. Estimates of existing species number range from 4 million to 100 million (with 10-15 million being a more commonly considered upper estimate).
Two Patterns of Speciation One species evolving fromOne anotherspecies with bothcompletely thenreplacing existing atanother same time
How Do New Species Arise? The key to speciation is reproductive isolation of populations **It is reproduction that makes more. organisms…so mess with reproductive success and you mess with population!
What does it mean to have reproductive isolation?It means that for whatever reason, one or moreorganisms are prevented from being able toreproduce with others in the population of thatspecies.1. Organism itself changes because of random mutation(s)2.Environment changes, causing separation of population intotwo or more subgroups of the original population.
Intrinsic Reproductive Isolating Mechanisms Something changes about the organisms in thepopulation that interferes with reproductive success but is not caused by geographic isolation! Maybe something about mating ritual Maybe something about sperm or egg Maybe something about time of year they mate Etc.
Extrinsic Reproductive Isolating MechanismsSomething happens to the ENVIRONMENT ofthe organism which interferes with reproductivesuccess.
Reproductive Isolation May Occur With or Without Geographic IsolationAllopatric speciationoccurs when geographicisolation creates a situationwhere reproduction can’toccur (an extrinsicmechanism).Sympatric speciationoccurs when reproductionis not inhibited bygeographic isolation butsomething about theindividual organisms of thepopulation changes andinterferes withreproductive capability. (anintrinsic mechanism)
Allopatric Speciation: A change in the environment Harris’ antelope squirrel White-tailed antelope squirrelTwo species of ground squirrel are believed to havedescended from a common ancestral population that wasseparated by formation of the Grand Canyon.
Intrinsic Reproductive Isolating Mechanisms Are Always Required for Speciation…Even when allopatric speciation has already occurred. Harris’ antelope squirrel White-tailed antelope squirrelIntrinsic mechanisms involve changes to the organisms that prevent interbreeding.In allopatric speciation, intrinsic mechanisms come into play oncepopulations are physically separated.In sympatric speciation, intrinsic mechanisms are the only ones involved.
Intrinsic Isolating Mechanisms (different habits within an overlapping range)
Intrinsic Isolating Mechanisms Continued:Courtship rituals,are critical formating within aspecies, butineffective forattracting membersof other species.
Mating Ritual Blue-footed BoobyMating Ritual Fiddler Crab Male Waving
Intrinsic Isolating Mechanisms Continued
Salamander Evolution Subspecies evolving At different Locations in California… ALL ARE SAME SPECIES
Speciation Occurs at Widely Differing RatesA slow rate of speciation is seen with a living horseshoecrab which has diversified into 13 species within the last300 million years.A rapid rate of speciation is seen in Galapagos fincheswhich have diversified into 13 species within the last100,000 years.
What else, other than isolatingMechanisms, can contribute to theformation of new species?
Sometimes simply CHANCE EVENTS can contribute to speciation! Genetic Drift any change in the allelic frequency in a population that is due chance events (floods, insects getting sprayed with insecticides, earthquakes, fires, etc)Allelic frequency: a measure of how common an allele is in a populationEspecially true in small populations because thechance of losing an allele completely becomes greater!
Type of Genetic DriftFounder Effect: when a small populationcolonizes a new habitat (separate from the restof that population) and interbreeds. Alleles thatwere once fairly uncommon in a population nowmay be more common because this subset of thepopulation is so small.Amish population:Dwarfism, Polydactyly
Migration may contribute to speciation as well!Transports genes to different gene pools ifthey mate while they are migrating. b/c their genes are isolated from their original population and the alleles enter a different gene pool.
Speciation Rates Generalists (eat many different foods, well-adapted to many different habitats) like the horseshoe crab, tend to remain as stable species.Specialists (eat more specific foodsand only adapted to live in certainhabitat), like the Galapagos finch, tendto be unstable as a species andMore likely to change!Speciation also becomes rapid when, newniches become available to the organisms,as seen with Galapagos islands.
Formation of New Species- Gradualism or Punctuated Equilibrium?GradualismPunctuatedEquilibrium
Does Evolution Create the Perfect Organism?No, only organisms which arebetter suited to survive andreproduce in thatenvironment at that time!
Species Come and GoBest estimates from the fossil record indicatethat greater than 99% of species that haveexisted are now extinct. A typical “lifetime” for a species is about 1 million years.
Mass Extinctions Are a Fact of Life
The Cretaceous/Tertiary Mass Extinction 65.5 mya Asteroid Impact Theory 85% of all species died Gary Larson
Are We Now Causing a Mass Extinction?
Are humans causing the 6th mass extinction? Current Mass Extinction Mass Extinction Humans and Extinction