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is defined as a group of organisms that are able to breed with one another and produce fertile offspring
a species will not be able to produce fertile offspring with members of other groups



is the emergence of a new species



gradual changes within a population over a umber of generations
-gradual adaptation of a population to its enviornement



broad patterns of evolutionnary change over long time periods
-largest scale changes that include the introduction of new groups



speciation acts to increase the diversity of life
usually a new species closely resembles its parent species (mircroevolution) but occasionally a new species will have enough change to define a new branch on the tree of life (macroevolution)
ex) the legs of land animals
the definition of species can take on many forms:
charles linnaues a swedish botanist developed the taxonomix system of naming and classifying in the 18th century and it is still used today
a binomial system assigning a genus and a species name to each organism
organisms were grouped as species based on physical similarities
ex) first and last name
rahel = specific, species -must be lower case
muster = lots of these, genus - must be capital letter and underlined


how to define a species

sometimes two organisms are of a different species but their apperance are very similar
-eastern spotted skunk and the western spotted skunk
they breed at different times of the year and even though they live close to one another they do not interbreed (temporal seperation)
humans appear very different from one another but still belong to the same species


the biological species concept

developed in 1942
defines a species as a group of populations whose members are bale to interbreed and produce fertile offspring
members of different species do not usually mate with one another but if they do their offspring are usually infertile
reproductive isolation occurs when the inability to produce fertile offspring assist in maintaining the gap in between different species
prevents gene flow between populations of different species
there are some problems with biological species concept
-prokaryokes do not reproduce sexually
we are unable to determine weather fossils were capable of interbreeding


other ways to define a species

1. morphological species concept
2. ecological species concept
3. phyogenetic species concept


morphological species concept

classification of a species based on measureable and observable physical traits
-shape, size, and other morphological features
-can be used for fossils and asexual organisms
-doesnt require info on interbreeding
-disavantage: relies on subjective criteria, some may disagree with what other people consider similar


ecological species concept

indentities species on the basis of ecological niche
-how they have adapted to a particular environment
-ex) two fish may be visibly similar to one another but distingishable based on diet or depth of water in which they live


phylogentic species concept

defined as the smallest group of individuals that share a common ansestor and form one branch on the tree of life
-determined based on comparisons of morphology and DNA sequences
-problem: agreeing on the amount of difference necessary to constitute a separate species


reproductive barriers and speciation

reproductive barriers: features a particular organism that makes it incapable of breeding with closely related species which live in the same geographical area
there are two types of reproductive barriers:
1. pre-zygotic reproductive barriers: prevent mating or fertilization between species
2. post-zygotic reproductive barriers: operate after hybrid zones are formed


pre-zygotic reporductive barriers (before zygote)

1. temporal isolation
2. habitat isolation
3. behvaioural isolation
4. mechanical isolation
5. gametic isolation


post- zygotic reproductive barriers (after zygote)

1. reduced hybrid viability
2. reduced hybrid fertility
3. hybrid breakdown


temporal isolation

occurs because two species mate at different times
can be different seasons, years, or time of the day
ex) a flowering plant have flowers which open at different times of the day


habitat isolation

two species living in the same general location
ex) two snakes living in westerm north america but one lives on land and the other in water
affects paarasites that are animal host species


behavioural isolation

little or no sexual attraction in between males and females of different species
special signals that work to attract mates of the same species do not work with other species
ex) courtship rituals are elaborate displays that must be preformed before mating can occur


mechanical isolation

female and male sex organs are not compatible
ex) spiraling of snail shells in different directions prevents mating


gametic isolation

the sexual act of mating may occur but the male and female gametes will not unite to form a zygote
ex) sea urchins release eggs and sperm into the sea but ferilization will occur only if species specific receptors are present on the egg and the sperm allowing them to attach to one another



union of gametes from two different species


reduced hybrid viability

most hybrid offspring do not survive
ex) salamander may hybribize but most ybrids do not complete development and those that do are very frail


reduced hybrid fertility

hybrid offspring of two different species reach maturity and are healthy but sterile
they are unable to facilitate gene flow between the two parent species
ex) a horse and a donkey can mate and produce a mule, the mule is sterile
-gene pool of the horse and the donkey remain seperate


hybrid breakdown

fist generation hybrid offspring are viable and fertile
when these hybrid mate with one another or with a memeber of one of the parent species the offspring are feeble or sterile


mechanisms of speciation

1. allopatric speciation
2. sympatric speciation
a) polyploidy
b) habitat differentiation
c) sexual selection


allopatric speciation

speciation that occurs as the result of introducing a geographical barrier
leads to the separation of a population with its gene pool from other populations of the same species
this can occur as the result of:
-the introduction of a mountain range which gradually splits two populations of organisms
-the formation of a land bridge diving marine populations in two
the size of the barrier required to create speciation depends on the species
-birds are not affected by the introduction of moutains or rivers
this likeihood of this occuring is higher when the population is small and isolated


sympatric speciation

a new species arises within the same geographical area as the parent species
how can this lead to reproductive isolation if the populations are not seperated from one another
three main ways:
a) polyploidy
b) habitat differentation
c) sexual selection



accidents during cell division result in extra sets of chromosomes
polyploidy: a cell that has greater than two complete chromosome sets
ex) a tetraploid plant arises from a diploid plant
teraploids cannot produce fertile offspring by mating with the parent plants
this would produce triploid offspring which are sterile
since there are only three chromosome sets they would not be able to form homologous pairs and seperate properly during meiosis
this means that the production of a tetraploid organism is instaneous speciation, produce in just one generation
most polyploid species result from mating of two different species


habitat differentiation

more likely to be the mechanism of speciation in animals
adaptations to different habitats in the same geographical area can lead to speciation because mating in between the two groups would become rare rare mating- isolated gene pools = speciation


sexual selection

choosing mates based on some physical characteristics
leads to reporductive isolation via seperated gene pools



when two sperated but closely related populations come back into contract with one another do reproductive barriers remain or will the two species be able to interbreed and become one?
hybrid zones: are regions in which members of different species meet and mate producing some hybrid offspring
these zones are studied by scientists in order to answer the above question
there are three possible outcomes for the hybrid zone over time:
1. reinforcement
2. fusion
3. stability



when hybrids are less fit than both parents species
expect that natural selection would strengthen/reinforce reproductive barriers
prevents the formation of unfit hybrids
(two species go in different directions)



reproductive barriers between two species are not strong
gene flow may occur in the hybrid zone reversing speciation
two hybridizing species fuse into one
(fuse and then come out as one)



hybrid zones are stable- best thrive
hybrids continue to be reproduced
some genes flow may occur between populations
each species maintains its own integrity
(species stay parallel to one another)


adaptive radiation

is the evolution of many diverse species from one common ancestor
-the adaptations of these species allow them to fill new habitats or roles in their community
-this occurs when a few organisms colonize new, unexploited areas or when environemental changes cause a number of extinctions opening up a number of habitats for new species
adaptive radiation is linked to new opportunity:
no competitors
new habitats and food sources
sometimes new structures evolve


rate of speciation

punctuated equilibrium
gradualism model


punctuated equilibrium

long periods occuring with little or no change, equilibrium, punctuated by abrupt episodes of speciation


gradualism model

species that have diverged gradually over long periods of time
-differences gradually evolve in a population as they become better adapted to their new environment
-new species evolve gradually from the ancestral population



major changes recorded in the history of life over large periods of time
ex) the evolution of flight in vertebrate lineages


earth is estimated to be

4.6 billion years old
all matter of the universe was initially present as one big mass
-the occurrence of a big bang 10-20 billion years ago blew this mass of matter apart


conditions on early earth

earth probably began as a molten mass
the atmosphere was likely thick with water vapor and contained other compounds released b volcanic eruptions:
-nitrogen, oxides, carbon dioxide, methane, ammonia, hygroden, hydrogen sulfide
-as the earth gradually cooled the water vapor condensed into oceans and the hydrogen gas escaped into space
-very intense ultraviolet light, lightening, and volcanic activity occurred on early earth
-the early earth also lacked oxygen: the primary necessity for life
- no molecular oxygen present


early earth

the earliest evidence of life on earth is from fossils such as stromatolite which are 3.5 billion years old
it is thought that life in an even simpler form may have arose as early as 3.9 billion years ago
how did life arise if it has been shown that life cannot arise from non- living matter


how did life arise

scientists have collected evidence and conducted experiments that led to the hypothesis that the conditions of early earth could have allowed the production of simple cells
these simple cells could have formed via the following four stages
1. abiotic synthesis of small organic molecules such as amino acids and nucelotides
2. joining of these small molecules into macromolcules ex) protiens and DNA
3. packaging of these marcomolecules into "protobionts" which are droplets surrounded by membranes that maintain an internal chemistry different than that of the external surrounds
4. the orgin of self-replicating molecules which eventually made inheritance possible


stanley millers experiments

miller was the first to show that amino acids and other organic molecules could e formed under conditions of early earth
organic molecules cannot be synthesized spontaneously because of the presence of atmospheric oxygen which oxidizes chemical bonds
the energy for this abiotic synthesis is thought to have come from lighting and intense UV radiation
the early oceans were thought to have been "primative soup" or a solution of organic molecules


stanley millers experiments- what he did

a flask of warm water represented the early sea
the water was heated so that some evaporated and moved into a second flask to simulate the early atmosphere
this atmosphere consisted of hydrogen, water vapor, methane, and ammonia-all gases thought in the 1950s to have made up the ancient atmosphere
electrodes discharged shocks into the flasks to simulate lightening
a condenser cooled the atmosphere, raining water and any dissolved particles back into the initial flask-the miniature sea
after one week miller found a variety of organic compounds in the solution including some amino acids