SGN 27 The Origin of Species Macroevolution describes
SGN 27 The Origin of Species Macroevolution describes the origination of new taxonomic groups (vs microevolution, which deals with change in allele frequency, and frequencies of genotype and phenotype) In macroevolution we see the emergence of new species and higher taxonomic groups What is a species??
What is a species? Biological species concept the largest set of populations in which genetic exchange is theoretically possible; species are reproductively isolated from other species, separated by reproductive barriers Each species is isolated by biological factors that prevent interbreeding How is a species different from a
population? Different species are typically sequestered genetically by more than one postzygotic or prezygotic barriers, which make it impossible to reproduce together, even if they occupy the same general area (so simple geographic separation is not prezygotic or postzygotic barrier, although as we will see it might lead to them forming) Barriers make successful fertilization or reproduction impossible Prezygotic before fertilization
Postzygotic after fertilization a Prezygotic barriers impede courtship, mating and related events (but not simply physical isolation) Types of barriers habitat, behavior, temporal, mechanical, gametic Not simply physical isolation, as cause of physical
isolation can be removed and two group as will once again interbreed PreZ and PostZ barriers cannot be removed two groups are different species Anole lizards Temporal, mechanical, gametic
Diurnal Nocturnal Postzygotic barriers results of fertilization (hybrid) will not thrive or
be fertile Reduced hybrid viability typically embryo not viable, or offspring are frail Reduced hybrid fertility cannot back breed, or meiosis malfunctions so
gametes not viable or offspring of hybrids not viable Hybrid breakdown offspring of hybrids have reduced fertility or viability Limitations of BSC
In many instances (extinct and many existing groups) information about interbreeding is lacking BSC inapplicable to asexually reproducing groups Is geographic isolation enough to consider nonbreeding groups different species? Grolar bear Alternative concepts of species
But problems with each Morphological (a species has a unique set of adaptations) species concept Genealogical/Phylogenetic (species has unique genetic history, based on genetic comparisons) species concept These happy face spiders look different, but since
they can interbreed, they are considered the same species: Theridion grallator. Regardless of how we define species, different species, separated by genetic barriers, exist by all three definitions. How do the barriers arise? What processes bring about speciation?
2 Primary modes of speciation how do barriers arise? Allopatric different country Sympatric same country Allopatric speciation occurs due to populations diverging into
geographically different ranges and therefore exposure to different environmental pressures Ring species (distributed around a geographic barrier) shows population at various stages in their gradual divergence from common ancestors Island species adaptive radiation (see below) is common
Adaptive radiation occurs because of allopatric speciation Many species (existing in different environments) might evolve from a common ancestor migrating into those different environments In regard to AS, geographic isolation alone may create separate populations but is not in itself a reproductive barrier defining species; natural selection under different conditions, genetic drift and mutation have to create genetic barriers Vicariance event or process that erects barrier to gene flow between two
populations and establishes them as different species Sympatric speciation a new species originates in the geographic midst of the parent species Genetic events can produce barriers to separate populations Mistakes in meiosis can produce polyploidy (extra sets of chromosomes) Autopolyploidy in plants and possibly other types of living things; more than 2 chromosome sets contributed by parents of the
same species Allopolyploidy two different species reproduce to make a polyploid hybrid Hybrid typically sterile but often vigorous and able to reproduce using asexual propagation; certain events can restore fertility in hybrids, which can then self fertilize or reproduce with other of the same type of polyploid with restored fertility Many important agricultural plants are polyploids (wheat, cotton, potatoes, tobacco, oats, etc.); when successful seems to increase size and vigor of plant and/or grain Sympatric speciation in animals occurs occasionally when groups take
on different behaviors Groups pursue different food sources in an environment, which leads to divergence of populations Nonrandom mating causes subgroups to emerge, and establishes reproductive barrier Tempo of speciation Gradualism model slow accumulation of many small changes leads to new species
Punctuated Equilibrium model species undergo most of their morphological modification as they first bud off from parent species, and then change little Initial species forming changes may take 10s of thousands of years, in a species that will then exist for millions of years unchanged due to stabilizing selection Probable that both can lead to speciation, as could a combination of the two From microevolution to speciation to macroevolution Microevolution change in populations allele frequency (nat sel, gen drift, etc.)
Speciation enough microevolution to make new species (allopatric or sympatric speciation erects reproductive barriers) Macroevolution level of change that is evident over the time scale of the fossil record; development of new taxonomic groups Macroevolution occurs when genetic divergence leads to reproductive barriers/isolation, and over vast tracts of time divergent change leads to the complexity and diversity we see in life today Vicariance
event or process that erects barrier to gene flow between two populations and establishes them as different species
What kind of events involving microevolution (change in alleles) lead to macroevolution (emergence of new species with significant differences)? What events lead to the emergence of evolutionary novelties? Gradual change in allopatric speciation Example: Older structures modified in increments Rapid change due to mutation Example: Evo-Devo
Older structures modified in increments to make more modern structures Structures might be refined to become better at the same thing; example - the
eye Structures might be coopted for another purpose (exaptations); example honeycombed bones and feathers of reptiles/birds Evo-Devo: genes that control development play a major role in evolution
Genes that program development control the rate, timing and spatial pattern of change in an organisms form as it is transfigured from a zygote to an adult Allometric growth growth rates of different parts are unequal and disproportionate, which gives body its specific shape Heterochrony evolutionary change in the rate or timing of developmental events; evolution of morphology that arises by a modification in allometric growth Can affect timing of reproductive maturity and other important developmental events (paedomorphosis or neotony - the
retention of juvenile features in the adult animal; the sexual maturity of an animal while it is still in a mainly larval state) Axolotl- retains juvenile characteristics but becomes reproductively mature Changes in genes that control placement and spatial arrangement of body parts (homeotic genes) can result in macroevolution
Hox genes and Evo-Devo Differential expression of the same gene (turned on in different tissue at different concentrations for different length of time) produces the forelimb in all vertebrates
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