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9/7/2014 Changing Environments Accelerate Evolution • Evolution is always occurring, just slowly – Bacteria and viruses change all the time, so we’re constantly coming up with new antibiotics, antivirals, and vaccines to treat them Speciation • New habitats open up opportunities for evolution • Leads to adaptive radiations – where many groups evolve from a single ancestor • i.e. after the fall of dinosaurs – many mammal groups evolved Chapter 24 Pg. 488 - 506 Species Allopatric Speciation • A species is a group of individuals capable of interbreeding and producing fertile offspring – Also defined in terms of reproductive isolation • Speciation is the formation of new species – anything that fragments a population and isolates small groups of individuals may cause it. – It occurs by… • Begins when a population is divided by a geographic barrier (physical separation) – Interbreeding is prevented – Common barriers include mountain ranges or rivers, but can also include regions without vital resources (no water, no food, volcanic lava, paved road, etc.) • Once isolated, gene frequencies in the two populations can diverge due to natural selection (environments may be slightly different), mutation, or genetic drift – If gene pools diverge enough, interbreeding will not occur if the barrier is removed – A reproductive barrier is formed – New species are formed • Allopatric speciation • Sympatric speciation • Adaptive radiation Basically: Geographic barrier isolation Reproductive barriers New species Sympatric Speciation Reproductive Differential evolution • Formation of new species without a geographic barrier; develop a genetic difference preventing them from interbreeding • May occur via… – – – – – – – Balanced polymorphism Polyploidy Hybridization Habitat isolation Behavioral isolation Temporal isolation Reproductive isolation 1 9/7/2014 Balanced Polymorphism • When among subpopulations, it may lead to speciation • Example: population of insects possess polymorphism for color; each color provides camouflage to different substrate – no camo means the insect is eaten. – Under these circumstances, only insects with the same color can associate and mate. – Thus, similarly colored insects are reproductively isolated from other subpopulations, and their gene pools diverge as an allopatric speciation. Hybridization • Occurs when two distinctly different forms of a species (or closely related species that are normally reproductively isolated) mate and produce progeny along a geographic boundary called a hybrid zone – May cause reduced hybrid viability (development may not occur), reduced hybrid fertility (hybrids may be sterile), and/or hybrid breakdown (when hybrids mate, offspring may be weak or sterile) • Sometimes the genetic variation of the hybrids is greater than either parent – It also permits the population of hybrids to evolve adaptations to environmental conditions in the hybrid zone beyond the range of either parent – Exposed to different selection pressures, hybrids eventually diverge from both parent populations Behavioral Isolation • Example: sticklebacks, small saltwater fish, have elaborate mating behavior. At breeding time, in response to increased sunlight, the males change color and develop a red underbelly. He builds a nest and courts the female with a dance that involves both partners. If either partner fails in any step of the mating dance, no mating occurs and no young are produced. • Example: male fireflies signal to females of their kind by blinking the lights on their tails in a particular pattern. Females respond only to characteristics of their own species, flashing back to attract males. If, for any reason, the female does not respond with the correct pattern, no mating occurs. The two animals become isolated from each other. Polyploidy • Possession of more than the normal 2 sets of chromosomes found in diploid (2n) cells • Often occurs in plants (only occasionally animals) – 3n (triploid) – 4n (tetraploid) – Higher numbers are also found • Occurs as a result of nondisjunction of all chromosomes during meiosis – produce two viable diploid gametes and two sterile gametes with no chromosomes – Tetraploid zygote = diploid sperm + diploid egg • Since normal meiosis in the tetraploid will continue to produce diploid gametes, reproductive isolation with other individuals in the population (and thus speciation) occurs immediately in a single generation Habitat Isolation • Two organisms live in the same area but rarely encounter each other • Example: two species of one genus of snake can be found in the same geographic area, but one inhabits water while the other is mainly terrestrial Temporal Isolation • Temporal refers to time • Example: a flowering plant colonizes a region with areas that are warm and sunny and areas that are cool and shady. Flowers in warmer regions become sexually mature sooner than flowers in cooler areas. This separates flowers in the two different environments into two separate populations. 2 9/7/2014 Reproductive Isolation • Closely related species may be unable to mate because of a variety of reasons – Things that prevent mating are called prezygotic barriers (mechanical isolation) • Differences in structural of genitalia may prevent insemination • Difference in flower shape may prevent pollination • Small male dog and large female dog may have difficulty mating Reproductive isolation: • Reproductive isolation is the existence of biological factors (barriers) that impede members of two species from producing viable, fertile offspring • Two types: 1) Prezygotic Isolation – prevent mating 2) Postzygotic Isolation- prevent a fertilized egg from developing a fertile adult – Things that prevent the production of fertile offspring once mating has occurred are called postzygotic barriers (gametic isolation) • Particular zygote may not be viable Prezygotic Isolation: Postzygotic Isolation: Prezyotic barriers include: 1. Temporal Isolation: species may breed at different times of day; different seasons or different years (ex. wood frog mate in march, leopard frog mate in april) 2. Behavior Isolation: use of signals or behaviors to attract mates that are unique to the species (ex. visual like firefly, auditory like crickets, pheromones) 3. Mechanical Isolation: species reproductive organs are anatomically incompatible (ex. shells of two species of snails, one has moving inward spiral, other has outward spiral) 4. Gamete/Genetic Isolation: gametes unable to fuse and form a gamete (sperm can’t survive in female reproductive tract) 5. Habitat Isolation: two species that may live in same geographical region but not in same habitat so they can’t mate aka competitive exclusion principle, each with its one niche (ex. one species of garter snakes live on land other liv in water( Postzygotic barriers include: 1. Reduced hybrid viability – when zygote is formed, genetic incompatibility caused development to cease 2. Reduced hybrid fertility – viable offspring unable to reproduce; horse (64 chromosomes) and a donkey (62 chromosomes) make a mule (63 chromosomes) 3. Hybrid breakdown – two species mate, produce viable, fertile hybrids but when hybrids mate their offspring are weak or sterile Adaptive Radiation Adaptive Radiation Examples • Relatively rapid evolution of many species from a single ancestor • Occurs when ancestral species is introduced to an area where diverse geographic or ecological conditions are available for colonization • Variants of ancestral species diverge as populations specialize for each set of conditions • The marsupials of Australia began with the colonization and subsequent adaptive radiation of a single ancestral species. • The 14 species of Darwin’s finches on the Galapagos Islands evolved from a single ancestral South American mainland species. • Adaptive radiations occurred after each of the 5 big mass extinctions. With up to 90% of species going extinct, the periods following extinctions provided numerous ecological opportunities for species to colonize. – Colonization was followed by competition, which, in turn, promoted speciation. 3 9/7/2014 Parapatric Speciation: • Occurs where populations share a common border; the borders are open to gene flow. • Hybrid zone – adjoining populations meet and reproduce hybrid offspring Rate of Speciation • Fossil record lacks transitional forms between two species • Is the fossil record incomplete or accurately reflects evolution as it occurs • Punctuated Equilibrium- long periods of stasis, punctuated by periods of rapid speciation, trigger by changes in environment • Gradualism- continuous evolution over long periods, gradually accumulate adaptations Extinction •Is the norm •Most species go extinct- but new ones survive •There have been at least 5 major mass extinctions 4