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Micro vs Macro Evolution • Microevolution – The process of change in allele frequencies of a population. • Macroevolution – The process by which species change over long periods of time. I. REPRODUCTIVE ISOLATING MECHANISMS In order for one population to become very different from another, they must be reproductively isolated. This means that there will no longer be a free exchange of alleles between the two populations. There are Prezygotic mechanisms that prevent mating or fertilization and Postzygotic mechanisms that prevent the development of a zygote. 1. PREZYGOTIC MECHANISMS a. Ecological Isolation (habitat) Two populations do not exchange alleles with each other because they are in different geographic places or at different places within the same ecosystem. eg. cheetah and tiger do not meet because one is in Africa and one in Asia cheetah range tiger range eg. Asiatic lion and Bengal tiger are both in northern India, but lion is on savannah and tiger in forest b. Temporal Isolation (seasonal, time) Two populations do not exchange alleles because they are only available to exchange alleles at different times of year or even of the day. eg. morning glory opens its flower at sunrise; cactus opens its flowers at sunset eg. purple finch mates in June so babies have access to lots of berry seeds goldfinch mate in August so that babies have access to lots of thistle seeds c. Behavioural isolation Two populations do not exchange alleles because they do not respond to each others mating rituals. eg. male grey crickets rub legs at 25 times a second male black crickets at 45 times a second the females of each species only responds to the sound made by the male of that same species d. Mechanical Isolation Two populations do not exchange alleles because of some physical barrier that prevents this. eg. many insects have modifications on their exoskeletons such that the male and female parts are a perfect 'lock-and-key' fit eg. orchids are shaped so that only certain beetles can reach the nectar and therefore pick up the pollen, that beetle will then go to another orchid of the same type to deposit the pollen e. Gametic isolation Two populations exchange sperm and eggs but chemical markers prevent the eggs from being fertilized by the 'wrong' sperm and so no alleles are exchanged. eg. wind blows the pollen of corn onto the flowers of milkweed, but the pollen can not grow down through the stigma because it does not possess the correct enzyme eg. clams and fish both shed eggs and sperm into the same water, but the clam sperm can not penetrate the fish eggs and vice versa, due to enzymes not being able to eat through zona pellucida (thick outer membrane around the ovum) 2. POST-ZYGOTIC ISOLATING MECHANISMS a. Zygotic mortality - even though the zygote is created, it fails to develop to maturity. b. Hybrid inviability- even though the hybrid is born, it does not live long or is not as healthy. c. Hybrid infertility - even though the hybrid is healthy and vigourous, it may not be able to reproduce. eg. donkey X horse mule eg. lion X tiger liger healthy and strong but mules are sterile healthy & strong; but ligers are sterile 3. SPECIATION When two populations become completely isolated and no longer exchange alleles, they are said to have formed separate species. Species – groups of individuals who are able to reproduce healthy, fertile offspring. Ex. Horse + donkey mule (not fertile) Thus parents were not from the same species. a. Allopatric speciation - this is a situation in which the two populations are geographically isolated prior to them becoming separate species. the population is isolated and then the changes occur due to new environments b. Sympatric speciation - this is a situation where the two populations remain in physical contact with each other but still stop exchanging alleles and become separate species. although not separated, the population undergoes different selection pressures and changes occur over time in allele frequencies • Allopatric Speciation example – https://www.youtube.com/watch?v=cSgulsydsQU Progression to Speciation • Allopatric or Sympatric factors • reproductive isolation (inability to mate and/or produce viable offspring) • Speciation (formation of a new species)