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Chapter 17

Chapter 17 . Microevolutionary Process. Where we left off. Darwin had come up with a mechanism for evolution called natural selection. It seemed to explain what had happened over long periods of time, based on the fossil evidence

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Chapter 17

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  1. Chapter 17 Microevolutionary Process

  2. Where we left off • Darwin had come up with a mechanism for evolution called natural selection. • It seemed to explain what had happened over long periods of time, based on the fossil evidence • He had inferred what it was working in populations but couldn’t prove it. • Don’t forget… • In the adult populations there is variation, which leads to unequal distribution of resources and uneven reproduction

  3. Microevolution looks at a single population • Evolution acts on populations • A population is a group of individuals that live in the same area and reproduce among each other • The variance in the adults of the population, as well as the environment, provide selection pressure • A population is smallest unit that can evolve • Ex. ~ a population of insects on one farm in Minnesota can evolve tolerance to insecticide but the insects in CA have not (for now)

  4. Quantifying Evolution with the Gene Pool… • Evolving is now defined as changes in a populations gene pool over time • A gene pool is the alleles available within a population • They are the only alleles an individual has to choose from unless in breeds outside of its population • If a population is NOT evolving then the gene pool remains constant

  5. How do you know when populations are evolving? • First you need to quantify the gene pool • Hardy-Weinberg Equilibrium was twice independently derived to show IF there is a difference. • Then look at two generations gene pools and see if there are SIGNIFICANTdifferences between the frequencies • p2 + 2pq +q2 =1

  6. Start with the assumption that ‘normal’ populations do not evolve • ** If these things are happening the populations is not evolving… how often do you think that happens?

  7. There are lots of reasons why equilibrium may not be met • Genetic Drift: (by chance) • Bottleneck effect • Founder effect: (new location) • Gene flow: (move to or from) • Mutation ** These things occurring cause adaptive changes… evolution

  8. Depending on success, selection can work in different ways

  9. Difference in sexes can also be selected • Since they put more effort and time into making offspring, females are the choosey ones. • Consequently males have to do something to attract the females… this leads to sexual dimorphism (males and females look different) • Male deer have antlers, peacocks have big tail feathers, etc. • The ‘prettier’ male gets to mate more often.

  10. So if the point is success, why is there still variation? • While survival is putting a pressure on one allele, there are other factors that favor variation. • If neither is too bad, having two alleles helps you change the phenotype on only 1 generation! • Heterozygote advantage is when having both types of alleles is an advantage… like sickle-cell anemia • Frequency dependent selection: this is a predation effect (like trying to find your keys). The more of one color/pattern the faster they are recognized by predators.

  11. Speciation (The rest of Ch17 in one lecture!)

  12. So what is a species • A species is a group of populations with the potential to interbreed and produce fertile offspring • This is the biological species concept and applies ONLY to organisms that reproduce sexually

  13. How do new species arise? • In order for two things to form a new species there must be something to separate them from their old species (REPRODUCTIVE BARRIER) • Can prevent fertilization (prezygotic) • Or produce non-viable offspring (postzygotic)

  14. PrezygoticBarriers 12 different species of fiddler crabs on the same beach in Panama could be distinguished by the display of waving their large cheliped, elevating the body, and moving around in their burrow

  15. Postzygotic Barriers Hybrid rice plant is in the center

  16. Which came first the chicken or the egg • Is it the reproductive barrier that makes them different species, or is it being different species that creates a barrier? • In some populations the barrier arises… like the Grand Canyon separating a species of squirrels, that once separated diverged from each other and are now two separate species (allopatric speciation) • In other populations the two are living right there and a mutation separates the two groups by creating a reproductive barrier (sympatric speciation)… tends to be more common in asexual species… why do you think that is?

  17. Is evolution fast or slow • It depends on where you are sitting • For me it seems SLOW… the gradualist model says that big changes are really just the accumulation of lots of small changes… like watching the time pass on a clock with no second hand • In retrospect… based on the history of the world… evolution can be seen a quite FAST… hence the theory of punctuated equilibrium, where species change a lot in a period of 50,000 years, then hardly at all for the next 3 million. (relatively fast but still slow compared to my lifetime… so which is it??? Hmmm)

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