Evolution of Populations: Genetic Variation and Natural Selection

1. CHAPTER 16 EVOLUTION OF POPULATIONS A. Darwin’s Ideas revisited - it was more than 50 years after Darwin started to develop his theory of evolution before biologists could determine how evolution takes place - about 1910, biologists realized that genes carry the information that determine traits - with this knowledge, they combined Mendel’s work on inheritance with Darwin’s work on evolutionary theory - today, we combine genetics, molecular biology, and evolutionary theory to explain how inheritable variation occurs and how natural selection operated on that variation

2. B. Gene Pools - to study evolution, biologists focus on one particular population - populations share a common group of genes called a gene pool - gene pools contain the combined genetic information of all the members of a particular population - they contain two or more alleles for each inheritable trait - the relative frequency of an allele is the number of times that allele occurs in a gene pool compared with the number of times other alleles occur

4. C. Sources of Genetic Variation Two main sources include: 1. mutations - any change in DNA sequence - occur due to mistakes in replication of DNA or from radiation or chemicals in the environment - may be limited to one or a few DNA bases or may affect long segments of a chromosome - do not always affect an organism’s phenotype, but many do - those that do may or may not affect an organism’s fitness, or its ability to survive and reproduce

5. 2. gene shuffling - most inheritable differences result from gene shuffling that occurs during the production of gametes - because each chromosome (all 23 pairs) move independently during meiosis, 8.4 million different combinations of genes can occur -crossing over further increases the number of different genotypes that can occur in offspring, making sexual reproduction a major source of variation within populations - sexual reproduction produces different phenotypes, but does not change the relative frequency of alleles in a population

6. Gene Shuffling

7. D. Single-gene and polygenic traits - the number of phenotypes produced for a given trait depends on how many genes control the trait a. Single-gene trait (Ex. widow’s peak) - controlled by a single gene with two alleles and only two outcomes; the widow’s peak or no widow’s peak

8. b. polygenic traits - most traits are controlled by two or more genes - each gene has two or more alleles - therefore, one polygenic trait can have many possible genotypes and thus, even more phenotypes Ex. height - a bell curve is typical of polygenic traits (shows normal distribution)

9. 16-2 Evolution as Genetic Change A. Natural Selection on Single-gene traits - leads to changes in allele frequencies and thus, to evolution See Fig. 16-5 pg. 397 Ex. organisms of one color may produce fewer offspring of other colors

13. B. Natural selection on polygenic traits - more complex - fitness varies from one end of the bell curve to the other; where fitness varies, natural selection can act - distribution of phenotypes are affected in 3 ways:

14. 1. directional selection - occurs when individuals at one end of the curve have higher fitness than individuals in the middle or at the other end - causes the curve to move as the character trait changes - evolution causes an increase in the number of organisms with the trait at one end of the curve (See Fig 16-6)

15. 2. stabilizing selection - occurs when organisms at the center of the curve have higher fitness than those at either end - keeps the curve at its current position, but narrows the overall graph (See Fig 16-7)

16. 3. disruptive selection - occurs when organisms at the upper and lower ends of the curve have higher fitness than those in the middle - selection acts more strongly against organisms of an intermediate type - if natural selection pressure is strong enough and lasts long enough, it can split the graph in two creating two distinct phenotypes (See Fig 16-8)

17. C. Genetic Drift - a random change in allele frequency that occurs in small populations - it occurs when: individuals that carry a particular allele may have more descendants than other individuals; may cause an allele to become more common in a population - may occur when groups of individuals colonize a new habitat; they may carry alleles in different relative frequencies than the larger populations they came from - the new population may become genetically different from the parent population (See Fig 16-9)

18. - the founder effect is a change in allele frequency that results from the migration of a small subgroup of a population Ex. fruit flies on the Hawaiian Islands

19. D. Evolution versus Genetic Frequency Are there any conditions under which evolution will not occur? The Hardy-Weinberg Principle - states that allele frequencies in a population will remain constant unless one or more factors cause those frequencies to change - the situation in which allele frequencies remain the same is called genetic equilibrium

20. Five conditions are necessary to maintain genetic equilibrium from generation to generation: 1. random mating - all members of a population have an equal opportunity to produce offspring and an equal chance of passing on its alleles -in natural populations, it is rarely random Ex. lions and wolves select mates on certain traits such as size or strength making genes for these traits not in equilibrium and under strong selection pressure

21. 2. large populations - important in maintaining genetic equilibrium because genetic drift has less effect on large populations 3. No movement into or out of the population -movement would bring new alleles into a population - the population gene pool must be kept together and kept separate from that of other populations

22. 4. No mutations -mutations can result in new alleles being introduced into the population and allele frequencies will change 5. No natural selection -all genotypes must have equal probabilities of survival and reproduction with no phenotype having a selective advantage over another

23. 16-3 The Process of Speciation -the formation of a new species A. Isolating Mechanisms - as new species evolve, populations become reproductively isolated from one another - reproductive isolation occurs when two populations cannot interbreed and produce fertile offspring - the populations develop separate gene pools and respond to natural selection or genetic drift as separate units

24. Ways in which reproductive isolation occurs: 1. behavioral isolation - occurs when two populations are capable of interbreeding but have differences in courtship rituals or other types of behavior Ex. Eastern and Western meadowlarks Fig 16-11 Western eastern - they have different mating songs even though their ranges overlap

25. 2. geographic isolation - two populations are separated by geographic barriers such as rivers, mountains, or bodies of water (See Fig 16-12 the Abert squirrel) - does not guarantee the formation of a new species - separated populations may mix due to lakes that become linked or land bridges that form temporarily between islands

26. 3. temporal isolation - two or more species reproduce at different times Ex. Closely related species of fireflies mate at different times of night or different species of plants have different flowering seasons

27. B. Testing Natural Selection in Nature - these processes can be observed in nature as was evidenced by Darwin and his observation of finches in the Galapogos Islands (See Fig 16-13) - tested finally by Peter and Rosemary Grant studying non hibernating finches

28. 1. Variation - the Grants identified and measured as many birds as possible on a single island and found great variation among inheritable traits 2. Natural Selection -individual birds with different beak sizes had different chances of survival during a drought - when food was scarce, finches with the largest beaks survived - beak size also played a role in mating behavior with big-beaked birds mating with other big-beaked birds - this resulted in an increase in beak size in that population (directional selection)

29. 3. Rapid Evolution - by documenting natural selection in the wild, the Grants provided evidence of the process of evolution; the beak size got bigger in the next generation of finches - they found natural selection takes place frequently and sometimes rapidly C. Speciation in Darwin’s Finches - speciation occurred in this way: 1. Founders arrived - a few finches traveled from S. America to one of the islands, survived, and reproduced

30. 2. Separation of populations - some birds crossed to a second island and the two populations no longer shared a gene pool 3. Changes in the gene pool - seed sizes on the second island favored birds with bigger beaks, therefore, the population on the second island evolved into one with bigger beaks

31. 4. Reproductive isolation - a few big-beaked birds crossed back to the first island and would not mate with the birds there; now there were separate gene pools and two separate species 5. Ecological Competition - as the two species on the first island complete for seeds, they continue to evolve. Some of the original birds from the second island may migrate to a third island 6. Continued Evolution -the process continued leading to the formation of 13 species of finches on the Galapogos Islands (See Fig 16-17)