Chapter 16 Population Genetics and Speciation

1. Chapter 16Population Genetics and Speciation Section 1: Genetic Equilibrium Section 2: Disruption of Genetic Equilibrium Section 3: Formation of Species Leopard Seal, Antarctica

2. Section 1: Genetic Equilibrium Population Genetics(Microevolution) Study of evolution from a genetic point of view • Every population has some genetic variation that influences fitness - Evolution is potentially a continuing process in all populations • Changes in selective factors in the environment will almost always be met by evolutionary responses - leads to shifts in the frequencies of genotypes in a population • Rapid changes will often exceed the capacity of a population to respond by evolution - decline could lead to extinction

3. Darwin’s finches • Peter and Rosemary Grant • The Galapagos Islands: normally dry • El Nino: increase rainfall, vegetation flourishes • La Nina: periods of drought • Reproductive success and survival of individuals differed between El Nino and La Nina years • Caused dramatic evolutionary change

4. Medium ground finch • Seeds, cracks with beak • La Nina (drought): • amount of seeds dropped, seeds became tougher • population dropped • 1400 in 1975 to 200 at end of 1977 • larger beaks could crack the larger seeds and survived better than those with smaller beaks • Average beak size increased

6. Within a population individuals vary in observable traits. Traits cover a range that can be represented by a bell curve

7. El Nino (wet) 1983 • Small seeds in abundance • Those with smaller beaks handled the smaller seeds better, able to survive, produce more offspring than those with larger beaks • Average beak size returned to a lower value

8. Population genetics studies the ways in which populations respond to such selective pressures with changes in allele frequencies

9. Genetic Variation • Mutation: random change in a gene, passed on to offspring • Recombination: reshuffling of genes • Independent assortment • Crossing-over • Random Pairing of gametes

10. Gene Pool: total genetic information available in a population When mate at random: all combinations of different alleles are possible Peccaries are small, tough relatives of the modern pig, whose lineage diverged about 40 million years ago. They live in southern Texas, Arizona, and New Mexico. The forces of evolution shape and change the composition of this gene pool and thus the nature of the population.

11. Allele Frequency = # of a certain allele / total number of alleles B: Long bristles on the bodies b: short bristles 15 Individual peccaries in the population 30 alleles If 6 alleles in this population are b, and 24 are B Then the frequencies of these alleles are: 6/30 of the genes in the gene pool are b – a frequency of 0.2 24/30 of the gene in the gene pool are B – a frequency of 0.8

12. Phenotype Frequency = # of individuals with a particular phenotype / total number of individuals in the population

13. Frequencies of alleles and genotypes remain constant from generation to generation in a population (No evolutionary change occurs through the process of sexual reproduction) Changes can result only from the action of additional forces on the gene pool of a population Ideal hypothetical population that is not evolving Set of Assumptions No net mutations occur Individuals neither enter nor leave the population The population is large Individuals mate randomly Selection does not occur *any exception to these can result in evolution Hardy-Weinberg Equilibrium

14. Section 2: Disruption of Genetic Equilibrium No Net Mutations Occur To be at Equilibrium • Spontaneous mutations occur constantly at very low rates • If exposed to mutagens, rates can increase significantly • Mutations can produce totally new alleles for a trait

15. To be at Equilibrium Individuals can neither enter nor leave a population • Size of the population must remain constant • If individuals move, genes move too Immigration: movement into a population Emigration: movement out of a population Gene flow: process of genes moving from one population to another

16. To be at Equilibrium Large Population Genetic Drift: phenomenon by which allele frequencies in a population change as a result of random events, or chance

17. To be at Equilibrium Individuals Mate Randomly Nonrandom Mating • Influenced by geographic proximity • Select a mate with similar traits:Assortative Mating • Sexual selection: females tend to choose males based on certain traits (Planet Earth: Jungles, Birds of Paradise)

18. Birds of Paradise-Planet Earth

19. To be at Equilibrium No Natural Selection Ongoing process in nature Some members of a population are more likely than others to survive and reproduce and thus contribute their genes to the next generation

20. Stabilizing Selection: individuals with the average form of a trait have the highest fitness

21. Disruptive Selection: individuals with either extreme variation of a trait have greater fitness than those with the average form of that trait

22. Directional Selection: individuals that display a more extreme form of a trait have a greater fitness than those with an average form

23. Black dots represent individuals that die before passing on their genes.

24. Hardy-Weinberg Conditions Animations http://nhscience.lonestar.edu/biol/hwe.html http://www.accessexcellence.org/AE/AEPC/WWC/1995/hardyweinberg.php

25. Hardy-Weinberg Equation Used to discover the probable genotype frequencies in a population and to track their changes from one generation to the next p2 + 2pq + q2 = 1 p + q = 1 p = frequency of the dominant allele (A) q = frequency of the recessive allele(a) p2 is the predicted frequency of homozygous dominant people (AA) 2pq is the predicted frequency of heterozygous people (Aa) q2 is the predicted frequency of homozygous recessive people (aa)

26. Section 3: Formation of Species Speciation: process of species formation Species: a single kind of organism. Members are morphologically similar (external structure and appearance) and can interbreed to produce fertile offspring. (biological species concept)

27. How do species give rise to other ones? 1. Geographic isolation: physical separation of members of a population.

28. No longer experience gene flow, so the gene pools of each separate population may begin to differ due to genetic drift, mutations and natural selection Allopatric Speciation happens when species arise as a result of geographic isolation “different homeland”

29. Reproductive Isolation: results from barriers to successful breeding between population groups in the same area. • Pre-zygotic: occurs before fertilization • active at different seasons or times of day • Post-zygotic: occurs after fertilization • zygote dies • F1 hybrids have reduced fertility or sterility • Example: mule from horse and donkey

30. Sympatric Speciation: when two subpopulations become reproductively isolated within the same geographic area 200 years ago, ancestors of apple maggot flies laid their eggs only on hawthorns. Today, these flies lay eggs on hawthorns and domestic apples. Females choose to lay eggs on fruit they grew up in and males look for mates on the fruit they grew up on. Gene flow between parts of the population that mate on different types of fruit is reduced. Led to new species

31. Rates of Speciation Happens at a regular, gradual rate Sudden, rapid change followed by little to no change (few million yrs) (few thousand years)