Chapter 12: Processes of evolution

1. Chapter 12: Processes of evolution Part 1

2. Making Waves in the Gene Pool • A group of individuals of the same species in a specified area is a population. • Individuals in a population have similar traits because they have similar genes (Genotype gives rise to phenotype). • All of the genes of a population make up the gene pool (or pool of genetic resources) of a population.

3. Making Waves in the Gene Pool • Even though all of the individuals in a population have similar genes, their genes are not identical. • Therefore, every trait in a population varies a little bit in the individuals of the population. • Different alleles (or forms) of genes are the reason behind this variation. • A trait with only two alleles has only two forms (or morphs) and is called dimorphic. • A trait with more than two alleles and therefore, forms or morphs, is called polymorphic. • When traits are determined by many genes (and different forms of each of these many genes), it will vary continuously among individuals in the population (like height or weight in humans) due to interaction among the genes. • Variations also exist due to the influence of environmental factors.

4. Making Waves in the Gene Pool • Add the fact that there are many processes that introduce variations in traits among individuals (see the next slide) and you see just how unlikely it is that another individual has or will ever have the exact genetic makeup and/or combination of variations of traits as you!!

5. Sources of Variation of Traits • Genetic Event Effect • Mutation Source of new alleles • Crossing over (Meiosis I) Introduces new combinations of alleles into chromosomes • Independent Assortment (Meiosis I) Mixes maternal and paternal chromosomes • Fertilization Combines alleles from two parents • Changes in chromosome number/structure Transposition, duplication, loss of chromosomes

6. Mutations • Mutation is the original source of new alleles and is the raw material of evolution. • Mutations are rare events, are spontaneous, and are unpredictable, so we cannot predict when a mutation will occur, where in the DNA it will occur, or which individual it will occur in. • But we can predict the average mutation rate of a species, which is the probability that a mutation will occur . • The average mutation rate in humans is 175 mutations per person per generation.

7. Mutations • A mutation that drastically alters the phenotype of an individual is called a lethal mutation because it results in death. (ex. Collagen mutation) • A mutation that does not affect survival or reproduction is called a neutral mutation. (ex. Earlobes) • Natural selection does act upon neutral mutations since they do not affect survival or reproduction. • This is the reason their accumulation can be used to determine the relatedness of organisms when looking at their DNA sequences.

8. Mutations • Sometimes, a change in the environment can favor a mutation that had previously been neutral or even harmful. • When this happens, natural selection increases the frequency of this mutated form of the gene in the gene pool of a population over time.

9. Allele Frequencies • Allele frequency is the abundance of an allele in a population. • Change in allele frequency is the same thing as change in a line of descent- EVOLUTION. • Microevolution refers to evolution within a population or species.

10. Allele Frequencies • We can determine how much a population has evolved by considering a theoretical genetic equilibrium, called Hardy-Weinberg equilibrium. • Remember, this is a theoretical equilibrium, it is never actually present in a population. • This equilibrium is what would would be true if NO EVOLUTION ever happened in a population. • Since all populations evolve, no real world population would ever be in Hardy-Weinberg equilibrium. • This is just a theoretical control population that we can compare real world populations to in order to determine just how much a population is evolving.

11. Allele Frequencies • In order for a population to be in Hardy-Weinberg equilibrium, five conditions must be met in order for there to be no evolution occurring: • 1) very large population • 2) no emigration or immigration • 3) no mutations • 4) random mating • 5) no natural selection • Since all five condition are never met by a real world population, real world populations are never in Hardy-Weinberg equilibrium; that’s why this is a theoretical equilibrium.

12. Hardy-Weinberg Equilibrium

13. Condition 5: No Natural Selection • Natural selection does occur in natural populations. • With natural selection, there is differential survival and reproduction of individuals in a population so that allele frequencies change over many generations. • We observe different patterns of selection depending upon selection pressures in the environment and the organisms involved. • There are three types of natural selection: • 1) directional selection • 2) stabilizing selection • 3) disruptive selection

14. Condition 5: No Natural Selection • Directional selection: allele frequencies shift in a consistent direction so that forms at one end of a range of phenotypic variation become more common. • In other words, out of a range of phenotypes, from one extreme to another, one of the extremes becomes the most common.Examples: peppered moths, rock pocket mice, warfarin-resistant rats, antibiotic resistant bacteria

15. Condition 5: No Natural Selection • Stabilizing selection: intermediate form becomes more frequent than the two extremes out of a range of possible phenotypes. • Examples: body weight of weaver birds

16. Condition 5: No Natural Selection • Disruptive selection: both extreme forms of a trait become more frequent, while the intermediate form is selected against • Example: black-bellied seedcracker (finches)

17. Condition 4: (Non) Random Mating • Natural selection occurs not only as a result of interactions between species and their environment, but also as a result of competition. • For example, some traits, though energetically costly and attractive to predators, persist in a population due to sexual selection. • In sexual selection, the most adaptive forms of a trait are the ones that help individuals defeat same-sex rivals for mates or are the ones most attractive to the opposite sex. • In sexual selection, the genetic winners are those that out-reproduce others in the population because hey are better at attracting a mate. • By choosing among mates, a male or female acts as a selective agent on its own species and, therefore, mating is not random.

18. Condition 4: (Non) Random Mating • For example, one sex of a species may “shop” for a mate among individuals of the opposite sex for those that display specific cues such as more colorful, larger, and more flashy body parts. • Even though these body parts may actually hinder the animal in escaping predators and may even attract predators, it also implies health and vigor to the opposite sex, which implies that its offspring will also be healthy and more vigorous. • Therefore, the individuals with these traits, even though they may act as a “handicap” in some aspects, actually results in these individuals producing more offspring, passing the genes for these traits on to the next generation and increasing the frequency of the alleles for these traits over generations.

19. Condition 4: (Non) Random Mating • Example: stalk-eyed flies, eyespots on male peacocks

20. Homework • On the next two slides you will find three scenarios and three graphs. • For each scenario, indicate whether stabilizing, directional, or disruptive selection is at work and explain why. • Indicate whether each graph (1, 2, and 3) represents stabilizing, directional, or disruptive selection and explain why.

21. Homework • In a population of deer, solid brown deer and spotted white/brown deer (intermediate phenotype) are camouflaged, while solid white deer are almost always seen and eaten by predators. • In the human population, very short (dwarfism) and very tall (gigantism) people suffer from genetic disorders and have shortened life spans, while average height people live longer life spans. • In a population of chickens, chickens that lay eggs with a medium thick shell are more desirable and are bred by humans because these eggs don’t break as easily. (Chickens that lay very thin shelled eggs aren’t bred because the eggs break very easily and those that lay very thick shelled aren’t bred because they are too hard to break).

22. Disruptive, Directional, or Stabilizing Selection? Homework 1 2 3

23. Homework • Locate and explain another example of sexual selection that results in nonrandom mating.