Chapter 16: “Population Genetics and Speciation”

Chapter 16: “Population Genetics and Speciation” Aka: Natural Selection meets Genetics

Section 16.1: Genetic Equilibrium • Line up in the front of the class from the shortest person to the tallest person. Let’s graph the frequencies of the heights in this class. • Amongst any population, there are always variations

5 4 3 2 1 Frequency 5’ 1” 2” 3” 4” 5” 6” 7” 8” 9” 10” 11” 6’ 1” 2” 3” Height

Normal distribution curve

Bell Curve • Graph showing frequencies of occurrences with the majority in the middle and curving out towards the extremes. • Gene frequencies within a population follow this curve.

Population Genetics • Study of how genetics plays a role in evolution **New definition of evolution: • it’s a gradual change in the genetic material within a population over time. • c/c with old definition: • Development of new types of organisms from pre-existing types of organisms over time

So what causes these variations in height amongst your classmates? • Variations – caused by: • Environmental factors (food availability, amount of sunlight, etc) • Mutations – gene and chromosomal • Recombinations (crossing over, independent assortment. • Random fusion of gametes - each brings their own set of genes

All the members of the same population contribute their genes into a gene pool. • If the people on the right side of the classroom during the height lineup were to all throw their genes into a gene pool, what would be the expected outcome of their offspring? • The tall trait would show up more often (phenotype frequency) thus increasing the presence of the tall gene (allele frequency).

Gene Pool 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. A gene pool is the sum of all the individual genes in a given population. A population is the smallest unit of living organisms that can undergo evolution.

Hardy and Weinberg * A mathematician and a physician who in 1908 developed a mathematical model to predict gene frequencies in future populations in an attempt to explain microevolution (change in genetic material of a population). Macroevolution (chapter 15) is change on an organismal level. • This is ONE equation they used: p + q = 1 where p = dominant allele where q = recessive allele p equals all of the alleles in individuals who are homozygous dominant (AA) and half of the alleles in people who are heterozygous (Aa) for this trait in a population. So, if 30% of a population was left handed, you could calculate what % must be right handed (70%). You would not know, however, what % was heterozygous right-handed with this equation.

Allele frequency – gene math • Amount of times a certain gene (allele) appears in a population. • If two parents are hybrids for hand preference, it’s 0.5: 0.5 (must add up to 1) • or 50:50 Rr Rr 2R and 2r • If one is a hybrid and the other is pure recessive, it’s .25:.75 or 25:75 Rr rr 1R & 3r • Let p = dominant trait (R) & q = recessive trait (r) p+q = 1

Predicting Genotype • Review – we will use the 4 o’clock flower as an example • Remember – this flower has an incomplete dominance inheritance pattern

In a population, there are 4 red four o’clock flowers and 4 pink ones (incomplete dominance) RR RR RR RRRrRrRrRr • Phenotype frequency = 4/8 Red & 4/8 Pink • or 0.5 : 0.5 • Allele frequency = R = 12/16 r = 4/16 • or 0.75 : 0.25 Let R = p and r = q p + q = 1 .75 + .25 = 1

Second generation: 5 Red, 2 pink and 1 white RR RR RR RR RRRr Rrrr Phenotype frequency = 5/8: 2/8: 1/8 or .625: .25: .125 Allele frequency = 12/16: 4/16 or .75: .25 .75 + .25 =1 **So the allele frequencies remain the same from generation to generation in a closed population because you are dipping into the same gene pool!

So what is the likelihood of two “R” genes (RR) being pulled from the gene pool? • If p = .75 • then it’s p2 = .75 x .75 = .5625 • p2 = homozygous dominant allele frequency • For two r genes, q = .25 • q2 = .25 x .25 = .0625 • q2 = homozygous recessive allele frequency

How about for a hybrid? • Frequencies of all types expected in the next generations must add up to 1.0 p + q = 1 • So 1.0 – frequency of RR – frequency of rr = frequency of Rr • 1.0 - .5625 - .0625 = 0.375 (frequency of Rr)

Hardy Weinberg Genetic Equilibrium equation (p + q)2 = 1 (p + q) (p + q) = 1 p2 + 2pq + q2 = 1 Equals the hybrid frequency Pure Recessive frequency Pure Dominant frequency p is defined as the frequency of the dominant allele and q as the frequency of the recessive allele for a trait controlled by a pair of alleles (A and a). In other words, p equals all of the alleles in individuals who are homozygous dominant (AA) and half of the alleles in people who are heterozygous (Aa) for this trait in a population.

Hardy Weinberg Equation of Genetic Equilibrium • Wilhelm Weinberg and Godfrey Hardy showed that allele frequencies inapopulation remain the same over timeunless acted upon by outside influences

Assumptions The allele frequencies of generations within a population will remain the same if: • large population - to insure no sampling error from one generation to the next • random mating - no assortative mating or mating by genotype • no mutations – allele frequencies don’t show a net change due to mutations • no migration between populations (gene flow) • no natural selection – all genotypes reproduce with equal success

So, what’s all this talk about changing frequencies and evolution? • Assumption #1 – phenotypes change over time due to reshuffling of gene pool • Assumption #2 – genotypes remain constant over time unless population evolves • Assumption #3 – if we can look at a stable population first, we can then note what a CHANGING population looks like, hence EVOLUTION • Assumption #4 - we can observe genotype frequencies in a population and track their changes from one generation to another • Assumption #5 – we can obtain genotype frequencies from phenotype frequencies using the H-W equation!!!

Section 16.2: Disruption of Hardy Weinberg equilibrium (results in evolution) • 1. Mutations – Occur spontaneously. Introduces new alleles into population • Harmful mutations are slowly selected out while beneficial mutations drive evolution.

2. Migration • Constant movement of animals w/i a population in and out • Immigration – movement into a population • Emigration – movement out of a population • Gene Flow – movement of genes from 1 population to another

3. Genetic Drift • Within some populations, certain allele frequencies change as a result of random events or chance rather than by natural selection • Polydactyly is very common among the Amish: Ellis-van Creveld syndrome • A small population inbreeding causes recessive alleles to come together more frequently

Genetic Drift occurs when a small population is isolated. • Dipping in to the same gene pool. • Gene frequency changes but not due to natural selection or adaptiveadvantage. Due to purely chance. • Danger occurs when only 1 allele is left. No variation w/in a population • Simulation

4. Nonrandom Mating • What if mating only occurs with people w/in one’s own village? • Dipping into the same gene pool could expose disorders due to recessive genes • Assortative mating – people tend to mate w/ others that are similar or dissimilar to them

Sexual Selection • Attractive phenotypes such as brightly colored feathers (peacock) • Red stripe and blue muzzle of mandrill baboon • Human male body odors different from female • Predators???

5. Natural Selection • Most significant factor to disrupt genetic equilibrium • Peppered moths in Industrial England – White trees became soot covered White on white – > in numbers White on dark - < in numbers Sooty on light - < in #’s

Patterns of Natural Selection

What pattern of natural selection is being demonstrating? Stabilizing selection – average form of a trait has the highest fitness • The extremes are more obvious to predators. • Follows bell curve but then favors average • In between colored moths would have been favored – when peppered, they would have blended in with lichens on trees

Directional Selection – • Extreme form is favored. Directs evolution away from the average. Black moths would have been favored if there were black trees Anteater with longest tongue gets to the deepest nests

Disruptive selection • Extremes are favored while the average is selected out. Both black and white moths would have been favored if there were both black and white trees

Stabilizing Directional Disruptive

Section 16.3: Formation of a Species – Speciation • Species – • a population of organisms that can successfully interbreed and cannot breed (produce a fertile offspring) with others out of their species. • Morphologically (internal & external structures), they are similar

What drives speciation? • Isolation – separates breeding populations • Geographic isolation – physical separation from original habitat. Development of a deep cannon, river changes, land floats, volcanic activity, etc • Isolation stops gene flow between two subpopulations. • Populations diverge and can no longer interbreed

Reproductive Isolation- caused by barriers to successful breeding between population groups in the same area • Prezygotic isolation – occurs before fertilization • Incompatible behaviors - different mating seasons & calls

Prezygotic isolation Leopard Frog Wood Frog Wood frogs mate in late March, Leopard frogs mate in mid April Since both are of the genus Rana, they can interbreed but do not

Postzygotic isolation • occurs after fertilization. Due to a failure of zygote to develop or infertility. • Wastes gametes

Rates of Speciation • Gradualism – evolution is a constant process and occurs at a steady rate • Punctuated equilibrium- Periods of stability separated by accelerated changes. • Species arise abruptly then have long periods of little change

Chapter 16: “Population Genetics and Speciation”