Chapter 19 THE EVOLUTION OF POPULATIONS BIOLOGY Chapter 19 THE EVOLUTION OF POPULATIONS 19.1 Population Evolution 19.2 Population Genetics 19.3 Adaptive Evolution

Figure 19.1 Living things may be single-celled or complex, multicellular organisms. They may be plants, animals, fungi, bacteria, or archaea. This diversity results from evolution.

Microevolution – change of a population over time Population evolution Mechanisms of inheritance were not understood when Darwin & Wallace developed their ideas of natural selection. Modern synthesis – relationship between natural selection and genetics (1940s) Microevolution – change of a population over time Macroevolution – processes that gave rise to new species & higher taxonomic groups with divergent characteristics

Change in frequency = evolution Population genetics The study of how selective forces change a population through changes in allele & genotypic frequencies. Allele frequency/gene frequency – the rate at which a specific allele appears within a population Change in frequency = evolution Gene pool – the sum of all alleles in a population Genetic drift – random change in gene pool with no advantage Founder effect – the event that starts change in allele frequencies

Hardy-weinberg principle of equilibrium Developed in the early 20th century by an English mathematician, Hardy, & German physician, Weinberg. A population’s allele & genotype frequencies are stable unless some kind of evolutionary force is acting upon the population. Assumptions: NO mutations, migrations, emigration, or selective pressure for or against genotype, plus an infinite population Gives us a mathematical model to estimate the alleles or genotypes in a stable population and compare to real population

Hardy-weinberg principle of equilibrium Different alleles = different variables p & q If only 2 alleles the p + q = 1 More interested in frequencies of genotypes, not alleles – the population’s genetic structure By observing phenotypes, can only know the genotype of the homozygous recessive individuals, so calculations can provide an estimate of remaining genotypes: p2 + 2pq + q2 = 1

Figure 19.2 When populations are in the Hardy-Weinberg equilibrium, the allelic frequency is stable from generation to generation and the distribution of alleles can be determined from the Hardy- Weinberg equation. If the allelic frequency measured in the field differs from the predicted value, scientists can make inferences about what evolutionary forces are at play.

Distribution of phenotypes among individuals = population variation 19.2 population genetics Distribution of phenotypes among individuals = population variation Influenced by several factors: Genetic Variance – diversity of alleles and genotypes within a population, inbreeding

Figure 19.3 The distribution of phenotypes in this litter of kittens illustrates population variation. (credit: Pieter Lanser)

Figure 19.4 Genetic drift in a population can lead to the elimination of an allele from a population by chance. Small populations are more susceptible to forces of genetic drift. In this example, rabbits with the brown coat color allele (B) are dominant over rabbits with the white coat color allele (b). In the first generation, the two alleles occur with equal frequency in the population, resulting in p and q values of .5. Only half of the individuals reproduce, resulting in a second generation with p and q values of .7 and .3, respectively. Only two individuals in the second generation reproduce, and by chance these individuals are homozygous dominant for brown coat color. As a result, in the third generation the recessive b allele is lost.

Figure 19.5 A chance event or catastrophe can reduce the genetic variability within a population. Magnifies genetic drift by drastically reducing population size – ex. Tornado or hurricane

Figure 19.6 Gene flow can occur when an individual travels from one geographic location to another. Introduces new genes into a population.

Simple mate choice – females prefer certain male characteristics 19.2 population genetics Mutations – changes in organisms DNA, drive diversity, accumulates in population over time. Nonrandom Mating – Simple mate choice – females prefer certain male characteristics Assortative mating – preference to mate with phenotypically similar individuals Physical location can influence nonrandom mating

Environmental variance - Figure 19.7 Some species – gender is determined by environment – temperature-dependent sex determination (TSD) Geographical variation – cline, variation across an ecological gradient The sex of the American alligator (Alligator mississippiensis) is determined by the temperature at which the eggs are incubated. Eggs incubated at 30°C produce females, and eggs incubated at 33°C produce males. (credit: Steve Hillebrand, USFWS)

Environmental variance - Figure 19.7 Some species – gender is determined by environment – temperature-dependent sex determination (TSD) Geographical variation – cline, variation across an ecological gradient The sex of the American alligator (Alligator mississippiensis) is determined by the temperature at which the eggs are incubated. Eggs incubated at 30°C produce females, and eggs incubated at 33°C produce males. (credit: Steve Hillebrand, USFWS)

19.3 Adaptive evolution Selecting for beneficial alleles (increasing frequency) and against deleterious alleles (decreasing frequency) But works on organism as a whole – so evolutionary fitness selects for individuals with greater contributions to the gene pool of the next generation. Relative fitness measures how an individual compares to the others in the population

Figure 19.8 Figure 19.8 Different types of natural selection can impact the distribution of phenotypes within a population. In (a) stabilizing selection, an average phenotype is favored. In (b) directional selection, a change in the environment shifts the spectrum of phenotypes observed. In (c) diversifying selection, two or more extreme phenotypes are selected for, while the average phenotype is selected against.

Frequency-dependent selection Figure 19.9 Favors phenotypes that are either common (positive) or rare (negative) Negative increases genetic variance & positive decreases it Like a game of rock-paper- scissors, orange beats blue, blue beats yellow, & yellow beats orange A yellow-throated side-blotched lizard is smaller than either the blue- throated or orange-throated males and appears a bit like the females of the species, allowing it to sneak copulations. (credit: “tinyfroglet”/Flickr)

Figure 19.10 Sexual dimorphism is when the male and the female of the same species look completely different. One gender selects mates from the other based on certain traits. peacocks and peahens, Argiope appensa spiders (the female spider is the large one) wood ducks. Handicap principle – large appendages or bright colors carry risks, survive risks = better fitness Good genes hypothesis – bright colors or ornaments indicates better ability to fight off disease or higher metabolisms, so they have better fitness

No perfect organism Natural selection is the driving force in evolution and can generate populations that are better adapted to survive and successfully reproduce in their environments. CAN NOT produce the perfect organism – only select on existing variation in a population Limited by existing genetic variance & new alleles that arise from mutations and gene flow Limited because it works at the level of the individual not the alleles – this could link some beneficial alleles to some detrimental ones – looks at the net affect of the combinations on the individual as a whole Constrained by relationships between different polymorphisms. Not all evolution is adaptive – natural selection selects fittest individuals, but gene flow and genetic drift do the opposite Evolution has no purpose – it is the sum of various forces influences the genes of a population This PowerPoint file is copyright 2011-2015, Rice University. All Rights Reserved.