Population Evolution

Population Evolution

Individuals don’t evolve, populations do. • As Charles Darwin and Aflred Wallace perceived long ago, individuals don’t evolve, populations do. • A population is a group of individuals of the same species in a specified area. • To under stand how it evolves, start with variations in the traits that characterizes it.

Variation in Populations • Individuals in a population have traits in common. • Morphological traits • Features like two feathered wings, or five forward-facing toes. • Physiological traits • Metabolic activities • Behavioral traits • Respond to stimuli • Individuals also have variation in the details of their shared traits.

Variations in Populations • For sexually reproducing species, a population is a group of individuals that are interbreeding, reproductively isolated from other species, and produce fertile offspring. • The offspring typically have two parents, and they have mixes of parental forms of traits.

Variation in Populations • Dimorphism • The persistence of two forms of a trait in a population. • For example: humans have sexual dimorphism in which males and females have specific characteristics, and can (usually) be told apart easily. • Polymorphism • The persistence of three or more forms of a trait. • For example: humans have polymorphism in skin color.

The Gene Pool • Genes encode information about heritable traits. • The individuals of a population inherit the same number and kind of genes (except for their gametes). • Together, they and their offspring represent a gene pool—a source of genetic resources.

The Gene Pool—Sources of Variation • For sexual reproducers, nearly all genes available in the shared pool have two or more slightly different molecular forms, or alleles. • Any individual might or might not inherit alleles for any trait. • This is the source of variations in phenotype. • Mutation also leads to new alleles. • New alleles may come as a result of genetic recombination during sexual reproduction.

Mutation Revisited • Mutations are a major source of variation in populations. • Most mutations are neutral and have no effect on the individual. • Some mutations lead to death. • These lethal mutations will not be passed onto future generations. • Every so often, a beneficial mutation will occur, and if it gives an organism an advantage, may increase their fitness. • This is an adaptation.

Stability and Change in Allele Frequencies • Researchers typically track allele frequencies, or the relative abundance of alleles of a given gene among all individuals of a population. • For example: Count the number of students with red hair, blond hair, brown hair, and black hair in class. • If there are 40 students in class, and 2 of them have red hair, 10 have blond hair, 25 have brown hair, and 3 have black hair, then the allele frequencies for each hair color will be: • 5% red hair • 25% blond hair • 62.5% brown hair • 7.5 % black hair

Stability and change in Allele Frequencies • Microevolution refers to small-scale changes in allele frequencies that arise as an outcome of mutation, natural selection, genetic drift or gene flow, or some combination of these. • An example of microevolution would be… • Zombies are taking over the world! To zombies, the brains of brunettes and blonds are the tastiest, so they eat them first. They will eat the brains of people with black hair occasionally, but only eat the brains of red-heads when they absolutely have to. (Environmental pressure). • As a result, the allele frequencies in this population will change. • Having red hair is now an adaptation! Those with red hair will be the most fit in the changed environment, and will have the most opportunities to have children. • If you were to take a survey of a 40-person classroom, the allele frequencies would be… • 85% red hair • 2% blond hair • 3% brown hair • 10% black hair Mmmm….blonds….

When is a population not evolving? • When a population stops evolving, they are under genetic equilibrium. • Genetic equilibrium is exceedingly rare in nature, and can lead to the extinction of a species. • Genetic Equilibrium can only be achieved if five conditions are met. • 1. There is no mutation. • 2. The population is infinitely large. • 3. The population is isolated from all other populations of the species (no gene flow). • 4. Mating is random. • 5. All individuals survive and produce the same number of offspring. • This scenario can never truly happen in nature, and is called the Hardy-Weinburg Formula.

The Hardy-Weinburg Formula • When scientists are studying a population undergoing change, they may utilize the Hardy-Weinburg formula to attempt to discern the reason behind that change. • Remember, the 5 conditions of equilibrium are: • 1. There is no mutation. • 2. The population is infinitely large. • 3. The population is isolated from all other populations of the species (no gene flow). • 4. Mating is random. • 5. All individuals survive and produce the same number of offspring. • If a scientist notices a change in allele frequencies, she can ask…are there new mutations? Is the population getting smaller? Is the population interbreeding with a new population? Are some mates more desirable than others? Are offspring dying at different rates or are more offspring being produced by some individuals than others? • The answers to these questions may give the scientist an idea about why allele frequencies are changing.

Natural Selection Revisited • Natural selection is the most influential process in microevolution. • Natural selection can cause populations to undergo… • Directional selection • Stabilizing selection • Disruptive selection

Directional Selection Suddenly, the environment runs out of the dragons favorite food, hot dogs, and the smaller dragons, who need to eat less, become more common than the large dragons, who need more. • In directional selection, allele frequencies shift in a consistent direction, so forms at one end of a phenotypic range become more common than mid-range forms. • For example: Dragons come in a variety of sizes, as evidenced by this bell curve. Suddenly, people develop a taste for dragon and begin to hunt them. Small dragons are easier to catch and eat, and so the larger dragons become more common. Both of these scenarios are examples of directional selection

The Peppered Moth • Populations of peppered moths are a classic example of directional selection. • The moths feed and mate at night and rest motionless on trees during the day. Their behavior and coloration (mottled gray to nearly black) camouflage them from day-flying, moth-eating birds. • In the 1850’s, the industrial revolution started in England, and factory smoke altered conditions in much of the countryside. • Before then, light moths were the most common form, and a dark form was rare. • Also, light-gray speckled lichens had grown thickly on tree trunks. Light moths but not dark moths that rested on the lichens were camouflaged.

The Peppered Moth • Lichens are sensitive to air pollution. Between 1848-1898, soot an other pollutants started to kill the lichens and darken the tree trunks. • The dark moth form was better camouflaged. • The dark moths became the most common form of moth until 1952 when pollution controls allowed lichens to make a comeback, and the light colored moths, once more, became more common. • Researchers hypothesized: If the original conditions favored light moths, then the changed conditions favored dark ones.

Pocket Mice • In the Sonoran Desert (that’s here!) there are two main colors of pocket mice: tawny and black. • Rock pocket mice are small mammals that spend the day in underground burrows and forage for seeds at night. Those who live in tawny-colored outcroppings of granite, are…well…tawny colored! • Those who live in the dark basalt of ancient lava flows (but the same area and same species) tend to be black. • We can expect that night-flying predatory birds are selective agents that affect fur color. This placed selective pressure on the Rock Pocket Mice living in the two separate environments causing the allele frequencies to change.

Resistance to Pesticides and Antibiotics • Pesticides can cause directional selection. • Typically a heritable aspect of body form, physiology, or behavior helps a few individuals to survive the first pesticide doses. • As the most resistant ones are favored, resistance become more common. • There are 450 species of pests that are now resistant to one or more types of pesticides—including bed bugs! Ewww. • Antibiotics can also cause directional selection. • Antibiotics are used to fight pathogenic bacteria, and have been notoriously overprescribed, overused, and not used correctly. • This has lead to the evolution of “super bugs”! We will talk more on those later! Bed bugs were virtually wiped out in America in the 1930’s thanks to the pesticide DDT. Research into recent infestations in large cities show that bed bugs are now resistant to many of the traditional pesticides used to the control them.

Stabilizing selection • With stabilizing selection, intermediate forms of a trait in a population are favored, and extreme forms are not. • This mode of selection can counter mutation, genetic drift, and gene flow. • For example: Baby dragons come in all sizes, as shown in this bell curve. • Small baby dragons aren’t strong enough to compete with their siblings to get food and they die. • Large baby dragons need too much food to survive, and they die. • This is stabilizing selection.

Sociable Weavers • Between 1993-2000, scientists captured, measured, tagged, released, and recaptured 70 to 100 percent of the birds living in communal nests during the breeding season. (Can we say alien abduction?!) • Their field studies supported a prediction that body mass is a trade-off between risks of starvation and predation. • Intermediate-mass birds have the selective advantage. • Foraging is not easy in this habitat, and lean birds do not store enough fat to avoid starvation. • The largest birds are more attractive to predators and not as good at escaping. Social weaver nest (above) and birds (below).

Disruptive selection • With disruptive selection, forms at both ends of the range of variation are favored, and intermediate forms are selected against. • For example: Dragons come in a variety of sizes as evidenced by this bell curve. • Small dragons are able to find food in the smallest rock crags, and large dragons are able to hunt big game for food. • Medium sized dragons compete aggressively with other predators to get food, and die more frequently. • This is disruptive selection.

Black-Bellied Seed Cracker • The black-bellied seed crackers of Cameroon come in two sizes and two sizes only—large billed or small billed, with nothing in between. • Factors that affect feeding performance are the key. Cameroon’s swamp forests flood in the wet season; lightning-sparked fires burn in the hot, dry season. Most plants are fire-resistant, grasslike sedges. One species produces hard seeds and the other, soft seeds. • All Camaroon seedcrackers prefer soft seeds, but birds with large bills are better at cracking hard ones. • In the dry season, the birds compete fiercely for scare seeds. • Birds with intermediate sizes are being selected against, and now all bills are either 12 or 15 mm wide. 12 mm beak size 15 mm beak size

Sexual selection • The individuals of many sexually reproducing species show a distinct male or female phenotype, or sexual dimorphism. • Often the males are larger and flashier than females. • Courtship rituals and male aggression are common. • These adaptations and behaviors seem puzzling. All take energy and time away from an individual’s survival activities. Why do they persist if they do not contribute directly to survival? The answer is sexual selection. • By this mode of natural selection, winners are the ones that are better at attracting mates and successfully reproducing compared to others of the population. Sexy and I know it!

Sexual Selection • By choosing mates, a male or female is a selective agent acting on its own species. • The selected males and females pass on their alleles to the next generation. • Flashy body parts and behaviors are often observable signs of health and vigor. Such traits may improve the odds of producing more healthy, vigorous offspring.

Genetic Drift—The Chance Changes • Genetic drift is a random change in allele frequencies over time, brought about by chance alone. • There are two types of genetic drift, the bottleneck effect and the founder effect. The Black Plague would have caused genetic drift in the European countries that it greatly affected.

Bottlenecks • Bottlenecks occur when there is a drastic reduction in population size is brought on by extreme pressure. • Suppose that contagious disease, habitat loss, or hunting nearly wipes out a population. • Even if moderate numbers of individuals survive a bottleneck, allele frequencies will have been altered at random. • In the 1890’s, hunters killed all but twenty of a large population of northern elephant seals. • Government restrictions allowed the population to recover to about 130,000 individuals. • Each is homozygous for all the genes analyzed so far! (No genetic variation).

The Founder Effect • The founder effect is when unpredictable genetic shifts occur after a few individuals establish a new population. • Genetic diversity might be greatly reduced relative to the original gene pool, • For example: If the people on Lost were stuck on a real island (rather than a metaphorical one), and decided to continue a population, that population would be incredibly good-looking as compared with the population in general.

Inbred Populations • Inbreeding is nonrandom mating among very close relatives, which share many identical alleles. • It leads to the homozygous condition, and can lower fitness if harmful recessive alleles are increasing in frequency. • For example: The Old Order Amish in Pennsylvania are moderately inbred, and they have a high frequency of a recessive allele that causes Ellis-Van Creveld syndrome, in which individuals have extra fingers, toes, or both. • The allele might have been rare when a few founders entered Pennsylvania, but now 1 in 8 individuals are heterozygous for the allele, and 1 in 200 are homozygous for it.

Gene Flow • Individuals of the same species don’t always stay put. • A population loses alleles when an individual leaves it for good—emigration. • A popualtion gains alleles when an individual permanently moves in—immigration. • In both cases, gene flow—the physical movement of alleles into and out of a population—occurs. • For example: According to Y-chromosome data, there may be as many as 16 million men in the world today who can claim to be descendants of Genghis Khan. • Genghis Khan slaughtered the populations of the cities and nations he took over, and kept the prettiest girls for himself. His son, Tushi, got the next pick. • His grandson, Kubilai Khan, established the Yuan Dynasty, and supposedly added 30 virgins to his harem every year of his reign. • Now that’s gene flow! Genghis Khan founded the Mongol Empire, the largest contiguous empire in history. He united the tribes of northeast Asia and his empire extended through Central Asia and China.

Population Evolution