Chapter 23The Evolution of Populations Benito Polanco Period 1
Vocabulary Microevolution: change in the allele frequencies in a population over generations. Geographic Variation: differences between the gene pools of geographically separate populations or population subgroups. Cline: a graded change in a character along a geographic axis. Population: a localized group of individuals of the same species that can interbreed, producing fertile offspring. Gene Pool: all of the alleles for all of the loci in all individuals in a population. Genetic Drift: chance events cause unpredictable fluctuations in allele frequencies from one generation to the next. Founder Effect: genetic drift that occurs when a few individuals become isolated form a larger population, with the result that the composition of the new population’s gene pool is not reflective of that of the original population.
Continued… Bottleneck Effect: genetic drift that occurs when the size of a population is reduced, as by a natural disaster or human actions. Typically, the surviving population is no longer genetically representative of the original population. Gene Flow: the transfer of alleles from one population to another, resulting form the movement of fertile individuals or their gametes. Relative Fitness: the contribution an individual makes to the gene pool of the next generation, relative to the contributions of other individuals in the population. Directional Selection: natural selection in which individuals at one end of the phenotypic range survive or reproduce more successfully than do other individuals. Disruptive Selection: natural selection in which individuals on both extremes of a phenotypic range survive or reproduce more successfully than do individuals with intermediate phenotypes.
Continued… Stabilizing Selection: neutral selection in which intermediate phenotypes survive or reproduce more successfully than do extreme phenotypes. Sexual Selection: a form of natural selection in which individuals with certain inherited characteristics are more likely than other individuals to obtain mates. Sexual Dimorphism: marked differences between the secondary sex characteristics of males and females. Heterozygoe Advantage: greater reproductive success of heterozygous individuals compared with homozygotes; tends to preserve variation in a gene pool. Neutral Variation: genetic variation that does not appear to provide a selective advantage or disadvantage.
What causes genetic variation? Genetic Variation: Everyone has genotypes that are reflected in phenotypic variations. There are some phenotypic variations that are not inherited- environmental influences. Ex: body builders don’t pass down their muscles to their offspring.
Fig. 23-2 (b) (a)
Continued… Variation within a Population Characters that vary within in a population can be discrete or quantative. Biologists measure genetic variation in a population at both the whole-gene level (gene variability-heterozygous). And Molecular level of DNA (nucleotide variability) is measured by comparing the DNA of 2 individuals in a population and then averaging the data from many such comparisons.
Continued… Variation between populations Species also exhibit geographic variation, differences in the genetic composition of separate populations. Researchers have observed differences in the karyotypes of isolated populations- the chromosomes have become fused- differ. Other examples occur as a cline- produced by a gradation in an environmental variable. Some clines are probably a result from natural selection- there has to be multiple alleles that exist for that given locus.
Continued… Mutations are changes in the nucleotide sequence of DNA which cause new genes and alleles to arise. Only mutations in cells that produce gametes can be passed to offspring. Sexual reproduction can shuffle existing alleles into new combinations. In organisms that reproduce sexually, recombination of alleles is more important than mutation in producing the genetic differences that make adaptation possible.
How does the Hardy-Weinberg equation demonstrate the evolution of a population? The gene pool of a population that is not evolving can be described by the Hardy-Weinberg principle. This principle states that the frequencies of alleles and genotypes in a population will remain constant from generation to generation. Instead of considering the possible allele combination from one genetic cross, our focus now is on the combination of alleles in all genetic crosses in a population.
Fig. 23-6 Alleles in the population Frequencies of alleles Gametes produced p = frequency of Each egg: Each sperm: CR allele = 0.8 q = frequency of 80% chance 80% chance 20% chance 20% chance CW allele = 0.2 The bin is holding the population’s gene pool for a locus. “Reproduction” occurs by selecting alleles at random from the bin. By viewing reproduction as a random selection of alleles from the bin, we are in effect assuming that mating occurs at random- all male-female matings are equally likely.
Continued… If p and q represent the relative frequencies of the only two possible alleles in a population at a particular locus, then p2 + 2pq + q2 = 1 where p2 and q2 represent the frequencies of the homozygous genotypes and 2pq represents the frequency of the heterozygous genotype
The conditions required for Hardy-Weinberg equilibrium. The requirements describes a hypothetical population that is not evolving.
No mutations- by altering alleles or (in large-scale changes) deleting or duplicating entire genes, mutations modify the gene pool.
Random mating - if individuals mate preferentially within a subset of the population, such as their close relatives, random mixing of gametes does not occur, and genotype frequencies change.
No Natural selection- differences in the survival and reproductive success of individuals carrying different genotypes can alter allele frequencies.
Extremely large population size- the smaller the population, the more likely it is that allele frequencies will fluctuate by chance from one generation to the next (genetic drift).
No gene flow- by moving alleles into or out of populations, gene flow can alter allele frequencies.
Natural Selection Only natural selection consistently results in adaptive evolution. Natural selection brings about adaptive evolution by acting on an organism’s phenotype. Relative fitness is the contribution an individual makes to the gene pool of the next generation, relative to the contributions of other individuals. Selection favors certain genotypes by acting on the phenotypes of certain organisms.
Continued… Three modes of selection:
Directional selection favors individuals at one end of the phenotypic range.
Disruptive selection favors individuals at both extremes of the phenotypic range.
Stabilizing selection favors intermediate variants and acts against extreme phenotypes.
Fig. 23-13 The graphs show how the frequencies of individuals with different fur colors change over time. the large white arrow symbolize selective pressures against certain phenotypes. Original population Frequency of individuals Original population Phenotypes (fur color) Evolved population (c) Stabilizing selection (b) Disruptive selection (a) Directional selection
Continued… Adaptations like a cuttlefish rapidly changing color and the remarkable jaws of a snake, etc. These can arise gradually over time as natural selection increases the frequencies of alleles that enhance survival and reproduction. (a) Color-changing ability in cuttlefish
Fig. 23-14 (a) Color-changing ability in cuttlefish Movable bones (b) Movable jaw bones in snakes
Continued… Charles Darwin was the first to explore the implications of sexual selection, a form of natural selection in which individuals with certain inherited characteristics are more likely than other individuals to obtain mates. This can result in sexual dimorphism which are marked differences like size, color and behavior.
Continued… Heterozygote advantage occurs when heterozygotes have a higher fitness than do both homozygotes. Sickle-cell disease is when the red blood cells of people become distorted in shape, which can lead to complication. However, heterozygotes are protected against the most severe effects of malaria- important to regions where malaria is a major killer.
Fig. 23-17 The frequency of sickle-cell allele in Africa is generally highest in areas where the malaria parasite is most common. Frequencies of the sickle-cell allele 0–2.5% 2.5–5.0% 5.0–7.5% Distribution of malaria caused by Plasmodium falciparum (a parasitic unicellular eukaryote) 7.5–10.0% 10.0–12.5% >12.5%
Evolution doesn’t produce organisms perfectly to fit their environment 1. Selection can act only on existing variations. Natural selection favors only the fittest phenotypes among those currently in the population, which may not be the ideal traits. New advantageous alleles do not arise on demand. 2. Evolution is limited by historical constraints. Each species has a legacy of descent with modification from ancestral forms. Evolution co-opts existing structures and adapts them to new situations. For example, evolution does not grow an extra pair of limbs for a new adaptation but rather it operates on the traits that it already has.
Continued… 3. Adaptations are often compromises. Each organisms must do many different things. A seal spends part of its time on rocks; it could probably walk better if it had legs instead of flippers, but then it could not swim nearly as well. Sometimes, structural reinforcement has been compromised for agility. 4. Chance, natural selection, and the environment interact. Chance events can affect the subsequent evolutionary history of populations- storm transporting individuals that are best suited to a new environment. the environment at a particular location may change unpredictably from year to year.
Fig. 23-19 The frog makes a sound to attract mates but it also attracts more unsavory animals like the bat.
Summary of Lizards Undergo Rapid Evolution After Introduction To A New Home In 1971, biologists moved five adult pairs of Italian wall lizards from their home island of Pod Kopiste, in the South Adriatic Sea, to the neighboring island of Pod Mrcaru. “Striking differences in head size and shape, increased bite strength and the development of new structures in the lizard’s digestive tracts were noted after only 36 years, which is an extremely short time scale,” says Duncan Irschick. Observed changes in head morphology were caused by adaptation to a different food source- 2/3’s of their food use to be plants. Change in diet also affected the population density and social structure of the Pod Mrcaru population. Because plants provide a larger and more predictable food supply, there were more lizards in a given area on Pod Mrcaru. Food was obtained through browsing rather than the active pursuit of prey, and the lizards had given up defending territories.