From Biol301

Contents 1 The Evolution of Populations: 445 - 463 1.1 Population Genetics 1.1.1 The Modern evolutionary synthesis integrated Darwinian selection and Mendelian inheritance 1.1.2 A Population’s Gene Pool is Defined by its Allele Frequencies 1.1.3 The Hardy – Weinberg Theorem Describes a Non-evolving Population 1.2 Causes of Microevolution 1.2.1 Microevolution is a Generation-to-generation Change in a Population’s Allele Frequencies 1.3 The Two Main Causes of Microevolution are Genetic Drift and Natural Selection 1.4 Genetic Variation, The Substrate for Natural Selection 1.4.1 Genetic Variation Occurs Within and Between Populations 1.4.2 Mutations and Sexual Recombination Generate Genetic Variation 1.4.3 Diploidy and Balanced Polymorphism Preserve Variation 1.5 A Closer Look at Natural Selection as the Mechanism of Adaptive Evolution 1.5.1 Evolutionary Fitness is the Relative Contribution an Individual makes to the Gene Pool of the Next Generation 1.5.2 The Effect of Selection on Varying Characteristics can be Directional, Diversifying, or Stabilizing 1.5.3 Natural Selection maintains Sexual Reproduction 1.5.4 Sexual Selection may Lead to Pronounced Secondary Differences between the Sexes 1.6 Natural Selection cannot Fashion Perfect Organisms

The Evolution of Populations: 445 - 463

Population Genetics

-Darwin’s Origin of Species did a good job convincing scientists that species were the result of evolution, but not that natural selection was the mechanism. He couldn’t explain how traits got passed down; “like begets like, but not exactly”. No one noticed that Mendel had come up with the answer. Page 445

The Modern evolutionary synthesis integrated Darwinian selection and Mendelian inheritance

-Since Mendel noted discrete genetic variation and Darwin noted continuous variables, most didn’t see the correlation between the two. However, with the birth of populations genetics which focused on extensive genetic variation within populations, the two theories were united. Page 446 -“Modern synthesis” was come with primarily by Dobzhanski (geneticist), Wright (geneticist), Mayr (biogeographer and taxonomist), Simpson (paleontologist) and Stebbins (botanist). It is called a synthesis because it is built on the foundations of many different disciplines. -“The modern synthesis emphasizes the importance of populations as the units of evolution, the central role of natural selection as the most important mechanism of evolution, and the idea of gradualism to explain how large changes can evolve as an accumulation of small changes occurring over long periods of time.” Page 446.

A Population’s Gene Pool is Defined by its Allele Frequencies

-a population is a localized group of individuals belonging to the same species. -a species is a group of populations whose individuals have the potential to interbreed and produce fertile offspring in nature. -isolation is particularly common for populations confind to widely separated islands, unconnected lakes, or mountain ranges separated by lowlands. -when looking at the area a population covers, those in the middle are more likely to be breeding with other members of that population that with other populations; therefore the center animals are more closely related to one another -the total aggregate of genes in a population at any one time is called the population’s gene pool. -if all members of a population are homozygous for the same allele, that allele is said to be fixed in the gene pool.

The Hardy – Weinberg Theorem Describes a Non-evolving Population

-this theorem states that the frequencies of alleles and genotypes in a population’s gene pool remain constant over the generations unless acted upon by agents other than Mendelian segregation and recombination of alleles. -p = (total number of Type A alleles / number of alleles in population), q = 1-p = (total number of type B alleles / number of alleles in population), -this system operates somewhat like shuffling a deck of cards: no matter how many times the deck is reshuffled to deal out new hands, the deck itself remains the same. Aces do not grow more numerous than jacks. -p2 + 2pq + q2 = 1 -the equation enables us to calculate frequencies of alleles in a gene pool if we know frequencies of genotypes, and vice versa. -the h-w theorem is important conceptually and historically because it shows how Mendel’s theory of inheritance plugs a hole in Darwin’s theory of natural selection. Natural selection requires genetic variation; it cannot act in a genetically uniform population. The H-W theorem explains how Mendelian inheritance preserves genetic variation from one generation to the next. -blending theory of genetics (which dominated before Mendel’s work was recognized) suggested that animals would mix into one uniform species. The H-W theorem corrected this thinking. -5 tenets of H-W theorem:

1. Very large population size.
2. No migration.
3. No net mutations
4. Random mating
5. No natural selection

Causes of Microevolution

Microevolution is a Generation-to-generation Change in a Population’s Allele Frequencies

-H-W gives us a standard from which to measure the deviance to know how a population is changing -microevolution the changing of frequencies of just a single locus.

The Two Main Causes of Microevolution are Genetic Drift and Natural Selection

-main factors that change the frequency of alleles: (cause departures from H-W theorem)

1. genetic drift
2. natural selection (always has a positive effect because selection targets unfavorable traits)
3. gene flow
4. mutation

-*Genetic Drift* -if a population is small enough, there might just be change in frequency by chance; 100 flips doesn’t always lead to 50 heads and 50 tails -the bottleneck effect: when something happens such that only a fraction of the population lives, then only a fraction of the diversity comes with it. This is modeled by marbles in a two-liter bottle –only so many colors can come out in a single dump. Example: cheetah, ice ages, hunting, three groups left, very little diversity. -the founder effect: when a few individuals from one population colonize a new area. They don’t bring with them all the variation, only a fraction of it. Example: people who founded Tristan da Cunha had eye problems; now they still have high rate of eye problems. -natural selection accumulates and maintains favorable genotypes in a population

_*Gene Flow*_: genetic change due to the migration of fertile individuals or gametes between populations. Gene glow tends to reduce differences between populations. If it is extensive enough, gene flow can eventually amalgamate neighboring populations into a single population with a common gene pool. -*Mutation*: A mutation is a change in an organism’s DNA. If some new allele produced by mutation increases its frequency by a significant amount in a population, it is not because mutation is generating the allele in abundance, but because individuals carrying the mutant allele are producing a disproportionate number of offspring as a result of natural selection or genetic drift. Mutation is the “raw material” for natural selection.

Genetic Variation, The Substrate for Natural Selection

Genetic Variation Occurs Within and Between Populations

• remember that only genetic aspects of natural selection can have evolutionary consequences
  • both quantitative and discrete characters contribute to variation within a population
  • quantitative variation usually indicates polygenic inheritance, an additive effect of two or more genes on a single phenotypic character
  • a population is said to be polymorphic if two or more morphs are represented in high enough frequencies
  • polymorphism applies only to discrete characteristics
• *Measuring Genetic Variation*
  • population geneticists measure on both the level of whole genes and at the molecular level
  • the gene diversity of a species is the average percent of these loci that are heterozygous.
  • population geneticists measure neucleotide diversity by comparing the nucleotide sequences of DNA samples from two individuals and then pooling the data from amny such comparisons of two individuals.
  • in humans gene diversity is 14%, nucleotide diversity is only about 0.1%
• *Variation between populations*
  • geographic variation is commonly shown: a difference in a species based on location of population. Genetic drift can cause this. Geo-variation can be localized too, within a population.
  • a cline is a graded change in some trait along a geographic axis. (ex: average body size of NA birds).

Mutations and Sexual Recombination Generate Genetic Variation

• mutation and sexual recombination create variation in the gene pool of a population
• *Mutation*:
  • new alleles originate only by mutation, or change in the nucleotide sequence of DNA.
  • only mutations that occur in cell lines that produce gametes can be passed along to offspring.
  • a random change is not likely to improve the genome any more than blindly firing a shotgun through the hood of a car is likely to improve engine performance.
  • mutations are more likely to be good when the environment is changing demands on the population
  • shorter the lifespan, the more quickly mutations arise; this is why bacteria can evolve so quickly (explosive asexual reproductive expansion)
• *Sexual Recombination*
  • sex shuffles alleles and deals them at random to determine individual genotypes
  • each zygote made by a mating pair has a unique assortment of alleles resulting from the random union of a sperm and an ovum

Diploidy and Balanced Polymorphism Preserve Variation

• *Diploidy*:
  • The diploid nature of most euks hides a considerable amount of genetic variation form selection in the form of recessive alleles in heterozygotes
  • Heterozygote protection maintains a huge pool of alleles that may not be suitable for present conditions but that could bring new benefits when the environment changes.
• *Balanced Polymorphism*:
  • the abilityo f natural selection to maintain stable frequencies of two or more phenotypic forms in a populations is called balanced polymorphism.
  • Natural selection preserves variation by two mechanisms.

1. One such mechanism is heterozygote advantage: If individuals who are hetero at a particular locus have greater survivorship and reproductive success than any type of homo, then two or more alleles will be maintained at that locus by natural selection. (Example: sicle cell = double recessive, malaria resistant = hetero). 2. Frequency-dependent selection: in which the survival and reproduction of any one morph declines if that phenotypic form becomes too common in the population. (Example: parasite and bird with resistance, when bird’s change, parasite is selected to change, and back and forth). -*Neutral Variation*: -trivial in their impact on reproductive success. The diversity of human fingerprints is an example. Relative frequencies of neutral variations is not affected by natural selection.

A Closer Look at Natural Selection as the Mechanism of Adaptive Evolution

-adaptive evolution is a blend of chance and sorting –chance in the creation of new genetic variations by mutation and sexual recombination, and sorting in the workings of selection as if it favors the propagation of some chance variations over others.

Evolutionary Fitness is the Relative Contribution an Individual makes to the Gene Pool of the Next Generation

-Darwinian fitness: the contribution an individual makes to the gene pool of the next generation relative to the contributions of other individuals. -relative fitness: the contribution of a genotype to the next generation compared to the contributions of alternative genotypes for the same locus. -the genotype that produces the most offspring is considered “1”; everything else is somewhere between 0 and 1 based on what percentage of offspring they make relative to the genotype that makes the most. -the relative fitness of an allele depends on the entire genetic context in which it works. For example, a gene that lets a turtle reach higher for food doesn’t make it more fit if it can’t walk to the food for some genetic reason.

The Effect of Selection on Varying Characteristics can be Directional, Diversifying, or Stabilizing

-Directional Selection: -most common during periods of environmental change or when members of a population migrate to some new habitat with different environmental conditions. -directional selection shifts the frequency curve for variations in some phenotypic character in one direction or the other by favoring what are initially relatively rare individuals that deviate from the average for that character. -Diversifying selection -occurs when environmental conditions are varied in a way that favors individuals on both extremes of a phenotypic range over intermediate phenotypes. -Stabilizing selection: -acts agains exterem phenotypes and favors the more common intermediate variants. -this mode of selection reduces variation and maintains the status quo for a particular phenotypic character

Natural Selection maintains Sexual Reproduction

-Sex is an evolutionary enigma; it is far inferior to asexual reproduction, yet we still have it. -sex is maintained by the vast majority of euks -the textbook explanation is that sex creates the genetic variation that ns works with; but this is hard to support because ns acts in the here in now, not planning ahead for mutations to select from -one hypothesis is the resistance to disease. This hypothesis predicts that the diversity of alleles would be particularly extensive at those gene loci that code for the molecules on cell surfaces to which pathogens attach; seems to be true. Sex provides a mechanism for chaning the locks that pathogens need to attack humans.

Sexual Selection may Lead to Pronounced Secondary Differences between the Sexes

-there are also marked differences, called secondary sexual characteristics that are not directly associated with reproduction -distinction between sexes is called sexual dimorphism -intrasexual sexual selection means within the same sex. It is best illustrated by males fighting for a mate –whoever loses doesn’t give to the next generation and gets selected for. -intersexual selection is usually seen in the form of a female picking a male mate. Darwin noted that sometimes it seems that females chose their mate on criteria that have nothing to do with fitness (i.e. peacock color). Some modern scientists think it is just that the characteristics she does chose on gives an overall impression of healthiness.

Natural Selection cannot Fashion Perfect Organisms

-four points show that evolution cannot form perfect organisms and therefore that non-perfect organism are evidence for evolution: 1. Evolution is limited by historical constraints: each species has a legacy of descent with modification. Evolution does not scrap ancestral anatomy and build each new complex structure from scratch. 2. Adaptations are often compromises: each organism must do many different things, so often the adaptation isn’t the best way to do any one thing (example: seal’s flippers). 3. Not all evolution is adaptive: chance probably affects the genetic structure of populations to a greater extent than was once believed. When an organism is moved to a new environment, the wind didn’t choose the best one, it was random. 4. Selection can only edit existing variations: natural selection favors only the fittest variations form the phenotypes that are available, which may not be the ideal traits. -natural selection operates on a “better than” basis