Ch. 15 Evolution

Individual organisms differ, and some of this variation is heritable Organisms produce more offspring than can survive, and many

that do survive do not reproduce Because more organisms are produced than can survive, they

compete for limited resources Each unique organism has different advantages and

disadvantages in the struggle for existence. Individuals best suited to their environment survive and reproduce most successfully. These organisms pass their heritable traits to their offspring. Other individuals die or leave fewer offspring. This process of natural selection causes species to change over time.

Species alive today are descended with modification from ancestral species that lived in the distant past. This process, by which diverse species evolved from common ancestors, unites all organisms on Earth into a single tree of life.

Summary of Charles Darwin’s Theory of Evolution

1. Mutations 2. Genetic shuffling that results from sexual

reproduction *The number of phenotypes produced for a

given trait depends on how many genes control the trait

1. single-gene trait = leads to 2 phenotypes, ex: widow’s peak, tongue rolling

2. polygenic trait = many possible genotypes and phenotypes, ex: height

Sources of Genetic Variation Look at Summary #1

Gene pool = consists of all genes, including all the different alleles, that are present in a population

Relative frequency = number of times that the allele occurs in a gene pool compared with the number of times other alleles for the same gene occur. (%)

Evolution = any change in the relative frequency of alleles in a population◦ If the relative frequency of an allele changes over

time, that means the population is evolving

Vocab.

A. Fossil record◦ Derived Traits = newly evolved features, such

as feathers, that do not appear in the fossils of common ancestors

◦ Ancestral Traits = primitive features, such as teeth and tails, that do appear in ancestral forms

Evidence of Evolution

B. Comparative Anatomy◦ Homologous structures = anatomically similar

structures inherited from a common ancestor Arm, foreleg, fin, wing

◦ Vestigial structures = structures that are reduced forms of functional structures in other organisms snake pelvis, kiwi wings, human appendix

◦ Analogous structures = can be used for the same purpose and can be superficially similar in construction but are not inherited from a common ancestor Bird wings and insect wings

Adaptation = a trait shaped by natural selection that increases an organism’s reproductive success

Fitness = measure of the relative contribution an individual trait makes to the next generation

Types of Adaptation Camouflage = morphological adaptations that

allow them to blend in with their envs. Mimicry = one species evolves to resemble

another species (kingsnake and coral snake) Antimicrobial resistance

Adaptation

Hardy-Weinberg principle = states that when allele frequencies in a population remain constant a population is in genetic equilibrium◦ P2 + 2pq + q2 = 1p = dominant, q = recessive◦ Homozygous dom. + heterozygous + homozygous recessive = 1

Genetic equilibrium = the situation in which allele frequencies remain constant

5 conditions are required to maintain genetic equilibrium from generation to generation◦ 1. Random mating◦ 2. Large population◦ 3. No movement into or out of the population (No interbreeding)◦ 4. No mutations◦ 5. No natural selection

Hardy-Weinberg Principle

A. Natural selection = process by which favorable traits that are heritable become more common in successive generations of a population of reproducing organisms◦ individuals with favorable phenotypes are more

likely to survive and reproduce than those with less favorable phenotypes

Sources of Evolutionary Change

Natural selection can affect the distributions of phenotypes in any of three ways:

1. Individuals at one end of the curve have higher fitness than individuals in the middle or at the other end◦ Ex. Small and medium-sized seeds start to run

low and only large seeds finches with large, thick beaks survive and reproduce so average beak size would increase

Directional Selection

2. Individuals near the center of the curve have higher fitness than individuals at either end of the curve

◦ Ex. Babies that are too small or too large are less likely to survive (average is selected for)

Stabilizing Selection

3. Individuals at the upper and lower ends of the curve have higher fitness than individuals near the middle

◦ Ex. Snakes live at edge of forest. Light and dark are selected for. Medium color is disadvantage

Disruptive Selection

Natural selection in which change in frequency of a trait is based on the ability to attract a mate

◦ ex. Brighter and bigger tails in peacocks◦ Larger size and antlers in deer

Sexual Selection

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What about behavior?

Gifts?

Random change in allele frequency due to chance

◦ More extreme in small populations

◦ May occur when a small group of individuals colonizes a new habitat

Genetic Drift

A situation in which allele frequencies change as a result of the migration of a small subgroup of a population

Founder Effect

Occurs when a population declines to a very low number and then rebounds

Bottleneck

Speciation = formation of new species◦ A population must diverge and then be

reproductively isolated

Reproductive isolation = when the members of 2 pops. cannot interbreed and produce fertile offspring

A physical barrier divides one population into 2 or more populations

Allopatric Speciation

A species evolves into a new species without a physical barrier

Sympatric Speciation

Prezygotic isolating mechanisms = operate before fertilization occurs◦Keep species separate

Postzygotic isolating mechanisms = operate after fertilization◦hybrid remains infertile (liger, mule)

1. Behavioral isolation = occurs when 2 pops. are capable of interbreeding but have differences in courtship rituals or other reproductive strategies that involve behavior

◦ Ex. Eastern and western meadowlarks have overlapping ranges but do not mate with each other because they have diff. mating songs

2. Geographic isolation = 2 pops. are separated by geographic barriers such as rivers, mountains, or bodies of water◦ Ex. Colorado river split pop. of Abert squirrels and

2 separate gene pools were formed to form a new species

3. Temporal isolation = 2 or more species reproduce at different times◦ Ex. 2 pops. of orchids release pollen on different

days so they cannot mate with each other, eventually they may become separate species

Pg. 439

Explain Adaptive radiation Explain Coevolution Explain Convergent evolution Explain Rate of Speciation

◦ Gradualism◦ Punctuated equilibrium

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