Change in the genetic makeup of a population over time.

Types of Evolution

Macroevolution

Changes in the kinds of animals and plants on earth that take place over long periods of time.

New forms replace old ones.

Evolutionary change on a grand scale.

Microevolution

Progressive change in gene frequencies.

Natural selection allows some individuals to produce more offspring, therefore a population contains more individuals with those genetic characteristics.

Change within a species.

Adaptation

Inherited trait that helps an organism survive its environment.

Types of adaptations

Structural: associated with structure and anatomy.

Ex. webbed feet, woodpecker’s tongue

Physiological: associated with functions.

Ex. poisons in snakes, ink of an octopus

Behavioral: associated with actions and responses to the environment.

Ex. bird migration, plants growing toward light

Evidence for Evolution by Natural Selection

Fossil Record

Fossil: preserved evidence of life long past

Some species are older than others; fossils are located in various strata.

Rates of evolution

Gradualism: gradual evolutionary change.

Punctuated equilibrium: species may exist for a long time more or less unchanged under the influence of stabilizing selection. Then an isolated population evolves into a new species that spreads out, overtaking former species.

Comparative Anatomy

Homologous Structures

Same evolutionary and embryonic origin, but occurring in different species.

Structures may perform the same or different functions.

Example: forelimbs of mammals

Analogous Structures

Similar functions but different development and structure.

Appear to be unrelated.

Examples: wings of birds and insects

Vestigial Structures

Apparently useless organs that are homologous to structures that serve important functions in other species.

 Comparative Embryology

Embryo contains organs not found in adult.

Examples

vestigial gill slits and tails seen in vertebrates (reptiles, birds, mammals)

tooth buds in whales and birds

Comparative Behavior

Threat behaviors: making oneself larger (erect fins, gill covers, feathers, furs).

Weapons (teeth, beaks, horns) displayed in attack position.

Comparative Biochemistry

DNA: gradual changes in DNA sequences of various molecules in organisms.

Cytochrome c: used in oxidative metabolism.

Globins: a, b hemoglobin, myoglobin.

Protein structure (human and chimp proteins 99% alike).

Biogeography

Geographical distributions of organisms.

different finches on each of Galapagos islands

marsupials in Australia

Convergent evolution

Evolution of the same adaptations in unrelated organisms.

Presumably the result of natural selection in similar environments

Examples

bandicoot and rabbit

wombat and woodchuck

Adaptive radiation

Descendents of an ancestral group in a particular place spread (radiate) to other areas and encounter new environmental conditions, thus evolving new adaptations.

Giving rise to new species adapted to new habitats and ways of life.

Causes of Evolution

Mutation: ultimate source of genetic variation, but not reliable by itself to produce population changes.

Mating preference: selects for certain genotype.

Genetic Drift: change in gene pool. in small populations, a greater likelihood that alleles will be lost because they may only be present in one individual which does not reproduce.

Founder effect: change in allele frequencies when new population arises from one or a few individuals.

Population bottleneck: number of individuals falls.

Endangered species

Isolated species

Gene flow: one phenotype is more favored for survival and reproduction.

Gene flow between adjacent populations means gene pools of populations share more alleles than those farther apart.

cline: a gradient of variation across an area.

Environmental features vary along gradients.

Tends to increase similarity between all populations of a species.

Natural selection

Main mechanism of evolution.

Process by which populations change in response to their environment as individuals with better adaptations leave more offspring.

Due to a differential reproduction of genotypes. individuals with some genotypes produce more offspring than those with other genetic combinations.

Stabilizing selection: average phenotype have advantage over either extreme (most common).

Directional selection: phenotype at one extreme have selective advantage. usually a result of environmental change.

Disruptive selection: extremes are favored relative to the intermediate.

Darwin’s Theory of Natural Selection

Natural Selection: the best adapted individuals in a population survive and produce offspring that are likewise well adapted

Many organisms overproduce.

Not all offspring survive.

Individuals in a population are different from each other (shows variation).

Different genetic traits are inherited from parents and passed to offspring.

Some traits allow individuals to live longer and produce more offspring. Traits poorly suited may cause the organism to die earlier and not have as many offspring.

Because some traits are more likely to be present and other traits are lost, the entire population will change to be better adapted. (“Survival of the Fittest”)

Speciation

Species: one or more populations that share a common gene pool. Organisms capable of breeding with one another but not with members of other such groups.

Live in reproductive isolation, resulting in genetic isolation.

Prezygotic isolation: barrier to reproduction that prevents mating.

Postzygotic isolation: keeps hybrid zygotes from developing or producing fertile offspring.

Speciation: formation of one or more new species from an existing species.

Species test

Reproductive isolation: members of 2 strains do not interbreed, thus there are no offspring.

Behavioral differences: example: no response to courting behavior.

Differences in protein structure: amino acid or structural differences.

Differences in DNA sequences: gene marker dissimilarity.

Sympatric speciation

Two populations in the same place.

Production of new species within a single population.

Often a result of a change in chromosome number.

Polyploidy: multiplication of normal chromosome number.

Autopolyploidy: chromosome number doubles followed by self-fertilization.

Allopolyploidy: two related species interbreed, producing an infertile hybrid offspring; chromosome number doubles to produce fertile organism.

Allopatric speciation

two populations in different places.

new species arises when a population becomes separated from the rest of its species, then changes enough to become a new species.

examples

founder effect

subject to a new set of selective pressures

Population Genetics

Populations: All the members of a species that occupy a particular area at the same time. Populations form breeding groups.

Study of frequencies of alleles in populations; relative proportions of alternative alleles.

1908; G.H. Hardy and W. Weinberg

Hardy-Weinberg law

Provides a theoretical model that does not change genetically from one generation to the next.

p = frequency of allele A.

q = frequency of allele a.

p + q = 1

p2 = frequency that A sperm will fertilize A egg.

q2 = frequency that a sperm will fertilize a egg.

2pq = frequency of Aa

p2 + 2pq+ q2= 1 (frequency of genotypes must add up to 1.

Allows for a standard for comparison with real populations.

Frequency of alleles and genotypes will remain the same through all successive generations if certain conditions are met.

Genetic polymorphism: occurrence in the same place at the same time of 2 or more genetic variants.

Male/ female

ABO blood types

Frequency dependent selection: phenotypes have different selective advantages depending on the frequency in the population.

Heterozygote advantage

often more heterozygote individuals at a locus than Hardy-Weinberg equilibrium predicts.

Heterozygotes are rare in a population with a high degree of inbreeding.

hybrid: matings between members of 2 different strains.

Conditions

No net mutation.

No mating preferences: sexual reproduction is required. (Sexual reproduction by itself is not enough to cause evolution.)

Large size: reduces sampling errors.

No gene flow: population must be isolated so no exchange of genes by migration of individuals or gametes with other populations.

No selection: no genotype has a reproductive advantage over another.

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