Evolution: theory of how modern organisms have descended from ancient organisms; a.k.a. "a change over time"

Evolution UnitCh. 15-17 in Miller & Levine Biology textbook

Charles Darwin is one of the many individuals who proposed a theory of evolution called natural selection. Born in 1809 In 1831, he began a round-the-world sea voyage on the H.H.S. Beagle. Of all the Beagle's ports of call, the one that influeneced Darwin the most was the Galapagos Islands off the coast of South America. In 1859, he published his book called On The Origin of Species.

In a century prior to Darwin, a few scientists questioned these beliefs and helped pave the way for Darwin's theory of natural selection...Geologists James Hutton and Charles Lyell helped scientists recognize that Earth is many millions of years old, and the processes that changed Earth in the past are the same processes that operate in the present.

Naturalist Jean-Baptiste Lamarck recognized that living things have changed over time and that organisms are somehow adapted to their environment. Lamarck proposed that by selective use or disuse of organs, organisms acquired or lost certain traits during their lifetime. These acquired traits could then be passed on to their offspring. Although his hypotheses of evolution are incorrect in several ways, he was one of the first scientists to develop a scientific hypothesis of evolution.

Economist Thomas Malthus reasoned that if the human population continued to grow unchecked, sooner or later there would be insufficient living space and food for everyone. Led Darwin to wonder what factor(s) determines survival and reproduction in a species?

Before Darwin, two ideas about life on Earth prevailed...1. Species are fixed, or permanent.2. The Earth is less than 10,000 years old and relatively unchanging.

Charles Darwin's Idea of Evolution by Natural Selection (a.k.a. Survival of the Fittest)

In order for natural selection to occur, four things must be true...

1. There is diversity among inherited traits.

2. There is an overproduction of offspring.

3. There is a struggle for existence.

4. There is differential reproductive success, meaning individuals with inherited characteristics well-suited to the environment leave more offspring on average than do other individuals.

Darwin proposed that over long periods, natural selection produces organisms that have different structures, establish different niches, or occupy different habitats. As a result, species today look different from their ancestors. Each living species has descended, with changes, from other species over time.

This idea also implies that all living organisms are related to one another. Darwin proposed that all species - living and extinct - were derived from a single common ancestor.

Charles Darwin's Idea of Descent with Modification

Today when studying the phylogeny of organisms, scientists use cladograms to represent the evolutionary relationships between organisms in Darwin's "Tree of Life."

The Fossil Record!

Most fossils are found in sedimentary rock layers, called strata.

The younger fossils are found in the uppermost rock layers.

Fossils form in many ways: buried in volcanic ash or sedimentary rock, frozen in ice, stuck in hardened tree sap (amber), imprints, casts, petrification, etc.

The oldest fossil is of a prokaryote that is 3.8 billion years old!

Based on traits common with mammals, it was long thought that birds were more closely related to mammals than to other vertebrates. When looking at various skull and hip characteristics, however, birds share more traits with certain lineages of dinosaurs. When fossil evidence arose that some dinosaurs (Archaeopteryx) had beak-like traits and feathers, it became clear that these bird traits were homologous with dinosaurs!

Evolution has left much evidence...

The forelimbs of all mammals consist of the same skeletal parts. However, the function of these forelimbs differ.

Such similar structures in species sharing a common ancestor are called homologous structures.

These structures are evidence of adaptations to an environment. An adaptation is any inherited characteristic that increases as orgnaism's chance of survival.

Similarities in Structure!

Homologous structures support other evidence that evolution is a remodeling process. Structures that originally functioned one way in an ancestrial species become modified as they take on a new functions.

Some of the most interesting homologous structures are those that have a major function in one species but are less important in a related species.

Vestigial structures are remnants of structures that may have had important functions in an ancestral species, but have no clear function in some of the modern descents.

Examples:Whale pelvis boneHuman appendixHuman tailboneGoosebumps

Many patterns of geographic distribution of life forms makes sense in an evolutionary context. The flying squirrel and the sugar glider do not share a recent common ancestor like one might think, yet they share many of the same physical characteristics. How can this be? Both species have adapted to similar environments on the two different continents. This is an example of convergent evolution.

Convergent evolution can lead to analogous structures, such as a bat's wing and a bird's wing - similar in function, but different in structure. Why is Australia home to pounched mammals (marsupials) while very few placental animals live there? The origin of new species is closely linked to the Earth's changing geography and environmental conditions. This is an example of divergent evolution.

Geographic Distribution

Similarities in Development

Embryos of closely related organisms often have similar stages of development.

All vertebrates have an embryonic stage in which pouches appear on the sides of the throat. These different vertebrates take on more distinctive features as development progresses. In fishes, most of the throat pouches develop into gills; in land vertebrates, these features are involved in the development of other structures, such as bones of the skull.

Molecular Biology

If two species have genes and proteins with sequences that match closely, biologists conclude that the sequences must have been inherited from a relatively recent common ancestor.

In contrast, the greater number of differences in DNA and protein sequences between species, the less likely they share as close a common ancestry.

Speciation is the creation of new species. Darwin proposed that natural selection and adaptation to an environment were at the basis of speciation.

Online youtube video of evolution in action - speciation of salamanders.

There are 13 species of finches unique to the Galapagos Islands. They most closely resemble one finch species living on the South American mainland. One finch came over to the islands and colonized. This species adapted to the various habitats of the islands and eventually diversified into the 13 species seen today. The most important condition for speciation is reproductive isolation between individuals. By being reproductively isolated, genetic mutations can occur in the different populations eventually leading to drastic changes.

Refer to the class activity for specific examples.

Three different ways individuals may be reproductively isolated are:

1. Geographic Separation - individuals are separated by a geographic barrier such as a mountain range, a body of water, a canyon, etc.

2. Temporal Separation - individuals are separated by the timing of their mating seasons.

3. Behavioral Separation - individuals are separated by difference in behavioral characteristics used in sexual selection.

Three important things about Natural Selection...

1. Although natural selection occurs through interactions between individual organisms and their environment, individuals do not evolve. A population is the smallest unit that can evolve.

2. Natural selection can only amplify or diminish heritable traits.

3. Environmental factors vary from place to place and from time to time. A trait that is favorable in one situation may be useless - or even detrimental - in different circumstances. Natural selection is always operating, but which traits are favored depends on the environment.

Why can't natural selection fashion the PERFECT organism?

1. Evolution is limited by historical restraints.

2. Adaptations are often compromises.

3. Chance and natural selection interact.

4. Selection can only edit existing variations.

The diagram to the left shows the gene pool for fur color in a population of mice.

Here, in a total of 50 alleles, 20 alleles are B (black), and 30 are b (brown). How many of each allele would be present in a total of 100 alleles?

A population is a group of individuals of the same species that interbreed. Because members of a population interbreed, they share a common group of genes called a gene pool.

The relative frequency of an allele is the number of times that the allele occurs in a gene pool, compared with the number of times other alleles for the same gene occur.

36 + 36 = 72 b + 48 b = 120 b out of 200 alleles = 120/200 = 3/5 = 60% b16 + 16 = 32 B + 48 B = 80 B out of 200 alleles = 80/200 = 2/5 = 40% B

IN GENETIC TERMS, evolution is any change in the relative frequency of alleles in a population...

on polygenic traits can affect the distributions of phenotypes in any of three ways: directional selection, disruptive selection, or stabilizing selection.

Natural Selection on single-gene traits can lead to changes in allele frequencies and thus to evolution

Initial Population:80% brown lizards10% red lizards10% black lizards

Generation 10:80% brown lizards0% red lizards20% black lizards

If red lizards are more visible to predators, they might be less likely to survive and reproduce, and the allele for red coloring might not become common.

What mechanisms can change a population's gene pool?

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The Bottleneck Effect:

The Founder Effect:

Genetic Drift is a change in the gene pool of a population due to chance. In small populations, the allele frequencies can vary erratically from generation to generation.

What mechanisms can change a population's gene pool?

What mechanisms can change a population's gene pool?

Gene Flow is the exchange of genes with another population.

Occurs when fertile offspring or their gametes migrate between populations.

For example, the wind carries pollen from one population of plants into a neighboring populatio of plants across the river.

Another example, two populations of bunny rabbits meet in the forest and begin to interbreed. The two populations can eventually become a single population of bunny rabbits with one common gene pool.

What mechanisms can change a population's gene pool?

Mutations are random changes in an individual's DNA.

If a mutation occurs in a gamete that is involved in fertilization, that mutation is passed on to the offspring of the individual. Thereby, the mutation enters into the gene pool of the population.

Mutations are the original source of genetic variation!

Are there any conditions under which evolution will not occur?

The Hardy-Weinberg Principle states that allele frequencies in a population will remain constant unless one or more factors cause those frequencies to change. The situation in which allele frequencies remain constant is called genetic equilibrium. So....under what conditions does the Hardy-Weinberg principle hold?

1. There must be random mating. 2. The population must be very large. 3. There can be no gene flow. 4. There can be no mutations. 5. There can be no natural selection.