Natural Selection

• Evolution is a change in a population over time.

• Natural selection is defined in terms of populations but occurs in terms of individuals.

• Much of what we know about evolution is based on the work of Charles Darwin

Darwin’s observations:

• Each species produces more offspring than can survive.

• These offspring compete with one another for the limited resources available to them.

• Organisms in every population vary.

• Evolutionary fitness is measured by reproductive success. The fittest offspring, or those with the most favorable traits, are the most likely to survive and therefore produce a second generation.

• These are the traits most likely to pass to subsequent generations. Fitness varies based on biotic and abiotic factors; different genetic variations can be selected for in different generations.

Lamarck and the Long Necks

• A theory of evolution in Darwin’s day was proposed by Jean-Baptiste de Lamarck.

• He proposed that acquired traits were inherited and passed on to offspring.

Evidence for Evolution

• Paleontology: paleontology revealed the various organisms and the major lines of evolution. Fossils can be dated by:

i. The age of the rocks where a fossil is found

ii. The rate of decay of isotopes including carbon-14

iii. Geographical data

• Biogeography, or the study of the distribution of flora (plants) and fauna (animals) in the environment: scientists found related species in widely separated regions of the world.

• Embryology, or the study of development of an organism: All the vertebrates—including fish, amphibians, reptiles, birds, and even mammals such as humans—show fishlike features called gill slits.

• Morphological homologies, or the study of the anatomy of various animals: scientists discovered that some animals have similar structures that serve different functions. Homologous structures, also point to a common ancestor. Analogous structures evolved totally independently of one another.

• Molecular Biology: The most compelling proof of all is the similarity at the molecular level.

• Continuing Evolution: Evolution is constantly occurring. We can see small changes in DNA and changes in the fossil record consistently

Common Ancestry

• Some original life-form is the common ancestor to all life.

• Charts called phylogenetic trees, or cladograms, are used to study relationships between organisms.

• Phylogenetic trees are built using data from the fossil record or molecular record. Cladograms are often drawn with even spacing between species, but phylogenetic trees are often drawn with different distances between species and as a result they look more like a tree with uneven branches.

• They always begin with the common ancestor and then branch out. Anytime there is a fork in the road, it is called a common ancestor node

Genetic Variability

• The differences in people are known as genetic variability.

• Natural selection occurs only if some individuals have more evolutionary fitness and can be selected. The more variations there are among a population, the more likely that a trait will exist that might be the perfect lifesaver.

• Genetic variation is the very foundation of evolution

The Peppered Moths

• Exactly half of them were dark and carried alleles for dark coloring. The other half were light and carried alleles for light coloring. This 1:1 ratio of phenotypes was observed until air pollution, due primarily to the burning of coal, changed the environment.

• The gene pool reached 90 percent dark alleles and only 10 percent light alleles. This occurred because the light moths didn’t stand a chance in an environment where they were so easy to spot. The dark moths, on the other hand, multiplied just as fast as they could.

Causes of Evolution

• Natural selection requires genetic variation and an environmental pressure.

• Biotic and abiotic factors can affect the direction of evolution.

• As long as a mutation does not kill an organism before it reproduces it may be passed on to the next generation.

• Survival of the fittest is the name of the game, and any trait that causes an individual to reproduce better gives that individual evolutionary fitness

• Females choose to mate with males that have a large and beautiful tail. This is an example of sexual selection.

• Genetic drift is something that causes a change in the genetics of a population, but it is not natural selection. Instead. It is also called the bottleneck effect or the founder effect, this occurs when only a few individuals are left to mate and regrow a population.

• Gene flow can occur between different populations of the same species if individuals migrate.

• The situation with our moths is an example of directional selection.

• Two other types of selection are stabilizing selection and disruptive selection. Stabilizing selection means that organisms in a population with extreme traits are eliminated.

• Another type of selection to be aware of is artificial selection. This is a type of selection where humans directly affect variation in other species.

Species Population

• In order to become different species, they would have to become reproductively isolated from each other.

• With different variation and different environmental pressures, they could each change in different ways and no longer be able to mate. This is called divergent evolution.

• Divergent evolution that occurs quickly after a period of stasis is called punctuated equilibrium.

• Pre-zygotic barriers prevent fertilization.

• A post-zygotic barrier is related to the inability of a hybrid organism to produce offspring.

• Convergent evolution is the process by which two unrelated and dissimilar species come to have similar traits, often because they have been exposed to similar selective pressures.

• There are two types of speciation:

• Allopatric speciation and sympatric speciation.

• Allopatric speciation simply means that a population becomes separated from the rest of the species by a geographic barrier so the two populations can’t interbreed.

Genetics

• Mendel’s laws can extend to the population level.

• The Hardy-Weinberg law states that even with all the shuffling of genes that goes on, the relative frequencies of genotypes in a population are constant over time.

• p2 + 2pq + q2 = 1

• In this equation, p2 represents the homozygous dominants, 2pq represents the heterozygotes, and q2 represents the homozygous recessives.

• It is important to understand the consequences on the population if any of the five conditions are not met:

  1. If the population is small, the population will be more susceptible to random environmental impact than if the population is large.

  2. If mutations are present in a population, new alleles will be introduced into the population and hence genetic equilibrium will be disturbed.

  3. If immigration or emigration is present, species entering or leaving a population will bring or remove alleles with them.

  4. If there is non-random mating, individuals will be selective in how they pick their mating partners based on a trait or traits.

  5. If there is natural selection, organisms better adapted to their environment will be more likely to survive and reproduce, and thus their alleles will be preferentially propagated to the next generation.

Origins of Life on Earth

• Two scientists, Alexander Oparin and J. B. S. Haldane, proposed that the primitive atmosphere contained mostly inorganic molecules and was rich in the following gases:

• methane (CH4),ammonia (NH3), hydrogen (H2), and water (H2O).

• There was almost no free oxygen (O2) in this early atmosphere.

• Stanley Miller and Harold Urey simulated the conditions of primitive Earth in a laboratory. They put the gases theorized to be abundant in the early atmosphere into a flask, struck them with electrical charges in order to mimic lightning, and organic compounds similar to amino acids appeared.

• The original life-forms were simply molecules of RNA. This is called the RNA-world hypothesis.