FRQ: Evolution

Nina Yang, AP Biology B, Mitchell

Rafi Wazir

Schindler's List - John Williams - Violin / Piano by Rafi Wazir

Darwin is considered the "father of evolutionary biology."

A Super Short(-ish) Biography of Charles Darwin

Darwin was a British scientist who laid the foundations of the theory of evolution and transformed the way we think about the natural world.

Charles Robert Darwin was born on 12 February 1809 in Shrewsbury, Shropshire into a wealthy and well-connected family. Darwin himself initially planned to follow a medical career, and studied at Edinburgh University but later switched to divinity at Cambridge. In 1831, he joined a five year scientific expedition on the survey ship HMS Beagle. At this time, most Europeans believed that the world was created by God in seven days as described in the bible. On the voyage, Darwin read Lyell's 'Principles of Geology,' and Lyell's argument was reinforced in Darwin's own mind by the rich variety of animal life and the geological features he saw during his voyage. The breakthrough in his ideas came in the Galapagos Islands, 500 miles west of South America. Darwin noticed that each island supported its own form of finch which were closely related but differed in important ways. On his return to England in 1836, Darwin tried to solve the riddles of these observations and the puzzle of how species evolve. Influenced by the ideas of Malthus, he proposed a theory of evolution occurring by the process of natural selection. Darwin worked on his theory for 20 years. In 1859 Darwin published 'On the Origin of Species by Means of Natural Selection'.

What are examples and evidence of his contributions to the field of evolutionary biology?

The non-constancy of species is how there is variation and diversity in each individual of a species or population, whether phenotypic or genotypic. As a result, the species or population changes over time from evolution and natural selection. For example, the peppered moths originally were mainly light-colored. After pollution from the Industrial Revolution caused the moths’ environment to darken, many of the lighter moths died from predation because they no longer camouflaged with their environment. On the other hand, the darker moths thrived for their ability to hide on the darkened trees. As a result, the population experienced directional selection from lighter colors to darker colors over time.

Branching evolution is how one species becomes two or more through divergent evolution. The resulting populations share a common ancestor, but they are no longer considered one species. We can see examples of this through homologous structures, traits shared between species because they are inherited from a common ancestor. For example, humans, cats, bats, and whales today have the same group of bones from shoulders down (to fingers and hands, not vertically down to feet). In fact, most tetrapods (including reptiles, birds, mammals, and amphibians) have this same bone structure. Although they are today used for different purposes, the actual bones remain a similarity between the organisms because they all share a common ancestor.

Occurrence of gradual changes in species is how species change slowly and gradually over time from gene mutations and selective pressures. Individuals do not evolve. However, little variations in phenotype and genotype begin to accumulate over generations, causing evolution to happen slowly but surely. We can use vestigial structures to prove change over time. For example, whales today still have hipbones, evidence that their ancestors used to be land-dwelling organisms that eventually lost function of their hips and legs as they began to live permanently in the sea. Also, some snakes also still have hipbones, also helping to prove that they evolved from ancestors that had limbs.

Finally, natural selection is “survival of the fittest,” where those most suited to their environment based on inherited traits tend to live longer and produce more offspring, thus passing on these favorable traits to the next generation. For example, the Galapagos finches all used to be one species that somehow moved from off the coast of South America to the Galapagos Islands. On these islands, selective pressures based on the available foods were present. Those with the largest beaks could crack open the larger seeds, while those with the smallest beaks could eat the softer ones. Those in the middle were unable to eat either, and died of starvation. In this case, selective pressures caused disruptive selection where the extremes were favorable in this environment. Thus, those that survived reproduced, passing on their traits for beak size on to their offspring. Another example would be the moths (explained when talking about the non-constancy of species). The lighter-coloured moths were selected against because of the change in environment. They were more easily spotted by birds, and so dark color became a favorable trait. The moths that were darker in color tended to survive longer and reproduce more.

(1/5) Origin of Species by Charles Darwin

Darwin's ideas have been enhanced and modified as new knowledge and technologies have become available.

How have the concepts of Hardy-Weinberg equilibrium and punctuated equilibrium been modified based on biologists' interpretations of Darwin's original contributions?

The Hardy-Weinberg theorem describes a hypothetical population that is not evolving. But in real populations, allele and genotype frequencies do change over time. This is because the following five conditions for non-evolving populations is rarely met for long in nature.

1. Extremely large population size. The smaller the population, the more likely genetic drift will occur, and the more likely allele freqeuncies will fluctuate from one generation to the next.

2. No gene flow. Gene flow can alter allele frequencies by transfering alleles between populations.

3. No mutations. Mutations can also modify the gene pool by introducing or removing genes, or by changing one allele into another.

4. Random mating. If individuals choose mates with certain genotypes, random mixing of gametes does not occur.

5. No natural selection. Differential survival and reproductive success of individuals will alter allele frequencies.

Hardy-Weinberg equilibrium can help add to Darwin’s theory of evolution, as these are all factors that cause evolution. Only when they do not occur do species not evolve and allelic frequencies remain unchanged. If even one condition occurs, then the species will undergo gradual changes, however slow they might be.

The equation p2+2pq+q2=1 represents a population that is in Hardy-Weinberg equilibrium. P is the frequency of the dominant allele, while q is the frequency of the recessive allele. As such, p2 is the frequency of homozygous dominant individuals, and q2 is the frequency of homozygous recessive individuals. Finally, 2pq represents the frequency of individuals who are heterozygous.

While natural populations rarely, if ever, are in true Hardy-Weinberg equilibrium, the rate of evolutionary change in these populations is often so slow that these populations appear to be close to equilibrium. As such, this enables us to obtain approximate estimates of allele and genotype frequencies.

Punctuated equilibrium is when a population’s genotypic and phenotypic traits remain relatively constant, until there is a sudden rapid change because of a shift in environmental factors. This goes against Darwin’s theory that populations evolve very slowly and gradually over a long period of time. We can see an example of this in the graphs below. The first shows punctuated equilibrium, where evolution happens very rapidly, remains relatively constant, and then evolves very rapidly again because of environmental changes. On the other hand, in the graph showing Darwin’s idea of evolution, there is a constant change in species over a long period of time.

These patterns may require an explanation outside the Darwinian model of gradual descent with modification. Darwin himself noted, "Although each species must have passed through numerous transitional stages, it is probably that the periods during which each underwent modification, though many and long as measured by years, have been short in comparison with the periods during which each remained in an unchanged condition."

Nina Yang

Hi :) Please do not call or email or anything. I just made those up :O