EVOLUTION
Definition
Evolution refers to the descent of organisms with modification over time; a cumulative inherited change in a population. Various biological mechanisms produce different species with great variety. Evolution explains the appearance of new species and modifications to species over long periods of time.
How do we know evolution exists?
There's quite a bit of evidence to support the theory of evolution. An example that's quite simple, that we've seen in class before, is that bacteria will grow resistant to antibiotics over time. That happens because, with each new generation of bacteria, ones with some kind of mutation that make them resistant to the antibiotic will survive, while those who don't have this mutation (the majority) will die. The few mutant bacteria that survived will be the only ones producing new bacteria and passing on their genes, thus which each new generation, the bacteria grow more and more resistant to the antibiotic. This can happen in a number of days for bacteria, but if we look at this process on a much larger scale for organisms with a much longer lifespan (like humans), it explains the gradual changes in population over time.
Another type of evidence for evolution is homology. This refers to similarities that result from common ancestry- for example, while human arms and whale flippers appear very different and clearly are from two very separately grouped organisms, since humans and whales share a common mammalian ancestor, they have the same bones that makeup arm / flipper structure. Sometimes this results in 'leftovers', called vestigial structures, another important piece of evidence to support evolutionary theory. For example, some snakes' skeletons still have remnants of pelvic and leg bones, since they developed from walking ancestors.
Thus, the similarities between seemingly distant organisms, as well as the clear ability of organisms to biologically adapt to their environment over many generations supports the idea of evolution that Darwin proposed, which is now widely excepted as an undeniable theory in the scientific world.
Natural Selection
This theory was first presented by Charles Darwin. Natural Selection refers to "a natural process that results in the survival and reproductive success of individuals or groups best adjusted to their environment and that leads to the perpetuation of genetic qualities best suited to that particular environment". In simpler terms, natural selection is when organisms with a certain trait survive in a particular environment, while those with another die off or are generally less successful. The result is that those with the favored trait are able to pass on their genes to future generations, whereas others with non-favored traits cannot (you can't have kids if you're dead). The population will over time have far more individuals with the favored trait than those with the non-favored trait.
The image is Darwin's "adaptive radiation", highlighting his theory of Natural Selection. The different fiches all branched off of one finch, but they each developed different beaks depending on the environment they were in. Natural selection favored certain beak types in each environment, eventually resulting in distinctively different kinds of finches.
Darwin's Arguments for Natural Selection:
- Observation: Individuals of a population often vary in traits
- Observation: Species produce more offspring than their environment can support
- Inference: Members of a population that have traits that give them a higher chance of survival in their environment leave more offspring
- Inference: When some individuals have a higher chance of survival and reproduction, it will result in a accumulation of their favorable traits in a population
Homology: similarity resulting from common ancestry
Homologous structures: structural similarities in organisms that point to common ancestry
Vestigial structures: "left over" anatomical feature that no longer has purpose (implies that at one point the structure served a purpose to an ancestor)
Different Kinds of Selection Overtime
Directional Selection
Selection favors one phenotype over another. This will result in a shift toward one trait that the environment promotes success with. In the beetle example, darker beetle color is favored- this could happen if a new kind of plant with much darker leaves emerges in their environment that those with darker shells are able to blend into better, thus not getting eaten as those with lighter shells would. Other examples of this would be the speed of a deer increasing over time as they are more frequently chased by increasingly fast predators, or the length of a giraffe's neck getting longer as those who can reach the nutrients of higher trees are able to pass on their lineage.
Stabilizing Selection
Selection favors an average phenotype over the extremes. This results in an increased number of individuals with the average, while those on either extreme will die off. An example of this could be (shown on the graph) if the color of leaves that these beetles live on is a middle shade of green- beetles that are very dark or very light would get eaten by predators more often, while those of a medium shade could blend in and pass on their genes to future generations.
Disruptive Selection
Selection favors the two extremes and not the average. This results in an increase of those with either extreme trait, whereas those with the average of 'middle' trait die off. This is much less common than the above two. An explanation for why this could happen in the beetle population could be if all the plants with medium green leaves died off suddenly, so now only dark green plants and light green plants remained. Those beetles with medium green shells could not blend into any plant in their environment and would be seen by predators more easily.
Genetic Drift: chance events that can alter gene frequencies to change unpredictably
- FOUNDER EFFECT: small group isolated from a great population, results in a different gene pool and (eventually) population
- BOTTLENECK EFFECT: sudden change in the environment that causes a drastic reduction of the size of the population. Alters the gene pool
A quick review of the classifications of life
Organisms are categorized into various groups. To the right is a list of these classifications, getting more specific as we go down. To start, there are three domains, archaea, bacteria, and eukarya. Grizzly bears, humans, a butterfly, or a flower, are part of the Eukarya domain. Following domain is kingdom, phylum, class, order, family, genus, and finally species. To help you understand and remember the classifications, the photo to the right presents the example of a grizzly bear.
SPECIATION- process by which one species splits into two or more species
Allopatric Speciation
- when gene flow is interrupted by a geographical barrier
Sympatric Speciation
- speciation that occurs in populations that are not geographically isolated
- can occur through: polyploidy, habitat differentiation, or sexual selection