In class on Friday, we discussed Darwin’s theory of natural selection. This lesson goes over questions 10-13 in the study guide. So first, we said that natural selection is the engine that drives changes in species, but there are misconceptions. The first is that “survival of the fittest” is not based on physical strength but it’s based on survival and reproductive success. And second, there is no goal in the process and then end state is not perfection, it’s just survival.
Then we talked about the four things that are needed in Darwin’s theory of natural selection. As you can see in the diagram, natural selection is the biggest arrow that leads to more species.
1. Genetic variation: there have to be some variations in individuals within a population. For example, in a hummingbird species, there has to be some hummingbirds with short beaks and some with long beaks.
2. Overproduction of offspring: the rate of reproduction is surpassing the amount of supplies, which leads to competition.
3. A struggle of existence: the competition for food and space created from the overproduction of offspring creates a struggle for survival.
4. Differential survival and reproduction: those organisms that win the struggle are able to reproduce more offspring with successful genes/traits, so over time the population changes.
Next, we went over the three different types of natural selection. These are also written on a handout that we got in class on Friday.
1. Directional: this shifts the range of variation, meaning that the species moves from one specific genotype and phenotype towards towards a different genotype and phenotype.
2. Stabilizing: this is when extreme forms of this species are eliminated and the intermediate phenotypes become dominant. For example, if there are three different colors of butterfly, white, red, and pink, the extremes of red and white will show up less than the intermediate of pink.
3. Diversifying: this is the opposite of stabilizing natural selection and the extremes become dominant over the intermediate. So, in the butterfly example, there would be more red and white butterflies instead of pink ones.
As you can see in the diagram, there is a smaller arrow below the one for natural selection that says random processes. Changes in species can also arise through random processes and there are four types.
1. Mutation: a mutation can arise by mistakes in the copying process of genes that changes the genetic code and if they are not lost, then they may increase in frequency in the population.
2. Genetic Drift: in large populations genetic composition stays the same, while in a smaller population it takes less time for the composition to change.
3. Bottleneck effect: when there is a drastic decrease in size within a population, genotypes will be lost.
4. Founder effect: when a small group of individuals are isolated from the population, genetic variety will be lost.
Now to go over the diagram in more detail. One important thing to notice is that over time, changes can lead to different species. But you can also have change that does not create new species. We have already gone over the bigger arrow (natural selection) and the smaller arrow (random processes). Those two arrows make up microevolution which occurs within one population. The splitting arrow represents macroevolution which is changes between populations that gives rise to new species. Written inside the splitting arrow are two types of isolation that lead to these two different species. One type is reproductive isolation, also known as sympatric isolation. And the second is geographic isolation, also known as allopatric isolation. (To see an example of how geographic isolation can lead to two different species, click on the link below and view the short video on salamanders.) To conclude, through natural selection, random processes, and isolation, population A, species X can split into population A, species X and population B, species Y and they can not interbreed.
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