Evolution 4-Darwin and Mechanisms of Natural Selection Flashcards
What letter did Darwin receive?
-In 1958, Darwin received a letter that changed everything.
-It was from Alfred Russel Wallace.
-Alfred Russel Wallace was a young naturalist working in the East Indies, who had written a short paper with a new idea.
-He asked Darwin to evaluate his ideas and pass it along for publication.
-Subsequently, Darwin had to hurry up, and on November 24th, 1859, Darwin published, “On the Origin of Species by Means of Natural Selection.”
Why was Darwin a reluctant revolutionary?
- Returned to England from his voyage in 1836, which went from 1831-1836.
- Wrote papers describing his collections & observations.
- Brought back thousands of specimens and fossils.
- Draft of his theory of species formation in 1844.
- Concerned about the impact of his ideas in a religious culture.
- Instructed his wife to publish this essay upon his death
- Reluctant to publish but didn’t want ideas to die with him.
What was the essence of Darwin’s ideas?
- Variation exists in natural populations
- Many more offspring are born each season than can possibly survive to maturity
- As a result, there is a struggle for existence - competition
- Characteristics that are beneficial in the struggle
for existence will tend to become more common in the population, changing the average characteristics of the population - adaptations - Over long periods of time, and given a steady input of new variation into a population, these processes lead to the emergence of new species
What was Darwin’s Theory of Evolution by Natural Selection (Observations)?
- Individuals within a species vary in many ways
- Some variability can be inherited
- Each generation produces way more offspring than can survive and passes on variations
- Populations tend to remain stable in size
What was Darwin’s Theory of Evolution by Natural Selection (Inferences)?
1.Members of the same species compete with each other for survival
2.Individuals with more favourable variation are more likely to survive. SURVIVAL IS NOT RANDOM
3.Survivors contribute proportionately more offspring to succeeding generations the favourable variation will become more common THIS IS NATURAL SELECTION
Why did people not believe it?
- Not everyone thought the earth was millions of years old
- Fossil record had HUGE gaps (geology and palaeontology are new sciences)
- Fossil record did not show transitional forms
- Didn’t know about genetics (believed in blending in traits, so how could new structures appear from nothing?)
What do we believe today?
- Life on earth has been dated at 3.5 billion years
- Far more fossils found which show progression (ex. 50 million year old whale ancestor with big hind limbs that today, are stubby, and non-functioning.
- We understand genetics, mutations, and recombination: this gives the most compelling evidence
- 1866- Gregor Mendel publishes work on peas (ignored until 1900s)
What were the two results or theories came from the idea of Natural Selection as well?
-Descent with Modification: Each living organism has descended, with changes from other species over time
-Common Descent: Were derived from common ancestors
Natural Selection Definition
A mechanism of evolution where by traits become more or less common in a population due to effects on survival and reproduction
What are the four types of natural selection?
1.Directional Selection
2.Stabilizing Selection
3.Disruptive Selection
4.Sexual Selection
What is Directional Selection?
Selection that favours an increase or decrease in the value of a trait from the current population
What happens during Directional Selection?
-Directional selection favours the traits (phenotypes) of one extreme over the other.
-This type of selection is common during times of environmental change or when a population migrates to a new habitat that has different environmental conditions and niches to exploit.
-Two examples of directional selection include the colouration of peppered moths, and the increase of antibiotic resistance in infection-causing bacteria.
What is Stabilizing Selection?
Selection that favours intermediate traits (phenotypes) and acts against extreme variants.
What happens during Stabilizing Selection?
-It favours an intermediate phenotype and acts against extreme variants of the phenotype.
-The most common phenotype-the intermediate form-is made more common in the population by removing the extreme forms.
-This type of selection reduces variation and improves the adaptation of the population to aspects of the environment that remain fairly constant.
-One example of stabilizing selection would be human babies being an average size: if they are too big, they can’t get through the birth canal, and if they are too small, they have low survivability.
What is Disruptive (Diversifying) Selection?
- Selection that favours two or more variations of a trait that differ from the current population average.
-It can lead to new species forming.
