For Basic Definitions
We will use a site that we will often turn to in this course. This was a site, Evo 101, developed in conjunction with an NSF Grant to the University of California at Berkeley. It offers material suitable for high school to graduate level courses. Thus it allow you to review basic concepts regardless of background.
https://evolution.berkeley.edu/evolution-101/mechanisms-the-processes-of-evolution/
Evolution definition on survey.
Darwin's definition of evolution? It is a trick question. Darwin hated that term. He did not allow it to be used except in latter reprints of his origin thesis. His theory was a theory of common descent. It is a theory that focuses on relatedness of all life. The term evolution however turns the focus on change, as in this definition from Wikipedia. “Evolution is a change in the heritable characteristics of biological populations over successive generations”
This is known as a more micro evolution definition and the focus on the mechanisms or how change or evolution occurs. This is the modern focus and our next study guide will focus on how we detect natural selection or drift as the agents of such change.
Below are Evo 101 websites that offer both types of definitions.
https://evolution.berkeley.edu/evolution-101/mechanisms-the-processes-of-evolution/
https://evolution.berkeley.edu/glossary/evolution/
Why distinguish between macro and micro evolution? Because, some argue, mechanisms might be different. For example consider extinction. This is a random process on the macro level that essentially resets evolutionary patterns for a large proportion or even the entire planet.
Natural selection
Visit this website. https://evolution.berkeley.edu/evolution-101/mechanisms-the-processes-of-evolution/natural-selection/
Examine the necessary conditions for natural selection to occur.
Note the website never really defines natural selection. The best it does is list the requirements of natural selection. We will use the definition below for now. It is a micro evolutionary definition.
Definition: Differential reproduction in response to some environmental pressure.
Darwin’s theory had two major tenets, one was that all living things (organisms) are related through common descent and the other was that natural selection was the main mechanism by which diversity in form and function among organisms although related, occurred.
In Darwin’s own words from the Origin of the species, published in 1859.
" I am fully convinced that species are not immutable; but that those belonging to what are called the same genera are lineal descendants of some other and generally extinct species, in the same manner as the acknowledged varieties of any one species are the descendants of that species. Furthermore, I am convinced that Natural Selection has been the main but not exclusive means of modification.”
You can see the importance Darwin placed on this mechanism, by noting that the extended title of the Origin of Species includes this mechanism.
the Origin of species by Means of Natural Selection of The Preservation of Favoured Races in the Struggle for Life First edition. Charles Darwin. http://www.talkorigins.org/faqs/origin.html This is a website that contains the full text of the first edition of Darwin's thesis in addition to number of historically important papers on evolutionary principles. It's a good resource to know about.
What is the connection between evolution and natural selection? It all depends on whether the conditions are not only met for natural selection to occur, but also what type of natural selection is occurring.
We can distinguish three kinds of natural selection, according to their effect on a character such as body size.
• Directional selection
Smaller individuals may have higher fitness (produce more offspring) than larger individuals. Natural selection is then directional: it favors smaller individuals and will, if the character is inherited, produce a decrease in average body size. Directional selection could, of course, also produce an evolutionary increase in body size if larger individuals had higher fitness.
• Stabilizing selection
Natural selection could be stabilizing. The average members of the population, with intermediate body sizes, may have higher fitness than the extremes.
• Disruptive selection
Natural selection could favor both extremes over the intermediate types. This is called disruptive selection.
Most studies that are monitoring natural selection as a force driving evolution are looking for directional selection. They are looking for changes from smaller to larger body sizes, for changes in viral protein structure from one type to another, etc. So this has been the type of selection most associated with evolutionary change.
Natural selection and fitness
Examine the definition of fitness at the website below.
It equates "fitness" among population members based on some judgment as to which individuals leave the most reproductive offspring.
How you determine this varies because unfortunately most offspring die or leave home. Also you may not be able, without DNA evidence, to associate offspring with proper parents.
Examples: Egg parasites are common. Cuckoldry is also common. A experimenter castrated half of the males maintaining mating territories in one population of blackbirds. All of the nests in the population produced young.
Although it is sometime possible to judge short term fitness, determining what that means for the long term or macro evolution "fitness" is more difficult.
Consider the example you were questioned about in the survey.
One pair of birds (species A) fosters tender loving care on the two chicks they have every breeding season. Unfortunately, despite their care, they, during their lifetime, produce only 4 chicks that reproduce.
In another related species B, pairs live only one season and have 12 -15 young.
You follow two pairs. One pair (Y) during their lifetime has 12 young but also leave 4 young that live to reproduce. The other pair (Z) has 15 young, 7 survive, but only 5 so far have reproduced themselves
a. Which pair of the two of species B would you judge most fit?
Z, Because they leave the most reproductive young.
b. Which strategy, breeding only once or multiply, would you judge most fit?
Both are equally fit in the long term. Why because both species exist at this point in time. There is really no way of judging fitness between species with very different life strategies. Which entity is more fit, the COVID virus or elephants? Long term fitness is most associated with persistence, that is how long a group survives. Trilobites survived for a longer period than dinosaurs, so judged more successful as a group than dinosaurs. https://evolution.berkeley.edu/the-arthropod-story/exoskeleton-and-the-jointed-limb/trilobites-excellent-exoskeletons/trilobites-extinct-but-not-a-failure/
Interestingly enough, both groups disappeared during major extinctions. It is interestingly to speculate if dinosaurs would still be among us, if that meteorite had not hit.
Although if we apply our short term criteria, breeding only once is more fit. It is often difficult not to think of characteristics that resemble those of humans as inherently more "fit". Many of you looked at multiple breeding in this way, citing the care parents could give to a smaller clutch, or the number of clutches a pair could have over many years. But again macro fitness is simply persistence, and micro fitness is related by agreement among scientists studying micro evolution, with the number of reproductive young.
c. How would you go about representing fitness mathematically?
In mathematical models, highest fitness is assigned a value of one. All other fitnesses are measured against the highest. So a phenotype yielding three young as opposed to a phenotype of leaving two young could lead to fitnesses expressed as 1 and 0.67.
Genetic drift
Darwin recognized genetic drift as a valid mechanism but did not think it was a major mechanism of change. A growing appreciation for a larger relative role of genetic drift in generating diversity is one of the major refinements by today’s evolutionists to Darwin’s theory. What the role is of genetic drift in adaptation is still being debated today.There are two schools: One holds that genetic drift is a major driver of adaptation, the other that drift simply shapes the variation available to natural selection and literally, there is no adaptation without selection. A lot depends on how you define adaptation and what you assume about the variability available to natural selection. More about this in the weeks to come.
For now simply visit this website for a definition of genetic drift.
Definition of genetic drift. https://evolution.berkeley.edu/sampling-error-and-evolution/
Sexual selection
Another revision to Darwin's view of evolution and natural selection is that now we consider sexual selection to be a subset of natural selection.
Darwin considered sexual selection as different from natural selection. His view of natural selection equated it with survival differentials between individuals. He separated the phenomena of sexual selection from natural selection. Sexual selection consists of competition for mates among individuals, usually males for females, and mate choice, usually by females among males. Darwin separated the two, because at times, it is not the individual that survives the longest that is preferred by females or even proves the better competitor for mates.
Sexual selection was not even discussed by Darwin, until his second text on evolution, The Descent of Man, and Selection in Relation to Sex.
"...My conviction of the power of sexual selection remains unshaken..." (C. Darwin, 1874, Preface) "We learn that the advantages which favored males derived from conquering other males in battle or courtship, and thus leaving a numerous progeny (male-male competition) are in the long run greater than those derived from rather more perfect adaptations to their conditions of life.We shall further see, and it could never have been anticipated, that the power to charm the female (female choice) has sometimes been more important than the power to conquer other males in battle" (Darwin, 1871, pg. 583)
We will examine female choice in more detail at the end of the term.
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Gene frequency
One way to examine natural selection or genetic drift at the micro level involves monitoring "gene" or allele frequency. It is important than that you understand what is meant by gene frequency and how to calculate gene or allele frequencies
Gene frequency=allele frequency=alternates that can exist at the same DNA site in the genome (old def.)
Dominant allele (R), uppercase or capital letters
Recessive alleles (r), lower case letters
Co-Dominant alleles (R1, R2, R3), numbers or subscripts.
An aside: (Beadle and Tatum proposed that for most "structural" or foundation genes, each gene directs the formation of one functional enzyme. For their work, Beadle and Tatum shared, with J. Lederberg, the 1958 Nobel Prize in Physiology or Medicine. Co-dominant alleles produce different varieties of functional enzyme. Recessive alleles produce no or nonfunctional enzyme.
Example, ABo blood types: AB type, two antigens, no antibodies---A or B types, one antigen, one antibody--- o type, no antigens, two antibodies. Type determined genetically, expressed before 3 months in the human embryo.)
So........ Count the number of alleles in the population of each kind. Divide by the number of total alleles for that site or "gene" in the population to obtain gene frequency.
An example:
Color is a co dominant trait (the heterozygotes differ phenotypically from both homozygotes). The pink individuals are heterozygotes. You sample a population of annual flowers in 2020 and find the following phenotypic frequencies:
Red Pink White
100 200 100Determine gene or allele frequency for the red (R1) and white (R2) alleles.
Allele frequency:
Freq. of R1= number of red alleles divided by the total number of alleles in the population = (2(100)+200)/(2(400))
Freq. of R1= 0.5
Freq. of R2=( 200+2(100))/(2(400))
Freq of (R2)= 0.5Homework
For homework, you will also be asked to revisit definitions of other terms that we will be using to examine the roles of natural selection and drift in shaping evolution in the coming weeks.
You will need to visit these web pages to answer homework questions. They are provided here and in the homework file because many browsers have problems with linking with hyper links supplied in Word docs.
Question 3: How would you begin to evaluate the fitness of the two different types of male crickets--------------- http://evolution.berkeley.edu/evolibrary/article/sneakermales_01
Question 5: On mutation and sexual reproduction (recombination). We are using these websites for basic definitions. We may re-examine some of the examples provided later.
Question 6: Artificial selection: Visit the website below for basic definition and examples. At this time, commit the mustard seed example to memory. Ignore the digging data example for now. https://evolution.berkeley.edu/evolibrary/article/evo_30