Genetics & Evolution Mendelian Genetics
When you think about genetics, what usually comes to mind? Many of us might quickly come to think of the progressive series of pictures of man growing from an ape to what we are now. As easy as it might be able to relate to this classic representation, especially because we share about 99.5% of our DNA with chimps, this depiction is very misleading. This depiction of growth is not what evolution is. To quote Michael Jon Jensem, “Evolution is not survival of the strongest, or failure of the weakest. Evolution is not fair; it's not predictable; it's not kind. Nor is it cruel, or chaotic, or unfair, for that matter. It's what happens when environmental pressures change.”
A major part of evolution is genetics, and because evolution is essentially the competition for survival, it is very important to know that those who survive pass on their genes to the next generation.
So, when boiling down everything, evolution can be thought as a change in gene frequency in a population over time. This change does not happen to individuals, but rather it happens to populations.
Obviously genetics is a major part of evolution, so what is important to know about genetics?
Genetics is the study of inheritance. What is inheritance though? Throughout the years many people have wondered how our children can look so similar to their parents. People understood how babies were made but lacked understanding of why babies came out the way they did. This is where Genetics kicks in:
In the 1860, Austrian monk Gregor Mendel began to discover patterns in inheritance. In his study, Mendel took a self-fertilizing pea and began to study and observe that physical characteristics of that peas offspring. Mendel took the offspring that had purple flowers and allowed them to self fertilize again. Mendel found that ¾ of that populations offspring had purple flowers and ¼ of them had white flowers. Mendel observed that each pea plant would have unique traits from their flower color to their pod size or type. Some of the traits that these plants would have were more dominant than others in his study. What is amazing about the study that Mendel made was that the ratio of dominant to recessive traits of the plant was 3:1. This means that the traits that were more dominant appeared 3 times more than the recessive (less common) ones.
In his conclusion to his experiment, Mendel came up with his law of independent assortment, which essentially says that all of the traits that he observed in the plants were independent of one another. This meant that just because one pea plant had one trait did not mean it had to have another. Mendel also concluded that each parent contributed a discrete unit for each trait. We know these “units” today as alleles/ genes. The second part of his conclusion was the law of segregation, which says that since every parent has 2 discrete units (genes/alleles) they must end up separating during egg/sperm formation.
How does Mendel’s study relate back to inheritance? Through his study, Mendel was able to figure out that traits are coded by genes. Each gene that an offspring would have was made up if 2 alleles. In every gene of an offspring, there must be 2 alleles, one from the mom and one from the dad. What is most amazing about Mendel’s study is that he did everything in his study without knowing what DNA, Genes, Alleles, and Chromosomes were. These gene combinations can be expressed in the offspring through various traits. These traits can be one of 3 things: dominant, recessive, or incomplete. Dominant alleles always mask or cover the traits of the recessive allele. Recessive alleles are only expressed when they are paired together, which is why they were only expressed in 25% of the offspring in his study. In incomplete alleles, neither the dominant or recessive trait takes precedence over the other. Today, we can predict the genotype/ phenotype of an offspring if we know the mom and dad’s genotype through a Punnett Square.
If Mendel did not know what any of these things were, how can we relate what he discovered to our knowledge of genetics today?
To start, DNA is deoxyribonucleic acid and is found in the nucleus of almost all of our cells. DNA is a very long molecule and is organized into individual units called chromosomes. Genes, are regions of DNA on a chromosome. Genes hold the code for each specific trait. Where the gene is located on a chromosome is what’s called the locus. Two alleles at a locus make a chromosome.
Mitosis Mitosis is the process in which an exact replica of a cell is made, this replica is called a daughter cell. The replication of cells (mitosis) is used for cell growth, cell replacement and asexual reproduction* but how do these cells replicate?
The first step of mitosis is the condensing and copying of DNA. Within the cell are two identical DNA molecules called sister chomratids, these sister chromatids line up in the middle of the cell and the spinal fibers pull them apart. At this stage, the cell pinches apart to form two new cells with identical DNA. To give you a better understanding of the steps of mitosis we have provided for you the figure below.
A gamete is a mature male or female germ cell usually possessing a haploid chromosome set and capable of initiating formation of a new diploid individual by fusion with a gamete of the opposite sex (merriamwebseter.com)
*Asexual reproduction is when an organism reproduces without a mate therefore foregoing meiosis. In asexual reproduction, the offspring is a clone of the parent and therefore results in low genetic variation in the species as a whole.
Meiosis
Meiosis is the process where gametes are made for sexual reproduction. There are many steps to meiosis, that’s why they are split up into different groups.
Before meiosis begins, genetic material is duplicated. First, keep in mind that each chromosome consists of two, closely associated sister chromatids. Then they swap “segments” and this process is called crossing over. This means two chromosomes pair up and exchange segments of their genetic material. This happens when the chromosomes line up in the middle of the cell and after they cross over, the homologous pairs separate, but sisters always stay together. Then they split to form two daughter cells (keep in mind the sisters stay together). The chromatids arrive at opposite sides of the cell and separate into four haploid cells. Each haploid is a gamete.
Molecular Genetics Cells make proteins for the cells, but how do they do this? DNA is copied onto a transcribed copy called messenger RNA (mRNA). Transcription is kind of like copying a recipe for making a certain kind of cookie. When the mRNA is in the cytoplasm, it finds a ribosome (rRNA) to help translate the code. The ribosomes then read, or translate, the mRNA. This is known as translation. Then transfer RNA (tRNA) picks up theamino acids. It strings the amino acids together at the ribosome, making a protein. The tRNA gets the amino acids from your cells that have amino acids floating around from digestion.
The genetic code is the set of rules by which information encoded in mRNA sequences is translated into proteins by living cells. The code defines a mapping between tri-nucleotide sequences, called codons, and amino acids. The four base pairs are guanine, adenine, thymine, and cytosine. Thymine and adenine usually pair up, and so do cytosine and guanine. During RNA transcription though, uracil replaces thymine and pairs with adenine. Occasionally though, an error can occur during this process, known as a mutation. A Mutation is a permanent alteration in an organism’s DNA. Mutations by replication errors can happen in two ways: point mutationswhich is a type of mutation that causes the replacement of a single base nucleotide with another nucleotide of the genetic material, DNA or RNA. The other way is frameshift mutations which is a genetic mutation caused by insertions, where an extra base is added into the gene region, or deletions, where a base is remover from the gene region Both insertions and deletions shift the reading frame. Mutations can also be caused by radiation or chemical damage to DNA. Radiation such as x-rays and exposure to UV rays like in tanning beds can increase your chances of getting a mutation. There are also many chemical mutagens, agents that increase the risk of getting mutations, such as tobacco smoke, burnt food, high temp cooking, and sodium azide.
Microevolution Microevolution is when the genes in a certain population change over time. Keep in mind that evolution does not occur in individuals, but in whole populations. Individuals are not able to change by themselves and in turn, cannot expect to survive within their population, that’s why the whole population has to evolve together. As a result of this, the same species are able to reproduce, but once they change, they are not able to reproduce. For example, a donkey and a horse can mate but the mule cannot reproduce because it does not have the “parts” to do so. A mule cannot mate with a horse and a mule cannot mate with a donkey. It just doesn’t work like that! The term “genetic fitness” is referring to how able someone is to reproduce. If you are able to reproduce more than once, you are genetically fit! Natural selection is the “process by which traits become more or less common in a population due to consistent effects upon the survival or reproduction of their bearers.” In other words, natural selection is when a trait becomes more or less popular because of different changes happening in an environment. Natural selection decreases genetic variation because it is narrowing down the selection. Sexual selection is another type of selection that animals use. This is when an animal is attracted to a mate of the opposite sex solely due to the fact that something they have appeals to them. An example of this is a Red Deer. The Red Deer has very large antlers, which is very attractive to the females in that population. The only problem is, is when a male has small antlers, they become less attractive to the females and therefore are not passing their genes on, causing a decrease in genetic variation.
Allele frequencies are always changing, sometimes by selection and sometimes by chance. Going along with this, gene flow is the movement of alleles between populations by inbreeding, migration, seed dispersal and cross-pollination. This is basically when one population joins another. An example of this in humans is world travel, which has increased gene flow. Humans have much easier access to world travel because of planes, trains, buses, and all the other sources of transportation. It’s very easy to take a plane to another country now. As a result of this, many people find someone they want to mate with essentially, and this is an example of gene flow: when one population joins another. Another way that causes allele frequencies to change is by genetic drift. Genetic drift is when an allele changes because of random sampling. This is proving that allele frequencies don’t always change by selection it can also be random. This can happen when organisms die by accident, any random events, and alleles being removed from the gene pool. A gene pool is simply the unique set of alleles in a species or population.
Certain things can happen that will cause the population size to decrease. One of these things is called the “bottleneck” effect. This is when a dramatic reduction in population size occurs due to factors like weather, disease, and overhunting. An example of this is the elephant seals. So many of the seals were being hunted and just about fifty were left. They were able to pass a law that forbade anyone to hunt the elephant seals and therefore, the population was able to grow back to about 50,000. Another thing that causes dramatic population decrease is the “founder effect.” The founder effect is when a few alleles start a new population. An example of this is the Amish settlement in Pennsylvania. These humans decided that they were going to only produce within their tiny population and this caused a very small settlement to take place. As a result of this, many of the children were inbred and were born with mutations.
A mutation is a permanent alteration in an organism’s DNA. This alteration can be big or small, positive, negative or indifferent and it’s always random. Having a mutation definitely increases the genetic variation there is in a gene pool. There are only two ways to get a mutation, by radiation or chemical damage to DNA and replication errors. Some of the mutagens that can happen are when someone gets an X-Ray while pregnant or being exposed to UV radiation (skin cancer). Some chemical mutagens are things like asbestos, tobacco smoke, high temperature cooking, and burnt food. A replication error can happen in two ways: base-pair substitution (point mutation) and a frameshift mutation. A point mutation causes the replacement of a single base nucleotide (Adenine, Thymine, Guanine, and Cytosine). This also includes insertions or deletions of one of these bases. An example of a point mutation is Hutchinson-Gilford Progeria Syndrome, which is when a child is born with older features than what they should have, like brittle bones and wrinkly skin. The mutation causes cells to die prematurely because of an unstable nuclear membrane. These poor children usually die of a heart attack before the age of 13. A frameshift mutation is an insertion or deletion in the gene region. An insertion would be an extra base added on, resulting in something like Down syndrome (which happens on the 21st chromosome). A deletion would be a removal of a base. Mutations are not always bad though. They can be an advantage, disadvantage, or make no difference (be indifferent).
Macroevolution Macroevolution is basically all about gene pools and what happens within them. Speciation is when new biological species are made; this is due to isolation and separation of one species, which turns them into two separate species. There are many barriers and things that hold back certain species from mating. Intrinsic isolating methods are one of them; this is when there is a major obstacle to interbreeding, such as a structural difference in species parts. Geographic barriers also have a hindrance in whether a specie can mate or not. This is when one species gets separated and after living in such different locations, cannot mate when brought back together. A good example of this is the is the now called “Nene goose.” The Nene started in Canada and tried to fly away for the winter, but got hit by a huge storm and were pushed to Hawaii. Adapting to the different climate of Hawaii was not too hard for these geese, but when brought back together with other Canadian geese, they were not able to mate because they had a geographic difference now that prevented them from mating. Extrinsic Isolating mechanisms are those that prevent species from mating due to what they look like. For example, if a male peacock loses the “eyes” on his feathers, many females will not want to mate because they don’t look very appealing to them anymore. Ecological isolation is even when a specie lives in the same place, they do not come in contact with each other, just because that’s how it is. The fossil record is something that just shows different species lived on the earth at different times, but it is not always accurate because it does not show the evolutionary change in a species over time. Temporal isolation is when one species breeds at different times, even if they come in contact, they can’t breed because they do it at different times. Behavioral isolation is when a specie is just not attracted to one another and therefore will not mate. Mechanical isolation is when two animals are not physically compatible, their parts just don’t match up. Gametic isolation is even if they are physically compatible an embryo will not from if the egg and sperm do not fuse properly. Hybrid infertility, this is even if the egg and sperm fuse, the offspring may not survive or if it does survive it will not be able to reproduce like a mule or a liger.
Mendelian Genetics
When you think about genetics, what usually comes to mind? Many of us might quickly come to think of the progressive series of pictures of man growing from an ape to what we are now. As easy as it might be able to relate to this classic representation, especially because we share about 99.5% of our DNA with chimps, this depiction is very misleading. This depiction of growth is not what evolution is. To quote Michael Jon Jensem, “Evolution is not survival of the strongest, or failure of the weakest. Evolution is not fair; it's not predictable; it's not kind. Nor is it cruel, or chaotic, or unfair, for that matter. It's what happens when environmental pressures change.”
A major part of evolution is genetics, and because evolution is essentially the competition for survival, it is very important to know that those who survive pass on their genes to the next generation.
So, when boiling down everything, evolution can be thought as a change in gene frequency in a population over time. This change does not happen to individuals, but rather it happens to populations.
Obviously genetics is a major part of evolution, so what is important to know about genetics?
Genetics is the study of inheritance. What is inheritance though? Throughout the years many people have wondered how our children can look so similar to their parents. People understood how babies were made but lacked understanding of why babies came out the way they did. This is where Genetics kicks in:
In the 1860, Austrian monk Gregor Mendel began to discover patterns in inheritance. In his study, Mendel took a self-fertilizing pea and began to study and observe that physical characteristics of that peas offspring. Mendel took the offspring that had purple flowers and allowed them to self fertilize again. Mendel found that ¾ of that populations offspring had purple flowers and ¼ of them had white flowers. Mendel observed that each pea plant would have unique traits from their flower color to their pod size or type. Some of the traits that these plants would have were more dominant than others in his study. What is amazing about the study that Mendel made was that the ratio of dominant to recessive traits of the plant was 3:1. This means that the traits that were more dominant appeared 3 times more than the recessive (less common) ones.
In his conclusion to his experiment, Mendel came up with his law of independent assortment, which essentially says that all of the traits that he observed in the plants were independent of one another. This meant that just because one pea plant had one trait did not mean it had to have another. Mendel also concluded that each parent contributed a discrete unit for each trait. We know these “units” today as alleles/ genes. The second part of his conclusion was the law of segregation, which says that since every parent has 2 discrete units (genes/alleles) they must end up separating during egg/sperm formation.
How does Mendel’s study relate back to inheritance? Through his study, Mendel was able to figure out that traits are coded by genes. Each gene that an offspring would have was made up if 2 alleles. In every gene of an offspring, there must be 2 alleles, one from the mom and one from the dad. What is most amazing about Mendel’s study is that he did everything in his study without knowing what DNA, Genes, Alleles, and Chromosomes were. These gene combinations can be expressed in the offspring through various traits. These traits can be one of 3 things: dominant, recessive, or incomplete. Dominant alleles always mask or cover the traits of the recessive allele. Recessive alleles are only expressed when they are paired together, which is why they were only expressed in 25% of the offspring in his study. In incomplete alleles, neither the dominant or recessive trait takes precedence over the other. Today, we can predict the genotype/ phenotype of an offspring if we know the mom and dad’s genotype through a Punnett Square.
If Mendel did not know what any of these things were, how can we relate what he discovered to our knowledge of genetics today?
To start, DNA is deoxyribonucleic acid and is found in the nucleus of almost all of our cells. DNA is a very long molecule and is organized into individual units called chromosomes. Genes, are regions of DNA on a chromosome. Genes hold the code for each specific trait. Where the gene is located on a chromosome is what’s called the locus. Two alleles at a locus make a chromosome.
Mitosis
Mitosis is the process in which an exact replica of a cell is made, this replica is called a daughter cell. The replication of cells (mitosis) is used for cell growth, cell replacement and asexual reproduction* but how do these cells replicate?
The first step of mitosis is the condensing and copying of DNA. Within the cell are two identical DNA molecules called sister chomratids, these sister chromatids line up in the middle of the cell and the spinal fibers pull them apart. At this stage, the cell pinches apart to form two new cells with identical DNA. To give you a better understanding of the steps of mitosis we have provided for you the figure below.
A gamete is a mature male or female germ cell usually possessing a haploid chromosome set and capable of initiating formation of a new diploid individual by fusion with a gamete of the opposite sex (merriamwebseter.com)
*Asexual reproduction is when an organism reproduces without a mate therefore foregoing meiosis.
In asexual reproduction, the offspring is a clone of the parent and therefore results in low genetic variation in the species as a whole.
Meiosis
Meiosis is the process where gametes are made for sexual reproduction. There are many steps to meiosis, that’s why they are split up into different groups.
Before meiosis begins, genetic material is duplicated. First, keep in mind that each chromosome consists of two, closely associated sister chromatids. Then they swap “segments” and this process is called crossing over. This means two chromosomes pair up and exchange segments of their genetic material. This happens when the chromosomes line up in the middle of the cell and after they cross over, the homologous pairs separate, but sisters always stay together. Then they split to form two daughter cells (keep in mind the sisters stay together). The chromatids arrive at opposite sides of the cell and separate into four haploid cells. Each haploid is a gamete.
Molecular Genetics
Cells make proteins for the cells, but how do they do this? DNA is copied onto a transcribed copy called messenger RNA (mRNA). Transcription is kind of like copying a recipe for making a certain kind of cookie. When the mRNA is in the cytoplasm, it finds a ribosome (rRNA) to help translate the code. The ribosomes then read, or translate, the mRNA. This is known as translation. Then transfer RNA (tRNA) picks up theamino acids. It strings the amino acids together at the ribosome, making a protein. The tRNA gets the amino acids from your cells that have amino acids floating around from digestion.
The genetic code is the set of rules by which information encoded in mRNA sequences is translated into proteins by living cells. The code defines a mapping between tri-nucleotide sequences, called codons, and amino acids. The four base pairs are guanine, adenine, thymine, and cytosine. Thymine and adenine usually pair up, and so do cytosine and guanine. During RNA transcription though, uracil replaces thymine and pairs with adenine.
Occasionally though, an error can occur during this process, known as a mutation. A Mutation is a permanent alteration in an organism’s DNA. Mutations by replication errors can happen in two ways: point mutationswhich is a type of mutation that causes the replacement of a single base nucleotide with another nucleotide of the genetic material, DNA or RNA. The other way is frameshift mutations which is a genetic mutation caused by insertions, where an extra base is added into the gene region, or deletions, where a base is remover from the gene region Both insertions and deletions shift the reading frame. Mutations can also be caused by radiation or chemical damage to DNA. Radiation such as x-rays and exposure to UV rays like in tanning beds can increase your chances of getting a mutation. There are also many chemical mutagens, agents that increase the risk of getting mutations, such as tobacco smoke, burnt food, high temp cooking, and sodium azide.
Microevolution
Microevolution is when the genes in a certain population change over time. Keep in mind that evolution does not occur in individuals, but in whole populations. Individuals are not able to change by themselves and in turn, cannot expect to survive within their population, that’s why the whole population has to evolve together. As a result of this, the same species are able to reproduce, but once they change, they are not able to reproduce. For example, a donkey and a horse can mate but the mule cannot reproduce because it does not have the “parts” to do so. A mule cannot mate with a horse and a mule cannot mate with a donkey. It just doesn’t work like that! The term “genetic fitness” is referring to how able someone is to reproduce. If you are able to reproduce more than once, you are genetically fit! Natural selection is the “process by which traits become more or less common in a population due to consistent effects upon the survival or reproduction of their bearers.” In other words, natural selection is when a trait becomes more or less popular because of different changes happening in an environment. Natural selection decreases genetic variation because it is narrowing down the selection. Sexual selection is another type of selection that animals use. This is when an animal is attracted to a mate of the opposite sex solely due to the fact that something they have appeals to them. An example of this is a Red Deer. The Red Deer has very large antlers, which is very attractive to the females in that population. The only problem is, is when a male has small antlers, they become less attractive to the females and therefore are not passing their genes on, causing a decrease in genetic variation.
Allele frequencies are always changing, sometimes by selection and sometimes by chance. Going along with this, gene flow is the movement of alleles between populations by inbreeding, migration, seed dispersal and cross-pollination. This is basically when one population joins another. An example of this in humans is world travel, which has increased gene flow. Humans have much easier access to world travel because of planes, trains, buses, and all the other sources of transportation. It’s very easy to take a plane to another country now. As a result of this, many people find someone they want to mate with essentially, and this is an example of gene flow: when one population joins another. Another way that causes allele frequencies to change is by genetic drift. Genetic drift is when an allele changes because of random sampling. This is proving that allele frequencies don’t always change by selection it can also be random. This can happen when organisms die by accident, any random events, and alleles being removed from the gene pool. A gene pool is simply the unique set of alleles in a species or population.
Certain things can happen that will cause the population size to decrease. One of these things is called the “bottleneck” effect. This is when a dramatic reduction in population size occurs due to factors like weather, disease, and overhunting. An example of this is the elephant seals. So many of the seals were being hunted and just about fifty were left. They were able to pass a law that forbade anyone to hunt the elephant seals and therefore, the population was able to grow back to about 50,000. Another thing that causes dramatic population decrease is the “founder effect.” The founder effect is when a few alleles start a new population. An example of this is the Amish settlement in Pennsylvania. These humans decided that they were going to only produce within their tiny population and this caused a very small settlement to take place. As a result of this, many of the children were inbred and were born with mutations.
A mutation is a permanent alteration in an organism’s DNA. This alteration can be big or small, positive, negative or indifferent and it’s always random. Having a mutation definitely increases the genetic variation there is in a gene pool. There are only two ways to get a mutation, by radiation or chemical damage to DNA and replication errors. Some of the mutagens that can happen are when someone gets an X-Ray while pregnant or being exposed to UV radiation (skin cancer). Some chemical mutagens are things like asbestos, tobacco smoke, high temperature cooking, and burnt food. A replication error can happen in two ways: base-pair substitution (point mutation) and a frameshift mutation. A point mutation causes the replacement of a single base nucleotide (Adenine, Thymine, Guanine, and Cytosine). This also includes insertions or deletions of one of these bases. An example of a point mutation is Hutchinson-Gilford Progeria Syndrome, which is when a child is born with older features than what they should have, like brittle bones and wrinkly skin. The mutation causes cells to die prematurely because of an unstable nuclear membrane. These poor children usually die of a heart attack before the age of 13. A frameshift mutation is an insertion or deletion in the gene region. An insertion would be an extra base added on, resulting in something like Down syndrome (which happens on the 21st chromosome). A deletion would be a removal of a base. Mutations are not always bad though. They can be an advantage, disadvantage, or make no difference (be indifferent).
Macroevolution
Macroevolution is basically all about gene pools and what happens within them. Speciation is when new biological species are made; this is due to isolation and separation of one species, which turns them into two separate species. There are many barriers and things that hold back certain species from mating. Intrinsic isolating methods are one of them; this is when there is a major obstacle to interbreeding, such as a structural difference in species parts. Geographic barriers also have a hindrance in whether a specie can mate or not. This is when one species gets separated and after living in such different locations, cannot mate when brought back together. A good example of this is the is the now called “Nene goose.” The Nene started in Canada and tried to fly away for the winter, but got hit by a huge storm and were pushed to Hawaii. Adapting to the different climate of Hawaii was not too hard for these geese, but when brought back together with other Canadian geese, they were not able to mate because they had a geographic difference now that prevented them from mating. Extrinsic Isolating mechanisms are those that prevent species from mating due to what they look like. For example, if a male peacock loses the “eyes” on his feathers, many females will not want to mate because they don’t look very appealing to them anymore. Ecological isolation is even when a specie lives in the same place, they do not come in contact with each other, just because that’s how it is. The fossil record is something that just shows different species lived on the earth at different times, but it is not always accurate because it does not show the evolutionary change in a species over time. Temporal isolation is when one species breeds at different times, even if they come in contact, they can’t breed because they do it at different times. Behavioral isolation is when a specie is just not attracted to one another and therefore will not mate. Mechanical isolation is when two animals are not physically compatible, their parts just don’t match up. Gametic isolation is even if they are physically compatible an embryo will not from if the egg and sperm do not fuse properly. Hybrid infertility, this is even if the egg and sperm fuse, the offspring may not survive or if it does survive it will not be able to reproduce like a mule or a liger.