Heredity and Evolution Notes

Introduction

Heredity refers to the passing of characteristics from one generation to the next. Evolution is defined as the gradual process by which a simple life form leads to the development of complex organisms over a period of time, spanning several generations.

Heredity

Heredity and evolution is the transmission of characters from parents to their children. An inherited trait that children get from their parents is a genetically determined feature that makes humans distinguishable from person to person. One of the most common examples of heredity and evolution is the free or attached ears, which we can see around us in humans.

Heredity derives from the Latin word Hereditatem, which means “condition of being an heir.”  It refers to the passing of characteristics or traits through parents’ genes to the offspring (i.e., from one generation to another.) Heredity is seen in sexual reproduction due to which the variations of inherited characteristics are high. The study of heredity and these genetic variations is called genetics.

Acquired Traits vs Inherited Traits

Acquired traits: Acquired traits are traits that cannot be inherited over generations or passed on the DNA as they are a result of changes in non-reproductive tissues. For example, situation 3, callus on fingers, increase in muscle mass to evade predators, etc.

Inherited traits: Inherited traits are those which are passed on from generation to generation and are caused due to changes in the DNA. For example, situation 1, eye color, hair color, etc.

 

Rules for the inheritance of traits:
Mendel’s contribution: The rules for inheritance of traits in human beings are related to the fact that both mother and father contribute an equal amount of genetic material i.e. DNA to their offspring. So an offspring will get two versions of that trait from the two parents. Mendel worked out rules for inheritance of these traits. Gregor Johann Mendel regarded as the ‘Father of Genetics’ performed his experiments with garden peas (Pisum sativum) in the garden behind his monastery. He observed a number of contrasting characters in garden peas and observed their inheritance.

ACCUMULATION OF VARIATION DURING REPRODUCTION

• The long term accumulation of variations may lead to gradual changes in the form or functions of organisms and may even lead to the formation of a new species over time. This process is known as evolution.
• In case an organism reproduces by asexual reproduction, one organism gives rise to two individuals which are similar in body design, but having subtle differences. These will in turn give rise to two individuals in the next generation. In this way, the four individuals formed will be different from each other.
• If sexual reproduction is involved, greater diversity will be generated in the offspring as compared to asexual reproduction where only minor differences would be generated due to small inaccuracies in DNA copying. Depending on the nature of variations, different individuals would have different kinds of advantages. The Selection of variants by environmental factors forms the basis for evolutionary processes.

Variations and its causes

Differences in the traits of parents and offsprings are known as variations. They are caused by mutations (errors in DNA copying), recombination, and random mating. Variations are mainly of two types:

1. Somatic Variation: This type of genetic variation occurs in somatic cells (all cells except reproductive cells). As they are not transmitted or inherited by the next generation, they’re also called acquired traits.
2. Gametic Variation: This genetic mutation occurs in germline cells (reproductive cells). The next generation may inherit these. Therefore, they’re also known as inherited traits.

Importance of variations

• They form the basis of heredity.
• They are essential as they contribute to evolution.
• It helps the organism to survive and adapt to its changing environment.

Some important terms

• Chromosomes: A thread like structure of nucleic acids and protein found in the nucleus of most living cells, carrying genetic information in the form of genes.
• DNA: A self-replicating material that is present in nearly all living organisms as the main constituent of  chromosomes. It is the carrier of genetic information.
• Gene: A unit of heredity which is transferred from a parent to offspring and is held to determine some characteristic of the offspring.
• Allele: Each of two or more alternative forms of a gene that arise by mutation and are found at the same place on a chromosome.
• Dominant trait: E dominant trait is an inherited characteristic that appears in an offspring if it is contributed from a parent through a dominant allele.
• Recessive trait: A recessive trait is being or produced by a form of a gene whose effect can be hidden by a dominant gene and which can produce a noticeable effect only when two copies of the gene are present.
• Homozygous: The presence of two identical alleles at a particular gene locus. A homozygous genotype may include two normal alleles or two allele that have the same variant.
• Heterozygous: The presence of two different alleles at a particular gene locus. A heterozygous genotype may include one normal allele and one mutated allele or two different mutated alleles.
• Genotype:The genotype of an organism is its complete set of genetic material. it also refers to the alleles or variants an individual carries in a particular gene or genetic location.
• Phenotype: It refers to the observable physical properties of an organisms appearance , development, development, and behavior. An organisms phenotype is determined by its genotype, which is the set of genes the organism carries, as well as by environmental influences upon these genes.

Mendel’s contribution to Genetics

Gregor Johann Mendel(1822-1884) is often known as the father of genetics because of his pioneering work on Pisum sativum (garden pea). His experiment studied seven pairs of contrasting traits and their inheritance:

1. Pea shape : Round or Wrinkled
2. Pea color : Green or Yellow
3. Pod shape : Constricted or Inflated
4. Pod color : Green or Yellow
5. Flower color : Purple or White
6. Plant size : Tall or Dwarf
7. Position of flowers : Axial or Terminal

Mendel these contrasting visible characters of garden peas – round/wrinkled seeds, tall/short plants, white/violet flowers and so on. He took pea plants with different characteristics for eg; – a tall plant and a short plant, produced progeny from them, and calculated the percentages of tall or short progeny.

Why Mendel choose the pea plant

• They  produces a large number of seeds
• They have shorter life cycle
• They are self-pollinating and thus cross pollination can easily performed
• There are various contrasting traits can be observed.

Mendel’s three laws of inheritance

Laws of Inheritance

Mendel proposed three laws:

• Law of Dominance
• The Law of Segregation
• Law of independent assortment

Law of Dominance

This law states that in a heterozygous condition, the allele whose characters are expressed over the other allele is called the dominant allele and the characters of this dominant allele are called dominant characters. The characters that appear in the F₁ generation are called as dominant characters. The recessive characters appear in the F₂ generation.

Law of Segregation

This law states that when two traits come together in onehybrid pair, the two characters do not mix with each other and are independent of each other. Each gamete receives one of the two alleles during meiosis of the chromosome.

Mendel’s law of segregations supports the phenotypic ratio of 3:1 i.e. the homozygous dominant and heterozygous offspring show dominant traits while the homozygous recessive shows the recessive trait.

Law of Independent Assortment

This means that at the time of gamete formation, the two genes segregate independently of each other as well as of other traits. Law of independent assortment emphasizes that there are separate genes for separate traits and characters and they influence and sort themselves independently of the other genes.

This law also says that at the time of gamete and zygote formation, the genes are independently passed on from the parents to the offspring.

Monohybrid vs. Dihybrid Cross

Monohybrid Cross

A monohybrid cross is a cross between f1 generation organisms (parents) that differ in a single contrasting trait. Here’s how you can use a Punnett square to predict the probable genetic outcomes of a monohybrid cross.

The experiment to observe the inheritance of one pair of contrasting characters was conducted as follows. Mendel chose a pure tall plant (genotype TT) and pure dwarf plant (genotype tt) pea plants and crossbred them to achieve a first (filial) generation. As shown in the figure, he obtained only tall plants. This indicated that only a single parental trait was observed instead of a mixture of two.

After then , the plants of the F generation were self pollinated to get F₂ generation. In this case, Mendel observed distribution of 75% tall plants and 25% dwarf plants (phenotypic ratio 3:1). This indicated that in the F generation, tall plants were expressed as a dominant trait and dwarf plant as recessive. However, in the F2 generation, the genotype ratio is 1:2:1 i.e Pure tall plant : Hybrid tall plant : Pure dwarf plant.

Dihybrid Cross

A dihybrid cross is a breeding experiment where two pairs of contrasting characters are crossed between two plants. Let’s understand this in more detail.

For this experiment, Mendel crossbred pea plants that produced round green seeds and yellow wrinkled seeds. He observed that in the Fx generation, he got round and yellow seed which indicated that yellow and round are dominant traits while green and wrinkled are recessive. He then proceeded to self pollinate the plants in Fx generation to achieve F2 generation. The results of this dihybrid cross as follows:

9:3:3:1 where,
9 – round and yellow seeds
3- wrinkled and yellow seeds
1- wrinkled and green seeds

Sex determination

• Sex determination is a complex system of genes, proteins coded by them and their functions on various target organs that cumulatively determine the development of sexual characteristics in an organism.
• The process of sex determination in an organism start with the development of the male and female reproductive structures right at the stage pf embryonic development.
• The sex determination system is switched on in undifferentiated zygote depending on the type of sex chromosomes present in it.

Chromosomes  

• Chromosomes are rod-shaped structures which become clearly visible during cell division in the cell. They are highly supercoiled DNA molecules that remain coiled around histone and non-histone proteins.
• The chromosomes are present as less coiled thread-like structures called chromatin in the nucleus of a non-dividing cell forming a network called chromatin reticulum.
• The chromosome number in a species remains constant .

Types of sex chromosomes in males and females

• There are 46 chromosomes in the human cells. Out of these, 44 chromosomes are autosomes and have genes for characters which are not related to the formation of reproductive organs. The rest of the two are called sex chromosomes.
• There are two types of sex chromosomes, X and Y. females have 44 autosomes and 2X chromosomes. Males have 44 autosomes and one X and one Y chromosome.
• When the X containing sperm fertilizes the ova , female offspring are formed. When the ovum is fertilized by Y containing sperm, male offspring are formed. Hence the sex of the child is determined by the type of sex chromosome contributed by the father.
• Human males have only one X chromosome while human females have two X chromosomes .
• Hence, in the early stages of development , one of the two X chromosomes is permanently inactivated in the somatic cells so that gene expression of one X chromosome out of the two is restricted after the successful development of the sex organs in females.

Chromosomes mechanism of sex determination

• Once the egg of a female gets fertilized by the sperm of a male, it results in the formation of zygote.
• This zygote is the first cell of a new life and this cell contains the content of both the male and female cells and this zygote will divide again and again until it eventually forms a baby.
• In sexually reproducing organisms besides morphological and behavioral differences between male and female sexes, the chromosomal differences also occurs.
• A German biologist, Henking(1891) firstly noted that half of the spermatozoa contained an extra chromosome which was called by him is X body. The significance of X body was involved in some way in determination of sex . later on it was well known that the body was a chromosome involved in sex determination so it is called sex chromosome.
• In human females, all chromosomes are paired and are in equal size. In male, all the chromosome are paired but the chromosome prepared with X chromosome was distinctly smaller and was called Y chromosome.
• In dioecious diploid organisms , following two systems of chromosomal determination of sex have been recognized :
  • Heterogametic males
  • Heterogametic females

Heterogametic males

• In this type, the female sex has two X chromosomes, while the male has only one X chromosome. Therefore, during the gametogenesis male produce two types of gametes, 50% with X chromosome while 50% without X chromosome.
• So a sex which produces two different type of gametes is called heterogametic sex. The heterogametic male are of following two types :
• XX-XO Type : in certain insects such as Hemiptera and Orthoptera, the female has two X chromosomes while male has only one X chromosome.
• XX-XO Type : in mammals, certain insects and some angiosperm plants, the female has two X chromosomes, while male has one X chromosome and one Y chromosome.

Heterogametic females :

•  In this type, male sex has two sex chromosome (X chromosome), while the female has only one X-chromosome. Therefore, during gametogenesis females produce two types of eggs/ova, 50% with X chromosome while 50% without X chromosome.
• ZO-ZZ Type : in birds, moths and butterflies, the female has one Z chromosome while male has two Z chromosomes.
• ZW-ZZ Type : in fishes, reptiles, some birds and insects, the female has one Z chromosome and one W chromosome, while male have two chromosomes.

Evolution

• The theory of evolution is based on the idea that all species are related and gradually change over time.
• Evolution relies on there being genetic variation in a population which affects the physical characteristics(phenotype) of an organism.
• Some of these characteristics may give the individual an advantage over other individuals which they can then pass on to their offspring.
• The mechanisms of evolution operate at the genomic level. Changes in DNA sequences affect the composition and expression of our genes, the basic units of inheritance.
• Our evolutionary history is written into our genome. The human genome looks the way it does because of all the genetic changes that affected our ancestors.
• When DNA and genes in different species look very similar, this is usually taken as evidence of them sharing ancestors.
• For example, humans and the fruit fly , Drosophila melanogaster, share much of their DNA. 75% of genes that cause diseases in humans are also found in the fruit fly.
• DNA accumulates changes over time. Some of these changes can be beneficial, and provide a selective advantage for an organism.
• Other changes may be harmful if they affect an important, everyday function. As a result some genes do not change much. They are said to be conserved.

A classic thought experiment to understand how evolution occurs. Consider a group of twelve red beetles that live in green bushes. They are able to sexually reproduce and are therefore expected to develop variations. Here are the possible outcomes:

Situation 1: Crows can eat these beetles by easily picking them out by color. As a result of color variations (due to sexual reproduction), green beetles start to appear and their population eventually increases. Crows are unable to see them so their population increases while that of red beetles decreases. This kind of variation gives a better chance of surviving.

Situation 2: A few blue beetles appear due to color variation. Crows can pick out the red and blue beetles by color and eat them. While there are more red beetles than blue ones initially, a natural calamity like an elephant stomping on mostly red ones. This allows the blue beetles to grow in population. This scenario does not provide any survival advantage but does improve diversity without adaptation.

Situation 3: An increase in the population of beetles leads to a disease in the bushes which eventually reduces the population of beetles in the absence of food. However, when the bushes are free of diseases in a few years, the beetle population grows back due to availability of food. This kind of change is not inherited.

Speciation

Speciation occurs when a group within a species separates from other members of its species and develops its own unique characteristics. The demands of a different environment or the characteristics of the members of the new group will differentiate the new species from their ancestors.

An example of speciation is the Galápagos finch. Different species of these birds live on different islands in the Galápagos archipelago, located in the Pacific Ocean off South America. The finches are isolated from one another by the ocean. Over millions of years, each species of finch developed a unique beak that is especially adapted to the kinds of food it eats. Some finches have large, blunt beaks that can crack the hard shells of nuts and seeds. Other finches have long, thin beaks that can probe into cactus flowers without the bird being poked by the  Because they are isolated, the birds don’t breed with one another and have therefore developed into unique species with unique characteristics. This is called allopatric speciation.

There are five types of speciation: allopatric, peripatric, parapatric, and sympatric and artificial.

Allopatric speciation occurs when a species separates into two separate groups which are isolated from one another. A physical barrier, such as a mountain range or a waterway, makes it impossible for them to breed with one another. Each species develops differently based on the demands of their unique habitat or the genetic characteristics of the group that are passed on to offspring.

When Arizona’s Grand Canyon formed, squirrels and other small mammals that had once been part of a single population could no longer contact and reproduce with each other across this new geographic barrier. They could no longer interbreed. The squirrel population underwent allopatric speciation. Today, two separate squirrel species inhabit the north and south rims of the canyon. On the other hand, birds and other species that could easily cross this barrier continued to interbreed and were not divided into separate populations.

When small groups of individuals break off from the larger group and form a new species, this is called peripatric speciation . As in allopatric speciation, physical barriers make it impossible for members of the groups to interbreed with one another. The main difference between allopatric speciation and peripatric speciation is that in peripatric speciation, one group is much smaller than the other. Unique characteristics of the smaller groups are passed on to future generations of the group, making those traits more common among that group and distinguishing it from the others.

In parapatric speciation , a species is spread out over a large geographic area. Although it is possible for any member of the species to mate with another member, individuals only mate with those in their own geographic region. Like allopatric and peripatric speciation, different habitats influence the development of different species in parapatric speciation. Instead of being separated by a physical barrier, the species are separated by differences in the same environment.

Parapatric speciation sometimes happens when part of an environment has been polluted. Mining activities leave waste with high amounts of metals like lead and zinc. These metals are absorbed into the soil, preventing most plants from growing. Some grasses, such as buffalo grass, can tolerate the metals. Buffalo grass, also known as vanilla grass, is native to Europe and Asia, but is now found throughout North and South America, too. Buffalo grass has become a unique species from the grasses that grow in areas not polluted by metals. Long distances can make it impractical to travel to reproduce with other members of the species. Buffalo grass seeds pass on the characteristics of the members in that region to offspring. Sometimes a species that is formed by parapatric speciation is especially suited to survive in a different kind of environment than the original species.

Sympatric speciation  is controversial. Some scientists don’t believe it exists. Sympatric speciation occurs when there are no physical barriers preventing any members of a species from mating with another, and all members are in close proximity to one another. A new species, perhaps based on a different food source or characteristic, seems to develop spontaneously. The theory is that some individuals become dependent on certain aspects of an environment—such as shelter or food sources—while others do not.

A possible example of sympatric speciation is the apple maggot, an insect that lays its eggs inside the fruit of an apple, causing it to rot. As the apple falls from the tree, the maggots dig in the ground before emerging as flies several months later. The apple maggot originally laid its eggs in the fruit of a relative of the apple—a fruit called a hawthorn. After apples were introduced to North America in the 19th century, a type of maggot developed that only lays its eggs in apples. The original hawthorn species still only lays its eggs in hawthorns. The two types of maggots are not different species yet, but many scientists believe they are undergoing the process of sympatric speciation.

Artificial speciation is the creation of new species by people. This is achieved through lab experiments, where scientists mostly research insects like fruit flies.

 

Darwin’s Theory of Evolution

Also known as “Theory of natural selection.”

Postulates of Darwin theory

1. Speciation (formation of species)– When Variations occur within a species, it can only be passed to the next generation.
2. The struggle of existence – An increase in the number of organisms, limited space, and food creates competition between the organisms.
3. Survival of the fittest or Natural selection- It is a process governed by nature in which organisms inherent characteristics that are fittest for their survival, such that they are adaptive to prevailing conditions.

Evolution and Classification: The organisms show certain features, like appearance and behaviour which are called characteristics for example; Plants can perform photosynthesis. The basic characteristics are shared by a large number of organisms. More characteristics which two species have in common more closely are related, if they are more closely related then they have common ancestors (explain the example of brother sister and cousins). Evolutionary relationships can be traced with the help of the following :

Homologous organs: Those organs which have the same basic structural design and developmental origin but perform different functions and appearance, for example; Forelimbs of frog, lizard, bird, bat and human beings. They have same design of bones but they perform different functions.

Analogous organs: Those organs which have different basic design and developmental origin but have similar appearance and perform a similar function, for example; wings of bat and bird. Wings of bat are folds of skin attached between fingers. But wing of birds are modified forelimbs.

Study of Fossils: Fossils are preserved remains of living organisms that lived in the past. When living organisms die their bodies decompose but some parts of their body may be in such an environment that they do not decompose for example; if a dead insect gets caught in hot mud it will not decompose quickly but the mud will harden and retain impressions of the body parts of the insects. These impressions are also called fossils: The age of fossil can be estimated in two ways :
The fossils that occur closer to earth surface are more recent to those found in deeper layers.
The second method is isotope dating i.e. detecting the ratio of different isotopes of the same element in the fossil material.

Significance of fossils: Fossils are formed layer by layer in the earths crust. The animals and plants which existed earlier are buried in the deeper layer which ones found in the upper layer. It is found that, deeper fossils have simpler structure than found than upper layer. Complete fossil record of animals like horse, camel, man has helped us to study the stages of evolution.

Evolution by stages:

The evolution of feathers is an example of evolution by stages such that earlier dinosaurs had feathers but couldn’t fly. However, later, birds used them for flying.

• Molecular Phylogeny: The changes in DNA during reproduction are also considered to be basic events of evolution. Organisms distantly related to each other carry a greater difference in their DNA.
• Evolution by artificial selection: This selects special phenotype characters to create organisms with enhanced characteristics, for example, producing different cabbage-like varieties such as red cabbage, broccoli, etc.

Some dinosaurs had feathers although they could not fly, this shows that birds are closely related to reptiles, since dinosaurs were reptiles Some dissimilar looking structures also evolved from common ancestors. The current example of such a process is wild cabbage plant from which different vegetables are generated by artificial selection rather than natural selection

• Selection of short distance between the leaves has led to formation of cabbage that, we eat.
• Selection for arrested flower development had led to broccoli,
• Selection for sterile flowers had made cauliflower,
• Selection for swollen-stem had formed kohlrabi.
• Selection for large leaves had formed leafy vegetable kale,
• Selection for colored leaves formed red cabbage.

therefore evolutionary relationships can be established by

• Study of Homologous organs
• Study of Analogous organs
• Study of fossils
• Changes in DNA during reproduction

Vestigial organs: These are rudimentary such that they have lost their functions through evolution—for example, Wisdom tooth, muscles of ears, etc.

Evolution and progress

Evolution should not be equated with progress, all animals have evolved and adapted to different environmental stressors that were placed upon them by nature.
We should remember that when we trace family trees of species, that there are multiple branches possibly existing at every stage of this process.

Evolution does not equate progress because:

• One species is not eliminated to give rise to a new one. The old species simply does not disappear after the emergence of a new one.
  We’ve seen that in the beetle example and that it all depends on the environment.
• The Newly generated species are not better or “superior” to the older ones, this newly generated species is a result of natural selection and genetic drift, just like it’s predecessor was a result of natural selection and genetic drift.
• The process of speciation requires that the male and female of the newly generated species are not able to reproduce with the original species. This does not equal superiority or progress in any way.
• Let us look at the example of Humans and Chimpanzees, both had a common ancestor 6-7 million years ago, that common ancestor was likely neither human nor chimpanzee, then both the species developed in their own way. It was not the case that both the species developed after the first step of separation.

Progress is not a good yardstick or measurement for analyzing evolution. Nor will it be correct to use progress as one.

• The continuing trend in evolution is that more and more complex bodies have emerged over the years.
• But, this does not imply that the older designs are inefficient or inferior, these organisms still survive, the world is filled with these simple animals with older designs, and one of the simplest forms of bacteria are found thriving even in the deep-sea thermal vents and the chilling ice in Antarctica.
• Humans are not some superior species at the peak of evolution, they simply are a species that is in the rich, vibrant and teeming spectrum of evolving life.

Human evolution

There is a great diversity of human forms and features across the planet, previously human were identified on the basis of their race and skin color was used as the distinguishing feature for race, but there is no biological basis to the concept of human races, all human beings are a single species.
Regardless of where we humans have lived, all of them have descended from Africa, the earliest members of Human species have been traced there, genetic footprint of our species can be traced back to Africa, we all have African roots.
Some of our ancestors left Africa while some stayed behind a couple thousand years ago, the migrants slowly spread across the planet, from Africa to West Asia, then to Eurasia, South Asia, East Asia, they traveled down the islands of Indonesia and the Philippines to Australia, and they crossed the Bering land Bridge(the Berring land bridge was a huge land bridge that connected modern Russia and North America about 20,000 years ago) to the America.
They came into being as an accident, like all other species on our planet and they were trying to survive and live the best they could. Excavation, time dating, determination of DNA sequence are used to study the human evolutionary relationship. The study shows that we all belong to a single species that was evolved in Africa that spread across the world Evolution is a continuous and gradual process, complicated organs did not evolve by a single DNA change but were formed by bit by bit change over generations for example; complex organs like eyes were created by bit by bit changes, in between the rudimentary eye in some insects also provided a fitness advantage. The structure of eye in all organisms is different enough to have evolutionary origins. Some organs even developed for one particular function but later become useful for quite a different function, e.g Feathers developed to provide warmth to the animal but later helped in flight.