Heredity and Evolution Notes - Mendel Laws, Variation, Sex Determination, Homologous Analogous Organs, Fossils and Human Evolution

• Heredity (inheritance) is the transmission of characters from parents to offspring.
• Evolution is the change in the characteristics of a species over several generations.

ACCUMULATION OF VARIATION DURING REPRODUCTION

• Inheritance from the previous generation provides a common basic body design and minute changes for the next generation. The second generation inherits these differences and newly created differences.
• In asexual reproduction, the resultant individuals get only very minor differences due to small inaccuracies in DNA copying. However, in sexual reproduction, greater diversity is generated.
• The variations in a species have unequal chances of surviving in the environment. Based on the nature of variations, different individuals have different kinds of advantages. For example, bacteria that can withstand heat will survive better in a heat wave.
• Selection of variants by environmental factors is the basis of evolution.

HEREDITY

Inherited Traits

• A child bears all the basic features of a human being. However, it does not look exactly like its parents.
• Human populations show great variation. For example, variation in ear lobes. Most individuals have free ear lobes (dominant trait), and some have attached ear lobes (recessive trait).

Rules for the Inheritance of Traits – Mendel’s Contributions

• Father and mother contribute equal amounts of genetic material to the child, i.e., each trait is influenced by paternal and maternal DNA. Thus, for each trait, there will be two versions in each child.
• Gregor Johann Mendel (1822–1884) worked out the main rules of such inheritance.
• Mendel used several contrasting visible characters of garden peas, e.g., round/wrinkled seeds, tall/short plants, white/violet flowers, etc.
• He crossed a tall plant and a short plant. In the first generation (F1 progeny), all plants were tall. There were no halfway characteristics (no ‘medium-height’ plants). This means that only one parental trait (tall) was expressed. The expressed trait is called dominant. The suppressed trait is called recessive.
• Mendel allowed F1 tall plants to reproduce by self-pollination. The second-generation (F2) progeny was 75% tall and one quarter (25%) short. This indicates that both traits (tall and short) were inherited in the F1 plants, but only the tallness trait was expressed. Thus, he proposed that two copies of a factor (now called genes) control traits in sexually reproducing organisms. They may be identical or different, based on the parentage.

Phenotypic ratio (Tall: Short) = 3:1

Genotypic ratio (TT: Tt: tt) = 1:2:1

• To confirm the genotypic ratio, each F2 plant is crossed with a pure recessive (tt) variety. This is called a test cross.
• TT and Tt are tall plants, while tt is a short plant. This means a single copy of ‘T’ can make the plant tall (dominant trait), but two copies of ‘t’ (tt) are needed to get a short plant (recessive trait).
• Cross of colored flowered and white flowered plants: Here, F1 produced all colored flowers. So, the colored flower is the dominant trait, and the white flower is recessive. F2 produced colored flowered plants and white flowered plants in a 3:1 ratio.

• When a tall plant with round seeds and a short plant with wrinkled seeds is crossed, the F1 progenies will be tall and round-seeded, i.e., tallness and round seeds are dominant traits.
• If the F1 progeny undergo self-pollination, the following types of F2 progeny are produced:

• New combinations are formed due to the independent inheritance of the tall/short trait and the round seed/wrinkled seed trait.
• Similarly, the formation of new combinations of traits in F2 occurs when factors controlling seed shape and seed color recombine to form a zygote, leading to F2 offspring.

How do these Traits get Expressed?

• DNA is the information source to make proteins in a cell.
• A section of DNA that provides information for one protein is called the gene for that protein.
• Genes control traits by producing proteins. For example:
• Plant height depends on a growth hormone, which is synthesized due to an enzyme (protein). This enzyme is synthesized due to a gene.

• Efficient enzyme → more hormone → tall plant.
• Alteration of the gene → less efficient enzyme → less hormone → short plant.

• According to Mendelian experiments, both parents contribute DNA equally (copies of the same genes) to the progeny. Thus, each pea plant inherits two sets of all genes. For this, each germ cell must have only one gene set.
• In Mendel’s experiment, the characteristics ‘R’ and ‘y’ were independently inherited because they are not linked. This indicates that each gene set is not in a single DNA thread (i.e., not in a whole gene set), but in separate independent pieces, each called a chromosome. Thus, each cell has two copies of each chromosome, one each from the male (paternal) and female (maternal) parents. Every germ cell takes one chromosome (maternal or paternal).
• When two germ cells combine, they restore the normal chromosome number in the progeny. This ensures the stability of the DNA of the species. Such a mechanism of inheritance is used by all sexually reproducing organisms. Asexually reproducing organisms also follow similar rules of inheritance.

Sex Determination

• There are different strategies for sex determination.
• Some species rely entirely on environmental cues. For example, in a few reptiles, the temperature at which fertilized eggs are kept determines whether they become male or female.
• In animals such as snails, individuals can change sex, indicating that their sex is not genetically determined.
• In human beings, the sex is genetically determined. The genes inherited from parents decide whether an individual will be a boy or a girl.

• All human chromosomes are not paired. Most human chromosomes have a maternal and a paternal copy, and have 22 such pairs. But one pair, sex chromosomes, is not always a perfect pair. Women have a perfect pair called XX, but men have a mismatched pair in which one is a normal-sized X while the other is short, called Y (XY).
• Half the children will be boys, and half will be girls.
• All children inherit an X chromosome from their mother.
• The sex of the children is determined by the father. A child who inherits an X chromosome from the father will be a girl, and one who inherits a Y chromosome will be a boy.

EVOLUTION

• Sexual reproduction and errors in DNA copying lead to variation. Its consequences are illustrated below:

• Consider a growing population of red beetles living in bushes with green leaves. Imagine that crows eat them.
• Here, the following situations may develop:

First Situation: Natural Selection

• A heritable color variation (green beetle) arises. Crows cannot see green beetles on the green leaves and cannot eat them. So, the number of green beetles increases, but the red beetles are eaten, and their number decreases.
• Here, the variation became common because it gave a survival advantage (naturally selected). The natural selection is exerted by the crows. If there are more crows, more red beetles would be eaten, so the proportion of green beetles increases. Thus, natural selection directs evolution. It results in adaptations in the beetle population to fit their environment better.

Second Situation: Genetic Drift

• A color variation (blue beetle) arises.
• Crows can see and eat both blue and red colored beetles on the green leaves.
• Initially, blue beetles are few. If an elephant stamps on the bushes, most of the beetles die. If the survived beetles are mostly blue, their proportion increases, and the population would be mostly blue beetles.
• Here, the color change has no survival advantage. Blue colored beetles accidentally survived and changed the common characteristic of the new population.
• In a very large population, such events do not have a major influence. But accidents in small populations can change the frequency of some genes. This is called genetic drift. It provides diversity without adaptations.

• In the first and second situations, a rare variation became a common characteristic in the population. This means the frequencies of inherited traits and their genes change over generations. This is the essence of the idea of evolution.

Third Situation: Non-genetic Variation

• The bushes are affected by a plant disease, reducing the amount of leaf material. Poor nourishment reduces the weight of beetles. Here, there is no genetic change.
• After some generations, if the plant disease is eliminated and leaf food is increased, the beetles’ weight increases again.

Acquired and Inherited Traits

• Reduced weight of beetles due to starvation will not change the DNA of the germ cells, i.e., it is not a heritable trait. So, it is not an example of evolution.
• Change in non-reproductive tissues cannot be passed on to the DNA of the germ cells. So, the experiences of an individual (acquired traits) during its lifetime do not inherit and cannot direct evolution.
• For example, breeding of mice produces all progeny with tails. If their tails are removed in each generation, they do not produce tailless progeny because it does not change the genes of germ cells.
• So, the study of heredity and genetics is essential to understand evolution.
• Charles Robert Darwin (1809–1882) set out on a five-year voyage to South America and proposed the idea of evolution of species by natural selection. But he could not explain the mechanism of evolution.

SPECIATION

• Minor genetic changes occurring in a population are called microevolution. They change the common characteristics of a species but cannot explain the formation of new species.
• Speciation is an evolutionary process of forming new species.
• If a population splits into two such that they cannot reproduce with each other, they are called two species. This can be explained using a beetle population.
• Consider a huge beetle population spread over a mountain range. Here, each beetle feeds mostly on nearby bushes. They do not travel far, so there will be sub-populations in neighborhoods.
• Most reproduction occurs within these sub-populations.
• In rare cases, some beetles might go from one site to another, or a crow picks up a beetle from one site to another. As a result, the genes of the migrant beetle enter the new population by reproduction and cause genetic drift (accidental genetic variation in a small population).
• If a large river comes between two sub-populations, they will be isolated. Thus, gene flow between them decreases.
• Over generations, genetic drift accumulates many changes in each sub-population. Also, natural selection operates in these different geographic locations. For example, in one sub-population, crows are eliminated by eagles, so the green beetle variation will not be selected. But in another sub-population, the number of crows is very high, so the green variation will be strongly selected.
• Genetic drift and natural selection together make isolated sub-populations more and more different. Eventually, members of these two groups will be incapable of reproducing with each other, even if they happen to meet. This can happen in many ways:
• If the DNA changes are significant (e.g., a change in the number of chromosomes), the germ cells of the two groups cannot fuse.
• A new variation may emerge in which green females will not mate with red males, but only with green males (natural selection for greenness). Thus, new species of beetles are generated.

EVOLUTION AND CLASSIFICATION

• Evolutionary relationships of the species can be worked out by identifying hierarchies of their characteristics.
• Characteristics are details of appearance or behavior; i.e., a particular form or function. For example, four limbs of animals, photosynthesis in plants, etc.
• Most organisms share some basic characteristics. Based on this, a hierarchy of classification is given below:

• If two species have more common characteristics, they are more closely related and will have a recent common ancestor. For example, a brother and a sister have immediate common ancestors (parents). But the common ancestors of first cousins are grandparents. Thus, the classification of species reflects their evolutionary relationship.
• Going backwards, we reach a single species (single common ancestor) at the beginning of evolutionary time.

Tracing Evolutionary Relationships

• Evolutionary relationships between different species are traced by identifying common characteristics.
• The organs having a similar basic structure but modified to perform different functions are called homologous organs. For example, mammals, birds, reptiles, and amphibians have four limbs. Their basic structure is similar but performs different functions.

• The organs having the same function but different structure and origin are called analogous organs. For example, wings of birds and bats.
• Squirrels and lizards have no wings. This does not mean birds and bats are more closely related.
• Wings of bats are skin folds stretched between elongated fingers, but a bird’s wings are a feathery covering along the arm. So, the design, structure, and components are different.
• However, the arms (forelimbs) of birds and bats can be considered homologous because they have nearly the same sets of bones. The forelimbs are modified into wings.

Fossils

• Fossils are preserved traces of organisms that lived in the past. They help us understand extinct species. For example:
• A dead insect caught in hot mud will not decompose quickly, and the mud will eventually harden and retain the impression of its body parts.
• The dinosaur skull fossil in the Narmada valley.
• Age of fossils can be estimated in two ways:
• Relative: Fossils closer to the surface are more recent than those in deeper layers.
• Detecting the ratios of different isotopes of the same element in the fossil.

Evolution by Stages

• Evolution of complex organs, such as the eye, is the result of the selection of several intermediate stages. For example, an intermediate stage, such as a rudimentary eye (e.g., eye spots of Planaria), gives a fitness advantage.
• Also, a change useful for one property can become useful for a different function. For example, feathers start out providing insulation in cold weather but later become useful for flight. Some dinosaurs (reptiles) had feathers but could not fly. Birds later adapted the feathers for flight. This means birds are very closely related to reptiles.
• All dissimilar structures evolved from a common ancestral design. This can be analyzed using fossils.
• Such evolutionary relationships can be understood with the help of wild cabbage.
• Humans started cultivating wild cabbage more than 2000 years ago and generated different vegetables from it by selection. This is artificial selection rather than natural selection.

• All these structures are descended from the same ancestor.
• Changes in DNA during reproduction are the basic events in evolution. Comparing the DNA of different species gives a direct estimate of how much the DNA has changed during speciation and helps identify where each change diverged from the other. This method is called molecular phylogeny. It is extensively used to define evolutionary relationships.

EVOLUTION SHOULD NOT BE EQUATED WITH ‘PROGRESS’

• There are multiple branches possible at each stage of tracing the family trees of species.
• If a new species emerges, it does not necessarily mean that the old species disappear. It depends on the environment. Also, the newly generated species may not be ‘better’ than the older one.
• Human beings did not evolve from chimpanzees. Rather, both humans and chimpanzees have a common ancestor. The two resultant species from that common ancestor evolved in separate ways to give rise to humans and chimpanzees.
• There is no real ‘progress’ in the idea of evolution.
• Evolution is simply the generation of diversity and the shaping of diversity by environmental selection.
• The only progressive trend in evolution is that more complex body designs have emerged over time. However, many older and simpler designs still survive. For example, bacteria inhabit habitats like hot springs, deep-sea thermal vents, and the ice in Antarctica. In other words, human beings are not the pinnacle of evolution but simply another species in the evolutionary process.

Human Evolution

• All humans are a single species.
• The earliest members of the human species, Homo sapiens, evolved in Africa.
• A couple of hundred thousand years ago, some human ancestors left Africa.
• The migrants slowly spread from Africa to West Asia, then to Central Asia, Eurasia, South Asia, and East Asia. They traveled down the islands of Indonesia and the Philippines to Australia and crossed the Bering land bridge to the Americas. This separation and mixing led to the evolution of modern humans.