Heredity and Evolution
In this chapter, we shall be studying the mechanism by which variations are created and inherited. The long-term consequences of the accumulation of variations are also an interesting point to be considered.
ACCUMULATION OF VARIATION DURING REPRODUCTION
- If one bacterium divides, and then the resultant two bacteria divide again, the four individual bacteria generated would be very similar. There would be only very minor differences between them, generated due to small inaccuracies in DNA copying. However, if sexual reproduction is involved, even greater diversity will be generated
- Depending on the nature of variations, different individuals would have different kinds of advantages. Bacteria that can withstand heat will survive better in a heat wave, as we have discussed earlier. Selection of variants by environmental factors forms the basis for evolutionary processes
Heredity is the process by which traits from parents are passed to offsprings through sexual or asexual reproduction.
Traits and characteristics are reliably
inherited to next generation to provide a common basic body design.
Example of inherited traits : The lowest part of the ear, called the earlobe, is closely attached to the side of the head in some of us, and not in others. Free and attached earlobes are two variants found in human populations.
• Differences in the characters (or traits)
among the individuals of a species.
• It helps in adaptation of organisms to the
It also forms the basis of heredity.
• It constitutes the raw material for organic
evolution and development of new species.
Rules for the Inheritance of Traits –Mendel’s Contributions
- The rules for inheritance of such traits in human beings are related to the fact that both the father and the mother contribute practically equal amounts of genetic material to the child. This means that each trait can be influenced by both paternal and maternal DNA. Thus, for each trait there will be two versions in each child. What will, then, the trait seen in the child be?
This means that each trait can be influenced by both paternal and maternal DNA. Thus, for each trait there will be two versions in each child. What will, then, the trait seen in the child be?
- Two copies of the trait are inherited in each sexually reproducing organism. These two may be identical, or may be different, depending on the parentage.
• Gregor Johann Mendel first studied inheritance of characters.
• He did his study on pea plant.
• Mendel crossed pea plants with one pair of contrasting traits to get F1 generation.
• The F, plants were allowed to self-pollinate. He concluded:
Characters were controlled by factors or genes which occur in pairs (alleles)
-When two different alleles are present together, the dominant one is expressed.
-Alleles segregated so that
each gamete gets only one
of the alleles.
•Mendel crossed pea plants considering two pairs of contrasting characters at a time to get F1 generation.
• The F, plants were allowed to self-pollinate.
-Pairs of contrasting characters
behave independently of the
-At the time of gamete
formation all genes assort
independently of each other.
How is the sex of a newborn individual determined?
- Different species use very different strategies for this.
- Some rely entirely on environmental cues. Thus, in some animals, the temperature at which fertilised eggs are kept determines whether the animals developing in the eggs will be male or female.
- In other animals, such as snails, individuals can change sex, indicating that sex is not genetically determined.
- However, in human beings, the sex of the individual is largely genetically determined. In other words, the genes inherited from our parents decide whether we will be boys or girls.
Sex determination in human beings
- Most human chromosomes have a maternal and a paternal copy, and we have 22 such pairs. But one pair, called the sex chromosomes, is odd in not always being a perfect pair. Women have a perfect pair of sex chromosomes, both called X. But men have a mismatched pair in which one is a normal-sized X while the other is a short one called Y. So women are XX, while men are XY.
- Women have a perfect pair of sex chromosomes, both called X. But men have a mismatched pair in which one is a normal-sized X while the other is a short one called Y. So women are XX, while men are XY.
- All children will inherit an X chromosome from their mother regardless of whether they are boys or girls. Thus, the sex of the children will be determined by what they inherit from their father. A child who inherits an X chromosome from her father will be a girl, and one who inherits a Y chromosome from him will be a boy.
Every life form that we see today has arisen by the accumulation of changes in their body pattern which possess them to survive better this is known as evolution.
) Natural Selection: A group of living beings adopts to fit their
(i) Genetic Drift: Provides diversity without any adaptations.
There is a random change in gene frequency.
Factors Resulting in Evolution
Acquired and Inherited Traits
• Characteristics of an organism that are not inherited but develop in response to the environment.
• These are not genetically controlled.
Characteristics of an organism that are caused by a change in the DNA and are inherited.
* These are controlled by specific genes.
• Origin of new species from the existing one
• Mechanisms of speciation: Accumulation of variation.
– Physical barriers.
EVOLUTION AND CLASSIFICATION
- Characteristics are details of appearance or behaviour; in other words, a particular form or a particular function. That we have four limbs is thus a characteristic. That plants can do photosynthesis is also a characteristic.
The more characteristics two species will have in common, the more closely they are related. And the more closely they are related, the more recently they will have had a common ancestor. An example will help.
- A brother and a sister are closely related. They have common ancestors in the first generation before them, namely, their parents. A girl and her first cousin are also related, but less than the girl and her brother. This is because cousins have common ancestors, their grandparents, in the second generation before them, not in the first one.
Tracing Evolutionary Relationships
1. Homologous organs: Organs having same basic structure but different functions.
- Example, consider the fact that mammals have four limbs, as do birds, reptiles and amphibians. The basic structure of the limbs is similar though it has been modified to perform different functions in various vertebrates. Such a homologous characteristic helps to identify an evolutionary relationship between apparently different species.
2. Analogous organs: Organs having
different basic structure but similar appearance and function.
Example :The wing of a bat and the wing of bird
3. Fossils: Remains or impressions of
dead animals or plants that lived in
the remote past are known as
fossils. The age of fossils can be
estimated by two ways:
(i) If we dig into the earth and start
finding fossils, it can be assumed
that the fossils closer to the surface
are more recent to those found in
(ii) By detecting the ratios of different
isotopes of the same element in the
Uses of fossils
- Racial history of plants and animals can be predicted.
- Past climatic conditions of the earth can be estimated.
- To measure the geological time.
Evolution by Stages
- Complex organs may have evolved because of the survival advantage of even the intermediate stages.
- Organs or features may be adapted to new functions during the course of evolution. For example, feathers are thought to have been initially evolved for warmth and later adapted for flight.
EVOLUTION SHOULD NOT BE EQUATED WITH ‘PROGRESS’
- Evolution cannot be said to ‘progress’ from ‘lower’ forms to ‘higher’ forms. Rather, evolution seems to have given rise to more complex body designs even while the simpler body designs continue to flourish.
- In fact, one of the simplest life forms – bacteria – inhabit the most inhospitable 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 yet another species in the teeming spectrum of evolving life.
Study of the evolution of human beings indicates that all of us belong to a single species that evolved in Africa and spread across the world in stages.