Evolution: Theories of Evolution, Jean Lamarck’s theories & Charles Darwin’s theory

Theories of Evolution

Organic Evolution

Organic evolution is the sum total of adaptive changes from pre–existing or old forms that has taken place over a long time resulting in diversity of forms, structures and functions among organism. The basis of evolution is that all organisms have pre–existing ancestors.

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Evidences of Evolution

Fossil record: A fossil is an impression of a plant or an animal that lived a very long time ago. The age of fossil is determined using radioisotope dating. Fossils are normally preserved in sedimentary rocks. Depending on the source, fossil records can be referred to as geological or paleontological or archaeological or historical record.

Geographical distribution: Based on the effect of climate on all living things, variations in their forms, structures and functions can occur. After several years of isolation, organisms of one climate tend to differ slightly from organisms of another climate.

Comparative anatomy: Evolution is obvious in anatomical comparison of vertebrates. The Pisces or fishes have simple heart with one auricle and one ventricle. The amphibians have two auricles and one ventricle. Reptiles have two auricles and a partially divided ventricle. Aves and mammals have two auricles and two ventricles.

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Embryological evidence: The embryo of man in the womb at different stages of development resembles the embryo of fish, amphibians and reptiles.

Evidence of vestigial organ: Vestigial organs are minute and incomplete organs that have no special function. Evolutionarily, these organs are believed to be once functioning e.g. appendix and rudimentary tail in man.

Evidence from domesticated animals: These include cats, dogs, hensetc which live with humans for many years.

Theories of Evolution

There are three prominent theories

Jean Lamarck’s theories

Jean Lamarck’s theories of use and disuse which states that Changes in the environment lead to changes in the species of organism.

The changes cause the organisms to form new structures or habits to adapt to environmental changes.

The organisms then develop specialized characters by use and disuse of organs.

Frequently used organs become well developed and the unused ones degenerate or become useless.

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The well-developed or dominantly acquired characters are inheritable.

Lamarck’s theory is unacceptable to modern scientists who have proved that only characters represented on genes are inheritable not physical ones got through use and disuse.

Charles Darwin’s theory

Charles Darwin’s theory of natural selection (survival of the fittest) which states that Species of organisms can produce large number of offspring to the environment with limited resources. This then results in competition among the offspring.

The survivors must have inherited the useful traits which are passed on to the offspring at reproduction. Those that could not survive the competition die off. As the population gradually becomes better adapted to the environment, new species emerge. This theory is widely acceptable to many scientists to date.

NOTE: Both Lamarck and Darwin recognized the importance of the environment.

Modern theories of evolution:

Based on the combination of natural selection (Darwin’s) and the genetic origin of variation. This theory states that:

There exist variations in the species population. Some of the variations have special survival advantages Individuals with favourable variations are more adaptive to their environment than others.

The individuals have to struggle for existence in the environment. The fittest contribute more offspring to the next generation than the unfit ones.The main causes of variations are mutation and recombination of genes.

Roles of mutation in evolution

Mutation plays a crucial and multifaceted role in the process of evolution, serving as a fundamental driver of biodiversity and adaptation in the natural world. These genetic alterations are not only responsible for shaping the diversity of life forms we observe today but also influencing the trajectory of species over geological time scales. Here, we delve into the various roles of mutation in evolution:

1. Source of Genetic Diversity: One of the most significant roles of mutation is its contribution to genetic diversity within populations. Mutations introduce novel genetic variants into a population’s gene pool, allowing species to explore a wider range of traits and characteristics. This diversity serves as the raw material upon which natural selection can act.

2. Adaptation to Changing Environments: Mutations provide the genetic basis for adaptation to changing environmental conditions. When a mutation confers a selective advantage in a particular environment, it can become more prevalent within a population over successive generations. This process, known as adaptive evolution, enables species to better thrive in their specific habitats.

3. Speciation: As mentioned, mutations in the genes of gametes can lead to the production of new species. Over time, accumulated genetic differences due to mutations can result in reproductive isolation between populations. This isolation is a crucial step in the formation of distinct species, as it prevents interbreeding and allows separate evolutionary trajectories to develop.

4. Evolutionary Arms Race: Mutations are central to the ongoing evolutionary arms race between species. For example, pathogens, such as bacteria or viruses, continuously mutate to evade the immune systems of their hosts. In response, hosts evolve their own mutations to counteract these threats. This co-evolutionary process drives the continual adaptation and counter-adaptation observed in nature.

5. Genetic Drift: In small populations, genetic drift can play a more prominent role than natural selection. Mutations can be responsible for genetic changes in these populations simply by chance, rather than through selective advantages. Over time, this can lead to significant genetic divergence within isolated populations.

6. Non-Adaptive Evolution: While some mutations confer adaptive advantages, many do not. Neutral mutations, those that neither enhance nor reduce an organism’s fitness, can accumulate over time. These neutral mutations can serve as molecular clocks, helping scientists estimate the timing of evolutionary events.

7. Innovation and Novelty: Occasionally, mutations can lead to entirely new traits or characteristics. These novel features can open up new ecological niches or exploit existing ones, driving innovation in the evolutionary process.

8. Genetic Conservation: In certain cases, mutations can lead to the conservation of certain genetic traits within a population. This can occur through stabilizing selection, where mutations that deviate from the existing trait are selectively disadvantaged, resulting in the preservation of the status quo.

In conclusion, mutation is a cornerstone of evolutionary biology, fueling the dynamic process of species diversification and adaptation. Its role extends from generating genetic diversity to driving the formation of new species and facilitating ongoing evolutionary responses to changing environments and ecological interactions. Ultimately, mutation is the creative force that has shaped the incredible diversity of life on Earth.