Evolutionary Biology and Genetics

‘Origin of Species’ is written by Darwin.

Darwin observed and documented variations in organisms but could not explain their origin or source. He explained over-production, survival of the fittest, and struggle for existence as key mechanisms of natural selection. Variations were known to exist, but their genetic basis (mutations and recombination) was discovered later.

The peppered moth (Biston betularia) is the classic example of industrial melanism, where dark-colored moths became more common during industrial pollution because they were better camouflaged against darkened trees. This occurred in England, not in the locations of the other animals listed.

The struggle for existence arises because organisms produce more offspring than the environment can support (high reproductive rate) but resources like food and space are limited. This leads to competition. Natural selection and variations are consequences or mechanisms, not the root cause.

Selaginella is a living fossil that has remained essentially unchanged for millions of years. Pinus and magnolia are more recent plants, and sunflower is an advanced flowering plant. Selaginella represents an ancient lineage that has survived with minimal change.

Gene recombination during sexual reproduction creates new combinations of existing genes, producing variations Darwin observed. While mutations create new alleles, recombination is the primary source of variations in populations. Natural selection acts on variations but doesn't create them.

Charles Darwin published 'On the Origin of Species' in 1859, presenting his theory of evolution by natural selection. This date is fundamental in biology history. 1530 is too early (pre-scientific revolution), 1795 and 1895 are incorrect.

Competition is most intense when organisms occupy the same niche (need the same resources) and are the same or very similar species. Different animals in the same niche compete, but same animals in the same niche have the most direct overlap of needs and resources.

Mutations are sudden, inheritable changes in genetic material. Gene recombination creates variations but is more predictable. Continuous variations are gradual, and acquired variations (Lamarckian concept) are not inheritable in the way Darwin meant.

Only mutations in germ cells (gametes - sperm and egg) can be passed to offspring and enter the gene pool. Mutations in somatic (body) cells affect only that individual and are not inherited. Plasma proteins are not genetic material.

Jean-Baptiste Lamarck proposed the theory of inheritance of acquired characteristics, which included the principle of use and disuse, stating that organs develop if used extensively and degenerate if not used.

Sibling species are morphologically similar or identical species that are reproductively isolated. They form through genetic divergence where mutations accumulate over time, eventually creating reproductive barriers even though the species appear similar. This process occurs through gradual genetic changes rather than other mechanisms like genetic drift or artificial selection.

A gene pool refers to the complete set of genetic information (alleles) present in all individuals of a population. It represents the total genetic diversity available for natural selection to act upon. Unlike genotype (individual genetic makeup) or genomics (study of genomes), the gene pool is specifically the aggregate genetic resources of a breeding population.

The germplasm theory proposed by August Weismann states that germ cells (reproductive cells) contain the hereditary information that is passed to offspring, while somatic cells (body cells) do not contribute to heredity. Vegetative reproduction in plants demonstrates this concept, as the new plant develops from germplasm rather than somatoplasm, supporting Weismann's theory of the continuity of the germplasm.

Hugo de Vries proposed the Mutation Theory of evolution in the early 20th century, suggesting that new species arise suddenly through large-scale mutations rather than gradual accumulation of small variations. His work with evening primrose (Oenothera) supported this idea, though modern evolutionary biology synthesizes mutation theory with Darwinian natural selection.

Thomas Robert Malthus wrote 'An Essay on the Principle of Population' in 1798, arguing that human populations grow geometrically while food production increases arithmetically. This work influenced Charles Darwin's thinking about natural selection, as it highlighted the struggle for existence due to limited resources.

Isolation is a key factor in speciation as it prevents gene flow between populations, allowing them to diverge genetically over time. Geographic, reproductive, ecological, or behavioral isolation mechanisms enable populations to evolve independently, eventually leading to the formation of new species. Without isolation, interbreeding would maintain genetic similarity.

Genetic drift is a random change in allele frequencies that has its most pronounced effects in small populations where chance events can significantly alter the genetic composition from one generation to the next. In large populations, random fluctuations tend to average out, but in small populations, drift can lead to fixation or loss of alleles regardless of their selective advantage.

Sympatric species are different species that occupy the same geographic area. This distinguishes them from allopatric species, which are separated by geographic barriers. Sympatric speciation occurs without physical isolation, often through mechanisms like ecological differentiation, polyploidy in plants, or behavioral reproductive isolation.

Joshua and Esther Lederberg's replica plating experiment demonstrated the pre-existing nature of bacterial mutations by showing that antibiotic-resistant mutants arise spontaneously in populations prior to exposure to the antibiotic.

When subpopulations at the extremes of a species' range become isolated by distance, they develop spatial isolation. This separation prevents gene flow, allowing genetic divergence to occur through drift, local adaptation, or mutation. Over time, this spatial separation can lead to speciation, particularly in species with wide geographic distributions.

Genetic drift causes random changes in allele frequencies from one generation to the next. These changes affect the gene pool of the offspring generation (next generation) as alleles are randomly sampled from the parental gene pool. Over multiple generations, drift can lead to fixation or loss of alleles, particularly in small populations where sampling error is most pronounced.

Ethological isolation is a form of reproductive isolation where differences in mating behaviors, courtship rituals, or other behavioral patterns prevent interbreeding between species. This behavioral barrier ensures that even if species occupy the same habitat, they do not recognize each other as mates or cannot successfully complete mating. Ecological isolation (A) refers to habitat separation, temporal isolation (C) to differences in breeding seasons/times, and mechanical isolation (D) to physical incompatibility.

Favism is a hemolytic condition triggered by eating fava beans, occurring in people with Glucose-6-phosphate dehydrogenase (G6PD) deficiency. G6PD is an enzyme crucial for maintaining red blood cell membrane integrity; its deficiency makes RBCs susceptible to oxidative damage from compounds in fava beans. Sickle cell anaemia (B) is a hemoglobin disorder, Alkaptonuria (C) is a metabolic disorder affecting tyrosine metabolism, and Mongolism (D) is an outdated term for Down syndrome (trisomy 21).