Darwin, Malthus, and Limiting Factors In 1798, Economist Thomas Malthus noted people were being born faster than people were dying, causing overcrowding. The forces that work against population growth, Malthus notes, were War, Famine, and disease. Darwin realized Malthus’s thinking applied even more to other organisms than to humans. As an example, a Maple tree produces thousands of seeds, but only a few survive to reproduce. Darwin wondered why.

Artificial Selection In order to explain what he was observing in Nature, Darwin turned to the idea of Artificial Selection. Plant and animal breeders knew that individual organisms within a given population varied in attributes like size, hardiness, and the amount of milk produced. The farmers told Darwin that some of this variation could be passed on to offspring in order to improve the stock.

In Artificial Selection, nature provides the variations, and humans select those variations they find useful. Darwin had no idea how heredity worked, or about heritable variation, but he did know that variation occurred within natural populations just as in domesticated plants and animals. Darwin’s breakthrough came when he realized that this variation provided the raw materials for evolution.

Natural Selection Darwin’s great contribution was to describe a process in nature – a scientific mechanism – that could operate like artificial selection. The struggle for existence – which organisms, within a species, survive and reproduce? Darwin hypothesized some of these organisms are better suited to survive in their environment than are others. These better suited organisms (ex. faster, stronger, or better camouflaged) would be the ones to pass on these ‘desirable’ traits.

Please note that physical attributes aren’t the only ones passed on – behaviours that favour reproductive success are also selected for and re-enforced in subsequent generations. Any heritable characteristic that improves the organism’s ability to survive and reproduce in an environment is called an adaptation. Darwin coined the phrase Survival of the Fittest to illustrate how differences in adaptions to the environment determine an organism’s fitness to survive and reproduce in that environment.

Natural Selection is the process by which organisms with variations most suited to local environments survive and leave more offspring. In both artificial and natural selection, only certain individuals in a population produce new individuals. In the case of Natural Selection, the environment – not a farmer or breeder – influences fitness. Natural selection occurs in any situation in which more individuals are born than can survive. There is natural heritable variation (variation and adaptation), and there is variable fitness among individuals in a given population (survival of the fittest). This is the long way of saying that Evolution is not random. Common descent – According to the principle of common descent, all species, whether living or extinct, are descended from ancient common ancestors.

Things natural selection doesn’t do It doesn’t make organisms ‘better’ It doesn’t make organisms perfect It doesn’t move in a fixed direction It doesn’t even guarantee the continued existence of a species Natural selection is also not the only mechanism driving evolution.

Evidence for Evolution Biogeography – This is the study of where organisms live now and where they, and their extinct ancestors, lived in the past.. Patterns in distribution of living and fossil species tell us how modern organisms evolved from their ancestors.. 1) The first is a pattern in which distantly related species develop similarities in similar environments. 2) The second is a pattern in which closely related species differentiate in slightly different climates.

To Darwin, the biogeography of the Galapagos species suggested the populations on the islands had evolved from mainland species. Over time, natural selection on the islands produced variation among the populations that resulted in different, but closely related, island species. On the other hand, similar habitats around the world are often home to organisms that fill similar niches, but are distantly related – the Rhea and Ostriches are examples. Both show similar characteristics and fill the same niches in their respective habitats, but are completely unrelated.

The Age of the Earth and the Fossil record Evolution takes time, a long time, and Darwin thought this pointed to an old Earth. Recent developments in the use of radiometric dating has shown the Earth is at least 4.5 billion years old. The fossil record shows that modern species, like whales, have evolved from ancient extinct ancestors.

Anatomy Homologous structures – Structures shared by related species and that have been inherited from a common ancestor are called homologous structures. Analogous structures, on the other hand, have similar functions but are inherited from unrelated species. Bee wings and Bird wings have similar function but completely different evolutionary pathways. Vestigial structures are structures that have been inherited from ancestors but have lost much or all of their original use due to different selection pressures acting on the descendants. The hipbones of bottlenose dolphins or boa constrictors are examples of vestigial structures.

Embryology & Genetics Similar patterns of embryological development provide further evidence that organisms have descended from a common ancestor. DNA, the universal genetic code, provides evidence of common descent.