Theories of Evolution
Evolution Evolution is an orderly succession of changes. Biological evolution is the change of populations of organisms over generation. Biogeography is the study of the geographic distribution of fossils and living organisms. Scientists use the knowledge obtained from biogeography to chronicle subtle changes in similar life forms to see how they evolved.
Charles Lyell Was the first major influence of the rise of evolutionary thought (early 1800 s) He proposed that to understand the beginning of the Earth scientists should examine known events of recent history to help to explain earlier conditions of the Earth Th d h h h h E h b This spurred the thoght that Earth may be millions of years old rather than thousands
Lyell s theory of Uniformitarianism Geological processes are so uniform that their rates and effects must balance out through time. Processes that build mountains must eventually be balanced by the erosion of those mountains. Geological change results from slow continuous processes not sudden events, and the earth must be very, very old!
Jean-Baptiste Lamarck French naturalist of early 1800 s He was the first to suggest that similar species descended from a common ancestor. Although much of his reasoning was incorrect, his work had much influence over the next generation of scientists and his idea of change over time would become central to many subsequent theories.
Lamarck, proposed that by selective use or disuse of ograns, organisms acquired or lost certain traits during their lifetime and that this process lead to change in a species over time. All organisms had a tendency toward complexity and perfection thus living more successfully in their environment. (Flying evolved from an urge to fly) by using or not using their bodies in particular ways organisms would eventually alter organs / limbs etc. (By trying to use front limbs to fly they would eventually become wings.) These acquired alterations (broken bills of birds) could be inherited.
Reverend Thomas Malthus Reverend Thomas Malthus suggested in 1798 that the human population had the potential to grow indefinitely by continuously doubling. He stated that it was environmental factors (war, disease, famine, etc) that prevented this from happening. He concluded that it was not in society s best interest t to help the poor because it would lead to too large a population and the eventual extinction of man due to the consumption of resources. This thought process concerning population growth would later have great influence in the beginnings of modern evolutionary theory.
Natural Selection In the mid 1800 s 2 scientists Charles Darwin and Alfred Wallace both proposed that t species were modified by natural selection. Natural Selection is the process in which organisms best suited to their environment reproduced more successfully Over time the proportion of those organisms with favorable traits would increase within a population.
Alfred Wallace Self-taught naturalist Contemporary to Darwin Wallace actually sent Darwin his memoirs advancing his theory of Natural Selection which was very similar to the work Darwin had been doing for more than 20 years.
Charles Darwin 1809 1882 Born to upper class his social status which h got him the job as naturalist on the MRS Beagle Voyage 5 years 1831 1836 Married Emma Wedgwood had 10 children ill most of his life
Voyage of the Beagle Darwin was seasick from the very first day of the voyage. He had to be left on land in one port as the ship mapped the shore of various islands and then picked up days later. While ashore Darwin collected fossils and specimens of life at each stop carefully cataloguing his data according to its geographic origins.
he realized through observations of the cliffs of Santiago that the earth s elevations seemed to change over very long periods of time millions of years as opposed to thousands. The fossils he observe red in these cliffs seemed to indicate that life forms had adapted d to the changes in the earth over time.
New discovery at Galapagos He noticed that the finches (songbirds) on the different islands there were fundamentally similar to each other but showed wide variations in their size, beaks and claws. For example, their beaks were different depending on the local food source. Darwin concluded that, because the islands are so distant from the mainland, the finches that had arrived there in the past had changed over time.
Darwin s Theory Darwin studied hundreds more animal and plant species. After nearly 30 years of research, in 1858 he proposed his theory of evolution by natural selection all the different species have evolved from simple life forms. Wallace actually sent Darwin his memoirs advancing his theory of Natural Selection which prompted Darwin to finish and publish his renowned work, The Origin of Species in 1859.
Still the ideas of Natural Selection were not widely accepted until the discovery of genes and mutations ti of the 20 th century made this explanation of evolution unavoidable Darwin stated his findings in two basic theories. Descent with Modification Modification by Natural Selection
Descent with Modification The newer forms of organisms appearing in the fossil record are actually the modified descendents of older species. This was similar to Lamarck s theory with the exception that Darwin also said that all species had descended d d from one or a few original life forms.
Modification by Natural Selection fittest survive and reproduce more readily Since populations p have the potential for unlimited reproduction and growth environmental factors must limit this potential. Populations of organisms adapt as the proportion of favorable traits increases. These are not acquired traits but inherited natural ones, and the environment selects the traits that will increase a population. Ex. - webbed feet and oily feathers are desirable near a watery environment / sharper clawed feet and a lighter airier plumage are more advantageous near a rougher interior terrain. Favorable traits give organisms an Favorable traits give organisms an adaptive advantage.
The Modern Theory of Natural Selection Individuals in a species show a wide range of variation. This variation is because of differences in genes. Individuals with characteristics most suited to the environment are more likely to survive and reproduce. The genes that allowed the individuals to be successful are passed to the offspring in the next generation.
Individuals that are poorly adapted to their environment are less likely to survive and reproduce. This means that their genes are less likely to be passed to the next generation. Given enough time, a species will gradually evolve losing unfavorable traits and gaining those that make them more successful. Similarly, it is possible that a species that is poorly adapted to its environment will not survive and will become extinct. Here are some of the factors that t can cause a species to become extinct: changes to the environment, such as a change in climate new diseases new predators new competitors
The Dodo was a large flightless bird first observed by European visitors to the Indian Ocean island of Mauritius around 1600. Mauritius was colonized by the Dutch, who over-hunted the bird for food, damaged its forest habitat and introduced other species, like pigs, cats and rats that ate the Dodo's young and eggs. Within eighty years the Dodo was extinct - entirely as a result of the activities of man.
Artificial selection Natural selection is nature's way of ensuring that the genes of those best suited to survive in a particular environment are passed on to the next generation, It works by random mutation. Since humans first began domesticating plant and animal species about 15,000 years ago, we have been doing something very different - deliberately selecting certain characteristics for breeding purposes, This is called artificial selection.
Selective breeding is when humans only breed from selected individuals in order to preserve and accentuate their desirable characteristics. Breeders, farmers, and horticulturists have used it for many centuries, in order to: produce new varieties of plant with higher food yields produce new varieties of fruit with better flavour produce new varieties of animal with higher meat, wool or milk yields produce new varieties of animal that are faster (for transport), stronger (for pulling ploughs or carts) or otherwise more useful to us (eg as pets) produce flowers with larger blooms or more colourful blooms produce disease-resistant or pest-resistant crops
One very familiar example of artificial breeding is dogs. Fossil evidence shows that all dogs have a common ancestor - the wild grey wolf. Over 14,000 years humans have 'designed' some 400 breeds of domestic dog, artificially selecting the looks and behavior of each breed.
Evidence of Evolution By examining genotypic and phenotypic evidence in modern organisms we can see evidence that evolution has occurred. As the world of genetic understanding grows so does our appreciation of the evidence of evolution.
Fossils One of the few animals for which we have a fairly complete evolutionary record is the horse, as all the main stages of horse evolution have been preserved in fossil form. Over 60 million years the horse evolved from a dog-sized rainforest-dwelling creature, into an animal adapted to plains-dwelling and standing up to 2 metres high. h In the process it traded-in its multi-toed feet, adapted for walking across the forest floor, for single-toed hooves, suited for running over open country.
Similarities in Embryology The early stages, embryos, of vertebrates are very similar. Many look remarkably similar during certain stages of development It is clear that groups of embryonic cells develop in the same order and in similar patterns to produce the tissues and organs of all vertebrates. This is a strong indicator of a common ancestry
Homologous and Analogous Structures: Homologous structures - develop from the same embryonic tissues but have different mature forms and often function. Example: - the beaks of Darwin s finches. These derived d from the same tissues and serve a common function for obtaining food, but in mature forms are different in shape and how they are used to gather food.
Other features that may not be so obvious are the forelimbs of vertebrates. Arms / Wings / Flippers They look very different and serve different function yet they are remarkably similar in skeletal structure. H l f t t l l Homologous features suggest clearly a common ancestor in recent geologic time
Structures that look similar and serve similar function but have no common structure or embryonic similarity are said to be analogous features. Because these features have dissimilar anatomy and embryonic development, they suggest a more remote common ancestor. The wings of birds and insects are prime examples of analogous structures. The look similar and serve a common function, yet are completely different anatomically and embryologically.
Vestigial Organs obvious evolutionary evidence are the structures we know to be vestigial i organs These are trace organs, often shrunken, that serve no function in present species but did have a function in their predecessors. Example - Sperm whales have pelvic bones and no hind legs.
A small percentage of them are born with vestigial leg bones and small protrusions where the legs would be. This is suggestive of the fact that these whales descended from an ancestor that lived on land. In the case of these vestigial organs the genes have been conserved rather than switched off as in the majority of the species.
The more similar the species the more recent in time is their common ancestor. the hemoglobin in the RBC s of many species are similar but not identical. They differ in amino acid sequencing. Humans and Gorillas differ in only 1 amino acid suggesting a more recent common ancestor. Humans and Frogs have hemoglobin that differs in 67 amino acids suggesting a more distant predecessor.
Patterns of Evolution Natural selection can ultimately lead to the formation of new species In studying the appearance of evolutionary changes and new species we see 3 distinct patterns arise. Co-Evolution Convergent Evolution Divergent Evolution
Coevolution: Coevolution is the joint change of two or more species in close interaction. Predators and their prey sometimes coevolve; parasites and their hosts often coevolve; plant-eating animals and the plants upon which they feed also coevolve. Coevolution is likely l to happen when different species have close ecological interactions ti with one another.
One example pe of coevolution o o is between ee plants and the animals that pollinate them. In tropical regions bats visit flowers to eat nectar. The fur on the bat's face and neck picks up pollen, which the bat transfers to the next flower it visits. The bats that feed at flowers have a slender muzzle and a long tongue with a brushed tip. These adaptations aid the bat in feeding. Flowers that have coevolved with bats are light in color and have a fruity odor, both traits which are attractive to the bats.
Convergent Evolution In convergent evolution unrelated species become more and more similar in appearance as they adapt to the same kind of environment. Convergent evolution is often referred to as parallel evolution Those different species with analogous structures are perfect examples of convergent evolution.
In convergent evolution the environment determines phenotypes of different species to be similar nonetheless. Sharks and porpoises have similar appearances, streamline bodies and fins, to aid them in their environment. However sharks are fish and porpoises p are mammals ; Both of completely different origins in recent geologic time.
Divergent Evolution Divergent evolution occurs when 2 or more similar species become more dissimilar. Divergence is almost always a response to differing habitats and often results in new species. One important type of divergence is adaptive radiation. This is an evolutionary process in which a single species gives rise to many new species. Darwin s finches are a perfect example. The process of divergence can be accelerated through The process of divergence can be accelerated through artificial selection. By breeding for selective traits we speed up a process that would normally take much longer if left to divergent evolution alone.