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Why was the evolution of large, slowly reproducing organisms preferred?

Why was the evolution of large, slowly reproducing organisms preferred?


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One of the very basic facts that Darwin pointed out was about the life span of organisms. Organisms with smaller lifespan reproduce quickly and hence variations are produced faster. This helps in faster evolution.

One of the main goals of living organisms is to reproduce, this fact being the backbone of neo-Darwinism. For organisms with longer life span, say humans, the amount of time required for new generations to come up, as well as the energy requirement is high, much higher if compared to bacteria. Then why did evolution of complexity arise if bacterial life forms could survive in harsh conditions and reproduce and mutate faster?


The simple answer is that the evolution of large, slowly reproducing organisms is not preferred: it is simply not selected against.

The key mistake in your thinking is this statement:

One of the main goals of living organisms is to reproduce

Most living organisms have no such goal, they simply take actions that have, historically, led to the continuation of their lineage.

In a complex world, there are many different strategies that can lead to survival and propagation of an organism, and cooperation between cells is one of them. Even simple bacteria often form large multicellular aggregates, which can provide physical advantages for their members over dissociated individuals, such as resistance to physical damage and formation of protected microclimates. The inside of your body is another such protected microclimate. So multicellularity can be quite advantageous.

As for limiting reproduction: remember that the closest competitors every new organism has are its own relatives, who occupy the same space and compete for the same resources. Even bacteria will often limit their reproduction when resources are limited. Reproduction also takes significant resources. A faster reproduction then, is also not necessarily advantageous for long-term survival.

Bottom line: neither large nor small, fast nor slow is preferred. Instead, the study of evolution predicts that we will see what we do see: a multiplicity of forms and strategies adapted for different niches in the ecosystem formed by their interactions with other species and the external environment.


Life cycles of animals

Invertebrate animals have a rich variety of life cycles, especially among those forms that undergo metamorphosis, a radical physical change. Butterflies, for instance, have a caterpillar stage ( larva), a dormant chrysalis stage ( pupa), and an adult stage ( imago). One remarkable aspect of this development is that, during the transition from caterpillar to adult, most of the caterpillar tissue disintegrates and is used as food, thereby providing energy for the next stage of development, which begins when certain small structures (imaginal disks) in the larva start growing into the adult form. Thus, the butterfly undergoes essentially two periods of growth and development (larva and pupa–adult) and two periods of small size (fertilized egg and imaginal disks). A somewhat similar phenomenon is found in sea urchins the larva, which is called a pluteus, has a small, wartlike bud that grows into the adult while the pluteus tissue disintegrates. In both examples it is as if the organism has two life histories, one built on the ruins of another.

Another life-cycle pattern found among certain invertebrates illustrates the principle that major differences between organisms are not always found in the physical appearance of the adult but in differences of the whole life history. In the coelenterate Obelia, for example, the egg develops into a colonial hydroid consisting of a series of branching Hydra-like organisms called polyps. Certain of these polyps become specialized (reproductive polyps) and bud off from the colony as free-swimming jellyfish ( medusae) that bear eggs and sperm. As with caterpillars and sea urchins, two distinct phases occur in the life cycle of Obelia: the sessile (anchored), branched polyps and the motile medusae. In some related coelenterates the medusa form has been totally lost, leaving only the polyp stage to bear eggs and sperm directly. In still other coelenterates the polyp stage has been lost, and the medusae produce other medusae directly, without the sessile stage. There are, furthermore, intermediate forms between the extremes.


An evolutionary roll of the dice explains why we're not perfect

If evolution selects for the fittest organisms, why do we still have imperfections? Scientists at the Milner Centre for Evolution at the University of Bath investigating this question have found that in species with small populations, chance events take precedence over natural selection, allowing imperfections to creep in.

Recent work by Alex Ho and Laurence Hurst from the Milner Centre for Evolution at the University of Bath analysed the genomes of a wide range of organisms, from mammals to single-celled algae. They compared the genetic instructions used by cells to make proteins -- specifically the code at the end of the gene that tells the cell to stop reading, called stop codons.

When making proteins, our DNA is read out in strings, with a stop codon at the end of a string to tell the cell to stop reading. In any given gene, most organisms have a choice of using one of three very similar stop codons, however, one of them (so called TAA) is much better than the others (called TGA and TAG) at making the cell machinery stop.

The researchers, publishing in Molecular Biology and Evolution, looked at why some genes use the less efficient stop codons, when evolution by natural selection should cause most genes to use the more efficient TAA codon.

They found that in species such as humans and other mammals, where populations are relatively small and reproduction is slow, selection favoured TAA in the most highly expressed genes. However mutations creating the less effective stop codons could increase in frequency because of chance events, the roll of the dice being more influential when populations are small. This results in a less efficient stop codon being found more often than would be expected, mostly in the less commonly used genes.

In contrast, in species with large, fast replicating populations, such as yeast or bacteria, chance is less important and so natural selection tended to "weed out" any less favourable mutations, resulting in TAA being very common.

The findings could help the design of new gene therapies for genetic diseases.

Professor Laurence Hurst, Director of the Milner Centre for Evolution, said: "Our total set of DNA seems very much more complicated than that of something like yeast. Humans have lots of enigmatic DNA between our genes and each of our genes can typically make many different products, whereas yeast genes tend to make just one.

"Our work shows that natural selection in humans is not very efficient and so our DNA ends up similar to an ancient rusting motor car -- just able to function, with all sorts of bad repairs and accretions built up over time. Yeast instead is more like an organism straight out of the showroom: the perfect machine."

Their results indicate that organisms, such as humans and other mammals, with relatively small population sizes, cannot sustain a perfect state over evolutionary time. It also supports the view that human DNA is error prone and poor quality, not as part of some complex machine for a complex organism, but instead because selection is too weak a force to stop our DNA from deteriorating.

Professor Hurst said: "These results matter because they help us understand that just because something is common, it doesn't mean it is the best. This helps both the understanding of, and therapeutics for, genetic diseases.

"For example, it suggests when making new genes for gene therapy, we should do what yeast do and use the best stop codon: TAA."


Earth’s Mineral Evolution

Evolution isn’t just for living organisms. Scientists at the Carnegie Institution have found that the mineral kingdom co-evolved with life, and that up to two thirds of the more than 4,000 known types of minerals on Earth can be directly or indirectly linked to biological activity. The finding, published in American Mineralogist, could aid scientists in the search for life on other planets.

Robert Hazen and Dominic Papineau of the Carnegie Institution’s Geophysical Laboratory, with six colleagues, reviewed the physical, chemical, and biological processes that gradually transformed about a dozen different primordial minerals in ancient interstellar dust grains to the thousands of mineral species on the present-day Earth. (Unlike biological species, each mineral species is defined by its characteristic chemical makeup and crystal structure.)

"It’s a different way of looking at minerals from more traditional approaches," says Hazen."Mineral evolution is obviously different from Darwinian evolution – minerals don’t mutate, reproduce or compete like living organisms. But we found both the variety and relative abundances of minerals have changed dramatically over more than 4.5 billion years of Earth’s history."

All the chemical elements were present from the start in the solar system’s primordial dust, but they formed comparatively few minerals. Only after large bodies such as the sun and planets congealed did there exist the extremes of temperature and pressure required to forge a large diversity of mineral species. Many elements were also too dispersed in the original dust clouds to be able to solidify into mineral crystals.

As the solar system took shape through "gravitational clumping" of small, undifferentiated bodies – fragments of which are found today in the form of meteorites – about 60 different minerals made their appearance. Larger, planet-sized bodies, especially those with volcanic activity and bearing significant amounts of water, could have given rise to several hundred new mineral species. Mars and Venus, which Hazen and coworkers estimate to have at least 500 different mineral species in their surface rocks, appear to have reached this stage in their mineral evolution.

However, only on Earth – at least in our solar system – did mineral evolution progress to the next stages. A key factor was the churning of the planet’s interior by plate tectonics, the process that drives the slow shifting continents and ocean basins over geological time. Unique to Earth, plate tectonics created new kinds of physical and chemical environments where minerals could form, and thereby boosted mineral diversity to more than a thousand types.

What ultimately had the biggest impact on mineral evolution, however, was the origin of life, approximately 4 billion years ago. "Of the approximately 4,300 known mineral species on Earth, perhaps two-thirds of them are biologically mediated," says Hazen." This is principally a consequence of our oxygen-rich atmosphere, which is a product of photosynthesis by microscopic algae." Many important minerals are oxidized weathering products, including ores of iron, copper and many other metals.

Microorganisms and plants also accelerated the production of diverse clay minerals. In the oceans, the evolution of organisms with shells and mineralized skeletons generated thick, layered deposits of minerals such as calcite, which would be rare on a lifeless planet.

"For at least 2.5 billion years, and possibly since the emergence of life, Earth’s mineralogy has evolved in parallel with biology," says Hazen. "One implication of this finding is that remote observations of the mineralogy of other moons and planets may provide crucial evidence for biological influences beyond Earth."

Stanford University geologist Gary Ernst called the study "breathtaking," saying that "the unique perspective presented in this paper may revolutionize the way Earth scientists regard minerals."


Why deep oceans gave life to the first big, complex organisms

In the beginning, life was small. For billions of years, all life on Earth was microscopic, consisting mostly of single cells. Then suddenly, about 570 million years ago, complex organisms including animals with soft, sponge-like bodies up to a meter long sprang to life. And for 15 million years, life at this size and complexity existed only in deep water.

Scientists have long questioned why these organisms appeared when and where they did: in the deep ocean, where light and food are scarce, in a time when oxygen in Earth's atmosphere was in particularly short supply. A new study from Stanford University, published Dec. 12 in the peer-reviewed Proceedings of the Royal Society B, suggests that the more stable temperatures of the ocean's depths allowed the burgeoning life forms to make the best use of limited oxygen supplies.

All of this matters in part because understanding the origins of these marine creatures from the Ediacaran period is about uncovering missing links in the evolution of life, and even our own species. "You can't have intelligent life without complex life," explained Tom Boag, lead author on the paper and a doctoral candidate in geological sciences at Stanford's School of Earth, Energy & Environmental Sciences (Stanford Earth).

The new research comes as part of a small but growing effort to apply knowledge of animal physiology to understand the fossil record in the context of a changing environment. The information could shed light on the kinds of organisms that will be able to survive in different environments in the future.

"Bringing in this data from physiology, treating the organisms as living, breathing things and trying to explain how they can make it through a day or a reproductive cycle is not a way that most paleontologists and geochemists have generally approached these questions," said Erik Sperling, senior author on the paper and an assistant professor of geological sciences.

Goldilocks and temperature change

Previously, scientists had theorized that animals have an optimum temperature at which they can thrive with the least amount of oxygen. According to the theory, oxygen requirements are higher at temperatures either colder or warmer than a happy medium. To test that theory in an animal reminiscent of those flourishing in the Ediacaran ocean depths, Boag measured the oxygen needs of sea anemones, whose gelatinous bodies and ability to breathe through the skin closely mimic the biology of fossils collected from the Ediacaran oceans.

"We assumed that their ability to tolerate low oxygen would get worse as the temperatures increased. That had been observed in more complex animals like fish and lobsters and crabs," Boag said. The scientists weren't sure whether colder temperatures would also strain the animals' tolerance. But indeed, the anemones needed more oxygen when temperatures in an experimental tank veered outside their comfort zone.

Together, these factors made Boag and his colleagues suspect that, like the anemones, Ediacaran life would also require stable temperatures to make the most efficient use of the ocean's limited oxygen supplies.

Refuge at depth

It would have been harder for Ediacaran animals to use the little oxygen present in cold, deep ocean waters than in warmer shallows because the gas diffuses into tissues more slowly in colder seawater. Animals in the cold have to expend a larger portion of their energy just to move oxygenated seawater through their bodies.

But what it lacked in useable oxygen, the deep Ediacaran ocean made up for with stability. In the shallows, the passing of the sun and seasons can deliver wild swings in temperature -- as much as 10 degrees Celsius in the modern ocean, compared to seasonal variations of less than 1 degree Celsius at depths below one kilometer (.62 mile). "Temperatures change much more rapidly on a daily and annual basis in shallow water," Sperling explained.

In a world with low oxygen levels, animals unable to regulate their own body temperature couldn't have withstood an environment that so regularly swung outside their Goldilocks temperature.

The Stanford team, in collaboration with colleagues at Yale University, propose that the need for a haven from such change may have determined where larger animals could evolve. "The only place where temperatures were consistent was in the deep ocean," Sperling said. In a world of limited oxygen, the newly evolving life needed to be as efficient as possible and that could only be achieved in the relatively stable depths. "That's why animals appeared there," he said.


Goldilocks and temperature change

Previously, scientists had theorized that animals have an optimum temperature at which they can thrive with the least amount of oxygen. According to the theory, oxygen requirements are higher at temperatures either colder or warmer than a happy medium. To test that theory in an animal reminiscent of those flourishing in the Ediacaran ocean depths, Boag measured the oxygen needs of sea anemones, whose gelatinous bodies and ability to breathe through the skin closely mimic the biology of fossils collected from the Ediacaran oceans.

“We assumed that their ability to tolerate low oxygen would get worse as the temperatures increased. That had been observed in more complex animals like fish and lobsters and crabs,” Boag said. The scientists weren’t sure whether colder temperatures would also strain the animals’ tolerance. But indeed, the anemones needed more oxygen when temperatures in an experimental tank veered outside their comfort zone.

Together, these factors made Boag and his colleagues suspect that, like the anemones, Ediacaran life would also require stable temperatures to make the most efficient use of the ocean’s limited oxygen supplies.


Why was the evolution of large, slowly reproducing organisms preferred? - Biology

A. Variation and Selection

1. Mechanisms the produce genetic variation in populations.

A mutation is a change in the nucleotide sequence of the DNA in a cell. There are many different kinds of mutations. Mutations can occur before, during, and after mitosis and meiosis. If a mutation occurs in cells that will make gametes by meiosis or during meiosis itself, it can be passed on to offspring and contribute to genetic variability of the population. Mutations are the sole source of genetic variability that can occur in asexual reproduction. Mutations are usually harmful or neutral to offspring but can occasionally be beneficial.

Mutations can result from the insertion , deletion , or substitution of one or a few nucleotides in a gene sequence. Small changes of this sort usually result from errors in DNA replication prior to cell division or from errors in the DNA repair that occurs in response to DNA damage. These small changes are generally known as "point mutations". If the small change occurs in a region of the gene that codes for an important part of its protein, the effect can be large, such as the mutation that causes sickle cell disease.

Mutations also result from gene rearrangements and other large changes in the DNA sequence of a chromosome. A translocation is movement of a segment of DNA from one place to another in a chromosome or between chromosomes. An inversion is a mutation in which a segment of DNA has flipped within a chromosome. A deletion is the loss of a segment of DNA. These large changes are relatively common, at least over long periods of time, and are abundant in genomes that have been sequenced.

Crossing over refers to the relatively frequent exchange of chromosome segments between paired homologous chromosomes that occurs during Prophase I. Often, this exchange produces little or no change in the order or number of genes on the chromsomes. It does mean that some of the genes originating on the maternal homologues get mixed in with genes on the paternal homologues, and vice versa. In other words, some of Mom's alleles get into Dad's homologues and vice versa.

Sometimes crossing over is unequal. One chromosome gets a longer piece of its homologue than the other chromosome gets in return. This can result in gene duplication in the chromosome that got more DNA. Gene duplication can give rise to new genes because the extra gene can sustain mutations while the duplicate gene continues to carry out its normal function. Analyses of the genomes of many organisms suggest that genes are often duplicated over evolutionary time. The groups of duplicated genes are referred to as "gene families", owing to the resemblance of their sequences and their origin by descent from a common ancestor gene.

Mutations occur during DNA replication prior to meiosis. Crossing over during metaphase I mixes alleles from different homologues into new combinations.

When meiosis is complete, the resulting eggs or sperm have a mixture of maternal and paternal chromosomes. This is because during anaphase I, the spindle accurately separates a complete set of 23 human chromosomes into each daughter cell but does not distinguish between the 23 from Mom and the 23 from Dad. Mom's and Dad's homologues are randomly intermixed during anaphase such that each egg or sperm cell has a nearly unique combination of Mom's and Dad's alleles. The number of combinations of 23 maternal and paternal homologues that can result from independent assortment is 2 23 , about 8 million. This leaves out variations caused by mutations or crossing over.

Fertilization randomly brings together two gametes produced in two different individuals. This means that for a particular man and woman, the number of unique combinations of genes that could occur in their offspring is 8 million times 8 million ( 64 trillion), not counting variation caused by crossing over and mutation. Random fertilization is a further mechanism that produces genetic variation in the process of sexual reproduction.

The genetic variation that results from mutations, meiosis, and fertilization cause the phenomenon with which we are all familiar: even in very large populations, such as the human population, every individual is genetically unique.

There are additional mechanisms that generate genetic variation. One is polyploidy, which occurs commonly in plants and leads to new species within one generation. Polyploidy events lead to organisms with more than two sets of chromosomes. More than half of wild plants are polyploid and sao are many domestic plants. On page 240 and 241, your text describes the polyploidy events that lead to modern wheat cultivars.

Another mechanism that produces genetic variation is the transfer of genes between species. This is common between different species of bacteria and may occur in eukaryotes as a result of virus infections in which the virus integrates some of its genes into cells that give rise to eggs or sperm.

The genetic variety produced by sexual reproduction offers many possibilities for how a population of organisms might change over time. The possibility that actually occurs is determined by selection. Those variants that are best suited to prevailing conditions produce more offspring than the others and their combinations of genes therefore tend to prevail in the population, at least until the selection regime changes and another combination of genes is preferred.

Selection is best seen as a filter through which a subset of the genetic variants in a population pass. Some genotypes make it, some don't.

B. Micro- and macro-evolution

Biological evolution is probably the biggest of all biological theories. It has been said that nothing in biology makes sense except in the light of evolution (Theodosius Dobzhansky, 1970). By the same token, modern cell biology and genetics have done much to make sense of biological evolution. Perhaps the first example of this was the discovery of genes by Gregor Mendel, which occurred subsequent to Charles Darwin's hypothesis of natural selection and provided an explanation for the inheritance of traits that were advantageous.

The magnitude of biological evolution has led to two perspectives on how it works: micro-evolution and macro-evolution. They are qualitatively the same but exhibit different scales of change and time.

Micro-evolution refers to small changes that occur quickly in a population of organisms. The diversity of dogs that has resulted from artifical selection for different physical traits is an example. Microevolution is easy to understand because we practice it in the selective breeding of animals and plants and because it happens in nature on a time scale that we can observe. Micro-evolution proceeds by the action of selection on the genetic diversity of a population, as we have discussed.

Macro-evolution refers generally to dramatic changes in the diversity of life on Earth over longer periods of time than humans can perceive. The Cambrian Explosion is an example of macroevolution (we will discuss this later).

Macro-evolution can be considered to consist of two parts: extinction and speciation. Extinction is easy to understand. It is the disappearance of a population when selection overwhelms it. Causes of extinction include, new diseases, new predators (e.g. man), climate changes (e.g. Ice Ages), habitat loss, geological processes (e.g. continental drift), and catastrophic events like asteroid impacts with the Earth.

Speciation is more difficult to understand and is discussed further later. Speciation is essentially lots of microevolution in populations that have become reproductively isolated, i.e. they can no longer share genetic diversity by interbreeding..

Micro and macro evolution are artificial concepts that are defined by a human perspective of time. Because of this, they don't really fit the growing body of evidence and hypotheses about evolution. For example, the Endosymbiont Hypothesis proposes that eukaryotes evolved from prokaryotic ancestors in a matter of moments. Was this micro or macro-evolution?

Speciation is the process by which new species arise from existing species. Two patterns for the process of speciation have been proposed: phyletic speciation and divergent speciation.


Goldilocks and temperature change

Previously, scientists had theorized that animals have an optimum temperature at which they can thrive with the least amount of oxygen. According to the theory, oxygen requirements are higher at temperatures either colder or warmer than a happy medium. To test that theory in an animal reminiscent of those flourishing in the Ediacaran ocean depths, Boag measured the oxygen needs of sea anemones, whose gelatinous bodies and ability to breathe through the skin closely mimic the biology of fossils collected from the Ediacaran oceans.

“We assumed that their ability to tolerate low oxygen would get worse as the temperatures increased. That had been observed in more complex animals like fish and lobsters and crabs,” Boag said. The scientists weren’t sure whether colder temperatures would also strain the animals’ tolerance. But indeed, the anemones needed more oxygen when temperatures in an experimental tank veered outside their comfort zone.

Together, these factors made Boag and his colleagues suspect that, like the anemones, Ediacaran life would also require stable temperatures to make the most efficient use of the ocean’s limited oxygen supplies.


Structural and Behavioral Adaptations

Adaptations are the result of evolution. Evolution is a change in a species over long periods of time.

Adaptations usually occur because a gene mutates or changes by accident! Some mutations can help an animal or plant survive better than others in the species without the mutation.

For example, imagine a bird species. One day a bird is born with a beak that is longer than the beak of other birds in the species. The longer beak helps the bird catch more food. Because the bird can catch more food, it is healthier than the other birds, lives longer and breeds more. The bird passes the gene for a longer beak on to its offspring. They also live longer and have more offspring and the gene continues to be inherited generation after generation.

Eventually the longer beak can be found in all of the species. This doesn't happen overnight. It takes thousands of years for a mutation to be found in an entire species.

Over time, animals that are better adapted to their environment survive and breed. Animals that are not well adapted to an environment may not survive.

The characteristics that help a species survive in an environment are passed on to future generations. Those characteristics that don't help the species survive slowly disappear.

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Types of Gradualism

Phyletic Gradualism

Seen in the top of the image below, phyletic gradualism suggests that the changes in animals occur steadily over time. The sloping lines suggest that the change in morphology shown happened slowly, through many generations of animals. While this image is entirely hypothetical, it could represent any group of animals.

For instance, if the two trees both represent hypothetical relationships between domestic dogs, the cladogram of phyletic gradualism on the top would tell a story of slow changes over time. At the bottom of the tree would be the wolf, or the closest ancestral relative. As the tree progresses upward, more generations of dog-like creatures were being born. Some of the lines, unfit for their environment, died out along the way. However, many lines made it to present day (the top of the tree). One the left might be the smallest dogs, such as the Dachshund and Rat Terrier. On the other side would be dogs which are now morphologically larger than wolves, such as Great Danes and Irish Wolf Hounds.

Gradualism suggests specifically that the changes in these animals came in small increments, over time. To get to a Dachshund, gradualism suggests, one must first breed a smaller and longer wolf. Every generation, the wolves get smaller and longer. Eventually, after many generations, you have a dog which no longer resembles a wolf at all. The same would occur in the other direction, with the animals slowly getting larger over time. Thus, the theory of gradualism proposes that changes occur in many small steps, over time. While this is an interesting theory, and probably true to some extent, fossil record analysis have shown that at least some organisms experience much more rapid change.

Punctuated gradualism

Punctuated equilibrium, seen in the lower half of the image above, suggests that instead of slow changes over time, change happens drastically and almost immediately. While several fossil records and studies of microevolution have shown that drastic change can happen quickly, it might not get the whole picture. Punctuated gradualism is a step between the two.

In an unpredictable manner, environmental changes can change the expression of an organism’s genetics. At times when there are large environmental upheavals, these “punctuated” change events can occur. Some organisms within a group, now able to utilize their genetic advantage, break off from the pack and forge their own evolutionary history. Punctuated gradualism is a middle-of-the-road approach to understanding evolutionary change.

Gradualism in the Environment

Gradualism, as a theory in general, started in geology. Scientists studying rock formations, such as the Grand Canyon, made the observation that the river within the canyon was carrying minute amounts of sediment from the canyon out to sea. While these deposits were small each year, over eons the Grand Canyon was carved.

Using the theory of gradualism, scientists can understand and infer more about the abiotic factors affecting biology. For example, every year the tectonic plates move a fraction of an inch. While this movement is negligible in our lifetimes (besides causing earthquakes), the movement of the continents over time has serious consequences for evolution as a whole. Measuring the movements of the continents, scientists have been able to model the Earth through the evolutionary history of organism. This not only gives us a greater understanding of the world around us, but shows us how seemingly different animals were once populating a mega-continent called Pangea.