Beagle , including stops in South America, Australia, and the southern tip of Africa. Wallace traveled to Brazil to collect insects in the Amazon rainforest from to and to the Malay Archipelago from to On these islands, Darwin observed species of organisms on different islands that were clearly similar, yet had distinct differences.
The species on the islands had a graded series of beak sizes and shapes with very small differences between the most similar. He observed that these finches closely resembled another finch species on the mainland of South America. Darwin imagined that the island species might be species modified from one of the original mainland species. Upon further study, he realized that the varied beaks of each finch helped the birds acquire a specific type of food. For example, seed-eating finches had stronger, thicker beaks for breaking seeds, and insect-eating finches had spear-like beaks for stabbing their prey.
Wallace and Darwin both observed similar patterns in other organisms and they independently developed the same explanation for how and why such changes could take place. Darwin called this mechanism natural selection.
For example, a population of giant tortoises found in the Galapagos Archipelago was observed by Darwin to have longer necks than those that lived on other islands with dry lowlands. Consequently, long-necked tortoises would be more likely to be reproductively successful and pass the long-necked trait to their offspring.
Typically, we think of biological evolution as changes in gene frequency within a population over time — if, say, birds with genes that produce wide beaks went from being rare to being common over multiple generations. But biological evolution also includes changes in DNA that does not code for genes and changes in heritable information not encoded in DNA at all.
In all of these cases, the modifications are heritable and can be passed on to the next generation — which is what really matters in evolution: long term change.
Compare these two examples of change in beetle populations. Which one is an example of evolution? Beetles on a diet Imagine the population experiences many years of drought in which there are few plants that the beetles can eat.
All the beetles have the same chances of survival and reproduction, but because of food restrictions, the beetles in later generations are smaller than the pre-drought generations of beetles.
Heritability : Some but not all variation is inherited. Causal mechanism of inheritance unknown in Darwin's time. Discovery by Gregor Mendel of genetics came later, and discovery of DNA came later still Heritable traits are coded in DNA and passed on to descendants Note that DNA is NOT a "blueprint" as commonly thought: it is a set of instructions for putting bodies together and maintaining them after they've been built Each little instruction is called a gene : a piece of code that helps the cell to build a protein Most genes have slightly different versions called alleles that produce different end products It is these alleles one copy for each gene per parent that is passed on to offspring Different combinations of alleles result in different traits being expressed that is, different phenotypes.
Depending on the particular combination of alleles an offspring gets, they might have the same trait as their mother, their father, or something different than either. Mutations are new variations in heritable traits, caused by miscopied DNA duplication of parts of genes; miswritten code; etc. Some mutations may be deleterious they result in harm to the organism Many mutations may be neutral they don't benefit the organism in an obvious way, nor hurt it A small number of mutations may wind up being beneficial the variation they produce allow it to do better somehow in the world Superfecundity : Organisms produced far more offspring than can possibly survive Application of demographer Thomas Malthus' reproductive excess concept to Nature Violated another previously-held belief: that Nature was perfect and everything had its place.
Thus, IF some variation gives the individual a slight advantage bigger, stronger, smaller, smarter, less tasty, whatever at surviving; and IF that variation is heritable; THEN there is a somewhat better than average chance that organisms with that variation will survive to bear the next generation. Over the long expanse of geologic time, the accumulation of these variations will change the population from one form to another: the origin of species.
Hence, Natural Selection is the differential survival and reproduction of variants in a population resulting in a net change in phenotype of the descendants. Short form: "Natural selection is the differential survival and reproduction of variants in a population. If Evolution can be summarized as "no one is identical to their parents", then Natural Selection can be summarized as "no one is identical to their siblings, either; plus, life's hard!
Key points of Natural Selection: Does NOT happen to individuals, only to populations lineages Analogous to "artificial selection" domestication , but operates: On all traits rather than a few humans can keep alive crops, farm animals, or pets that might otherwise die in the wild; obviously, wild plants and animals don't have that help!
Over vast amounts of geologic time, rather than just a few generations Does NOT require simple things evolving into complex: sometimes a simplified mutation of a structure might be advantageous than the ancestral complex one hence, vestigial organs Cannot evolve towards something with a goal in mind; only favors variations that are advantageous at the time of selection.
Phrase not in the earlier editions of the Origin , nor was it coined by Darwin. It is being better suited to the environment in some fashion relative to other members of your population. From Darwin and Wallace, we get the beginnings of modern evolutionary theory.
It has five major components: Evolution is descent with modification : that is, the anatomical traits and other features of populations change over time from generation to generation These modifications occur relatively slowly on average: small incremental changes added up over many generations Populations may diverge into two or more distinct lineages which may or may not produce their own descendant branches All species share a common ancestry: thus, the shape of the history of lineages can be seen as a Tree of Life Much although not all evolutionary change is due to natural selection, which is the sole process for producing adaptations.
Patterns and Processes: Macroevolution With the discovery of evolution by natural selection, biologists from Darwin and Wallace's time onward have documented many different patterns and processes in evolution. Sometimes they refer to "microevolution" changes within an species and "macroevolution" patterns on the larger scale; changes from one species to another, or between different lineages of ancestors and descendants.
It is important to remember that "micro-" vs "macro-" is just a matter of scale and perception: at the level of individuals and populations, there is just variability, heritability, and superfecundity.
The most important pattern: the Tree of Life. Darwin and Wallace demonstrated the reality of Divergence through Time and Common Ancestry : Divergence from common ancestors Two or more distinct variations in an ancestral population convey their own advantage against the rest of the population Over time, these two or more variations will become more distinct from each other If they diverge enough, they will no longer be able to mate with each other: will be different species Divergence can also occur perhaps more commonly!
It is important to understand that genes and individuals do not evolve, only populations as a whole evolve. The process looks like this: Genes mutate and those mutations have consequences for the individuals within a species.
Those individuals either thrive or die out due to their genetics. As a result, populations change evolve over time. Many students confuse natural selection with descent with modification, so it's worth repeating, and further clarifying, that natural selection is part of the process of evolution, but not the process itself.
Natural selection comes into play, according to Darwin, when a species as a whole adapts to its environment, thanks to its specific genetic makeup. Say at some point in time two species of wolves lived in the Arctic: those with short, thin fur and those with long, thick fur. Those wolves with long, thick fur were genetically capable of living in the cold. Those with short, thin fur were not. Therefore, those wolves whose genetics allowed them to live successfully in their environment lived longer, bred more frequently, and passed on their genetics.
They were "naturally selected" to thrive. Those wolves who were not genetically adapted to the cold eventually died out. Furthermore, natural selection doesn't create variation or give rise to new genetic traits—it selects for the genes already present in a population. In other words, the Arctic environment in which our wolves lived did not prompt a series of genetic traits that didn't already live in certain of the wolf individuals.
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