evolution i - theories of evolution Flashcards
what is biological evolution?
biological evolution is defined as descent with modification through the mechanism of natural selection and refers to the cumulative changes that occur in a population from generation to generation over time.
this leads to differences in populations & explains the origin of all organisms that exist today or have ever existed.
it encompasses both microevolution & macroevolution
define microevolution
microevolution is the form of evolutionary change that involves small-scale evolutionary change within the species level. it is caused by changes in allele/genotype frequencies that occur within a population of a particular species over a few generations.
we are concerned with patterns of change within a population/species, and is also a short-term perspective.
define macroevolution
macroevolution is the large scale phenotypic changes in populations that generally warrant their placement in taxonomic groups at the species level or higher.
studies of macroevolution seek to discover & explain major changes in species diversity through time, such as during adaptive radiation and mass extinction
what are the features of darwin-wallace’s theory of evolution?
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- organisms have great potential to reproduce (all organisms produce large numbers of offspring)
- environmental restrictions/constancy in numbers (despite tendency to increase numbers due to overproduction of offsrping, most populations maintain relatively constant numbers, due to limited resources, diseases & predation etc)
- struggle for existence/survival (based on the above 2 points, with more individuals than the environment can support, competition is inevitable - organisms thus face a constant struggle for existence)
- variation within a population (sexually produced offspring show individual variations - no 2 offspring are identical.
- survival of the fittest by natural selection (offspring that are better adapted to survive in the struggle for existence are more likely to survive long enough to breed)
- differential reproduction leading to reproductive success (those that survive to breed are likely to produce offspring similar to themselves - advantageous traits are likely to be passed on to the next generation, resulting in adaptation)
- formation of new species (over many generations, the proportion of indviduals possessing the advantageous traits increases while the proportion of those lacking the characteristics decreases, leading to evolution of the population. a new species is only formed after the development & accumulation of several characterstics and adaptive mechanisms in a particular population over many generations)
define natural selection
natural selection is the process by which the environment selects for well-adapted individuals, with inherited traits that are best suited to an environment, which are more likely to survive to reproduce (i.e. differential survival & reproduction).
there is hence reproductive success of fitter individuals over those that are less fit.
it increases the frequency of favourable genotypes/alleles in the gene pool, and the resultant population becomes adapted to its particular environment
what is an adaptation
an evolutionary modification that improves the chances of survival and reproductive success in a given environment
what evidence is there for darwin’s theory of evolution?
- homologies (anatomical & molecular)
- fossil evidence
- biogeography
- direct observations of evolutionary change
what is a homology?
structural/anatomical similarities in homologous structures between species suggest that they descended from a common ancestor with that feature.
such a condition is known as homology, which is the similarity in characteristics resulting from a shared ancestry
3 types of anatomical homologies?
- homologous structures (e.g. homology of the forelimb & hindlimb bones of vertebrates, which have the same basic pentadactyl pattern)
- early embryonic development (it is observed that at some point in development, all vertebrates bear striking anatomical similarities - a post anal tail & paired pharyngeal pouches - known as developmental homologies)
- vestigial structures (refer to structures that have no apparent function, but resemble structures present in their ancestors. e.g. modern whales have vestigial hind-limb bones that can be traced back to ancestral whales)
how can homologies be used as evidence for evolution?
many organisms exhibit organs that are similar in structure to those in a recent ancestor. this reflects evolutionary relatedness. descent with modification leads to a variety of forms that are adapted for very different function. the presence of homologies between species imply that they descended from a common ancestor
what is divergent evolution?
the tracing of a species back to the common ancestor can indicate how these organisms have diversified and diverged to colonize the different environments. such a form of evolution is called divergent evolution
how can molecular data be used as evidence for evolution?
- can analyse differences in conserved sequences of nucleotides in DNA/RNA & amino acids in proteins
- can compare structures of molecules from different speies (e.g. ribosomal RNA)
- as descendants of an ancestral species evolve independently, they will accumulate more and more differences in DNA - the more distantly related 2 species are, the greater number of sequence differences will be in their DNA
how can fossils be used as evidence for evolution?
fossil are the preserved remains of once-living organisms typically left in sedimentary rocks. fossil records allow us to study the history of particular groups and shows the evolutionary changes that have occurred over time in various groups of organisms.
evidence to support evolution include:
- there is a greater variety of fossils in younger rocks, and there is emergence of new species and extinction of others -> indicates the times at which species originated and became extinct
- there are progressive changes in the structures of organisms & increased complexity of structures of fossilized organisms in younger rocks than oldest rocks -> shows descent with modfication through changes in homologus structures due to environmental selection pressures.
what is biogeography and the 2 types of biogeography?
biogeography is the study of the range and geographical distribution of extinct & modern species of organisms in different places throughout the world.
2 types of biogeography: island biogeography & continent biogeography
how can island biogeography be used as evidence for evolution?
- islands generally have many species of plants & animals that are endemic (found nowhere else in the world)
- closely related species sharing similar characteristics tend to be found in the same geographic region, and most island species are closely related to species from the nearest mainland/neighbour island(s) -> implies that these species could have descended with modification from a common mainland ancestor
- islands are colonized by species from the nearest mainland -> colonists eventually give rise to new species as they adapt to their new environments
what is convergent evolution?
species from different evolutionary branches may come to resemble each other if they have similar ecologiacl roles & natural selection has shaped similar adaptations. this is called convergent evolution, and the similarity between 2 specieis due to convergent rather than divergent evolution is called analogy.
how can continent biogeography be used as evidence for evolution?
similarities between individual members of marsupials & placentals are a result of convergent evolution. these mammals have similar forms but different ancestry, because they evolved separately in different, isolated areas because of similar selective pressures in similar environments
