Evidence and Mechanisms of Evolution
Homologous Organs
The presence of organs with the same structural patterns but different functions in species that live in different environments is strong evidence of a common ancestor. These organs, called homologous organs, are the result of divergent evolution or adaptive radiation. Groups of individuals coming from the same species but living in different environments develop different functions or adaptations as a result of different selective pressures.
Vestigial Organs
Vestigial organs are the remains of organs that are no longer functional.
Analogous Organs
Analogous organs are organs that fulfill the same function although they have different evolutionary origins. They are proof of convergent evolution. Convergent evolution is the process whereby organisms that are not related develop similar characteristics when they are under the same selective pressure.
Fossil Evidence
Fossils clearly prove that organisms on Earth have changed over time. Fossils of related species from different periods show clear changes and can be useful to understand the evolution of a related group of organisms. In some cases, fossils with intermediate characteristics are found, and they can prove the evolution and the common origin of organisms that today are classified in different groups.
Embryonic Evidence
Some embryos from different species are similar, showing the evolutionary relationship between them.
Biogeographical Evidence
The geographical distribution of animal and plant species can be explained according to the theories of evolution. This was one of the strongest pieces of evidence used by Wallace to support his theory.
Molecular Evidence
The comparison of molecules found in different species reveals that the greater the molecular similarity between them, the closer their evolutionary relationship. All living things share the same types of biomolecules and perform similar basic chemical reactions (e.g., cellular respiration, protein synthesis). By comparing the sequences of DNA and proteins in different species, the evolutionary relationship between them can be established.
Neo-Darwinism
Neo-Darwinism, also called the modern evolutionary synthesis, is a theory that integrates Darwinian evolution and genetics. This theory was developed by Huxley, Dobzhansky, and Mayr, among others, between 1937 and 1948. Genetics explains the mechanisms that cause variety among individuals. This variability is due to:
- Sexual reproduction, which gives rise to gene combinations that are different from those of the parents.
- Genetic recombination, which happens during meiosis.
- Mutations, which cause changes in genes.
According to this theory, natural selection and mutation are complementary processes, and evolution is the result of the combined action of these two processes. Individuals in any population are exposed to mutations randomly. Some mutations will give the individuals an advantage in a certain environment and will be transmitted to the offspring. Some others represent a disadvantage and will finally disappear.
Stages in the Process of Speciation
- Production of evolutionary changes in populations. This happens because natural selection favors some individuals and prejudices others.
- Genetic isolation of a new population. For a new species to evolve, the members of the new population must not be able to reproduce with the original population. This genetic isolation can occur because of different types of barriers:
- Geographical barriers prevent physical contact between populations.
- Sexual barriers can be due to anatomical differences that prevent mating or a lack of synchronization between fertile periods.
- Chromosomal barriers are changes in the structure or number of chromosomes that prevent the affected individuals from having offspring with the rest of the population.
- Physiological barriers are incompatibilities in the functioning of the gametes, which prevent fertilization.
- Ethological barriers are caused by the appearance of new types of behaviors that result in the rejection of some individuals by others.
- Gradual differentiation. After isolation, the population accumulates changes due to new mutations and little by little becomes more and more different from the original population.
- Speciation. Over time, genetic changes produce such significant physical differences that it is no longer possible for the two populations to produce fertile offspring. Although the isolation barriers may have disappeared, these species can no longer reproduce together.
Types of Speciation
There are two types of speciation: sympatric and allopatric.
Sympatric Speciation
Sympatric speciation occurs when populations living in the same geographical region become reproductively isolated. Factors such as chromosomal changes (polyploidy), asexual reproduction, and non-random mating can alter gene flow. This type of speciation is more common in plants than in animals.
Allopatric Speciation
Allopatric speciation can occur when a population is split into two or more isolated groups by a geographical barrier. Over time, the gene pools of the split groups become different, and they are unable to interbreed (i.e., they become reproductively incompatible). This usually occurs when smaller groups are isolated from the larger population. Natural selection may also be a factor if the environment is slightly different from that of the parent population, creating selective pressures.
Adaptive Radiation
Adaptive radiation is the diversification of a common ancestral species into a variety of differently adapted species.
Divergent and Convergent Evolution
Divergent evolution is a pattern of evolution in which species that were once similar to an ancestral species diverge, or become increasingly distinct.
Convergent evolution is a pattern of evolution in which similar traits arise because different species have independently adapted to similar environmental conditions.
Coevolution
Coevolution is the process of reciprocal evolutionary change that occurs between pairs of species or among groups of species as they interact with one another. The activity of each species that participates in the interaction applies selective pressure on the others. Coevolution is one of the primary methods by which biological communities are organized. It can lead to very specialized relationships between species, such as those between pollinator and plant, between predator and prey, and between parasite and host.