Evolution notes
MDCAT Biology
Evolution is the gradual genetic change in populations over successive generations. This chapter explains the concept of evolution, evidence for common ancestry, Lamarckism, Darwinism, natural selection, speciation, and the genetic basis of evolution in Neo-Darwinism.
Concept of Evolution
Evolution is the gradual change in the genetic characteristics of a population over successive generations. It occurs in populations, not in individual organisms. Changes in allele frequencies may arise through mutation, recombination, natural selection, genetic drift and gene flow.
Evolution explains the diversity of organisms and the appearance of adaptations. An adaptation is an inherited feature that improves an organism’s ability to survive and reproduce in a particular environment. Different organisms may become adapted to different ecological niches, which can eventually lead to speciation.
A species is a group of similar organisms that can interbreed naturally and produce fertile offspring. It is the basic unit of biological classification. Speciation is the formation of new species, usually when populations become reproductively isolated and accumulate genetic differences.
- Evolution is defined as gradual genetic change in populations over successive generations.
- The basic unit of classification is the species.
- Adaptation means an inherited feature that increases survival or reproductive success in a particular environment.
- Speciation is the formation of new species through the accumulation of differences and reproductive isolation.
- Evolution affects populations, while natural selection acts on variations present in individual organisms.
- A niche is the functional role and position of an organism in its environment.
- Evolution does not mean that every organism becomes more complex or perfect. It means better adaptation to a particular environment.
Development of Evolutionary Thought
Early explanations of life often included the idea that species were fixed and had not changed since their creation. During Aristotle’s time, many people believed in the fixity of species and spontaneous generation. Spontaneous generation was the belief that living organisms could arise from non-living matter.
The theory of special creation, also called the theory of new creation in some textbooks, stated that organisms were separately created and did not evolve from common ancestors. This theory was associated with Louis Agassiz. Later, geology and palaeontology showed that different organisms dominated during different geological time periods.
Palaeontologists study fossils and use them to arrange organisms in a sequence according to geological time. The fossil record showed that many organisms lived in the past and later became extinct. It also provided evidence that life has changed through time.
- Aristotle’s period was associated with the ideas of fixity of species and spontaneous generation.
- Louis Agassiz composed or supported the theory of new creation, also called special creation.
- A palaeontologist studies fossils and the history of life through geological time.
- Different organisms dominated the Earth during different geological periods.
- The fossil record provides evidence that organisms have changed and become extinct through time.
- The oldest mineral discovered so far is zircon, dating back about 4.4 billion years.
- A productive scientific theory explains observations, makes testable predictions and allows different hypotheses to be suggested. A theory that discourages different hypotheses is not considered productive.
Evidence for Common Ancestry
Common ancestry means that different organisms descended from an ancestral population. Several independent lines of evidence support this idea. These include fossils, comparative anatomy, comparative embryology, vestigial organs, biochemical similarities and geographical distribution.
Homologous organs have the same basic evolutionary origin but may perform different functions. For example, the forelimbs of humans, dogs, bats and whales contain similar bones arranged in a common pattern. A bird’s wing is homologous to a dog’s front leg because both are modified vertebrate forelimbs.
Analogous organs perform similar functions but have different evolutionary origins and structural plans. The wings of a bird and an insect are analogous. They evolved independently in unrelated groups and show convergent evolution. Convergent evolution occurs when unrelated organisms develop similar features because they face similar environmental conditions.
- Homologous organs have a common evolutionary origin but may have different functions.
- A bird’s wing is homologous to a dog’s front leg.
- Analogous organs have different evolutionary origins but perform similar functions.
- Analogous organs provide an example of convergent evolution.
- Comparative embryology can show similarities in the early development of different organisms.
- Vestigial organs are reduced structures that have little or no important function in the present organism.
- Similarities in DNA and proteins, such as cytochrome c in humans and aerobic bacteria, support common ancestry.
- In humans, embryonic gill pouches develop into structures including parts associated with the Eustachian tubes and middle ear, rather than functioning as gills.
Darwin and Natural Selection
Charles Darwin was born in Shrewsbury in western England. He travelled on HMS Beagle and observed variations among organisms in different environments. Darwin returned to Great Britain in 1836. His observations helped him develop the theory of natural selection.
Darwin was greatly influenced by Thomas Malthus’s Essay on Population. Malthus argued that populations can increase faster than the food and resources available to them. Therefore, organisms compete for limited resources.
Darwin published On the Origin of Species by Means of Natural Selection. The book mainly explains how new species can arise, so it is mainly concerned with speciation. Darwin’s theory did not explain the genetic mechanism by which variations are inherited because the principles of genetics had not yet been incorporated into it.
- Charles Darwin was born at Shrewsbury in western England.
- Darwin returned to Great Britain in 1836 after his voyage on HMS Beagle.
- Darwin was influenced by Thomas Malthus’s Essay on Population.
- Darwin’s book is commonly referred to as On the Origin of Species by Means of Natural Selection.
- Darwin’s theory of natural selection is mainly concerned with adaptation and speciation.
- Natural selection requires variation, overproduction, competition and differential survival and reproduction.
- Individuals with favourable inherited variations leave more offspring on average.
- Natural selection acts on phenotypes, but its long-term result is a change in allele frequencies in populations.
Lamarckism
Jean-Baptiste Lamarck proposed a theory of evolution based on the inheritance of acquired characteristics. He hypothesized that organisms evolved because their organs were used or not used according to environmental needs. Structures used frequently became stronger or more developed, while unused structures became reduced.
Lamarck believed that changes acquired during an organism’s lifetime could be passed to its offspring. His traditional example was the long neck of the giraffe. According to Lamarck, repeated stretching to reach leaves caused the neck to become longer, and this acquired feature was inherited by later generations.
Modern genetics does not support the general inheritance of acquired characteristics in the Lamarckian sense. Changes in body cells acquired during life usually do not alter the DNA of reproductive cells and therefore are not transmitted as inherited traits. However, Lamarck’s emphasis on adaptation and the effect of environment was historically important.
- Lamarck hypothesized that organisms evolved through inheritance of acquired characteristics.
- The use and disuse theory was proposed by Lamarck.
- According to use and disuse, frequently used organs become stronger, while unused organs become reduced.
- Lamarck believed that acquired changes could be inherited by offspring.
- Lamarck’s giraffe explanation involved repeated stretching of the neck to obtain leaves.
- Lamarckism differs from Darwinism because Lamarck proposed that environmental needs directly produced inherited changes.
- Modern genetics does not generally support the inheritance of acquired bodily characteristics.
Darwinism Compared with Lamarckism
Darwinism states that populations contain heritable variations before selection occurs. Organisms with favourable variations survive and reproduce more successfully. Over many generations, these variations become more common in the population.
Lamarckism states that environmental conditions create a need, the organism develops a suitable characteristic through use, and the acquired characteristic is inherited. Darwinism therefore explains adaptation through differential reproduction, whereas Lamarckism explains it through the inheritance of acquired changes.
For example, Darwin would explain the giraffe’s long neck by the survival and reproduction of giraffes that already possessed slightly longer necks. Lamarck would explain it by the stretching and use of the neck during the giraffe’s lifetime.
- Darwinism begins with pre-existing heritable variations in a population.
- Lamarckism proposes that acquired characteristics are inherited.
- Darwinism: natural selection preserves favourable inherited variations.
- Lamarckism: use and disuse directly modify organs according to need.
- Darwinism explains evolution through differential survival and reproduction.
- Darwinism is supported by genetics, while the general Lamarckian inheritance of acquired traits is not supported by modern genetics.
- Both theories attempted to explain adaptation and evolutionary change.
Modern Evolutionary Theory and Genetic Change
Neo-Darwinism combines Darwin’s natural selection with genetics. It emphasizes the role of genetics in explaining how evolution works. According to Neo-Darwinism, mutation and recombination produce heritable variation, while natural selection changes the frequency of favourable alleles in populations.
Mutation is a sudden change in genetic material. Recombination occurs during sexual reproduction and produces new combinations of existing alleles. Gene flow occurs when individuals or their gametes move between populations and introduce alleles into a new population.
Genetic drift is a change in allele frequency that occurs by chance. Its effect is strongest in small populations. A population bottleneck and the founder effect are examples of situations in which chance can greatly change allele frequencies.
- Neo-Darwinism emphasizes the role of genetics in explaining evolution.
- Mutation produces new alleles and may introduce new genetic variation.
- Recombination produces new combinations of alleles during sexual reproduction.
- Natural selection changes allele frequencies by favouring some inherited variations.
- Genetic drift is a chance change in the frequency of alleles.
- The effect of genetic drift is generally greater in small populations.
- Gene flow is the movement of alleles between populations.
- Outbreeding increases heterozygosity by bringing together alleles from genetically different individuals.
Hardy-Weinberg Principle
The Hardy-Weinberg principle describes a population in which allele and genotype frequencies remain constant from generation to generation. Such a population is in genetic equilibrium. The principle provides a reference point for identifying whether evolution is occurring.
For two alleles, the frequency of the dominant allele is represented by p and the frequency of the recessive allele by q. The allele equation is p + q = 1. The genotype equation is p² + 2pq + q² = 1, where p² represents homozygous dominant individuals, 2pq represents heterozygous individuals and q² represents homozygous recessive individuals.
Hardy-Weinberg equilibrium requires a very large population, random mating, no mutation, no migration, no natural selection and no genetic drift. If any of these conditions is disturbed, allele frequencies may change and evolution may occur.
- Hardy-Weinberg equilibrium means that allele and genotype frequencies remain constant across generations.
- The allele frequency equation is p + q = 1.
- The genotype frequency equation is p² + 2pq + q² = 1.
- p² is the frequency of the homozygous dominant genotype.
- 2pq is the frequency of the heterozygous genotype.
- q² is the frequency of the homozygous recessive genotype.
- If 16% of individuals show a recessive trait, q² = 0.16, q = 0.4 and p = 0.6.
- If 35% of mice are white and white is caused by aa, the frequency of the aa genotype is 0.35. Under Hardy-Weinberg assumptions, q = √0.35, p = 1 − q, and the heterozygous frequency is 2pq.
Key terms
- Evolution
- The gradual genetic change in populations over successive generations.
- Population
- A group of organisms of the same species living in the same area and capable of interbreeding.
- Species
- A group of similar organisms that can interbreed naturally and produce fertile offspring.
- Adaptation
- An inherited feature that improves survival or reproductive success in a particular environment.
- Speciation
- The formation of new species from existing populations.
- Natural selection
- The process in which individuals with favourable inherited variations leave more offspring than others.
- Lamarckism
- The theory that organisms evolve through use and disuse of organs and inheritance of acquired characteristics.
- Darwinism
- The theory that natural selection acts on inherited variations and causes evolutionary change.
- Neo-Darwinism
- The modern evolutionary theory that combines natural selection with genetics.
- Homologous organs
- Structures with a common evolutionary origin that may perform different functions.
- Analogous organs
- Structures with different evolutionary origins that perform similar functions.
- Convergent evolution
- The independent development of similar features in unrelated organisms facing similar conditions.
- Genetic drift
- A chance change in allele frequency, especially in a small population.
- Mutation
- A change in genetic material that may create a new allele.
- Gene flow
- The movement of alleles from one population to another.
- Heterozygosity
- The condition in which individuals or a population possess different alleles at a gene locus.
- Hardy-Weinberg equilibrium
- A condition in which allele and genotype frequencies remain constant from generation to generation.
- Allele frequency
- The proportion of a particular allele in the gene pool of a population.
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Biology shortcuts
Recognising acellular organisms
If an entity has no cellular organisation and depends on a host for replication, identify it as a virus. Viruses are non-cellular and are not placed among cellular organisms.
- Check whether it has cytoplasm, ribosomes and independent metabolism.
- Viruses contain nucleic acid inside a protein coat but lack cellular structure.
- Example: An infectious particle without cytoplasm or ribosomes is a virus.
- Answer: Non-cellular virus.
This shortcut does not apply to bacteria, which are cellular prokaryotes.
Separating prokaryotic and eukaryotic cells
Use ribosomes as the common feature. Both cell types have ribosomes, but only eukaryotes have a membrane-bound nucleus and membrane-bound organelles.
- Look for the feature present in both groups.
- Ribosomes occur in prokaryotes and eukaryotes for protein synthesis.
- Example: Which structure is common to both? Ribosomes.
- Answer: Ribosomes.
Do not use mitochondria, chloroplasts or a membrane-bound nucleus as common features.
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