A4.1 Evolution and Speciation Flashcards
What is the evidence for evolution?
The study of fossils, selective breeding of domesticated plants & animals, comparative anatomy of animals, nucleotide & amino acid sequences, & geographical distribution of organisms act as evidence for evolution
What are the main points in Darwin’s theory of evolution by natural selection?
- In nature, there is a tendency to produce more offspring than can be supported by environment. This ‘overproduction’ eventually leads to competition for natural resources e.g. food & water, space… As a result, pop size remains fairly constant.
- Individuals within a pop aren’t identical but exhibit variations in their characteristics.
- Individuals with variations that help them adapt better environment are more likely to survive, reproduce & pass on positive variations to their offspring. Similarly, individuals with less favourable variations are less likely to survive. This ‘selection’ by nature = natural selection.
- Eventually, in a particular environment, individuals with more favourable variations will form a larger proportion of pop.
- Over many generations, natural selection leads to pops adapted for specific environments. Barriers to reproduction or reproductive isolation may lead to formation of a new species.
What are the main points in Lamarck’s theory of evolution?
- Change in environment led to use of certain organs & disuse of others among organisms.
- Organs that were used more would increase in size over lifetime of organisms. Similarly, organs that weren’t used over lifetime of organism would shrink.
- These changes acquired over lifetime of an individual would be passed on to offspring.
What is natural selection?
The process in which diff environments influences survival & reproduction of organisms due to presence of variations or adaptations.
What is evolution? And why is Lamarckism no longer accepted?
It is now known that evolution is a change in inherited characteristics of a pop. This is treason why Lamarckism is no longer accepted, as in most cases, acquired changes don’t affect germ cells (cells that develop into gametes) & hence aren’t passed on from one generation to another.
What is the evidence which shows evolution has occurred?
- Universality of DNA
- Molecular phylogeny
- Selective breeding - explain the mechanism
- Homologous structures - pentadactyl limb
- Convergent evolution
How is universality of DNA evidence for evolution?
In all living organisms, building blocks of life, DNA, RNA & proteins, are universal in nature. All living organisms have same nucleotide bases & amino acids. This indicates a common origin of life.
How is molecular phylogeny evidence for evolution?
One way of proving the common origin of life is comparative analysis of the sequences of the bases in DNA & RNA, & amino acids in proteins to infer evolutionary history. This is known as molecular phylogeny & it can be used to determine how closely two species are related to each other at a molecular level. More similarity in genes & proteins indicates more closely related species. Changes in the genome (& amino acids specified) are due to mutations. Rate of mutation of a gene is fairly constant & leads to diff in DNA, which accumulate over time. So number of diff between sequence of bases of a gene in two species increases with time. This forms basis of molecular clock, which measures time from changes in DNA. E.g. species with fewer diff in their genomes (& amino acids) would have recently split or diverged from each other.
How is selective breeding evidence for evolution?
Unlike natural selection, where organisms better adapted to environment survive, reproduce & pass on traits to their offspring, in selective breeding, humans decide favourable variations. We select organisms with desirable characteristics & use them as next generation of parents. Over generations, humans continually select organisms with desirable characteristics, removing those with less desirable traits, leading to a rapid genetic change in pop. Selective breeding isn’t new to humans. For centuries, we have been breeding plants & animals for genetic characteristics that are beneficial to humans. E.g. farmers often selectively breed cows to increase milk production or hens to increase egg size. Similarly, they may selectively breed crops to increase disease resistance & yield.
Selective breeding thus brings rapid changes in genetic make-up of a pop, eventually leading to evolution.
How did Darwin use selective breeding to prove natural selection?
Darwin extrapolated his understanding of selective breeding to frame his theory of natural selection. He drew parallels between process of selective breeding due to human intervention & natural selection by environment. He stated that while selective breeding was a more rapid process, in both cases, selection of favourable characteristics over generations, led to evolution.
How is homologous structures (pentadactyl limb)
evidence for evolution?
Studies show that body structures of some organisms are fundamentally similar. Homologous structures have same basic structural plan, indicating a common ancestry. E.g. pentadactyl (five-fingered) limb of vertebrates. Although limbs are used for diff functions, there is a striking similarity in their structures & general arrangements of humerus, radius, ulna, carpals & digits. Homologous structures indicate divergent evolution. Homologous structures (or organs) thus act as evidence for evolution as they indicate a common evolutionary organ.
What is divergent evolution?
Divergent evolution occurs when organisms arising from same ancestral species adapt to diff environmental conditions according to pressures of natural selection.
How is convergent evolution evidence for evolution?
If you compared wings of birds & insects, you would note that while they have same function, they are very dissimilar in structure. Analogous structures are body parts that have same or similar function in diff groups of organisms but have diff structures. Analogous structures don’t indicate evolutionary relationships but they are result of a process known as convergent evolution. Convergent evolution occurs when distantly related groups of organisms face similar environmental conditions & adapt in similar ways. Insect wings & bird wings evolved when their diff ancestors independently adapted to a similar mode of life, that is, flight.
What is a species?
When similar organisms can mate with each other & produce viable, fertile offspring, they are considered to be members of same species. In other words, reproductive compatibility defines a species. This holds true for most plants, animals & fungi.
What is speciation?
Speciation = process by which new species arise. It involves splitting of one ancestral species into two or more descendent species which are genetically diff from each other & can no longer interbreed.
What does a population of a species share?
A pop of species shares a gene pool or a collection of variants of genes in species.
What is a gene pool?
A gene pool is the sum of all the alleles of all the genes in a population.
How does speciation effect the gene pool of a population? And what is this lead to?
Speciation results in modifying original gene pool into separate gene pools in a way that interbreeding is prevented, that is, speciation leads to genetic separation. Speciation increases total number of species on Earth. On the other hand, extinction or dying out of species reduces total number of species on Earth.
Does evolutionary changes always lead to speciation?
Note that gradual evolutionary changes may not lead to formation of a new species. There could be evolutionary changes in gene pool prior to speciation. In speciation make-up of gene pool must change sufficiently to define a new species. Changes should be such that it is impossible for members of two or more descendant pop to interbreed.
What is reproductive isolation?
Reproductive isolation refers to barriers that prevent pop of same species from interbreeding and/or producing fertile offspring. Reproductive isolation could result from geographical barriers such as:
- physical barriers like mountain ranges or river
- large distances between pop
- human-made barriers e.g. large roads or dams.
How does reproductive isolation lead to speciation?
- large pop of individuals occupies a habitat.
- formation of a new geographical barrier divides original pop into two smaller pop: A & B.
- geographical barrier prevents members of the two pop from mixing & interbreeding.
- In diff environments, evolutionary forces e.g. natural selection, mutation & genetic drift act independently on gene pools of two pop for many generations.
- Due to action of evolutionary forces, genes that provide better adaptation to environmental challenges are selected over others & transmitted through generations. In other words, natural selection leads to differential reproduction & in a way differential selection.
- Differential selection eventually leads to genetic separation.
- resulting behavioural and/or physiological changes prevent pop from mating.
- Even if barrier is now removed, members of two pop can’t interbreed.
- Two new species A & B are formed.
What is selection differential?
Selection differential = diff between mean of a qualitative character in a whole population & mean of individuals selected to reproduce in next generation. It’s a measure of association between a trait value & its ability to confer reproductive success or increase survival rates.
What is the case study for selection differential?
Chimpanzees & bonobos are African apes that live in close proximity with each other. While chimpanzees are distributed across equatorial Africa, bonobos are restricted to a region south of the Congo River. Chimpanzees are larger, more aggressive & live in male-dominated groups. Bonobos are more slender, more playful & have female-dominated groups.
Chimpanzees & bonobos belonged to a common ancestral species living in rainforests of DRC. Formation of Congo River, deepest river in world, nearly 1.5-2 mn yrs ago, divided ancestral pop into two groups. Group north of river faced intense competition for resources. They had to compete not only with each other, but also with ancestral pop of gorillas. Aggressive tendencies were evolutionarily selected. Over time, this group evolved to the present day chimpanzees. To south of Congo River, where resources were plentiful, apes evolved to become slender & more cooperative, eventually giving rise to the bonobos. Thus, development of a geographical barrier & differential selection over the course of time led to reproductive isolation. This, in turn, led to speciation.
What are the two types of speciation?
- allopatric speciation
- sympatric speciation
