Lecture 7 - Basis of natural selection

evolution definition

evolution = a change in gene frequencies over time

species have changed in a Darwinian sense of "decent with modification" with all species sharing a common ancestor

evolution types

1. mutation - the source of genetic variation

   • homozygous - two strands of DNA, on the DNA there is a copy of a gene and homozygous means the genetic mutation is on both strands of DNA

   • it is recessive so both mutations need to be present for an effect to occur

2. natural selection - the sifting of variation to produce adaptations

3. genetic drift - random sampling of individuals over time

4. founder effects - small group of individuals "bud off" or survive by chance

phylogenetic trees

shows common ancestry links

evolution evidence

1. the fossil record

2. observations of similarities and differences between living species

3. evolution in action - observing changes in real time

the fossil record

• burgess shale - fossil collection from 500 million years ago

  • shale (sedimentary rock) is so fine it preserves soft body parts

• intermediate forms are present continuously in the fossil record

• order of fossils also suggests evolutionary relationships

problem with fossil record

however, some species today appear "unchanged" from when their evolutionary history began

observations of similarities and differences between living species

• analogous

• homologous

• vestigial structures

analogous

• analogous = structure looks similar but have very different evolutionary history

  • different variants of solutions to the same problems

    • for example, the problem of flight - birds, moths, bats, all look the same (wings) but don't share an evolutionary background, they have evolved to fix the same problem

homologous

• homologous = same variants of solutions for the same problem

  • similar evolutionary background but share a common ancestor

    • for example, the five-fingered limb has evolved over bats, moles, dolphins, monkeys, horse, pigs etc

vestigial structures

vestigial structures - where limbs are no longer adaptive, they have been lost but remains can still be seen in the skeleton

• shows evolutionary descent

  • example is the appendix - in earlier times we needed it due to a high cellulose diet, however since we have evolved, we no longer need this and it can be removed safely

phylogenetic approaches

definition = understanding evolution by comparing features of extant species

build phylogeny based on molecular data - similarity in DNA of species

2 changes are more likely to have occurred than 4 changes - more parsimonious

• parsimony - reconstruct phylogenetic history through determining independent changes in the least amount of steps

morphology definition

morphology = studying the form and structure of animals and making inferences of evolution

problems with evolutionary classifications based on external features

1. discriminating features

2. convergent evolution - independent evolution of similar features in species (like dolphin and great white shark)

genetic code

 

the genetic code, also known as the universal homology - the transcription between base triples in the DNA and amino acids in proteins is universal to all life

evolution in action - observing changes in real time

evolution can be produced experimentally - selective breading ie a pea plant, leads to rapid shifts

means there are many types of one species, like dog breeds

generators of variation

1. random mutations

2. recombination’s (via sex)

3. gene flow

random mutations

• born with 300 mutations which at least 1 or 2 are potentially harmful if they are homozygous

• over 10,000 human genes have been described that in their mutated form causes diseases or defects

• variation is the building block for evolution

recombination’s via sex

• all genetic variation owes its origin ultimately to mutation

• however, in the short term, recombination accounts for a great deal of the genetic variation in wild populations

• through the process of gamete production, there is a reconstructor of chromosomes

• most populations are a finite size and random fluctuations in allele frequencies can result in the replacement of alleles (non-adaptive evolution) = genetic drift

gene flow

• transfer of genetic material from one population to another

effective population size

number of adult individuals in the population that actually reproduce

founder effects

• genetic drift = the bottleneck effect

• populations that go through a restricted size are said to have gone through a bottleneck

  • this can happen when new populations are founded

process of natural selection

1. huge numbers of individuals are produced

2. insufficient resources for all, so many die before reproducing

3. individuals vary in attributes

4. individuals with attributes better suited to the environment are less likely to die

5. these individuals will be over-represented in the next generation