Darwin and Mendel

Two halves of one answer

Two people working in the same century produced the two ideas that modern biology rests on, and neither of them knew what the other had. Darwin could see what happens to variation. More organisms are born than can survive, those whose differences suit their surroundings are likelier to live and breed, and so those differences become more common. Mendel could see how characteristics are passed on. Working with pea plants, he showed that they pass in separate units rather than blending together. ⭐ EACH OF THOSE IS HALF OF ONE ANSWER. Darwin could not say where variation came from or why it was not simply averaged away. Mendel was not asking about species changing at all. ⚠️ The halves fit together perfectly, and it took decades before anybody set them side by side. So the question to ask of any piece of this topic is: which half of the problem does it address?

What each one could and could not do

Set the two side by side and the shape of the problem becomes obvious. Each column is strong exactly where the other is silent. Both were working in the nineteenth century, and neither was building on the other.

Order how the two ideas met

Put these stages in the order they happened.

• Two separate pieces of work appear in the same century, one on species and one on pea plants
• The work on inheritance attracts very little attention at the time
• That work is rediscovered by other scientists years later
• The separate units of inheritance are identified with genes
• Genes are shown to be carried on chromosomes, completing the link

The objection that had real force

Besides religious objection, there was a scientific reason the theory was hard to accept at first. What was it?

• If characteristics blended when passed on, any useful new difference would be diluted within a few generations, and Darwin had no answer to that
• There were no fossils of any kind at the time
• He had never travelled or seen the organisms he wrote about
• Nobody had suggested that species change before

Words for this topic

Five terms that let you write about the history as an argument rather than as a story.

Find the sentence that explains the delay

A student writes about why the theory took so long to be accepted. Select the ONE sentence that gives a scientific reason rather than restating the objection or judging people.

• Many people did not want to accept the theory when it was published.
• People at the time were not as clever as scientists today.
• Without any account of how characteristics are passed on, critics could argue that a useful new difference would be diluted away within a few generations, and there was no answer to that until inheritance was understood.
• Some people objected on religious grounds.
• The theory is now accepted by scientists.

Which address inheritance

Three of these five address how characteristics are passed on, rather than what happens to differences once they exist. Choose the three.

• Showing that characteristics pass in separate units rather than blending
• Identifying those units with genes
• Showing that genes are carried on chromosomes
• Observing that more organisms are born than can survive
• Observing that organisms suited to their surroundings are likelier to breed

One question for this whole topic

Questions here ask about contributions, about evidence, or about why acceptance was slow. ⭐ Ask which half of the problem a thing addresses: where variation comes from and how it survives being passed on, or what happens to it afterwards. Then, if the question is about acceptance, give BOTH kinds of reason: the objections people raised, and the genuine gap in the theory itself. ⚠️ An answer that explains the delay only by objection has missed the science, and an answer that treats people of the past as foolish has missed the point entirely.

Match each thing to what it supplied

Two people, two halves, one machine

Leave biology aside. A workshop machine breaks, and two people try to fix it in different rooms, not knowing about each other. The first works out exactly what the machine is doing wrong. Every time it runs, something slips, and she can describe the slip in detail and predict when it will happen. What she cannot find is the part causing it. The second is in the stores, cataloguing a box of odd components nobody has identified. He works out precisely what each one does. He is not thinking about the machine at all; he has never seen it run. ⭐ Between them, the problem is solved. Neither of them can solve it. ⚠️ And here is the part worth sitting with. The delay that follows is not caused by missing knowledge. Everything needed is already written down, in two places, by two people who will never meet. ⭐⭐ When somebody finally reads both notebooks, nothing new is discovered at all. Two things already known are simply put next to each other. That is a surprisingly common shape in the history of science, and it is exactly what happened here.

Complete the history facts

Darwin explained what happens to differences once they exist, through a process called natural _____. What he could not explain was where new differences came from, or why a useful one was not diluted away, because nothing was then known about _____. Mendel, working with pea plants, showed that characteristics are passed on in separate _____ rather than blending together, but his work went largely unnoticed for years. Those units were later identified with _____, which were then shown to be carried on _____. The differences that selection acts on are now known to arise from changes in _____.

Build the evaluation

Complete an answer to a question about why the theory was accepted slowly.

Darwin and Mendel quick-fire

Five quick decisions. Three lives.

Plan an answer about acceptance

A question asks why the theory of evolution by natural selection was not accepted quickly. Work through your plan.

• How should the answer open?
• What must the answer include besides the scientific gap?
• How should it finish?

Explain why two halves were needed

Explain the contributions of Darwin and of Mendel, and why neither explanation was complete on its own.

• Explain what Darwin could account for, using the idea of variation
• Explain what Mendel showed about how characteristics are passed on
• Explain the objection that a useful difference would be diluted away, and why it had force
• Explain how the two halves were eventually joined, naming genes and chromosomes
• Finish by giving both a scientific and a non-scientific reason why acceptance took time